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Help with back focus math

Refractor Imaging
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5 replies to this topic

#1 Deep Blue

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Posted 23 July 2021 - 10:43 PM

Guys,

 

I need your help to figure out my back focus math.  I have a GT71 with a Flat 6AIII, a ASI533MC pro, a ZWO filter drawer and an adapter 42 to 48.

 

This is what I have so far:

 

According to the WO website the Flattener on a GT71 requires a back focus of 64.1mm.

 

The camera has 17.5 mm 

The ZWO filter drawer has 21 mm.

The adapter is 16.5 mm.

 

These adds up to 55 mm (coincidence??)

 

Does this means that I need to compensate (adjust) the flattener to 9.1 to reach my 64.1?  Am I missing something?

 

Thanks so much guys for your comments.

 

DB

 

 


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#2 Cyborg84

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Posted 23 July 2021 - 11:32 PM

I have a similar setup - from my experience you are on the right track!

 

The only thing that you didnt account for is the lengthing of the backfocus necessary due to your filter.

(this will add ~0.5mm - 1.0mm depending on your filter thickness).

 

Since backfocus distances / thicknesses, etc are not super exact to the 0.1mm, I would start at the 9 mm point and take some images.

 

Compare the stars with the ones in this post: https://www.cloudyni.../#entry10504529 next and then fine tune your backfocus accordingly.

 

Since I don't have a rotator in my imaging train, I also end up using some thin spacers to adjust the camera angle, along with adjusting the flattener.



#3 Redbetter

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Posted 24 July 2021 - 02:36 AM

Wrong forum, the Beginner's forum excludes imaging.  A moderator will move this to an appropriate forum so that you can get the help you need.



#4 GUS.K

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Posted 24 July 2021 - 05:36 AM

Moving to Beginners DSO imaging.


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#5 idclimber

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Posted 24 July 2021 - 07:37 AM

These adding up to 55mm is not a coincidence. The ZWO website shows this combination. The filter drawer is designed to take up the same space as the filter wheel. The 16.5mm spacer is the same space as the OAG and can also be substituted. 

 

The reason is 55mm is a common backspace for many reducers and flatteners. ZWO is just providing the parts to make this work for most imagers. 

 

However, some reducers and flatteners have a larger specification for back focus and will need additional spacers. This is especially common and needed for scopes with full frame and larger sensors. The reducer on my SCT is something like 140mm and allows room for an external focuser and other accessories. This is by design. 



#6 Deep Blue

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Posted 24 July 2021 - 05:53 PM

Guys,

 

Thanks so much for the comments.

 

DB




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