#21  
Old 10-10-2010, 09:57 AM
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peter_4059 (Peter)
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Grady,

Sorry - I was mistaken - the dx and dy are pixels not arc seconds. You should get dx and dy movements in pixels from DSS. Your first (reference) sub should show 0 for dx and dy and if you order the images by time in DSS you should see dx and/or dy steadily increasing in each sub. If you take the last sub this will show the total drift over the entire imaging session. Divide each of these numbers by the total integrated imaging time in minutes and you get drift in pixels per minute.

Rob,

You can go back and calculate your drift this way if you keep your old subs - just load them up in deep sky stacker!


Peter
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  #22  
Old 10-10-2010, 10:07 AM
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Here's the reference to this technique to measure flexure...

http://deepskystacker.wikispaces.com...ential+flexure
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  #23  
Old 10-10-2010, 10:11 AM
stanlite (Grady)
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then its 1.24 pixels in dy and .33 in dx lol
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Old 10-10-2010, 10:23 AM
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So that means your star will be elongated by one and a quarter pixels in the y direction for each minute of sub exposure duration. Which way do you have the camera orientated - ie is x lined up with RA or DEC?

The other thing to check - is this the same amount of drift regardless of where the scope is pointing - ie is it worse when the scope is pointing east/west/directly overhead etc.

Last edited by peter_4059; 10-10-2010 at 08:42 PM.
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Old 10-10-2010, 01:44 PM
stanlite (Grady)
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if i am remembering correct my dx axis was lined up with DEC and dy was lined up with RA.

also on the same night i imaged Tarantula neb and didn't have anywhere near the same problem. dy drift of about 0.4 and dx drift of 0.08 i think.
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  #26  
Old 10-10-2010, 07:29 PM
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Given you are piggybacked and using a dslr I'd be looking at the Konus OTA focuser first. Try to work out where the scope is pointing when the drift is worse - DeepSkyStacker live gives you the dx and dy on the fly and is a good tool if you want to try the scope in a few different positions to check the drift.
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