astrodave
Posts: 5
Joined: Thu Apr 19, 2012 8:38 pm

Re: Guiding a telescope

Thu Apr 19, 2012 9:57 pm

Hi everyone

I hope to get a pi soon, I will be looking at using it to control my telescope mount. so I need t write a programme to control motor speed and direction and also have some kind of coordinate goto. I have absolutely no programming experience. the mount is driven by a 24v DC motor.Any advice would be greatly appreiciate

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scep
Posts: 1062
Joined: Sun Nov 20, 2011 8:53 am

Re: Guiding a telescope

Thu Apr 19, 2012 10:06 pm

Hi,

there's some good info here@

http://www.raspberrypi.org/for.....ontroller/

and other interesting stuff here:

http://www.raspberrypi.org/for.....otography/

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jbeale
Posts: 3700
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: Guiding a telescope

Thu Apr 19, 2012 10:13 pm

I'd start by reading this thread:

http://www.raspberrypi.org/for.....controller

and check out the openDrive project:


Chris.Rowland
Posts: 239
Joined: Thu Jan 12, 2012 5:45 pm

Re: Guiding a telescope

Thu Apr 19, 2012 10:46 pm

Do you mean guiding as in collecting an image of a star field, detecting any movement and issuing guide commands to a mount or working out conversions from a star position in Ra and Dec to motor axis positions and drive rates?

They are different problems, but both non-trivial.

There's another problem, to get star positions and send commands to an existing scope control system. It feels slightly easier but needs a star charting program to get the star positions. I'm not sure what is available, somebody managed to compile Stellarium but it may not have run very well.

astrodave
Posts: 5
Joined: Thu Apr 19, 2012 8:38 pm

Re: Guiding a telescope

Thu Apr 19, 2012 11:01 pm

Hi Chris

The guiding is done just by setting the speed of the motor to match the rotational speed of the earth sidereal time. which keeps the star stationary in the viewfinder for long exposure photos,I currently control the speed using a potentiometer and measure the voltage to set the speed, this I would like to control with the pi, the goto part uses a celestial longitude and latitude coordinates which I hope to use something similar to a setting circle all be it a virtual one.I think guiding using a second telescope with a camera and sending commands to the RA and Dec motors is a little to adventurous for my home made mount(just yet) Thanks for your interest

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scep
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Re: Guiding a telescope

Thu Apr 19, 2012 11:02 pm

[moved to projects]

Chris.Rowland
Posts: 239
Joined: Thu Jan 12, 2012 5:45 pm

Re: Guiding a telescope

Fri Apr 20, 2012 8:03 am

Hi Dave,

Sorry for the confusion, what you are describing is normally termed tracking. The simplest way to do sidereal tracking is to use a stepper motor. Then all that's needed is to keep the step rate constant.  With a DC motor you need some sort of feedback to make sure that the speed is correct. this is usually done with an encoder on the motor shaft and a microcontroller motor controller.

I've done this with a home made AltAz mounted scope using two stepper motors and Mel Bartels scope control software. This runs on an old laptop using DOS with the motors controlled using the parallel port. It's all on the internet.

I'd suggest changing to stepper motors because they are easier to control and use a separate microcontroller to handle the low level motor control - step rate and position in steps.  Then a Pi could be used to convert the axis positions to celestial coordinates and provide the user interface.  I'm not sure that a Pi could run the motors directly because there's so much more going on and getting the timing right could be a bit of a challenge.

Hope this helps.

astrodave
Posts: 5
Joined: Thu Apr 19, 2012 8:38 pm

Re: Guiding a telescope

Fri Apr 20, 2012 9:33 pm

Hi Chris, sorry for the confusion. I ment traking. I have tried a stepper motor and found the steps were to defined at slow speeds even with a gear box. I used a controler called a stepper bee+. the stepper motor has 200 steps per rotation. I have the mount tracking corectly with a dc motor and managed a 4 minuet exposure using my DSLR. just thought it would be good to be able to slew to an object automaticaly. but I'm open to sugestion. Thanks for the intrest

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jbeale
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Re: Guiding a telescope

Fri Apr 20, 2012 9:38 pm

I don't know about the stepper bee+ specifically, but if you have a micro-stepping controller I would think the effective step motion could be made very small indeed. Other things to consider is if you're near some mechanical resonance that the step frequency is exciting, and whether the current drive is higher than needed, with excessive torque steps generating vibrations.

astrodave
Posts: 5
Joined: Thu Apr 19, 2012 8:38 pm

Re: Guiding a telescope

Fri Apr 20, 2012 10:07 pm

Hi with the setup I have, gear boxes and timing gear drive belts, to acheive the rotational speed I require , I have to have around 60 miliseconds between steps and this ticking action dose not give me a smooth enough motion for imaging

Chris.Rowland
Posts: 239
Joined: Thu Jan 12, 2012 5:45 pm

Re: Guiding a telescope

Sat Apr 21, 2012 8:53 am

60 millisecs per step give 0.9 arc seconds per step and the steps will be visible.  Using a micro stepper controller with, say, 8 micro steps per step will get the step size to 0.11 arc seconds which should be fine.

I don't know about the stepper bee+ but have heard that the Easy Stepper supports microstepping and I believe that people have used this to run scopes.

There's a vast amount about this on the internet.

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