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ASIAir and a coma corrector


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Hello, the focal length setting in ASIAIR when using a skywatcher 150 PDS with its coma corrector....do I need to adjust the focal length setting because of the corrector and if so how do I work out what that number will be? The out of the box focal length is 750 which makes it an f4? but with the corrector its F3.6 which means the focal length setting has shortened?  

Many thanks 

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The 150PDS is f5 (750/150) and the modified focal length depends on the the specific coma corrector you use. There are coma correctors which don’t modify the focal length and others which do. The Sky-Watcher least expensive one has a x0.9 focal length multiplier action so 750mm reduces to 675mm.

Edited by Avocette
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50 minutes ago, Avocette said:

The 150PDS is f5 (750/150) and the modified focal length depends on the the specific coma corrector you use. There are coma correctors which don’t modify the focal length and others which do. The Sky-Watcher least expensive one has a x0.9 focal length multiplier action so 750mm reduces to 675mm.

Ahh, yes, you make it look so easy, which with hindsight it is! Thanks 

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If you don't know it, input 0 for the FL when you start the app. Once it does a plate solve/preview it will work out the focal length automatically.

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1 hour ago, Elp said:

If you don't know it, input 0 for the FL when you start the app. Once it does a plate solve/preview it will work out the focal length automatically.

This is what I do all the time incase of subtle changes when sorting my back spacing. My Skywatcher 72ed is 420mm FL when I add my ovl FF and subsequent backspacing my AAP works out my FL to 433mm.

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