Stepper motor selection for Sampson eq mount

[backman (Clive Stewart)]

Hi guys I was wondering if someone can help me with stepper motor selection for a Sampson EQ mount. I acquired the mount on the weekend and I am in the process of refurbishing it. The mount doesn't have any drive at all not even manual drive, so I thought I would do something different for the drive system. I was thinking of either going with a friction drive or cogged belt drive as I want to eliminate PE as much as possible as the mount will be used for Astrophotography, and I don't want to go the traditional worm drive for that reason. The mount will be pier mounted which has 240v and 12v available. And I can get the parts machined locally for the friction or belt drive.
So my My questions are: has anyone here made a friction drive or Belt drive for an Eq mount like this? Which would be better? What ratios do I need for the drive? Do I need to use a particular Spec/make/model Stepper motor to get the best results? and finally can I use The sound Stepper system to drive this mount? How do Get the sound stepper system or Do I need to make the components myself? I no there are lots of Questions Guys But I thought this would be a good place to start. Oh and by the way an 8" Newt will be mounted on it. for now! Any help would be greatly appreciated. Clive

[Barrykgerdes]

Hi Clive

There are two ways of looking at a project like that.
1. As an execise in your mechanical ingenuity to design and build a drive mechanism to do a job that is much easier done in other ways.
2. As a way to turn an old design equatorial mount into a mount for photography

If the aim is way 1. you could start up by reading up on the Bartel's system. I built a friction and toothed belt drive for a Dobsonian mount using the drive motors and controller for a Tech 2000 system about 20 years ago but it was not stable or smooth enough for photography.

However if the aim is to do astro photography you will probably need to spend enough money to have purchased a PME mount by the time you get to mounting that telescope on it.

The Sampson mount in its day was great for astronomy work but no where near as good as the simpler modern off the shelf equipment such as an EQ6 which is about the minimum required to support a short tube 8" Newtonian.

Barry

[bojan]

Yes, Barry is quite correct.

I used friction drive on my 10" F5.6 dobson (with Bartels software), I tried photography (30 sec) and found no issues re PE (the potential problem was field rotation, and expecting that, I aimed at objects near equator / meridian intersection (where the field rotation is minimal).
See here for mechanical details.

SoundStepper is an adequate replacement for Bartels software (which runs on DOS and w97 in cmd mode only), it has some limitations (no provisions yet for mechanical imperfections like non-orthogonality of mount shafts, no encoders interface) but those are not really significant, as they are important only for high precision GoTo.
Electronics for SoundStepper you must build yourself.. I would recommend to you this approach, as discussed in thread..

Steppers are not critical, try ebay (NEMA17, 200 or 400s/rev.. for Allegro driver 4-10 ohms, bipolar perform better than higher resistance - higher voltage types.. again, for details have a look at the abovementioned thread)

Ratios.. well you have to calculate/measure them yourself.
I would recommend ~1:12 or more for main shaft, as this reduces the PE generated in previous steps by factor of 12 (provided wheels are concentric). You will have to measure the actual ratio yourself, as it will not be exactly the desired number (due to tolerances of the turning machine, and to some extend it will depend on applied pressure, due to material elasticity)
The rest may be optional - additional 1:4 ~ 1:8, in combination with microstepping (16 if Allegro drivers are used) should be adequate.

Material used for friction drive components should be brass (or steel) for wheels, steel for driver shafts

[backman (Clive Stewart)]

Thanks very Much for the comments and information guys very helpful.
I like a good challenge so I will continue with my initial Plans to use the sampson. Clive

[Poita (Peter)]

Just as an aside, would soundstepper be able to drive a stepper for another project I have... driving a 16mm film projector, i need to advance the film a frame at a time under computer control.

[bojan]

Originally Posted by Poita

Just as an aside, would soundstepper be able to drive a stepper for another project I have... driving a 16mm film projector, i need to advance the film a frame at a time under computer control.

Yes of course.. it can drive 4 motors at the same time.
You just have to enter the appropriate reduction ratio for "tracking"

[torana68 (Roger)]

do you have a photo of the mount?

[backman (Clive Stewart)]

Hi Roger, I can put up a picture if you want. Do you Know a bit about the Sampson mounts?

Cheers Clive

[torana68 (Roger)]

just interested to see which one of the variations you have