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Motorized Dobsonian

ATM DIY Dob
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#1 mwaidelich

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Posted 24 January 2023 - 08:38 PM

I am re-posting in this forum as it seems closer to what I am interested in. I need some advice. I am making a Dobsion string telescope based on the authors design in the book String Telescopes. I am primarily interested in visual astronomy, but don’t want to eliminate the possibility of astrophotography.  I have ground a very light 11 ¼” mirror and want to take advantage of its weight (5 ½ lbs.) to make a lightweight, portable scope with GOTO and tracking. I am looking at Astro-Gadget’s AZStarPro controller. I am not worried about building the mount but am finding little info on motor drive specifics.
The controller uses type 17 or 23 NEMA stepper motors. I want to mount a rubber roller on the motor shafts and have that drive the alt and az axes with pressure on some as yet unspecified surface material. I like this approach because of its simplicity, lack of backlash and light weight. This scope will be fairly light.  Highe’s strut, string and total optical assembly weighed 23 lbs. My mirror is 2 lbs. lighter.
Can this work? Do I need gearboxes on the motors? What am I failing to consider? Thanks for your advice!

-Marc



#2 TOMDEY

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Posted 24 January 2023 - 09:34 PM

Hi, Marc... Have you made other telescopes before? How far along are you on this one? Ultralight, Alt Az, Dobsonian, String --- those all disfavor imaging applications other than the moon. Although possible to attmept other targets, it's riddled with problematic gremlins.    Tom 


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#3 drneilmb

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Posted 24 January 2023 - 10:05 PM

I doubt that you can make an 11.25" portable reflector that weighs 23 pounds as your first telescope.

When I was designing my third telescope, a 12 inch lightweight dob, I got curious about what was possible and I collected information on many, many telescopes.

https://docs.google....jjhIXr9y2ccCtPA

What I found was that weight in pounds divided by aperture in inches squared fell in a fairly narrow range of 0.1 to 0.6 where smaller values denote greater "efficiency". Commercial tube dobs are around 0.5-0.6. Regular amateur truss dobs are around 0.3-0.4. Below 0.3 are ultralight telescopes both amateur and commercial. Getting below 0.2 is a tremendous feat (the most "efficient" scope on the list right now is a 16" f4.5 that weighs 28 pounds at a value of 0.109).

String telescopes are not among the most weight efficient in my data. At your aperture, a 23 pounds scope would have an efficiency value of 0.18 which is world-class. It is physically possible to make an 11.25" reflector that light, but it is also an engineering triumph to do so.

You might be a triumphantly good telescope designer, but I think that you would be better served to aim for an efficiency around 0.3 or a weight around 35 pounds ... just like Tom Dey's 36 incher that weighs 400 pounds. ;)

-Neil
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#4 mwaidelich

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Posted 25 January 2023 - 08:16 PM

Thanks for the replies! I think I wasn't clear in my post about my objectives with this scope. Lightweight is good but not the driving force for decisions. The scope I am basing this on weighs 35 lbs. for a 12 1/2" f5. I think I will be close to that for my 11 1/4" f5.3 and that is fine. My first scope was i 6" equatorial and I ground the mirror and built the mount from machined plumbing parts and wood.. I am not worried about building this dob mount. What I am most interested in is drive details. I want to use a friction drive and am interested in ideas about materials and gear ratios. I have seen a lot of approaches to this and wonder what people are currently thinking. Thanks



#5 gordtulloch

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Posted 25 January 2023 - 10:39 PM

I am re-posting in this forum as it seems closer to what I am interested in. I need some advice. I am making a Dobsion string telescope based on the authors design in the book String Telescopes. I am primarily interested in visual astronomy, but don’t want to eliminate the possibility of astrophotography.  I have ground a very light 11 ¼” mirror and want to take advantage of its weight (5 ½ lbs.) to make a lightweight, portable scope with GOTO and tracking. I am looking at Astro-Gadget’s AZStarPro controller. I am not worried about building the mount but am finding little info on motor drive specifics.
The controller uses type 17 or 23 NEMA stepper motors. I want to mount a rubber roller on the motor shafts and have that drive the alt and az axes with pressure on some as yet unspecified surface material. I like this approach because of its simplicity, lack of backlash and light weight. This scope will be fairly light.  Highe’s strut, string and total optical assembly weighed 23 lbs. My mirror is 2 lbs. lighter.
Can this work? Do I need gearboxes on the motors? What am I failing to consider? Thanks for your advice!

-Marc

I'd encouraged you to consider the OnStep controller for your purpose, and take advantage of the web site which has a very large gallery of example scopes built using the controller:

 

https://onstep.groups.io/g/main/wiki

 

In particular have a look at the Wemos R32 with CNC V3 version of the hardware, I'm retrofitting a CG5 mount with that setup now and it's super light and portable.

 

 


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#6 mwaidelich

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Posted 26 January 2023 - 12:17 PM

Thanks for the reply. Onstep is right at the edge on my comfort zone. What would be the advantages over the Astro-Gadget controller?



#7 gordtulloch

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Posted 26 January 2023 - 07:16 PM

Thanks for the reply. Onstep is right at the edge on my comfort zone. What would be the advantages over the Astro-Gadget controller?

Half the price (and includes motors) for one. Although $198 isn't a lot either, say another $50 landed for motors. Support from AstroGadget might be an issue, seems like a one person shop so easy to get orphaned vs a community supported Open Source solution. I would be really surprised if the AstroGadget isn't Open Source under the covers.

 

On the flip side you need to build or buy a case, and adding wifi or bluetooth means a more complex setup. The latter is irrelevent if you're planning on using something like an RPI or ASIAir as a computer (since they provide the Wifi/Bluetooth).  If that's past your comfort then yeah you should probably go for the commercial device. Do look at the gallery on the OnStep wiki for different drive setups tho.



#8 tommm

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Posted 27 January 2023 - 01:39 PM

 Chad, on OnSteps groups.io site, sells Onstep assemblies commercially. He also is in the final stages of finishing a complete wireless hand controller. He 3D prints the cases which are included. He knows OnStep well, and you could also post any questions on the groups.io site. He mainly sells kits for a specific mount, but he may sell you a controller and handpad for your scope.  NEMA 11 motors would work fine for an 11.5" scope.  They have more than enough torque with the gear reduction you will require. You can of course use NEMA 17 too, or NEMA23 but that would be large overkill, adding unnecessary weight.




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