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Autoguiding Parameters

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#1 orlyandico

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Posted 20 November 2012 - 10:30 AM

I finally did some math around autoguiding parameters, so here goes..

1) assume a guide scope focal length of 200mm (finder guider) and a Meade DSI guide camera, because.. that's what I have. The pixel scale of the guider is 9.7" / pixel

2) assume the seeing limit is 1.5" (for most people it will be around 2.5" to 3") - 1.5" is 0.16 pixels on the guide scope

3) by the above reckoning, a Min Move of less than 0.16 is worthless, because you'd be chasing the seeing. Yes or no?

4) if I decide to be conservative and choose say a 0.25 pixel min move, that means my main scope cannot be more than 4X the focal length of my guide scope.. (assuming similar-size pixels, which in my case is also true)

5) which means the famous "the guide scope can be 1/10th the focal length of the main scope" isn't really reliable...

Comments?

#2 averen

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Posted 24 November 2012 - 01:19 PM

3) by the above reckoning, a Min Move of less than 0.16 is worthless, because you'd be chasing the seeing. Yes or no?


This depends on your Auto guider exposure time. If you use a longer exposure time this will allow the star to "bounce around" because of the seeing and you'll be left with a star that better approximates the actual position of the real star, thus negating some of the affects of seeing. So you can guide at lower arcseconds/pixel than the seeing provided that you allow your exposure time to be long enough that the seeing averages out over the star.

4) if I decide to be conservative and choose say a 0.25 pixel min move, that means my main scope cannot be more than 4X the focal length of my guide scope.. (assuming similar-size pixels, which in my case is also true)

5) which means the famous "the guide scope can be 1/10th the focal length of the main scope" isn't really reliable...


The above helps to answer this. But there are a lot of variables at play here that can affect your guide scope and main scope sizing. I previously imaged with a 1625mm scope that put my arcseconds per pixel around .68 (highly over sampling). I used a 200mm guide scope with great success (1/8th the FL of the main scope). I was using an SSAG to guide with and if I recall that put my arcseconds/pixel with the guider around 5 arcseconds/pixel. I ended up selling the longer FL scope because it was too slow and not a great match for my camera without binning.

I would focus more on the arcseconds/pixel difference between the two scope rather than the focal length. But if both cameras have the same sized pixels then this really comes out to mean the same thing.

Jared

#3 orlyandico

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Posted 24 November 2012 - 04:16 PM

Thanks Jared. My guide scope and main scope have (more or less) the same pixel size, so it does even out.

The challenge now is declination misbehavior, but it seems my "rule of thumb" kind of works well. I am using a 200mm guide scope and imaging at 1900mm (C9.25 with a refractor reducer/flattener) and I can get round stars when the everything works.






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