This long post is about my experience modifying my 1.5"-diam. shafts Astrola GEM to include better latitude adjust, Byers drives, an electric focuser, and a hand controller. I am the original owner and the mount has been well cared-for over the years. Ye Olde US-made mid-market cast aluminum GEM mounts are no longer being manufactured (Parks was the last manufacturer, I think). Rather than choose between a hyperbucks AstroPhysics or a cheap Chinese mount replete with plastic parts, I wanted to keep my Astrola but make it perform the way I wished. BTW, contrary to some posts I've read, the old US mounts can be quite good indeed. For example, the SealMaster bearings used in the Astrola mounts are first-rate. These older designs tends to be slightly more "bouncy" for the weight than modern semi-equivalents, such as Losmandy's offerings, because some (like Cave's) lack thrust bearings and because there is a larger offset between the cradle and the "tee" (allows for bigger RA gears, though). Outfitted properly, the old Astrola mounts can be quite suitable visually for telescopes lighter than about 60-70 lb (mine is at the limit) and--fixed up for auto guiding with better drives--photographically for 35-40 lb or less. Find a well-cared for mount, use a little of your skills, and you too can have a "classic" mount that performs well. Ignore the negativity: this comes from people who unrealistically expect a fifty year-old mount to perform like new without any upgrade or even servicing.
Many of you have significant machinist skills and tools, and I greatly admire your handiwork. However, while "handy," I am not in your category, and I expect most of us are not. Part of this post is to show that such skills are not necessary for decent-looking results, albeit not in the class of fine machining. In addition to the usual hand tools generally found at home, I required additional easy-to-find tools. These included: a table saw equipped with a metal cutting blade, a drill press and an x-y stage (Proxxon) to "convert" the drill press into a small mill, small mill bits (no larger than the 1/2' chuck on my drill press), good-quality taps in the following sizes: 6-32, 8-32, 8-24, 10-24, and 1/4-20, and a 2" hole cutter (the kind used for wood will work on aluminum if slowly but will screech horribly--wear ear protection). Also, I required various inexpensive tools to prepare "keystone" wire connectors. Regarding materials, metal of all sizes can be found on eBay and Amazon, as can connectors, fasteners, etc. I am fortunate to have a Fastenal store nearby, which stocks pretty much every machine screw size/head imaginable. Also, I acquired plastic for the drive housings from TAP Plastics. They were very helpful regarding forming techniques, BTW. Also, when you are milling small slots, that x-y stage must REALLY be bolted down firmly to your drill press. Try it with a piece of scrap first. Finally, it is crucial that you wear eye protection goggles when cutting or drilling, especially when using the table saw. The metal chips absolutely WILL get underneath your eyeglasses--plain safety glasses are not sealed well enough. Please heed this warning.
One of the two major problems with the Astola mounts is the poor implementation of the drives, especially the RA drive. The clutches are undersized and finicky, and worse the RA worm is mounted within an aluminum "U" that flexes when the polar axis is rotated. I elected to change both drives out. Byers is now selling off the very last of his wonderful drives. I was fortunate to have acquired new two drives in 2014: a 9" 360-tooth gear for the RA axis and a 7.1" 252-tooth gear for the Dec axis. They arrived, and to my surprise and delight they were perfect suck fits to my axes, hooray! I might have preferred the 359-tooth RA gear but they were already sold out and gone. Nevertheless, the acquisition of a 360-tooth gear means I could, if the fancy struck, rather easily convert my drives to DC stepper motors using a Losmandy controller. I nevertheless decided upon old-school AC synchronous motors due to their lack of failure-prone and ... eventually ... near impossible-to-replace electronic components, and the fact that I have unlimited AC power in my backyard. I also acquired a used Lumicon AC drive corrector to modify the AC motor speed a bit, although this would really only be necessary for astroimaging.
Oh, CN only allows 500KB images per-post, so I will add posts with the figures I reference here. Figure 1 shows my 10" f/6.6 telescope with the completed drives. I've since completed the electric focus module, and I will provide this image too shortly. I removed the original Cave Dec drive and chose to mount the Dec housing below the "tee" for better balance. That is a standard Cave weight you see and the additional weight is a 2" I.D. 5 lb barbell weight with a bushing made from 2" OD PVC. It works well. The entire tube assembly, including the rather heavy Parallax rotating rings, weighs 68 lb w/o eyepiece but with (not shown) a 2 lb sliding balance weight on the bottom of the tube on the opposite side of the tube from the focuser. The wrapped-up wires are for the electric focuser. In addition to the Lumicon drive corrector, you may notice the AC/DC converter. This provides power to my Argo Navis DSCs, whose encoders are contained within the two drive housings. BTW, the handle on the top plate really helps hold the tube for carrying. I do not find it at all difficult to lift and secure the tube to the mount (I am in my sixties!). Per other posts here and on the cave-astrola.com website, I cleaned, primed, and painted the mount and pier, and also cleaned up the brass fittings with Brasso (avail at Wal-Mart but, apparently, not at Target).
The drive plates began as a 3/8" thick 12" x 10" rectangle for the RA and 10" x 8.5" for the Dec plate. These were both tight fits, and I would encourage the builder to carefully lay out all components before sizing these plates, making special allowance for movement (adjustment) of the worm gear block. Figure 2 shows the RA drive plate. I cut both drive mounting plates from 3/8" aluminum plate, just like the originals but of course larger. I used many straight cuts, working my way around the semicircle, and then filed the curve smooth. I used a standard wall mounting plate with 4 keystone connectors to attach the sundry wired connections: (1) from the Lumicon drive corrector, (2) RA encoders to the Argo Navis, (3) Dec encoder to the Argo Navis, and (4) space for an ST-1 autoguider port. All the external wires are loomed. Also, note the stainless steel latitude adjust turnbuckle that connects between the RA plate and the pier. This allows for precise latitude adjustment, and fixes the other of the two major problems with the Astrola mount: latitude adjust. The turnbuckle geometry is arranged to avoid imposing any push-pull load on the mounting plate machine screws threaded into the polar housing. These are 8-32 and threaded into aluminum, so stripping these by overtightening the turnbuckle would otherwise be a risk. The turnbuckle eyes were opened somewhat for ease-of-removal using a vise and pry bar.
Figure 3 shows the back of the RA plate. The main gear is removed to expose the construction. Note the encoder wheel and belt (belt supplier is Bando). Length of the belt is critical to get the right fit. The black steel-sheet mounting bracket was salvaged from my old, external JMI implementation. I used socket head machine screws for attachment of components. All my external wiring connections are via junction block for two reasons: (1) inadvertent pulls on external wiring would not damage the internal wiring and (2) it makes continuity testing easy. #2 proved very useful. Also note the three 8-32 machine screws that attach the RA plate to the polar axis and the slotted "RA Motor Block," which allows for adjustment of the worm to the gear. The lock washers are necessary to maintain adjustment, BTW. The RA and Dec motors are actually powered separately but a single double-throw switch turns them both "on" at once. Very important for safety, and unlike the original Cave wiring, I have grounded both RA and Dec power inputs to case. That is the back of the "on" light located just below the "PWR ON" label, just like with the original Cave! Finally, those of you with sharp eyes will note the RA motor is wired incorrectly. Byers' instructions were inadequate on this point but I fixed it and added the necessary capacitor (see the Hurst Motor specs on the web for better instructions). While I allowed space (lower left-hand corner of the plate), I have not yet wired in the ST-1 auto guider port. I may be asking you all for help on this forum soon! The RA & Dec drives and the electric focuser are all wired into the hand controller, BTW.
Figure 3, "RA_Drive" (next post) shows the drive installed without the plastic cover. The bottom of the cover is constructed from 1/4" aluminum plate with 3/4"x3/4" aluminum standoffs for strength at the corners and ends. I attached all these and the plastic too with 6-32 flathead machine screws. I used the same knurled brass screws from the Cave original housings to attach the standoffs to the mounting plate, and they screw and unscrew easily by hand. I used 3/8" thick plastic from TAP plastics for the cover, the plastic being mortised using a table saw before forming to fit the underhang on the aluminum standoff visible in Figure 3. Also, see that the clearance between the standoff and the gear is quite tight. Just for fun, yes, that is the original Cave-supplied Dec controller box wound around the polar axis, it's long since been removed. I used opaque black plastic with a mottled surface to best replicate the look of the original Cave mount (see Figure 1), although I was tempted to use clear plastic instead, the better to admire the innards. Molding the plastic is critical and I recommend you take your time constructing the mold for best-fit. I sized and cut plastic edge trim gardeners use to delineate the edges of lawns screwed to plywood for my mold. I fit the mold carefully to what you see in Figure 3 except with the main gear out. You will need to soften the plastic flat in the oven, then quickly form it about the mold. Also, get the necessary advice on temperatures and softening times from your local TAP Plastics store, they are helpful. Plastic outgasses poisonously when heated. I did use my kitchen oven but I opened up all the windows, then cleaned the oven assiduously with oven cleaner afterward. Oh, you-know-who was out for the day, and 'wondered how the over got so clean. 'Better to ask forgiveness than permission. After forming, the plastic required an additional standoff at the top for strength.
Now you've done it, the cover is too tight and the main gear rubs! You might use the table saw to shave away a bit of the standoffs if these are the problem. Or, if it is the plastic cover, you can use a Dremel tool to carefully remove a bit of plastic from the inside surface. I needed to do just this with a slightly too-tight Dec housing cover.
Note too that the thicknesses of the housings are also critical, especially the RA housing thickness. By careful measurement, I was able to still install the mechanical 6" setting circle on the RA axis, although that and the locking collar made for a close fit. Yeah (laugh), they are undersized now, and with the DSCs I never use them.
I attached the Dec housing to the aluminum Astrola Dec casting using a large split collar that fits around the bottom of the housing. I drilled and tapped this collar in the collar-thickness direction to attach the Dec mounting plate. Note that you can warp the somewhat thin aluminum casting if you tighten the collar too tightly. Tighten it some, then note when the axis begins to bind, then back off. I'm sure it will still be more than plenty tight. I have since experienced zero slippage whatever.
Figure 4 shows the electric focuser mounted to the tube. The wingnut atop the plastic housing loosens and the motor assembly inside slides to engage the JMI NGF-1 focuser knob. I bonded a disk of felt to the focuser knob using contact cement for half the clutch. The other half, likewise bonded to a DC 4 RPM motor (Servo City) is cut from a wine bottle cork. There is a 9V battery (3 RPM at 9V) inside the housing. It and the motor are mounted on a piece of aluminum plate that slides back and forth in the plastic case. The wiring exits through the bottom of the plastic case, through the aluminum telescope tube but then underneath the flock board (Protostar) to an exit point midway between the rotating tube rings. A 4-wire connector is used to connect the focuser to the hand controller wiring. Yes, I suppose in retrospect I could have wired the electric focuser directly to my DC power supply but the battery seems to last forever. The plastic box (Radio Shack) is mounted to the tube using cork standoffs that are sized to position the center of the cork aligned with the center of the focuser knob. BTW, the focuser works very well. I use it for high-power observing of the planets with this scope's superb optics.
Figure 5 shows the hand controller. It fits nicely in the palm of my hand! Check out Figure 3 again to see the RA & Dec drive controller wiring to the hand controller. The focuser is a separate 4-conducter wire included in the same looming as the RA & Dec drive controller wires. Regarding the wiring, it will save you time to check the continuity for each connection. In particular, the keystone connectors don't always crimp properly.
How does it all work? Very well indeed. First, the idea of putting casters on the pier legs was a stroke of genius, whoever thought of it (why did it take so long with suitcases?). It is just about the same effort to roll the telescope mount out from the garage as it is to "wheelbarrow" out my 14.5" Dobsonian to the patio. The Dobsonian is rock-steady but the Astrola mount has a fundamental vibration mode that damps out in about 2.5 s. This becomes quite tolerable even at x480 with the electric focuser. The Byers drives are a massive improvement over the original drives. Movement is silk-smooth and the telescope stays where you put it with almost indiscernible backlash. With the RA drive on, the target just hangs in the center of the field, motionless and wonderful. I haven't evaluated the periodic error but, based on the extreme stability of the drive (as compared with the original, whose tracking was at least acceptable visually) at high power, I expect it to be quite low. The hand controller works perfectly. However, unlike the Cave drives, the speed-of-motion of the Byers drives is now more suitable to astro-imaging than for visual slewing or scan patterns to find targets. Nevertheless, it is fun, for example, to "tour" the moon with the hand controller. The Lumicon AC drive corrector works well and the AC-to-DC power supply has eliminated the monstrous appetite for AA cells by my hungry Argo Navis unit.
Was it worth it? If you enjoy modifying equipment for satisfaction and to avoid paying through the nose for similar performance, yes! If you want a "classic" look in a telescope that will outperform essentially anything else of comparable aperture and size, yes! If you wholly lack mechanical/electrical skills altogether but instead prefer to spend your way to bliss, no. I am not sure if I'll ever do astro-imaging (my Bortle 6 skies suck!). For someone who is absolutely sure to stick with visual-only, you might instead consider adding the Byers (or somewhat equivalent, see the Optic-Craft Machining website) 7.1" on the RA axis only. I selected the 9" drive in-part because the worm is supported on both ends. Flexure for the one-end version that came with the 7.1" shouldn't affect pure visual tracking much if at all. Such a setup would give you excellent tracking but of course no (or original Cave) Dec control.
I'll write an honest comparison of my GEM-mounted 10" and my 14.5" Dobsonian at some point, stay tuned, and of course ...
Happy observing always,