May 12, 2017

EQUATORIAL MOUNT after Chris Heapy legacy

WORKSHOP PROJECTS

https://stargazerslounge.com/topic/105383-diy-homemade-telescope-mount-project/

http://astro.neutral.org/homemade-diy-gem-telescope-mount.shtml?utm_source=ForumPost&utm_medium=Forum&utm_campaign=Mount

http://www.mini-lathe.org.uk/making-telescope-worm-wheels-gears-mini-lathe.shtml

http://astro.neutral.org/mount-costs2.htm

http://astro.neutral.org/telescope-mount-photos/

EQUATORIAL MOUNT

http://nsa.kpu-m.ac.jp/gijutu/kousaku/easyweb.easynet.co.uk/chrish/eq.htm

Construction Notes For a German Equatorial Astronomical Telescope Mount

Introduction

This project concerns the construction of an equatorial telescope mounting of the classical German design. This type of mounting is equally suitable for mounting Newtonian, cassegrain/ Schmidt-Cassegrain, and also refractor telescopes, though of course each of these differing optical designs will require a different height tripod for comfortable viewing. Realising I had the tools to make such a mount I tried (and failed) to locate a suitable published design on the 'net. I was not looking for the usual DIY wood and pipe-fittings mount but a professional, high-precision engineered mount, and the only option therefore appeared to be to design it myself. Whilst a large number of home machinists are able to construct such a mount the problem many of us face with designing a complex project like scratch is 'Where to Start?' Hopefully, the description of my prototype together with plans and photographs will encourage those amateur astronomers with a home workshop to have a go at making this mount. I should point out that I'm not an engineer by profession, engineering and pottering around in my workshop is simply one of my interests - albeit one that I've been involved in for 30-odd years.
Design Considerations:
At the outset my mount design had three main priorities; it had to be reasonably easy to make with the standard type of tools found in the home workshop and from easily available materials (i.e., no castings necessary), it needed to be very stable, the common failing of all but the best - and most expensive - mounts that you can buy, and it's drive system should be as accurate as any commercially available drive of equivalent capacity (that's a tough one!). The prototype features many of my own ideas which experience suggested might be worthwhile to include. Some of the design aspects are a little, shall we say, 'unconventional'.
Looking at some commercial mountings I often wonder why they are made the way they are, simple (cheap) design modifications would have improved them considerably. Cheaper imported equatorial mounts (as supplied with 60mm refractors and 4-1/2" reflectors) are usually hopeless in every respect and require no further mention, but even the cheaper end of the 'mid-range' mounts (the Vixen SP/GP for example) suffer from under-sized bearings, and insufficient spacing between bearing pairs to provide the stability which is so essential. Further, the castings tend to be light-weight and the RA/DEC shafts are too thin. The drive gears are frequently far too small at only 2-3 inches diameter to provide accurate tracking, and the whole construction generally exhibits backlash, excessive clearances, instability and insufficient support sub-structures. However, they do sometimes look attractive. Top of the range mounts (like the AP 1200) do not exhibit these defects - but fetch a correspondingly high price. Somewhat lower in the price range - the Losmandy G11 for example - is also a good quality mount, and this home-made design is probably similar in terms of load-carrying capacity. The overall stability of any mount depends on correctly spaced and supported bearings - in fact, at the fundamental level that's really all the telescope mount is - two sets of bearings. My design therefore makes sure that these bearings are both widely spaced with large chunks of metal in the right places to support them.
My philosophy throughout was to reduce machining costs wherever possible (I make my own tools and cutters, and I buy metal off-cuts), to maintain accuracy where it's required, and to use machining short-cuts only if it makes the job easier without compromising function. By making it yourself you can put as much time and effort into individual pieces as you feel like, there's no deadline to beat. This is the principle reason by which you can make something better than a commercial item - you don't have to worry about the labour costs or the time involved. If you make the effort then the end product is (IMHO) a very fine mount that you will be proud to both own and use. It will, with some care in it's machining, out-perform commercial mounts costing thousands of pounds, and you will have the satisfaction of saying to the curious onlooker at starparties 'What make is it? - Oh, I made it myself' (grin!). I would estimate that a payload of something between 50-75lbs (6" refractor/14" SCT/12" Newt) could be carried with sufficient precision and stability (if protected from wind) for astrophotography - perhaps the most demanding of all jobs for an astronomical telescope mount.
Workshop Requirements:
This mount can be constructed in a modest home workshop equipped with a reasonably good quality lathe (mine is a Myford S7, 3-1/2" CH x 18" between centers), a drilling machine (1/2" capacity), and preferably (but not essentially) a basic vertical milling machine of the mill-drill type. All the necessary milling could be done in the lather with a vertical slide attachment, the only drawback being the increased set-up time to convert it to perform each operation. A dividing head or rotary table will also be needed should you elect to cut your own gears, although these could be purchased instead, and the rotary table will also make the job of machining some other main body parts much easier - parts such as the curved edge on the ALT plate. You will also need the usual selection of hand tools such as drills, files, a few reamers, taps and dies etc., and also measuring equipment (a micrometer, 6" vernier caliper and 12" rule). I guess if you have a workshop you will already own most if not all of these items. Note that whilst most of the given dimensions are Imperial (or decimal fractions thereof) the screws I use are Metric, this is because it's now almost impossible to obtain the range of Imperial threaded screws in stainless steel - in the UK anyway. If you have a similarly equipped home workshop then making this mount should not present any insurmountable (!) problems.
A quick run-down of the major design features:

The mount is designed to be portable, the DEC shaft extension is easily removed, and the head separates from the tripod. I don't know the final gross weight of the head yet (it's approx 75lbs at the moment - a bit of a grunt to move it in one piece but not too bad).
All parts are made either from aluminium alloy or stainless steel (free-machining variety for preference) with the odd bits of bronze and carbon steel. All bolts in the final version are metric stainless steel hex-socket (Allen) bolts, so the mount should be fairly weather resistant.
If you wish to add a hard black anodised finish to the alloy parts instructions can be found here. The process is simple and will add a touch of professionalism to the finished job.
I've included a substantial range of latitude adjustment (0 - 58 degrees approximately, though the RA worm mounting may need repositioning should you need to set it below 25 degrees).
Both the ALT and AZ adjustments for polar aligning the mount are made through worm drives offering backlash-free, precise and predictable movements. Anyone who's tried using conventional push-pull tangent adjusters or (worse) a commercial equatorial wedge for precise polar alignment will instantly appreciate the value of this feature.
The polar axis is supported using twin-opposed angular contact bearings at the bottom end (to take the weight) and a precision-bored bronze plain bearing at the top end. The latter is of adequate length to retain precise alignment and resist wear. This arrangement has much in common with the spindle design of many older precision lathes. I may replace the front bearing with another twin contact bearing in the drawings, it offering a heavier load carrying capacity. The choice is yours. The DEC bearings are a pair of plain ball bearings in the prototype, though rollers could be substituted for these.
The main telescope drives are through conventional worm gears mounted on both declination (DEC) and Polar (RA) shafts. Both gears are 360-tooth 6.4" diameter with matching (lapped) worms, an arrangement that lends itself well to computerised pointing. A larger RA gear could be substituted with a few design modifications (see notes on drive accuracy 2.1.1 for implications of using gears of different sizes).
The RA and DEC drive worms can be quickly disengaged for protection of the gears whilst the head is transported.
Adjustable slip-clutches with quick-locking knobs are used on both axes, these allow permanent engagement of the worm drives.
Both R.A. and DEC. shafts are 1" precision ground steel, the large baseline between each pair of bearings and minimal overhang is what ensures good stability. I see no need to increase the diameter of these axles at this point, I had thought of using 1-1/4" or even 1-1/2" but this would only add weight with little benefit in terms of additional stability. Larger support structures would be required for shafts of larger cross-section than 1".
This mount is a perfect match for the 'Stasis' and 'Stubby' tripods (described in the Telescopes section - here is a direct link to the tripod article.
I haven't finalised the motor drive details yet. For computerised pointing I will need to install a pair of stepper motors, but in the mean time I am using DC servo motors with electronics removed from a scrapped medical infusion pump.

1. The body and bearing supports

1.1 Baseplate: The mount baseplate was turned from an offcut of 6-1/2" aluminium alloy barstock, a recess being machined on the underside to fit onto the 6" diameter circular 'Stasis' and 'Stubby' tripod tops, upon which it can rotate for alignment purposes. A 1" diameter bronze (or stainless steel) pivot extends below the baseplate and locates in a central hole of the tripod top. The pivot is threaded internally to accept the tripod center tensioning bar (which screws into it from underneath). Fully tightening the tensioning bar using the hand-knob clamps the mount onto the tripod top, whilst only partially tightening it allows the head to rotate under control of the AZ worm drive. The baseplate has a cutaway on one side (both sides on the prototype) to accommodate mounting the azimuth worm adjuster. Machining the baseplate is a simple turning job, but just make sure you machine the recess on the underside and also the bore for the pivot at the same setup to ensure the two are concentric. Skim the top and outside edge afterwards.
1.2 Box section main body:
The main body parts are cut from 1/2" and 1" thick aluminium plate and bolted together. A bandsaw is a great time-saver when cutting out these parts but an ordinary hacksaw will tear through the 1/2" alloy quick enough if you use plenty of cutting oil on the blade. This box design is quite simple but very strong when the two clamp knobs are tightened up. The only difficulty making this part might be in machining the semi-circular altitude adjusting plate and the matching curved spacer that separates the two side-plates, for these a rotary table for use on the milling machine (or possibly lathe) is the best option.
When making the side plates, mark the outline and hole positions on just one piece then cut both out a little over-size, drill holes through both and bolt them together, then finish machining them as a pair. This will ensure that everything that is symmetrical on the two plate (i.e., screw holes, pivot holes, plate edges, slots etc) end up true, square and parallel. I used my mill-drill for this job, a flycutter for the edges and a 5/16" endmill for the curved slot. Using a 5/16" endmill almost inevitably results in the slot being cut a few thou wider than the nominal 5/16", which is OK because a little clearance here is needed. To cut the slot (which locates the 5/16" guide pin) it's best to plan in advance based on the tools you have. In my case I used a rotary table which is equipped with a central 5/16" plain-shank bolt for holding workpieces concentric to the table center. Therefore, I first drilled and reamed the adjuster plate pivot holes 5/16" diameter and used this as a register to mount the workpiece on the table, the curved slot was machined, afterwards the pivot hole was opened up to 5/8" diameter. On the lathe (perhaps using a milling spindle) you could do an even better job by clamping the two side plates to the faceplate mounted on parallels (to give clearance for the cutter which needs to go right through), and bore the adjuster plate pivot hole 5/8" diameter and then also cut the slot at one setting. This will ensure the curved slot and pivot hole are accurately concentric and the parts will therefore slide without binding.
A 1" thick spacer, again of plate aluminium alloy, is fitted between the side plates and forms the channel in which the latitude adjusting plate slides. This spacer also carries the latitude adjustment worm should you decide to include it. On my prototype (and in the plans) the top surface of the spacer(s) is shown as being curved to match the curvature of the altitude plate, 1/16" clearance being allowed between the two. This gives the maximum support possible (that is, surface area of contact) for the side-plates but is perhaps otherwise an unnecessary elaboration - I guess you could just as easily cut it straight thus making a sort of truncated triangle. If you have a rotary table of course it's just as easy to make it curved, and because it can be machined at the same time as the latitude plate (which carries the Polar shaft sub-assembly) no extra effort is invloved.
Drilling the hole for the worm shaft which passes right through this angular workpiece may be a problem if you have little machining experience, this is because of the acute angle the hole makes with the front face. In such circumstances it is difficult to get the drill bit started in the right place. The answer is to use a stout center drill first, which will not deflect, and then follow this up with a slot-drill (a 2-flute milling cutter) to open the hole out sufficiently to get the point of the drill in. The normal drill will then go through straight without wandering off line. Be careful when it breaks through the opposite side as it might grab - the work needs to be securely held. Note that the 1" spacer(s) are machined to outline as a single piece, then drilled, tapped and bolted whilst held in place between the two side plates. It is then removed and cut into the two pieces as shown in the drawings (this gap provides room for the actual worm thread) and then it's re-assembled once more. The aformentioned 3/8" hole that carries the ALT worm shaft is then bored right through the longer part and the drill continued on to mark the position for the recess on the smaller part (this accommodates the far end of the ALT worm shaft). Before removing the part from the drilling machine follow up the 3/8" drill with a 1/2" slot-drill (or endmill) to form the short counter-bore in which a bronze bush sits (again. refer to the drawing).
The smaller of the two spacer pieces is then finish machined separately, it is drilled through 5mm and tapped 6mm for the backlash adjustment screw. This short spacer is designed to pivot under control of an 8mm setscrew threaded up through the baseplate below it (the two side screws need to be loosened to make this adjustment, which should be very infrequent). When tightened, the 8mm setscrew pushes the worm section into close engagement with the worm segment in the periphery of the altitude plate. The amount of movement allowable is limited to about 1/16" but is sufficient for the purpose. To provide for more movement it would have been necessary to make the bronze bush that supports the hand-knob end of the worm shaft adjustable, the prototype used a bush with an eccentric bore which could be turned (lifting that end of the shaft), however, in practice this wasn't required. All that is needed is to drill the 5/16" hole through the bush rather than ream it, this will make the bore slightly oversize and thus provide enough play for it not to bind with the shaft slightly off-axis. This is not particularly good engineering practice, but a little play here (and it will be very little) will have no detrimental effect on the way the ALT worm operates.
Because it pivots, the lower and rear edges of this part have to be machined away a little so they don't foul the baseplate or protrude from the rear. Look at the drawing to gain an idea of how this works - the exact shape is not critical of course, it's just to provide sufficient clearance.
The latitude plate pivot is turned from 5/8" diameter stainless steel, nutted on one end, and with a large hand-knob on the other to lock it in position. The plain shank of a smaller diameter (5/16") nutted guide-pin runs in the curved slots in the two side-plates, this provides additional security and locks the assembly immovably together when tightened. The exact positioning of this 5/16" hole through the semi-circular ALT plate may be anywhere along a line which is the correct radius from the center of the 5/8" pivot hole, and this will determine the limits of the latitude that can be set. In the prototype I drilled several of these holes which offered the maximum rage of adjustment possible.
1.3 Latitude adjusting plate (and worm drive):
From experience with the Meade SuperWedge I know the importance of having easy and precise adjustments in ALT and AZ for polar aligning purposes, and for this reason I have used worm drives for both. There are few things more annoying when polar aligning than having unpredictable movements due to backlash and excessive clearances when you're trying to align within a few arcseconds accuracy, so it's well worth the effort of adding this feature. For a mount on a permanent pier this is perhaps unnecessary as precise alignment is done only the once and a simple jacking screw could be used, but for a portable mount which needs aligning each time it's set up you'll find it very convenient.
The drawings show the details of the two mount-alignment worm drives, it is actually only necessary to provide for about 10 degrees total movement as the worm drive is only used for final fine adjustments, although the prototype has a greater range (in fact, on 'Stubby' the Azimuth worm was machined all around the cirumferance of the tripod top). It's easy to machine the short segments of worm rack using a simple cutter made from silver steel (drill-rod), one advantage of these cutters being that you can make the shank long enough to provide plenty of clearance for the drill chuck or whatever is used to hold it - I used a milling chuck. These cutters are easy and quick to make - although you do need a rotary table or other method of indexing the work. The dimensions of the one used for this job (and also the altitude adjusting plate) are also given in the drawings. If you have no experience making, hardening and tempering cutters from carbon tool-steel then take a look in the 'Techniques' section accessible from the main menu, there you'll find basic instructions. For these cutters it's necessary to turn a disk so that it's edge is the same profile as the gear teeth, then use a small endmill, approximately 3/16" diameter to make the cutting edges. I've found it useful to make the shank separately, the cutters having a hole through the middle and are held onto the shank with a nut. Great precision is not required for these worms (in complete contrast to the main drive worm gears). Adjustment is provided for gear/worm engagement and also to remove end-float on both ALT and AZ worms.
The 1" thick semi-circular latitude adjusting plate could be sawn and filed to shape (rather you than me...) though a rotary table would make the job far easier - and this would also facilitate engraving the latitude scale as it can be done at the same time. Alternatively, if you have cross-slide mounted milling spindle you could do this job in the lathe with the workpiece bolted onto the faceplate. The latitude worm and worm-gear requires a full turn of the hand-knob to move the head 2 degrees in elevation. I'd thought of using a higher TPI worm for finer adjustment but as this one has to jack up the head + payload (potentially 150lbs mass) I thought I'd err on the safe side and use a 10 tpi Acme worm. The scope should be in approximate balance before making any such adjustments to avoid excessive tooth pressures, the worm segment is cut into aluminium and therefore it will not withstand lifting heavy weights without risking rapid wear. The segment of worm gear machined into the semi-circular periphery of the plate is achieved with the 5/8" diameter cutter made to the dimensions on the drawings (sheet 1) and then hardened and tempered. The picture below shows the machining setup for cutting the worm teeth. The worm pitch-angle is accommodated by tilting the rotary table using packing under one side of it's base. Again, this does not need to be precise as the worm will soon bed-in through use. If you are a beginner, don't be intimidated by the prospect of machining this worm gearing as the job is quite straightforward, and having done it once you will find many other applications for the technique in other projects.

	Cutting a worm segment for the latitude adjustment control. The cutter is home-made.
	This is a different home-made cutter having a separate head which screws onto a mandrel (shown screwed on part way), cutter heads can be interchanged and only a small amount of the expensive tool-steel is used.
	The same cutter head shown above. You can see better how the rake angles are produced using (in this case) a 3/16" endmill. Off to the right is a second cutter which fits the same mandrel.

Other pictures below show the arrangement for mounting the worm between the two main body side-plates. End-float is taken care of by adjusting tension on a round-nosed grubscrew which bears on a countersink in the end of the worm shaft (upper picture, with the hex key in place), a second screw backs onto it and so locks it in place. To reduce wear, a bronze thrust washer is used between the end of the worm thread section and the larger of the two 1" spacing pieces. At the back you can see the underside of the latitude adjusting plate and the worm segment. The middle picture shows the other end of the worm where the hand-knob is located. The smaller 1" spacer block (the one holding the grubscrew) is allowed to pivot on the two screws that secure it between the side plates, and an 8 mm grubscrew threaded through the underside of the base is used to jack up the front of this block thus forcing the worm into contact with the gear segment. It's perhaps easier to look at the drawings to understand what's going on! On the plate you can also see engravings which are at 1 degree intervals, these indicate the latitude settings. In the lower photo you can see the fiducial line on a small plate set to indicate the angle of the polar axis relative to the base. The plate is movable so it can be set to read correctly, then it's locked in place with two small screws. To calibrate it, I set the mount up on a surface plate and adjusted the angle of the Polar axis to 50 degrees using a bevel gauge (any flat surface will do if you don't have a genuine surface plate - a formica faced worktop is usually reasonably flat, and you can use an ordinary protractor in place of the bevel gauge). I then loosened the two screws and adjusted the plate to read 50 degrees on the scale. Before final assembly the gear housing is packed with grease and it should then require no further maintenance.

	The altitude worm drive, the plate at the rear has the worm segment driven by the worm to be seen between the main body side-plates.
	Another view of the worm drive, the hand-knob is not attached in this photo.
	Latitude scale (set at 53 degrees), adjustment hand-knob at lower left.

1.4 Azimuth Adjustment Mechanism (worm drive)
The mechanical arrangement for the AZ worm drive adjuster rather depends upon the tripod or pier-top used to support the mount. For the prototypes, the 'Stasis' and 'Stubby' tripod tops are simple 6" diameter disks of aluminium alloy. Because the tripod and mount are expected to be portable they need to break down to smaller parts for transport, so it's necessary to arrange for the worm drive to be disconnected quickly and easily. A hinged bracket has been used which is bolted permanently to the main body assembly and this carries the worm. The worm gear segment is cut directly into the periphery of the tripod top using much the same method as for the latitude plate. A clamp knob then controls the engagement of worm and gear segment, releasing this knob allows the worm to swing clear so that the mount can be removed from the tripod.

	Azimuth worm adjuster (for polar alignment), fitted to mount. Note the worm segment machined into the tripod top.
	Azimuth worm adjuster; shown assembled.
	Azimuth worm adjuster; component parts.

The worm is 5/8" diameter, 12 tpi (details of thread form given in the drawings), and like the ALT worm is best cut from stainless steel or brass so it won't rust. For the mating gear segment I guess you can get away with using the same cutter that you used for the altitude plate, the differences in cutter dimensions being relatively small and modifications can be made to the worm when machining it (it would be necessary to make the thread-form narrower). The 12 tpi provides 180 teeth/rev so, as with the altitude adjuster, a full turn of the knob gives 360/180 = two degrees movement.
Rather than depending on using the exact dimensions for the bracket shown in the drawings it's perhaps easier to mark off from the job in hand. That's how I made mine, you can use the given dimensions as a guide to overall sizes. So make the worm bracket first, then the fixed hinge part and assemble the pair together. The hinged bracket needs to position the worm correctly to fully engage the gear segment, so offer the worm+bracket up by hand and it will be clear where the screw holes need to go (pre-drill the bracket for the two 5mm mounting screws and mark the position for the threaded holes on the main body). There are two springs between the worm carrier and the part of the bracket bolted to the base, these assist the worm to swing clear.
1.5 Polar Axis Plate, Axle and Bearings:
or the prototype I used a length of 1" precision-ground steel for both RA and DEC axles. If you can get it, stainless steel would be better, though precision ground is tricky to get hold of. Ordinary 1" diameter drawn bar is not really suitable as it's neither straight nor round - and it's likely to be 0.001"-0.002" out on diameter too. It's up to you to choose the exact size of axle you use, I used 1" PGMS because I had this size in stock and it was also compatible with the bearings I had. If your lathe is up to it, you could use a larger diameter and turn it down to suit. Of principle concern is that the axles *fit* the bearings with no slop whatsoever. On the lower end of my RA shaft there is a plain-turned portion to fit the bearing inner race (having a 3/4" bore) and a threaded portion on the very end for the pre-load locking collars. The top end of the shaft is also threaded for the flange used to mate the RA shaft and DEC shaft assemblies.
Bearing Choice:
There are several different types of bearing that could be used to support the axles, and I gave it some thought as to which arrangement I would use. The degrees of freedom to control are: End-float, lateral play, and running friction. Ideally, the bearings should be adjustable so that each of these can be reduced to an acceptable minimum. The alternative bearings I considered are listed below:

Twin plain bronze bearings (or 1" 'Oilite' bushes): Properly executed these offer very good accuracy, but they will require careful lubrication and maintenance to avoid premature wear (unless 'Oilite' pre-lubed are used). Load-carrying capacity is likely a little less than ball bearings, friction will be a bit higher, and separate ball (or roller) thrust washers should be used to control end-float.
Tapered shaft with tapered cone bronze bearings: excellent accuracy, built-in adjustment for fit and control of end-float, difficult for the amateur to machine within the necessary tolerances.
Standard ball bearings: Also a good choice, offering very low friction. The fairly large bearings used here for the DEC shaft can be pre-loaded axially to remove end-float without causing undue friction or otherwise stressing the bearings. Accurate bearing housings will need to be made.
Plain roller bearings: Have very good resistance to lateral shear but zero resistance to end-float, so will require separate thrust washers. They will need very accurately bored housings which are dead parallel or they will bind. Caged rollers with an inner race should be used otherwise the polar axle would need to be hardened and ground.
Taper rollers: At first sight might be considered the ideal answer, but in fact they exhibit high friction when all play is adjusted out, and are difficult to adjust with the required precision anyway. These bearings are usually setup with a significant clearance which is *not* what you need in this application.
Twin-opposed angular contact ball races: Now these *are* the things to use, they have excellent resistance to both lateral shear loading and also are adjustable for control of end-float. Being ball bearings, they also have low friction characteristics. They do require bearing housings somewhat larger than other types.

Any combination of the above would actually work of course, but given a free hand (which we have) it's as well to choose the best option commensurate with costs - angular contact bearings are not cheap. My solution was to use a twin-opposed angular contact bearing at the lower end of the Polar axis, this also controlled end-float in the RA shaft with the use of threaded locking collars. At the top end of the RA shaft I used a plain precision-bored bronze bearing. That exact combination is used for the spindle of the Myford S7 range of lathes (the Super 7's having bronze cone bearings). If you think about it, the mechanical requirements of a lathe spindle are very similar to the demands imposed upon the Polar axis, so it comes as no surprise that this arrangement turns out to be eminently suitable. If you are in any doubt as to whether you can successfully parallel bore the bearing housings then there may be some merit in using a twin-row, self-aligning ball-bearing as the upper RA bearing, this will accommodate a small degree of mis-alignment yet still produce a free-turning shaft.

(Left)A twin-opposed angular contact ball-bearing. These are ideal for the Polar axis.
(Right) A twin-row self-aligning bearing.

The demands on the DEC axis are arguably less, this axis is not required to continuously rotate at precise sidereal rate, it either remains locked or the DEC motor is used simply to correct tracking errors when taking long exposure photographs. This changes if the mount is computerised for all-sky pointing, both drives then need encoders and are required to point accurately. The prototype calls for a pair of plain ball bearings for the DEC shaft, and these are pre-loaded axially to control end-float. The telescope weight can pull this axle in either direction depending upon it's orientation. In practice I've not found a problem with this bearing arrangement, there's no slop or binding, but I confess I've yet to fully load the mount to it's weight limit (though it will be when I put my 40Kg 12" Newt on it!)
Construction of the Polar axle bearing plate is mainly concerned with boring the two bearing housings to ensure the assembly falls within tolerances and are parallel. Perhaps the best way of achieving this is to clamp the plate together with assembled bearing housings (just the outside of them machined to form), onto the lathe cross-slide and use a between-centers boring bar. This method will assure parallel bores which are also square to base plate. In addition, because the bore through the lower housing was larger to accommodate the angular contact bearing, this end is placed nearer the headstock so that counterboring could be done using a boring head mounted in the lathe spindle bore. One good feature of flat alloy plate is that it's internal stressed tend to be very low, so it stays flat after chunks have been machined out of it. This is not the case with bright mild steel flats which readily warp out of shape as internal stresses are released. I offer this as a warning should you be contemplating substituting steel for the alloy. You *might* get away with annealing the steel barstock first but I wouldn't depend on it. The same warning applies to using a welded construction in place of the bolts, unless you are very skilled welding *will* cause warping.
When the bores are complete the outer faces of the bearing shells can be skimmed at the same time using a fly-cutter held in the lathe chuck to ensure they are square to the base and flat.
The RA axis lock is essentially the same in outline as one of the bearing housings, but split horizontally with a hinge on one side and a locking knob on the other. This part should be bored out at the same setup used for the bearings. Perhaps the easiest way is to machine to outline (make sure the base is flat as this is the bolting face) and then machine the bore about 1/4" undersize (i.e., 3/4" diameter). Split the housing with a slitting saw (which saves having to clean the cut surfaces up), or a hacksaw and then trim the edges flat. Make the hinge (note that the 'top' half will need to be rounded off near the hinge to allow it to pivot), and follow this by drilling and tapping for the lock knob. The part can then be bolted into position on the polar axis plate and bored out together with the two bearing housings. Afterwards, a 0.01" skim off one side of the split edge will allow it close that little bit more and thus grip onto the RA shaft - only light pressure is needed to clamp the shaft immovably.
1.6 Declination Axis Plate and Bearings
The DEC plate assembly and bearing housings are very similar to the RA assembly, consisting of a baseplate, two bearing housings and the axis lock. Machining methods are therefore the same. The one critical thing you must get right is that the DEC plate (and bearings of course) *must* result in the DEC shaft being orthogonal (exactly 90 degrees) to the Polar axis. To this end, I used a large diameter flange to join the two sub-assemblies together held by 6 radially spaced bolts. I strongly advise you to bolt the plate onto the lathe cross-slide and use a between-centers boring bar for machining the DEC bearing housings, this ensures the DEC axis is parallel to the baseplate - something that is not so critical for the Polar axis. For attaching to the polar axis the flange has a threaded portion, and also a plain-bored portion which acts as the register. ALL of these threads must be screw-cut on a lathe and not die-cut. It is the plain-bored portion that assures good axial alignment of the flange with the polar axle, all the threaded part really does is hold the two parts together, it adds nothing to alignment accuracy so the thread should not be a tight fit. The plain-bored portion on the other hand should be a very close fit indeed, the 1" diameter shaft should be a firm push-fit in the bore (which needs to be around 0.0005-0.001" oversize). Make certain there are no burrs on the polar shaft caused by the threading operation when test fitting, and make sure the alloy flange is COOL too (Pushing the end of the cold Polar shaft into the bore of the warm flange just after it's been bored will likely cause it to shrink-fit, and you'll then spend an interesting hour or so trying to figure how to get it out again without damaging it!) It would be a wise precaution to setup the DEC shaft plus attached flange in the lathe, and using a fixed steady for support just skim the end to make absolutely certain it's face is square.
The DEC shaft has a removable extension piece made from 1" stainless steel to carry the counter-weight(s), this makes it a bit easier to transport the mount. Both the Polar and DEC shafts will be encased in shrouds eventually which will protect them from corrosion, but the DEC extension will be exposed to the elements quite a lot so needs to be stainless. The main DEC worm drive gear and it's slip-clutch are mounted on a stub-axle which screws onto the lower end of the DEC shaft. This stub axle is bored through to allow the DEC shaft extension to be attached, and it also serves the function of pre-loading the DEC ball-bearings (using a spacer). It is imperative that all these threads be carefully screw-cut to ensure they are concentric, and that the stub axle threads in particular are a good close fit. The DEC worm gear is mounted on this stub axle so it must run concentrically.
1.7 Telescope Mounting Plate
On my prototype I have utilised a length of dovetail section which accepts the standard LX200 accessories I make. This was mainly for convenience, my 90mm Vixen refractor simply slides into place as it does piggyback on the LX200, and I have ample stock of various sized rings and camera adapter plates.
1.8 Telescope Mounting rings
My 12" Newtonian uses the original rings supplied by Orion Optics (UK) attached to two dovetail mounting blocks. Not ideal but they work OK. I have plans to make rotating rings for the Newtonian so that the eyepiece can easily be placed in a comfortable viewing position. Conventional rotating rings are bulky and heavy, but I have a prototype construction which is neither - and is also easy to make.

2. Making The Telescope Drives

2.1 R.A. and DEC Worm Drives: Because machining worms and worm-wheels have wider applications other than for telescope drives, the machining technique is described under a separate title within the ENGINEERING PROJECTS section. A direct link to it is Here.
Amateur astrophotographers will be only too aware of the need for an accurate telescope drive which exhibits low periodic error, taking good images through the telescope is all but impossible without one. If you do not want to make your own worm gears you can certainly buy them purpose-made for the job. Byers gears have a reputation for fine accurate gears and you would need one of about 6" diameter (minimum) to do the mount justice. The DIY gear is about this size and one of the advantages of making it yourself is that, once the auxiliary jig and cutting tools are made, you can make a second large gear for the DEC axis for little additional effort or cost - something of a luxury if you have to buy them.
To track the stars across the heavens the RA (Polar) axis needs to turn at exactly sidereal rate, equivalent to about 1 turn in 23hrs 56mins. This requires high-ratio gearing which cannot be provided by the main worm gear alone, so intermediate gearing is usually required. The DIY gear has 360 teeth so the worm must complete one revolution every 239.333 seconds for the correct speed. My current thoughts on options for attaining this were:

Use an intermediate reduction worm drive (say, 180-tooth), this would need a DC motor turning at 1 rev /1.32963 secs.
Use the same intermediate worm plus a 60 rev/min synchronous motor running at 45.125Hz driven by variable frequency oscillator (VFO).
Drive the worm with a stepper motor, either direct or through the intermediate 180 tooth worm.
Substitute a spur gear train for the intermediate worm (like the GP mounts, but in this case it would need to be a lengthy compound train to achieve the necessary high ratio).

Of these options I don't really want to use spur gears at all, they are bound to introduce backlash (maybe a lot of it in the case of the long compound train). The AC synchronous motor is a good option from an accuracy viewpoint, and speed is readily controlled through an R-C 555 timer circuit or (better) a crystal oscillator/pulse-divider circuit. The stepper motor option requires a more complex driver circuit, but (in theory) it could drive the primary worm direct at the slow speed necessary. However, direct drive would be jerky (called 'cogging') even if used in half-step mode so some sort of intermediate reduction would still be necessary for a smooth drive. There is a technical solution to this problem in the form of a specialised controller that 'micro-steps' the motor, the current sequence to the windings is not sent as 'on-off' square-wave pulses but as a moving sine waveform, in this way the discrete 'steps' are eliminated. Designing such a controler is beyond me. At the moment I have on the bench some Swiss DC geared motors which can be made to run accurately at the required rate (45 rev/min) with speed finely adjustable using a simple potentiometer, so this is what I'll try first. The intermediate worm will still be required (machining details of this are given below), though I may have to use a pair of spurs to off-set the secondary worm to get working clearance.
addendum: to test the drive gears I have since utilised the original electronics from the scrap infusion pump the DC geared motors came out of. The speed is very well regulated thanks to the sophisticated feedback circuits but there is no control for guiding/centering.
2.1.1 A note on worm gear accuracy
For a worm gear of 6.5" diameter, movements (measured at the pitch circle of the gear - i.e., along the circumference) of the following distances translate to movements in angular pointing:

Gear Diameter (inches):	6	8	12
Gear Circumference:	18.84956	25.13274	37.69911
1 Degree:	0.05236	0.06981	0.10472
1 arc-minute:	0.00087	0.00116	0.00175
1 arc-second:	0.000015	0.000019	0.000029
Accuracy for +/- 5 arc-sec	0.000073	0.000097	0.000145

what these figures tell is that a pitch error of just 0.001" (one thousandth of an inch) is equivalent to a pointing inaccuracy of a whopping 57 arc-seconds - or about 1 arc-minute! Put another way, to get +/- 5 arc second accuracy from the drive mechanics requires the tooth pitch of the worm gears to be accurate to about 0.0001", or one ten-thousandth of an inch, and that doesn't account for any inaccuracies in the worm itself. This is a degree of accuracy that falls within the realms of lapping and not machining (cutting) metal. No gear cutting process will produce anywhere near the required accuracy, only lapping (fine grinding) gets close. Even should the desired accuracy be attained the accuracy of the resultant gear would be fragile indeed, wear or physical damage to the extent of just 1/10,000" on any particular tooth face will cause a tracking deviation of 5 arc-seconds.
This is the reason that large diameter gears are normally to be preferred - the larger the diameter then the larger the acceptable pitch error. Despite this daunting target for our small 6.5" gears, with careful work and using the gear-hobbing technique followed by lapping it is possible to approach this degree of accuacy - at least initially. It should also be obvious that some sort of electronic motor control (PEC) is highly desirable to compensate for minor drive errors.
Next, lets consider partial worm/worm-gear disengagement. The included angle of the worm and gear teeth is 20 deg, from basic trig we find that if the worm separates from the gear by 1 thou (0.001"), and if the tooth faces remain in contact this will amount to a 0.36 thou (TAN(20)x1) rotational movement of worm gear, which in turn is equivalent to around 20 arc-seconds angular deflection. For this reason springs are used to ensure that the worm is held in full contact with the worm gear.
Minor eccentricity of the main worm gear is of less concern where the worm is allowed to float - as it is in the design presented here. If the worm were fixed rather than floating then the amount of eccentricity would induce an error of the same extent as described for worm/worm-gear disengagement (say for e.g., 10 thou disengagement = 3.5 arc-min) . However, the difference in this case is that the total amplitude of the error would be distributed over one full revolution of the gear - 24hrs - so it's effect is negligible.
Eccentricity of the worm is of more concern because the frequency of error would be less, for our 360-tooth gear it would be a sinusidal error with a period of about 4 minutes. Certain machining techniques will reduce the possibility of generating such eccentricity in the worm, namely, completing all machining operations on the worm (thread, external diameter and bearing surfaces) at one setup, and if these instructions are followed there will be no problem. In any case, the floating worm carrier will prevent disengagement errors.
If there are any intermediate spur gears connecting the motor to the worm then these too are a potential source of periodic errors. With a compound train of spur gears the further back towards the motor the error lies then the higher the frequency will be (and if errors exist in more than one gear it may be a complex form rather than a simple sine wave). Fortunately however, the amplitude of the error will be also be divided by the ratio of the gears. Good quality cut brass gears with an involute tooth form are quite adequate.
Apart from the systematic errors (or 'Periodic Errors') there are the inevitable random errors induced by machining defects. By using the 'hobbing' process to make the gears, followed by lapping the worm and gear together over a period of hours, this goes a long way towards eliminating random errors. This process reduces the tooth-tooth variation which is present in any 'cut' gear made on a dividing head, and also smoothes out any local surface irregularities. Observation of the performance of the prototype gears shown here suggest the mechanical PE is exceptionally low, the only negative aspect is that the worm and worm-gear are such a perfect match that friction between the two can be very high without adequate lubrication.
2.2 Slip-clutches
Both main worm gears benefit greatly from having slip-clutches together with a locking mechanism. This feature enables the drive to remain engaged whilst the telescope is slewed manually, and it also protects the worm gear. The design I've used sandwiches the main worm gear between a 'fixed' friction plate (that is, fixed to the shaft) and a spring-loaded 'pressure plate'. The pressure plate is in two parts, one (the backing plate having 3 dowel pins) is fixed to the shaft whilst the other slides on the three pins with spring pressure acting against a backing plate to keep it in close contact with the worm gear. Care needs to be taken in machining these parts, particularly in ensuring that the plate bores are exactly square to their faces, and remain so when the clutch components are fixed to the shafts. This is because the friction plate also serves to maintain alignment of the main worm gear with the shaft. It might be a good idea to clamp the finished plate(s) onto the polar axis and mount this in lathe complete, and then give it's face a skim to true it up. Both the friction and pressure plates are 4-1/2" diameter machined aluminium, faced with 1/16" hard brass sheet epoxied in place. I use heavy oil between plates and worm gear to prevent any possibility of the damaging the aluminium surface of the gear wheel, there's still plenty of friction available from the large surface area to prevent the scope slipping. The amount of friction can be quickly adjusted using three screws which incorporate spring-loaded plungers. In normal use the scope can be manually slewed around whilst the drive remains running. A seperate locking screw is added to each clutch to instantly lock the clutch and gear together, thus guarding against any possibility of slippage during a long exposure photograph. This latter feature should be used with caution though - a sudden shock (as in for example, catching your head on the end of the tube) would be transmitted directly to the worm gear. A really severe jolt with the clutch locked would cause the worm to jump out of engagement and slip across the teeth of the worm-gear - not something you want to happen! It's preferable to get your telescope in good balance and rely on the slip-clutch for protection.

	Details of the RA slip-clutch.
	The DEC gear and slip-clutch.
	The DEC gear and slip-clutch sub-assembly, dismounted.
	The mount is definitely gaining in weight....

The DEC worm gear, pressure plate (and backing plate) and the friction plate are all mounted onto a hollow stub axle which screws onto the end of the DEC shaft. The DEC shaft extention (upon which the counterweights are mounted) passes through the center of this stub axle and screws directly into the end of the DEC shaft, which is threaded internally for this purpose. The external threads on the end of the DEC shaft, and the internal thread in the stub axle need to be high precision and so are carefully screwcut in the lathe until they are a 'good' fit without being too tight. The stub axle screws on almost all the way, but there is adjustment here for aligning the worm and worm gear. Once it is aligned the DEC worm gear sub-assembly is fixed in place with a single 8mm set screw acting on a 1/4" copper slug.
2.2.1 Machining Notes:
The drawings show details of the DEC slip-clutch assembly, but in practice the following instructions apply for this and other jobs where a similar clamp onto a threaded shaft is needed. Drill a radial hole in the friction plate right through 15/64" and then ream 1/4". Follow this with a 7mm drill part way (about 3/16" longer than the length of the setscrew) and tap 8mm fine. Put the reamer through again to clean any burrs off. You need a short length of 1/4" copper rod, heat it to cherry red and quench to anneal it, then pop it into the 1/4" reamed hole - it should be close fit in the bore. Now screw the DEC worm gear sub-assembly onto the DEC axle to the approximate position required. Use a piece of slightly undersized 1/4" diameter steel rod (just polish a thou or two off ordinary 1/4" rod with Emery cloth) with a flat, chamfered end as a punch, place it on top the copper slug and give the end a single whack with 4lb hammer - keep everything vertical and square so the hammer doesn't slip off. This will bed the end of the copper slug into the thread, and will also expand it in the bore so it fits snugly. Grind the end of your 8mm setscrew flat and chamfer it, then pop it into the threaded hole. When the setscrew is tightened down on the copper slug it will grip the thread very securely without the least chance of deforming it, yet to remove the DEC sub-assembly it's a simple matter of releasing the setscrew and unscrewing it. The copper slug will not fall out nor will it turn around in the bore. It will help the slug to bed into the thread if you tighten the setscrew a bit, work the the DEC sub-assembly around, then repeat this process a few more times.
2.3 Worm Mounting
My initial thoughts concerning the worm mount are to allow it to float in one axis only (towards and away from the edge of the worm gear), and to hold it in close contact with the main gear using a pair of springs. There are reasonable arguments both for and against this arrangement: if it is allowed to float it may conceivably 'rise up' on the 20-deg lands of the gear teeth thus causing tracking errors, whilst if it's fixed in one position the worm cannot follow accurately if the gear/worm are slightly eccentric, again a potential source of tracking errors. For better or worse, I'm going with the floating arrangement.
What is important is that, apart from the freedom of a single axis of movement we have allowed it, the worm should otherwise be held very rigidly. There's little point in making a highly accurate gear only to allow the worm to move around in it's bearings of it's own free will, or the carrier to flex, even very small movements will significantly affect the pointing accuracy and tracking. At the same time, we would like friction in the system to be reduced to a minimum so that the torque demands on the motor are kept within reason. In addition, having taken the trouble to make the worm and worm gear exactly match by lapping them together, we need to ensure that the two engage correctly. Those criteria - rigidity, free running, and precise alignment - are very difficult satisfy in combination.
The first two can be taken care of simply by careful machining, making sure that all mating surfaces are square and flat, and that holes for bearings are reamed whilst the components are held together in their final position. The third requirement is satisfied by having a rigid sub-assembly to carry the worm, but allowing it to be positioned as required before locking it in place. If you have made your own gear and worm according to the method outlined then at least you know that the worm only needs to be exactly orthogonal to the worm gear to assure perfect engagement - this is a help when machining the worm carrier because it's not necessary to account for any angular offsets.
2.3.1 R.A. Worm
The main worm bearings are plain 3/8" diameter reamed bronze bearings. They are perhaps easier to make (accurately) than housings for ball bearings as the pair of bronze bearings can simply be reamed right through. They also offer more support within the restricted space available. A ball-bearing with a 3/8" bore would be 0.75-1" diameter, requiring a housing something like 1.25-1.5" wide. A low-profile roller bearing is an alternative. One bad point to watch for - bronze bearings will produce large friction problems unless they are exactly parallel. The worm carrier pivot is a stainless steel shaft 5/16" diameter running in a reamed hole through both the carrier and mounting plate, I didn't think this needed a bronze bearing because it needs to move only very little, and some molybdenum grease on the shaft will prevent any wear. The worm carrier only pivots some 0.1" forwards and backwards, and it is arranged such that the worm is in perfect engagement when the carrier is at exactly 90 degrees to the mounting plate. The main RA gear can be moved up or down on the polar shaft by adjusting the position of the friction plate (the fixed component of the slip-clutch), so if you don't drill the 5/16" pivot hole in quite the right place then don't panic! The DEC worm gear is also adjustable but in this case exact position is achieved by turning the whole sub-assembly (worm gear and slip-clutch and threaded stub axle), this moves it up and down the DEC shaft.
The hobbing process will have produced a nicely concentric gear, there may be a little eccentricity after mounting the gears on the shafts although I do not envisage more than a couple of thou (0.002") runout. Even if it is substantially worse than this the pivoting carrier will keep the worm in mesh with the gear, the fact that the carrier's movement is an arc rather than being linear is not going to be significant over the small range of movement.
In the photos below you can see details of the the RA worm mounting. A jacking screw is used to lift the worm against the spring tension and hold it away from the gear wheel. This is primarily for transportation purposes (it prevents damage to the gears in the event of accidental impact), but it's also very useful when balancing the scope because with the worm disengaged the polar axis rotates completely free. Without this feature the friction from the slip-clutch would make it very difficult to determine when the mount was in correct balance.
Worm end-float is adjustable using a screw which presses against 2 hardened steel balls, one ball sits in a deep countersink in the end of the worm shaft, the other in a countersink in the end of the grubscrew - one hardened ball in contact with the other produces a low-friction rotation point. In the prototype shown, the carrier is fabricated in two parts using press-fit bronze bearings, it's important to assemble the parts complete with the bronze bearings fitted (their bores 1/64" undersize) and then ream right through 3/8" (use a machine reamer for preference, you'll find it cuts drawn bronze better than a hand-reamer would).
One end of the worm shaft is bored and threaded internally 1/4" x 32 TPI to accept a stub-axle onto which a brass gear is mounted, this is an intermediate gear to transfer the drive from the motor (which is mounted on a plate attached to the front of the carrier). The position of the worm carrier and motor mounting does limit the available lattitude adjustment however, so if your latitude demands that you set this below approximately 25 degrees the only solution would be to move the entire worm/motor mounting assembly, perhaps bolting it onto the side of the polar axis plate (i.e., 90 degrees to the position shown).

	RA worm with transfer gear to connect to motor.
	Front of RA worm carrier sub-assembly, note the jacking screw used to dis-engage the worm during transport.
	Rear of the worm carrier sub-assembly, two springs are used to ensure worm and worm gear mesh.
	Side view of RA worm sub-assembly secured to mount, springs lie neatly out of the way.

2.3.2 DEC Worm
The DEC worm mounting is similar to the R.A. mounting, there are a few differences in the way it's attached to the DEC bearing plate but otherwise the machining operations are the same. Other than for fast slewing, the DEC motor needs do little more than provide clockwise & counter-clockwise rotation of the DEC shaft at 1x sidereal rate, this is for drive-correction only. To perform this function successfully though it is essential that there be no backlash in the drive mechanism, it's difficult trying to guide if backlash makes the drive unpredictable (also, looking to future additions, an auto-guider would perform much better with minimal backlash).

	DEC worm carrier sub-assembly.
	DEC worm and motor drive.

2.4 Checking Alignment
There are a series of checks that can be done once most of the construction of the mount is completed. You will need a Dial Test Indicator (DTI) , a magnetic base, and preferably a 24" surface plate (though in absence of this it should be possible to make use of the lathe bed or milling table).
Test number one is to determine if the DEC axis is at 90 degrees to the Polar axis. This is crucial if later we are to rely on setting circles (manual or electronic) to find objects in the sky. With the mount supported on parallels, the DTI should be used to take a reading on the DEC shaft just forward of the lower DEC bearing, the polar axis is then rotated 180 degrees and a second reading taken at the other end of the DEC shaft. These two readings should agree if the DEC shaft is exactly at 90 degrees.

3. Electronics

3.1 Motors
There are several options for powering the drive(s), but whatever the choice it's necessary to control the motor speed with great accuracy. Perhaps the easiest way is to use an AC synchronous motor running off mains 50Hz (or 60Hz) cycles, although a variable-frequency oscillator in place of the mains 50Hz would provide more flexible control of the drive rate. Another option is to use stepper motors. This requires a controller circuit to provide the correct sequence of output currents to the motor windings, but it will give excellent control features including slewing speeds several times that of sidreal rate for centering objects. Rotation speed of the stepper motor usually controlled by a crystal oscillator circuit, divided down a 4MHz pulse rate. A micro-stepping drive circuit (which ramps the voltage to adjacent coils up and down in sine-wave fashion) would be advantageous because the worm could be driven directly without an intermediate gearbox, which is otherwise necessary to prevent 'cogging' (jerky movement due to the discrete steps). A DC sevo-motor together with the appropriate feedback circuits is perhaps even better, it having a greater dynamic speed range than a stepper (which loses torque as the revs are increased). The control circuit for a servo-motor is even more complex though, the feedback mechanism requires shaft encoders to monitor the speed and position of the motor shaft, the controller having to respond to these signals rapidly to modulate the motor current.
To get the mount up and running ASAP I rigged up a DC servo-motor (shown below) but using only a simple 0-100 Ohm potentiometer to control drive rate. I'm sure this will result in a drive rate that varied dependent on temperature, voltage and also loading - no good for photography then, but at least it allows me to use the mount while construction of a stepper motor controller proceeds (I've a 12" f/4 Newt arriving in a few weeks and nothing to mount it on!). The motor is a Swiss-made Escap geared motor with a built-in gearbox providing a drive reduction of 500:1, so all I needed to do was drive the transfer gear directly with the motor's output gear*. The motor seems to perform quite well, though I had to use 24 volts to produce a reasonably constant drive speed with the required torque. Another intermediate option I can try if this doesn't work is to use one of the stepper motors I already have, the controller I made for it is very basic, being designed for computer control it just provides the sequence of current pulses, it relies on a clock input to control speed. However, I happen to have a function generator which can generate accurately timed pulses to control the drive rate, not exactly a simple or economic system but it should work.
*Note: I have since added a compound reduction gear and used a larger transfer gear. In addition, the original control boards for the motors appear to be working (they were from medical instrumentation - constant rate infusion pumps), so I've used one of these to power the motor instead of the 12V battery. The original controllers use sophisticated circuitry to ensure a very accurate and constant motor speed even during varying torque loading, exactly what's needed for the scope drive.
The mount had first 'light' (?) tonight (7/2/99) to check the drive. Overall, it's performance is excellent. I did notice an intermittent periodic error (about 5 arc-sec, 2-3 second cycles), and I've isolated this to the first driven spur gear. I had soft-soldered a pair of brass gears together for the compound gearing and I think some of the solder got into the teeth. I'd run the gear for a few hours and thought that had cleaned it out - but obviously not completely. I cannot detect any other error with the 90mm refractor using a reticle eyepiece at 160x, the seeing was not good enough to support any higher power.

DC servo motor in position - I may well replace this with a stepper later.

3.2 Motor drive circuits

3.3 Drive Corrector/Hand paddle
A more sophisticated system will be described later.
3.4 Computer Control
Digital setting circles are a fairly easy option, I should think that any of the commercial units will fit OK. My only problem with these is with the price - here in the UK they retail around 400UKP, hardly a bargain but I don't think there is a simple DIY option for making your own (I've seen a few designs using mouse encoders and so on). There are certainly PC-dependent systems, and the excellent computer control system developed by Mel Bartel should be readily adaptable. The COAA design is another possibility, originally designed for the GP/DX series mounts it too is adaptable to other mounts. However I would really prefer a self-contained unit for this particular mount, it was conceived as being portable (and it is) so I feel that adding a PC would detract a bit from this ideal.
The Vixen SkySensor 2000PC *may* be adaptable, but to do so will require either intermediate gearing or it may be possible to configure the controller for a different drive rate. I know there is a setting for drive rate but I don't know whether it can accommodate a 360:1 worm. The SS2K was designed for the Vixen 144-tooth worm, so a 2.5:1 ratio change is needed - a 45:18 spur gear combination would do it, the reduced ratio should not be too much trouble for the motors because the 6.5" diameter worm gear has a lower torque requirement than the 75mm Vixen gear. Again, the SS2K is not cheap but it does offer a self-contained handset which has fairly sophisticated functions. Yet another off-the-shelf option would be those made by AWR. They offer a very nice micro-stepping drive controller and can also supply suitable motors, but my impression of the 'Intelligent Handset' to go with it is that it's 'work in progress' right now. It's got better control over the drives but the GOTO (coordinates only, no built-in database) and alignment options (one-star aligment only) make it less attractive than the SS2K. It's also inordinately expensive if purchased together with all the options.

4. Miscellaneous Accessories

4.1 Adding a Polar Alignment Scope
At the outset I thought the compromises entailed in adding a polar borescope were not worth the advantages, A split DEC shaft would be required and my mount has a longer Polar shaft than normally seen so arranging suitable optics would be difficult. in any case, the answer is quite simple - the machined polar bearing plate offers a true surface on which to mount a larger aperture (and higher power) sighting telescope. An alignment using a polar finder scope will get you close to good alignment, but not close enough for astrophotography, for that you will need to 'drift align' to fine-tune the mount alignment. For visual observations then, where you just want objects to stay in the eyepiece's FOV a polar scope is fine, and this can equally be done with the scope attached off-axis, all that is needed is a way of mounting the finder such that it remains parallel with the polar shaft. A fairly long dovetail bracket (say, 6" of machined dovetail) would be adequate to ensure good accuracy. I have a spare 4x32 rifle scope but, whilst bright enough, the magnification is too low to be useful (and I can't see the fine reticle in the dark!). I happened to drop across a Polar scope originally designed for a different mount and it was easy enough to make a simple bracket to carry it far enough off-axis to avoid the DEC assembly. You can see this in the pictures below (Section 4.2 Protective Casings for RA and DEC shafts), and it certainly works just fine.

The view through the polar alignment scope just misses the RA gear.

4.2 Protective casings for the Main Drive Gears
These are definitely going to be needed if the mount is to be transported regularly, and also desirable even if the mount is to be used on a permanent pier. They not only prevent the ingress of dust and grit (which will stick to the lube), but also guard against accidental impact damage. The precise figure of the teeth on the main gear wheels are particularly delicate and you will want to protect their fine finish - one ding in the gear teeth and accurate tracking will be severely marred at that point (see the note on gear accuracy in 2.1.1 which emphasises this point). Remember also to use the worm jacking screw during transport, this isolates the worm away from the gear and reduces the danger of a sudden impact damaging the gear teeth.
The gear casings are a bit of a pain to make, I tried fabricating one from bits of sheet metal and tubing but I didn't like the result. In the end I turned one out of the nearest sized available piece of round stock I had - which was a 7-1/2" diameter disk 1-1/8" thick alloy. A bit of a waste because most of the metal was machined away (which I tried to avoid by fabricating it) and I still can't believe how much swarf the job produced but I suppose the cost was offset by the time saved. It's held in place by three 4mm c'sunk stainless screws threaded into the front bearing housing. Before final assembly I covered the gear teeth with molly grease knowing this won't get contaminated as it's fully enclosed.

	Inside the R.A. Gear casing.
	With the polar shaft re-fitted the gear teeth are totally enclosed.

4.3 Protective casings for the R.A. and DEC Shafts
The open framework of the DEC and RA sub-assemblies are covered with a lightweight aluminium casing to protect the shafts from corrosion (on the prototype the shafts are PGMS not stainless). The tops of the casings are made from 1/8" wall alloy tube cut in half with a slitting saw, then two side plates of 1/16" alloy sheet were attached to the sides. If you happen to have the right diameter tube it's an easy matter to place the length of tube on the milling table aligned lengthways using one of the T-slots, and use a flat bar passed down the center bolted to the table at the ends (not too tight otherwise it will distort the tube), then run the slitting saw all around at center height of the tube. You can then run a small endmill around the inner edge to form a step which just helps alignment when fixing it to the mount. If you don't have any suitable tube then just use a single sheet of 1/16" alloy wrapped around and held by small screws at the sides, the 1/16" sheet is easy to bend using hand pressure alone. Spotting the positions for the clearance holes (the various clamp knobs) will be a bit more tricky if you use sheet material, but it will do no harm if they are a bit oversize. If I were to do this I would use an over-size sheet, make the holes, then fix the sheet in place. It would then be simple to run a scriber around and trim off the excess.

	DEC Shaft casing.
	Polar Shaft casing.
	Casings are finally anodised black.
	Here the mount is shown setup for photographing the 1999 Leonid meteor shower, two standard camera brackets are used at the ends, the center one is a universal camera bracket.