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what to look for when setting back focus up

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#1 iwols

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Posted 10 July 2020 - 06:54 AM

hi guys just wondered what to look for when setting back distance up on an imaging train ,is it the sharpness of the focus?



#2 spokeshave

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Posted 10 July 2020 - 07:11 AM

It depends on the imaging train. If you are not using a reducer/flattener, then backfocus is not critical at all. You only need to locate the camera image plane within the focus range of the scope.

 

On the other hand, if you are using a reducer/flattener, then backfocus - which is the distance from the camera image plane to the reducer/flattener - is critical. A reducer/flattener, as the name implies, flattens the field for a scope that has an otherwise curved field. A curved field means that different parts of the image plane will be in focus at different focal distances. So if the center of the image is focused, the corners will not be, and vice-versa.

 

Field flatteners are optimized to have the image plane a fairly precise distance away (backfocus). If the distance is not correct, the field will not be flat. So in optimizing backfocus, you are looking for sharpness of focus both in the center of the image and in the corners. 

 

Tim


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#3 iwols

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Posted 10 July 2020 - 07:15 AM

thanks tim im just adding a reducer to the ccd filter wheel and oagwaytogo.gif  if its slightly out do i just get a lack of focus then and that can be anywhere on the image?



#4 spokeshave

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Posted 10 July 2020 - 07:45 AM

It depends in whether you are short or long on the backfocus. In general, though, if you get the center focused, the outer parts of the image will be out of focus and aberrated of the spacing is wrong. Typically, the stars in the corner (when the center is focused) are either elongated radially or tangentially. 

 

Tim


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#5 iwols

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Posted 10 July 2020 - 07:48 AM

thanks timwaytogo.gif



#6 imtl

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Posted 10 July 2020 - 07:58 AM

When your backfocus is too long your stars will look like this:

post-38168-0-40757000-1591723894.jpg

 

If the backfocus spacing is too short your stars will look like this:

 

post-38168-0-25209200-1591723855.jpg

 

Eyal


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#7 iwols

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Posted 10 July 2020 - 08:46 AM

When your backfocus is too long your stars will look like this:

post-38168-0-40757000-1591723894.jpg

 

If the backfocus spacing is too short your stars will look like this:

 

post-38168-0-25209200-1591723855.jpg

 

Eyal

thanks that explains itwaytogo.gif




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