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Edge HD8+ASI 1600 w. reducer back focus 160 mm. Can it be correct?

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#1 lemmu

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Posted 21 February 2020 - 04:32 AM

According Celestron the back focus should be around 105. mm. I'm achieving focus at 160 mm with spacers. Can it be right? Does anyone else has the same back focus? Check the pic  IMGP4405.JPG . Opinions appreciated.



#2 sharkmelley

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Posted 21 February 2020 - 05:05 AM

I notice this is your first post - welcome to Cloudy Nights!

 

No doubt there is a wide range of spacings where you can achieve focus but only one spacing (i.e. 105mm) will give you the specified 0.7x reduction.  For best results use the reducer at the specified spacing.

 

Also, 160mm is quite extreme - I assume you are trying to focus on stars or an object many miles away and you are not making the mistake of testing focus on an object much closer?

 

Mark


Edited by sharkmelley, 21 February 2020 - 06:05 AM.


#3 pyrasanth

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Posted 21 February 2020 - 06:54 AM

Achieving focus is different from achieving focus at the optimal result for field flatness and round star images. There is a significant difference on the edge optics if you go further out than the prescribed recommended distance. Celestron did not say you can't go further out but your results could vary from mediocre to really bad.

 

Not so bad for visual but very bad for a CCD or CMOS sensor that expects a flat field.




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