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Re: Building a computer controlled telescope.

Subject: Re: Building a computer controlled telescope.
From: Chris L Peterson
Date: Fri, 02 Sep 2005 13:37:25 GMT
Newsgroups: sci.astro.amateur
On Fri, 02 Sep 2005 22:54:53 +1000, none <""gcain\"@(none)"> wrote:

>Is the RoboFocus anything more than a server or stepper motor hooked up 
>to the focusing ring/knob of the scope?

That is essentially what the RoboFocus is. The RoboFocus controller
keeps track of the step count (the focus position) in non-volatile
memory, effectively closing the loop on position (technically it is open
loop, but can be considered closed loop because it doesn't skip or lose
its position). You would want to make sure your focuser could do the

A conventional RC type servo won't work well, since it doesn't have fine
enough resolution. Depending on your scope type, you need positioning
resolution somewhere between hundreds of steps per turn (SCT) to
thousands of steps per turn (rack-and-pinion).

You may have to refocus as often as every 30 minutes or so, depending on
what the temperature is doing. Focusing needs to be closed loop- you
base the focus position on the quality of the image. The algorithms to
do this are not complicated, but do require a moderately bright star in
the field. This means you might have to move the telescope away from
your object of interest just to focus, so your code might need to have
the ability to examine a catalog and figure out where to go. Most mounts
are not accurate enough in their goto systems to place an object
reliably in the center of an imaging chip without some assistance from
software to manage a  pointing model. Writing such a program is quite

Internet controlled telescopes come in two flavors: real time, where the
operator aims, focuses, and exposes, and job-based, where the operator
submits a job request and the scope services it when convenient,
depending on weather, time, and other activity. The latter is a much
more efficient approach, but perhaps less exciting to the user.

In any case, the basic code to do most of this is pretty simple. Much
more complex (and important) is the hardware and software needed to
protect the telescope. I had to modify my scope (LX200) by adding
external switches to put absolute limits on movement. In addition, I
have mechanical failsafes for my observatory roof and for detecting bad
weather. Of course, you can get away with less if you will always be
present at the scope when others are controlling it remotely.

Good luck- let us know how things work out.


Chris L Peterson
Cloudbait Observatory

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