Topics

Equatorial platform resolution


Jesse Lichtenberg
 

Hello everybody,
 
I was hoping for some advice on stepper motor resolution for a new project and thought I'd see what you all thought since you were so helpful with my OnStep project.
 
I'm wanting to try some very rudimentary astrophotography with my dobs and an equatorial platform. I am not expecting much - I'd be thrilled with 10 second subs. 5 seconds, even.
 
One of the dobs runs OnStep, but obviously won't be tracking when on the platform. Using Howard's excellent spreadsheet, I'm looking at a calculated resolution of 0.03 arc-seconds, but more realistically estimated at 0.38 arc-seconds due to the high use of microsteps (256) and inherent inaccuracy. My setup is a 200 step motor, 99:05:1 planetary gearbox, and 8:1 belt drive gear reduction. I'd prefer not to rely on microsteps for 'accuracy' since that's not really their actual intent (more for smoothing).
 
I have one design idea in my head that is a bit out-there for me, but basically would increase my belt-driven gear reduction to 512:1. That's a resolution of 0.128 arc-seconds with no microstepping at all. Adding microstepping gets it significantly farther, in theory.
 
Is it even worth going this 512:1 route? I think it'd be fun, but perhaps that level of precision is wasted on a homemade equatorial platform for imaging with a dob that was never designed for imaging. Am I wasting my time here and better off just building it as I've designed, with an estimated 0.38 arc-second resolution? Image scale is small at 0.9ish. I'm not ready to dump money into a GEM build or CCD camera yet. I'll have maybe $100 in the equatorial table when all said and done.
 
Thank you all!


Howard Dutton
 

I'm not following, you're talking about a drive for an equatorial platform right?  Off topic but ok...

So what is the final reduction that the base, rollers i guess, have in combination with the curved sector?  That is, how many turns of those rollers to rotate the curved sector 360 degrees (if it were continuous.)  Knowing the sector radius and roller radius can provide the answer to that.


Oscar Lithgow
 

Hello everybody,
 
I was hoping for some advice on stepper motor resolution for a new project and thought I'd see what you all thought since you were so helpful with my OnStep project.
 
I'm wanting to try some very rudimentary astrophotography with my dobs and an equatorial platform. I am not expecting much - I'd be thrilled with 10 second subs. 5 seconds, even.
 
One of the dobs runs OnStep, but obviously won't be tracking when on the platform. Using Howard's excellent spreadsheet, I'm looking at a calculated resolution of 0.03 arc-seconds, but more realistically estimated at 0.38 arc-seconds due to the high use of microsteps (256) and inherent inaccuracy. My setup is a 200 step motor, 99:05:1 planetary gearbox, and 8:1 belt drive gear reduction. I'd prefer not to rely on microsteps for 'accuracy' since that's not really their actual intent (more for smoothing).
 
I have one design idea in my head that is a bit out-there for me, but basically would increase my belt-driven gear reduction to 512:1. That's a resolution of 0.128 arc-seconds with no microstepping at all. Adding microstepping gets it significantly farther, in theory.
 
Is it even worth going this 512:1 route? I think it'd be fun, but perhaps that level of precision is wasted on a homemade equatorial platform for imaging with a dob that was never designed for imaging. Am I wasting my time here and better off just building it as I've designed, with an estimated 0.38 arc-second resolution? Image scale is small at 0.9ish. I'm not ready to dump money into a GEM build or CCD camera yet. I'll have maybe $100 in the equatorial table when all said and done.
 
Thank you all!



As an EQ platform builder I can advise you not to worry too much about your drive resolution (I think in your case you have more than enough), but more about your platform’s tracking surfaces smoothness.

Every micrometer of imperfections on your tracking surface will be greatly amplified on the eyepiece, especially on high magnifications. Also the alignment is very important, and particularly on eq platforms is very difficult to get a near perfect polar alignment, cause of the design.

I have improved my platform by smoothing all the tracking surfaces. I was able to bring my exposure times form 1-2 sec, to up to a constant 8 sec without trails on high mags (225X +). And that is with simple geared DC motor with speed control for driving.

 

What type of platform design do you have?

 

I am also thinking a way to use onstep to implement PEC or autoguiding on a eq platform, by installing a motor to in a wat that could Tilt the platfrom angle by a few degrees (+-1) or so, corresponding to the DEC adjustments. That way one could convert a simple tracking platform to a really capable astrophotography platform, even for BIG DOBS!


Jesse Lichtenberg
 

Definitely not on topic for OnStep Howard, but somewhat related so I very much appreciate the both of you humoring me here. I've asked for some advice from ATM and Arduino folks but you all are the only ones I'm aware of who are pretty familiar with both. A brief description of the build and some photos are here. It is not quite finished though - I want to add a bit more to the gearing design and reinforce the axis of rotation. It is a simple 'Gee' type Poncet platform. I toyed around with vertical north segments but I think this is more simple.

I'm not driving the base with a powered roller on the north or south bearings (they are all passive). Instead there is an arm towards the rear of the platform on which a gear is mounted along the polar axis. A stepper will drive this with either a GT2 belt or I might go with a brass worm/wheel setup (still tinkering there).

Oscar, what you're saying about the tracking surfaces makes perfect sense. I think perhaps that I may be wasting time and energy in designing a more accurate system when the 3D printed bearings rolling on stainless steel rods will just throw a wrench in things. I will print them in high detail and sand/polish them afterwards, but I think you are correct that they are indeed the weakest point. They are designed for my latitude (down to a hundred thousandth of a degree), but hobbyist 3D printers aren't exactly known for producing extremely precise results. 

I love your idea of using OnStep for PEC or autoguiding! You're far beyond my skills there but I'm hoping to learn more as this project progresses.

Thank you both!