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Re: Building a computer controlled telescope.

Subject: Re: Building a computer controlled telescope.
From: none <""gcain\"@(none)">
Date: Fri, 02 Sep 2005 22:54:53 +1000
Newsgroups: sci.astro.amateur
Roger Hamlett wrote:
"none" <""gcain\"@(none)"> wrote in message news:4317eceb$1@xxxxxxxxxxxxxxxxxxxxxxxxx

Hi Guys,

I have recently decided to build a telescope, that is controlled by an old laptop I have. The idea is to eventually have the device hooked up to a website so people (from a different hemisphere, or just ones without a telescope) can login and browse the sky down here; plus it will keep me entertained while I build it! :P

Since I have basically no knowledge of astronomy or telescopes please forgive my ignorance. I'm a programmer and enjoy playing around with electronics so I figured hooking something like this up would be hours of fun and be a nice way to enter the astronomy field as a hobby.

So far I have been browsing the web and found many great sites with some fairly good information on construction of both the telescope and the electronics.

The only question I haven't been able to find an answer to is one about focusing. I will have the Telescope hooked up to a small camera (model yet to be decided on so any suggestions welcome!). Will there be a need to refocus if I move from one location in the sky to another? I have had conflicting answers to this, and it seriouly affects some of my design decisions.

Does anyone have any suggestions or links to sites on how to hook a camera to a telescope?

Any information that you can give on this project will be most appreciated.

Thanks to all in advance!

You should consider having the ability to refocus. Not necessarily 'if I move from one location in the sky to another', but to handle temperature changes. The telescope, may also show some movement across the sky as well (depends on the design). Look at a RoboFocus (or build your own - email me if you are interested in this approach). The problem is that for example, if you have a scope that is (say) a F/6 unit. The 'critical focus zone', is just 79um long. For a F/4 scope, the CFZ, is just 35um long. Now firstly, positioning the focusser to this level of accuracy is _hard_. Focussers that work fine for eyepiece use, are often unacceptably coarse for imaging. When you use a scope through an eyepiece, your eye will compensate for fractional focussing errors, but the imager, does not do this. Now aluminium, typically (depending on the alloy actually used), has a thermal expansion co-efficient of 23.1um/M/C. So even a simple scope like a 6" F/6 Newtonian, built of this material, will show focussing errors for a 3C change in temperature. In the case of the typical SCT, which has a larger CFZ, because of being F/10 (220um), but unfortunately, the expansion in the tube is optically magnified by the configuration, and shows problems for a similar change. Cardboard tubed Newtonians, are pretty good in this regard, but will show long term drifts from changes in humidity, and even on these, the changes over days and weeks, will need to be dealt with...

Best Wishes


Hi Roger,

Is the RoboFocus anything more than a server or stepper motor hooked up to the focusing ring/knob of the scope? I had a look at the RoboFocus website and it seems, from the pictures and brief descriptions, that they have simply built a bracket and then used a belt to link the motor and the focusing together.

If this is the case, then building one might be something that's very achievable.

Regards,
Garreth

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