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Can a DIY GEM achieve AP quality tracking?

ATM mount DIY
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#1 matt_astro_tx



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Posted 24 January 2021 - 08:48 AM

Good morning all. I’ve got an 8” f/8 newt on a homemade GEM that I built almost 30 years ago as a teenager. It’s a pipe mount using the threads as bearings. Not terribly smooth and not able to be upgraded with a drive system due to the configuration.

I’ve recently gotten the itch to improve the mount ala the classic GEM pipe mount designs out there using pipe reducer bushings and PVC as free rotating bearings, etc. and adding a homemade Arduino geared RA tracking system. (No Dec. I’m not looking for GOTO.) I work in aerospace and this will be a great engineering challenge and opportunity to captivate my five boy’s interest.

But I really want the end result to be capable of AP (5-10 minute subs would be a win for me).

I can’t find an affordable supplier for matched worm drive gear sets (looking for references) so I’m looking toward belt drive or spur gears with a micro stepper motor.

My question is, how tight do my tolerances really need to be to achieve decent amateur AP? Is this a pipe dream (intended)? Should I just save $1500 to buy a sky watcher mount? I’d love a pro mount, but that’s not really what I’m after. I want the engineering challenge.

I am a carpenter and have a shop. But I’m not a machinist nor do I have access to machine tools (or the desire to get into it).

Has anyone out there built a pipe GEM with RA tracking and been able to get 5 minute subs?

Attached a photo of my current setup and a mock-up of new bearings. 

Thanks for your thoughts!

Attached Thumbnails

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  • 81DA189F-5FFB-43C0-AA77-FCA606F5B681.jpeg

Edited by matt_astro_tx, 24 January 2021 - 06:52 PM.

#2 zakry3323


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Posted 24 January 2021 - 09:52 AM

I've made barndoor trackers that could get me 30sec subs at a 50mm focal length with pretty decent looking stars. This would certainly be a next level challenge! Following this thread to see what you work out, and what suggestions you receive :) 

#3 Richard O'Neill

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Posted 24 January 2021 - 10:50 AM

"I want the engineering challenge."



Then my advice is to carry on as your goal is certainly doable. Once the drive is able to keep a star centered in the field of view for the duration of an exposure, you'll be ready for AP. After all, getting there is far more than half the fun. It's all about the journey. waytogo.gif



Edited by Richard O'Neill, 24 January 2021 - 10:50 AM.

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#4 matt_astro_tx



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Posted 24 January 2021 - 11:18 AM

Thanks Richard! Definitely agree.

I should have clarified a little; I want the engineering challenge, but I don’t want to spend a year tweaking tolerances and learning machining skills to get there. It will be fun to build regardless, but what degree of tolerance is demanded of a mount for <5 min subs (I’m backing off my goal a little). Hoping someone on here has experience with pipe mount AP.


Truthfully I guess it will be determined by how long I can shoot without trailing. Right now it’s 1.5 seconds.

Edited by matt_astro_tx, 24 January 2021 - 11:21 AM.

#5 hcf


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Posted 24 January 2021 - 12:14 PM

My advice is not to rely on just tracking but also plan for guiding. Guiding is a blessing for DIY mounts which might otherwise have accuracy problems. It does add a significant software interfacing component to your plans.


If you use the ONSTEP framework, for your controller, you can use existing software to control and guide your mount.




If you are confident doing the software work yourself, you can probably have a simpler design than ONSTEP,  and have more control on features/bugs etc.


I motorized a manual EQ mount and added RA guiding to it as described here.



As a reference point, with tracking but without guiding I can get 30s shots with a 400mm FL scope and a DSLR. With guiding I can get 5min shots.

Edited by hcf, 24 January 2021 - 12:28 PM.

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#6 Richard O'Neill

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Posted 24 January 2021 - 12:32 PM

 It all depends on your goals. Required tracking accuracy is related to focal ratio and time. High focal ratios require proportionally greater tracking accuracy than low focal ratios.


  I assume you will be working at prime focus, which puts constraints on the brightness level of objects that can be recorded in 5-10 minutes.

Edited by Richard O'Neill, 24 January 2021 - 12:48 PM.

#7 gregj888



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Posted 24 January 2021 - 01:16 PM

It is both difficult and expensive to get a mount capable of AP tracking without guiding.  It can be done, look at what Dan Gray does with the Sidereal Tech mounts including the Alt-Alt and Alt-Az mounts.


Much less expensive to add a small inexpensive camera and guide.


Will add a +1 for Onstep (or Sidereal Tech) controllers.    You may also want to look at pillow block bearings at least for RA.


Food for thought stuff:

- there were some 180:1 harmonic drives on Ebay a month or s ago, might be OK with guiding.

- you can make a worm yourself, pretty easy with a lathe or mill if you use a plastic. 

             1/2" thick, run a tap against it say 18 TPI.

             Then get a piece of Acme all-thread say SAME tpi x 1/2", and cut some slot in the threads at one end. 

             Use that end like the tap to form the Acme thread in the big disk (A regular tap is sharper but the Acme form is better, IMHO).

             Us the uncut all thread as a work

             This isn't a Byers, but can be made to work.  You can do the same with Aluminum, best on a lathe (google for details "DIY Worm Gear"


There are a lot of old mounts, you may be able to cannibalize one of them, check you local club... know we have stuff like this in the Portland club and I have an 8" Meade SCT Quartz mount here (don't think it's worth shipping but you would be welcome to it). 

Edited by gregj888, 24 January 2021 - 01:17 PM.

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#8 VTstar


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Posted 26 January 2021 - 09:27 PM

I added guiding to my barn door mount and it drastically improved my guiding times. I used an Arduino and wrote an Ascom driver using the template in visual studio and just used some basic serial commands to tell the Arduino to speed up or slow down (right ascension only). It worked well with phd2. I really enjoyed the fun of seeing the mount come to life. But I have been playing with it for years, but it’s what I like to do, so I’m ok with it, and it’s not my day job.

#9 Beeham


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Posted 27 January 2021 - 07:20 AM

I think it's totally feasible, and I endorse other posters' recommendation for guiding as well.


One way to conceptualize the challenge: you don't need parts that are dimensionally perfect (difficult to do w/out machine tools), you just need fits that are tight so as to avoid mechanical slop. You can guide-scope your way out of a lot of problems, in principle, but if the mechanism isn't rigid and the motion tightly coupled to the the action of the motors, it won't work.


The good news is that tight fits are totally within the scope of hand work and ordinary shop tools.  I've produced parts accurate to 0.001" using drill press and table saw...it just takes a bit of engineering, and you may deviate from the manufacturers' recommended instructions in some cases.


I say go for it!

#10 steveastrouk



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Posted 27 January 2021 - 08:41 AM

Add position feedback, from an encoder like a Renishaw Atom.  They aren't cheap, but if you want precision, that's the way I'd go

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