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Cave GEM mods, including Byers drives

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41 replies to this topic

#26 charles genovese

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Posted 09 August 2019 - 06:27 AM

Also- I was able to fit the Meade digital setting circles in.

 

Charles



#27 65&Counting

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Posted 09 August 2019 - 11:12 AM

Oh - that's just fantastic! Great work Charles!



#28 photoracer18

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Posted 09 August 2019 - 02:23 PM

Photoracer18 thanks so much!

 

I'm really glad you brought up the Starmaster 11 5/16" because I have two of 'em - one older with the attached setting circle (really neat) and another without but both synchronous.

 

Thinking of doing exactly what you suggest and this helps greatly as to options.

 

I've read some posts on CN about adding steppers and I've spoken with Ed about it (recently) but he's about done with it all - have you posted anything on CN about the stepper conversion for this gearset? I'd love to get the stepper motors before it's too late and think Ed still has 'em - he told me I'd have to figure out the electronics & that he has someone do that for him as he's a machinist and not electrician - made me feel better :-)

 

I'd hate to keep adding parts if I can't make 'em work together.

This particular mount was farmed out to someone in PA to build by Barry Greiner of D&G for the late Dr. Green's 6" F15 Jagaers he was putting together. The Byers control box was capable of being modified for a stepper drive in the RA. It uses the original Meade 12v DC tangent arm in the DEC. There are stepper controller boards available. You need to use one capable of micro-stepping and includes some kind of gear reduction box on the end of the motor.


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#29 photoracer18

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Posted 09 August 2019 - 02:28 PM

As for another comment the Cave 12-12.5 scopes generally came on mounts with 1.5" shafts unless it was ordered with an observatory class mount. The one that came on the 12.5" Cass/Newt I have was an early pre-roller bearing mount with a 1.5" RA and a 2" DEC axis (pre-1964 or there about). It was actually the first type of observatory mount as it came with the rectangular aluminum alloy pier.


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#30 dhferguson

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Posted 05 September 2019 - 05:00 PM

Cheers, again,

 

One of you asked about encoders in this or a previous thread, and I want to comment on my experience adding encoders to my 1.5" axes Astrola mount. I delayed responding to that post because I wanted to understand certain pointing errors first.

 

To begin, back before any modifications, I attempted to use the OEM 6" setting circles to find various objects. After a relatively quick-but-crude drift polar alignment (perhaps a degree or so off), I would find the setting circles would misread by several degrees--from 1 deg to 6 deg--from true angles. I brushed this off as due to several errors: setting circle reading error, coning, polar misalignment, and perhaps mount axes orthogonality and there this problem sat for years until I installed encoders and Argo Navis DSCs.

 

I ordered my encoders and Argo Navis from JMI. Incidentally, JMI has been acquired by another firm and I don't know if they still sell these but the same encoders (5K) are also available direct from Argo Navis. Anyway, the kit consisted of the encoders, the Argo Navis unit, the necessary connector cables, one mounting bracket for each axis, and the gearing.

 

First, the gearing consisted of a 2:1 ratio (two 360 degree rotations of each encoder for one 360 degree rotation of each axis), connected by a Bando (brand) belt. Note that the length of this belt is critical should you decide to duplicate my setup for your mount, so measure carefully. The Bando belts are only available from a Bando distributor and are relatively inexpensive but come with a relatively large restocking fee. For this reason, I would order several sizes that bracket what you need, and perhaps add a few spares. Anyway, two toothed wheels were supplied by JMI for each axis. One of these, made entirely from metal, fit nicely to the encoder shaft and tightened via set screw. The other, 1.5" inches in diameter, slipped over the axis shaft and tightened with two set screws. The latter gear's inner piece was made of plastic, I assume because that is what was available in the rather odd 1.5" size. I don't know from where JMI got the toothed wheels.

 

The brackets supplied by JMI were made of sheet steel and were, frankly, of poor design. The declination axis bracket came with adhesive attachment pads (not nearly secure enough) and a rubberized hose clamp (jeez!). Due to this ad hoc design, I required several hours to properly align and shim it for it's original stock usage. The RA axis came only with adhesive pads, so I drilled two small holes into the original cast cover and attached it with screws. Once installed on the original covers, everything worked fine.

 

The implementation shown in the images I previously supplied with the new Byers drives is similar. I reused the original toothed wheels and encoders but had to order new Bando toothed belts because the dimensions were different. And, of course, I also made new encoder brackets to attach the encoders to (this time), the mounting plates (as opposed to the covers). As a result, my encoders are now fully enclosed within the covers instead of mounted outside, as in the original OEM implementation. Please measure carefully when you lay out your cover because there is none too much length on the axes--particularly on the RA axis--to install encoders and also accommodate the thickness of (for looks?) the original Cave setting circles. The new implementation also works fine, although the user-input sign(s) of the direction-of-encoder-motion in the Argo Navis unit may change, I don't remember.

 

I repeated my original pointing measurements and discovered both large offset (still up to 6 deg!) and coning (~2 deg in diameter to start) errors. The offset errors can be dealt with rather easily: simply find an "alignment star" relatively close to one's target and use the "permanent GEM alignment" routine as discussed in the Argo Navis manual. This only requires one alignment star so it can be done in less than a minute.

 

Limiting the coning error, however, was much more problematic. I have Parallax tube rings circa 1995. These are the solid and unopenable machined rotating rings that must be slid up the tube to be installed. Although both expensive and heavy, I regard them as a wonderful accessory that allows me to observe in comfort from any tube orientation. However, Parallax does NOT recommend DSCs with these rings for reasons that will be come clear. My Parallax rings were originally specified for a BakerTube (like a Sonotube) that was 12.375" O.D. As I had replaced this tube with an aluminum 12" O.D. Hastings Irrigation tube, shimming was required. My first attempt shimming attempt involved using a 1/8" aluminum strip between each tube ring and the tube. This sort-of-worked, and resulted in the aforementioned unacceptably large 2 deg diameter coning error.

 

Unhappily, I removed everything from the tube and replaced the aluminum shims with 1/16" ABS plastic bands (from TAP Plastics) wrapped around the tube until a solid fit was achieved, a bit more than two turns, then I glued the wrapped layers together with model airplane glue. After completely reassembling my tube, I also carefully "squared the focuser" and recollimated it. These steps indeed resulted in a tight fit and a big improvement in coning, to 0.35 deg diameter maximum error. This error was both acceptable as-is (the FOV in my low-power eyepiece is 1.1 deg) and can be further calibrated out if necessary. To give you a sense of the tolerances involved, my Cave OEM saddle--ring-to-ring--is roughly 2 ft long. A 0.35 degree error is thus a ~1/8" misalignment between the optical boresight and somewhere in each ring; i.e., quite small indeed. While it would certainly be possible to shim for even better misalignment, even re-collimating the telescope would affect coning, so I decided further shimming would be too much effort for too little reward.

 

I also noticed something about the outer ring races: they (one in particular) were never quite round. This manifests in a slight ellipticity to the coning which cannot be shimmed out (see attached image). I attempted to adjust the centering via the six screws drilled through the outer races. In their implementation, Parallax uses these screws to push on teflon (or nylon?) inserts that run in a groove on the inner ring. Due to the ellipticity of the outer rings, it is impossible to achieve a perfectly smooth tube rotation; however, I did come acceptably close. If too loose, the tube can essentially "rattle" within the outer rings, which of course increases pointing error. If too tight though, the rings become too stiff to move easily, and so there is a happy medium. However, due to tightness, the screws cannot be used to "true up" the outer rings into a circle because they are then too tight.

 

The attached image shows my coning analysis from three stars. I removed the offset errors and "clocked" the tube in the rings to make measurements around all 360 deg of clocking.  The radial axis is in deg (i.e., 1 deg max) and the digits 1-though-6 represent clockings of noon, 2PM, 4PM, etc. I fit an ellipse to these data and, based on the ellipse, computed a table of offsets for my Argo Navis unit from one "clocking" of the tube to any other, at least when it's necessary to be more precise than 0.35 deg. Soooo, I've learned how to live with my modest offset and coning errors. 

 

Happy observing always,

 

Don

Attached Thumbnails

  • Coning.jpg

Edited by dhferguson, 05 September 2019 - 05:12 PM.

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#31 photoracer18

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Posted 09 September 2019 - 03:40 PM

Oh - any chance you want to thin your collection of one 2.5" drive? That's next on my list since the 1.5" isn't made for a 12.5"

The early Cave 12.5" Cass/Newt Convertible I have came with an early observatory mount version that had a 1.5" RA axis and a 2" DEC axis mounted on the aluminum rectangular pier. However the RA drive looks like a mess of optional parts way different than any Cave mount I worked on over the years.

 

Cave drive..jpg


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#32 KentTolley

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Posted 10 September 2019 - 09:33 AM

Can you show the rectangular pier.  I've only seen the oversized round ones on the 12.5"ers



#33 photoracer18

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Posted 10 September 2019 - 02:37 PM

When Gary Hand made me a deal I could not refuse on the 12.5 + mount he kept the pier. Go to the cave-astrola.com website and scroll down the pictures to the 8" Cassegrain and it looks exactly like the middle picture with the round cap on top of the pier.


Edited by photoracer18, 10 September 2019 - 02:37 PM.


#34 KentTolley

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Posted 10 September 2019 - 03:08 PM

Ok thanks.   Of course I had seen this permanent pier but thought you were talking about something else.  



#35 photoracer18

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Posted 11 September 2019 - 03:49 PM

I got the cap with it so I could build it with a 6" steel pier and I have a set of cast iron HD Edmund pier legs. If I can talk Gary out of the pier that would be good. If not I can build something. Sometime in Spring 2020 I will start this project staring with the OTA because I can always carry it with either my custom DS-16 or the Schaefer AT-9. I plan on mounting the AT-9 on the used Sky Shed pier I recently acquired on a concrete slab (suitably deepened in the center) I will pour in the back yard. But that is next year. I still would like it to be used in the Classical Cassegrain mode but that will require expert optical testing and likely a replacement secondary if it proves too trouble some. I figure there is a reason that the late owner seemed to only have used it in Newtonian mode and since the primary was replaced by one from Optical Craftsman around 6-7 years after it was installed under the dome it may not match the original Cave Cass secondary (although the FL of the OC mirror is 72.15" I don't really know what the Cass focal ratio was as built.


Edited by photoracer18, 11 September 2019 - 03:56 PM.


#36 semiosteve

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Posted 12 September 2019 - 10:01 AM

Regarding the prior excellent post on adding encoders, I see that the ArgNavis folks have their own option for adding encoders to Cave mounts.

 

http://www.wildcard-...c_describe.html

 

Anyone here familiar with this option? I am considering it for my 8" Cave RFT.

 

Clrskz

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#37 65&Counting

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Posted 12 September 2019 - 10:12 AM

The original Byers motors have been replaced with a SyTech system.  A 10" f/4.5 doesn't look very big on it.

Also note it can be used as a binocular mount LOL

Charles

Oh thats sooooooo nice Charles!!!

 

At some point (like whenever available) I gotta get me one of these!!!



#38 dhferguson

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Posted 12 September 2019 - 11:13 AM

Hello Clsrkz,

 

Yes, the link you supplied shows a schematic of parts that are very similar if not identical to the system I purchased from JMI. It would appear JMI was Argo Navis' distributor in the US before the former sold out. Note that the encoders are 5K (evidently, the same that came with my JMI system), what with a  2:1 cogged wheel reduction in my case yields 10K per rotation axis. If the parts are indeed identical or closely so, I expect your experience to be very similar to my initial installation (unmodded Astrola mount) experience. BTW, no DSC unit corrects for coning error: you are likely to experience this and must measure and then decide how to deal with it. Having Parallax and not CAVE OEM rotating rings, I don't know what the magnitude of your initial coning error is likely to be (others chime in, please).

 

Happy observing always,

 

Don



#39 photoracer18

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Posted 16 September 2019 - 06:01 PM

The early Cave 12.5" Cass/Newt Convertible I have came with an early observatory mount version that had a 1.5" RA axis and a 2" DEC axis mounted on the aluminum rectangular pier. However the RA drive looks like a mess of optional parts way different than any Cave mount I worked on over the years.

 

attachicon.gif Cave drive..jpg

I have decided that I am going to replace this Rube Goldberg contraption with the 7" Byers Starmaster drive I have in a box when I start on this project next year.


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#40 semiosteve

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Posted 21 September 2019 - 02:56 PM

OK so I would just be happy to find the appropriate 10 teeth spur gear for the original Cave RA Bristol Motor 447 on my Cave 8" RFT.

 

Prior threads on Cave restoration refer to Boston Gear BG127 - which does not in fact fit the shaft on the original 447 motor - DUH.

 

Given all the trouble I am having just to get one gear replacement, I am in awe of the folks on this thread replacing the entire drive chain !!



#41 semiosteve

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Posted 22 September 2019 - 02:22 PM

OK so I would just be happy to find the appropriate 10 teeth spur gear for the original Cave RA Bristol Motor 447 on my Cave 8" RFT.

 

Prior threads on Cave restoration refer to Boston Gear BG127 - which does not in fact fit the shaft on the original 447 motor - DUH.

 

Given all the trouble I am having just to get one gear replacement, I am in awe of the folks on this thread replacing the entire drive chain !!

Looks like the Bristol Motor wants a .125in bore...that's a Boston Gear G96 not G127 - in case anyone else lands up going down this path.

 

sv



#42 tim53

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Posted 24 September 2019 - 09:19 AM

If you have a lathe, there's a "simple" way to decrease the bore size.  Turn a piece of bar stock (a brass bolt would do) so that the gear can be press-fit onto it.  Face off the excess.  Drill a hole of the right diameter in the center of the new sleeve you've just made.  Reverse the assembly in the chuck and face off the remnant excess bar stock/bolt.

 

I did this when I had adapted my Sitech servo goto kit to my early 90s non-goto EM-500 mount, because the output shafts on the pittmann motors are smaller than the ones on the Tak steppers.  I've since returned all the Tak hardware to the mount.  All I had to do was put the gears back in the lathe and use a piece of bar stock bigger than the hole in the sleeve, but smaller than the sleeve, mounted in the tail stock and used as a press to push the sleeve out.

 

-Tim.




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