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New Dob Owner

John K.
 

Hi everyone, I am new to the hobby, and recently purchased my first scope.  It is a used, but in new condition, Orion XT10 Plus dob.

I really wanted to get a goto, but the price was way outside my budget.  I found this group, and have a few questions about converting my scope.

I have some limited CAD ability, and I have been 3D printing for many years.  I have started to design some mods for my scope that I think will work, but I wanted to see if anyone has any input or suggestions.

I am thinking about using 3d Printed pulleys for GT2 belts to drive both the Alt and Az.  I think the mount would be compatible with a new bracket for the optical tube that I could design and 3D print, and would have an integrated pulley.  It would be large, around 240 teeth for the Alt, and 320 teeth for the Az. I am thinking about using a 9mm wide gt2 belt using either Nema 17 or 23 steppers.

Does this sound do-able?  

Any input would be appreciated.

Khalid Baheyeldin
 

Before you start, read the showcase page, and scroll down to Dobsonian.

There are quite a few similar to what you want, and you can get ideas from there.

After you have a better idea on what people did, you can ask questions.

John K.
 
Edited

Thanks, I had not seen those yet.

What I am thinking looks a bit like a hybrid between two of those, so I think it might work.  I am wanting to keep as much of the original mount intact as I can.

I will continue to work on the design and refine the mechanical stuff, and worry about the electronics later on.

John K.
 

I have been diving into the messages, and I think I now have a pretty good handle on what will be involved.  

I have finalized my initial prototypes, and I am printing a new bracket with a 320 tooth GT2 pulley.  I'll post a picture when it is done.

Of the following 2 options, would there be any benefit to one over the other?  They would give the same resolution

1.  A 20 tooth pulley on the stepper, with a 50:1 gearbox
2.  A 40 tooth pulley on the stepper, with a 100:1 gearbox





John K.
 
Edited

The alt bracket is done, the total printing time was 19 hours.  I have it mounted on the scope, and I think it will work.  I just need to figure out stepper locations and belt lengths needed.  I am planning on making some sort of  tension-ing sytem so that I can easily take the tube off the base for transport.  Here are some photos:

Khalid Baheyeldin
 

Innovative design for sure. I have not seen anyone take this approach before.

The only drawback I can think of, is that you have to put the belts on before
every session, tighten it, and then take them off after. Make sure you have
spares in case they break.

John K.
 

I will definitely order a couple of spares.  I am also thinking about someway to modify it so that the pulley stays in permanently.

Butchf
 

Very nice, will you share the STL for the Pulley/

John K.
 

Definitely, as soon as I have it all working.

Dave Schwartz
 

However, the benefit is that the belt only ever makes a partial rotation so you can build them out of GT2 belt 'strip' with joiners (several examples of that on any 3D printer) and open-ended belt strip is cheap.

For tensioning, I would use an idler pulley on an arm with a spring to tension. This way it can back off and skip if the telescope is moved by accident, since the OTA will have far greater leverage on the belt than the usual worm intermediate belt.

On 2019-09-29 1:12 p.m., Khalid Baheyeldin wrote:
Innovative design for sure. I have not seen anyone take this approach before.

The only drawback I can think of, is that you have to put the belts on before
every session, tighten it, and then take them off after. Make sure you have
spares in case they break.

John K.
 

Thanks, some sort of spring tensioning is exactly what I was thinking.

I can get away with an open belt for the Alt.  With the limitations of the design I am going to use for Az, I think I will need a closed loop belt for that one.

Dave Schwartz
 

It will be very challenging to find a continuous belt in such a large size.

On 2019-09-30 11:54 a.m., tj_slice via Groups.Io wrote:
Thanks, some sort of spring tensioning is exactly what I was thinking.

I can get away with an open belt for the Alt.  With the limitations of the design I am going to use for Az, I think I will need a closed loop belt for that one.

John K.
 

I ordered the belts from Misumi USA.  They are 9mm wide GT2 belts.  One is 810mm long, and the other is 950mm.  With shipping, they were $33.

I am finalizing my electronic choices.  With a 100:1 nema 17, the lowest amp motor I can find is 1.68, so I guess I will have to use DRV8825 drivers?

Does all of this look good for the MKS Gen-L?

Ebay 100:1 Nema 17 Steppers
Ebay DRV8825 drivers
Ebay Wifi - D1 Mini

I still need to decide on a power supply.  I am open to suggestions.

Khalid Baheyeldin
 

You are better off with other drivers, since the DRV8825 vibrate a lot.

The amps for this motor are high, and the only drivers that can drive it is the S109.

John K.
 

Thanks.  Will the DRV8825 work at all?  I can't find the S109 anywhere except ebay from China or Hong Kong, so those would be 3 to 4 weeks in shipping.

If the 8825 will work, I can use them initially, and swap them out later for the S109.

Khalid Baheyeldin
 

Depending on how you define 'work'.
The DRV8825 has enough amps for the 1.67A motors.
But it is notorious for vibration which makes tracking inaccurate (there are other factors involved).
See this video https://www.youtube.com/watch?v=I8Kb_BMNIaA

The S109 has been used for large motors and large mounts, and the proven to be superb.

There are also the TMC5160, which can do up to 3A RMS, but they lack the upward pointing pins that the TMC2130 SPI V1.2 has (that latter one can only do 1.2A).

The TMC5160 very new in OnStep, so I would not feel comfortable recommending them yet.

Try AliExpress, maybe with some additional shipping, you can get them fast.

John K.
 

Ok, will do.  Thanks for all of your help, I certainly appreciate it!!!

John K.
 

I've got everything ordered except the steppers and a power supply.  I got the S109s from AliExpress, and delivery shows in 2 weeks.

I've narrowed it down between 2 steppers.  One of them advertises a 'high precision' gearbox, but is about twice the price.

100:1 Nema 17 from stepperonline

High Precision 100:1 Nema 17 from stepperonline

The higher precision one shows about 4 times the torque and 25 arcmin backlash, vs 1 deg for the cheaper one.

For this application, is the high precision stepper worth the added cost?

Khalid Baheyeldin
 

On Tue, Oct 1, 2019 at 01:50 PM, <tj_slice@...> wrote:
I've narrowed it down between 2 steppers.  One of them advertises a 'high precision' gearbox, but is about twice the price.

100:1 Nema 17 from stepperonline

High Precision 100:1 Nema 17 from stepperonline

The higher precision one shows about 4 times the torque and 25 arcmin backlash, vs 1 deg for the cheaper one.

For this application, is the high precision stepper worth the added cost?
I find it odd that 'high precision' is 25 arc minutes, when my 18:1 NEMA11 has 110 - 130 arc SECONDS.
This is not from a spec sheet, it is what I found out pointing to a terrestrial object.
But it is a Vexta made by Oriental Motors of Japan and uses spur gears, so not a fair comparison.

My opinion: Since this is an Alt-Az and for visual only, backlash should not be a big issue, and
it can be compensated for within OnStep.

Can some people who used 100:1 with Alt-Az mounts please comment with your experience?

T.K
 

Really nice work.
I'm working on my Onstep design for my SkyWatcher 200p classic for nearly a year.
I didn't think that a 3d printed bracket can handle the OTA weight. Now I should consider it. Can you tell about this print properties? (material, layer thickness, etc)
My design is based on 3 sets of 9:1 pulleys (135 and 15 teeth) that results a total reduction of 729:1 for each axis. It's become complex, large and heavey due to use of several alumunium sheets in design and changing position of support bearings of altitude. So after watching your approch, I decided to redesign my assembly.
Here is link of my design in Fusion 360: Onstep Assembly Alt
Do you think about a clutch for manual pointing the OTA? Is it possible to point to celestial pole before turning Onstep on without a clutch? Wouldn't it hurt motors and drivers?
I will keep eye on your results.