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ZWO backfocus question

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#1 Craig_

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Posted 12 August 2020 - 11:11 PM

I was reading https://astronomy-im...tions-55mm.html which sets out how to achieve 55mm backfocus with common combinations of equipment using ZWO cameras. One thing that caught my eye is that when using the filter wheel (with or without OAG) on a small format sensor it actually sets out 56mm of backfocus, with the fine print saying "Add 1mm based on optical path difference of the filter". I am curious though - when looking at the filter drawer backfocus combo, it only achieves 55mm, seemingly making no allowance for the filter thickness if it's added to the optical path via a drawer as oppose to a wheel? Anyone know why?

 

With my current filter solution (screwing it into an extender ring in the optical path AFTER the field flattener) backfocus did not appear to change with or without the filter so I assume that any difference it made is within the margin of error for the scope backfocus requirement (which is 55mm from the end of the extender ring that screws into the field flattener.) Just curious if this would, for some reason, be different with an EFW as ZWO seems to suggest? My line of thought being currently I have 55mm of physical backfocus plus a 1.85mm thickness filter in the optical path, but if an EFW went in there instead I'd actually be at 56mm plus a 1.85mm filter, possibly pushing me outside the margin of error that I currently seem to operate within?

 

Hopefully this question makes sense.. Thanks



#2 bobzeq25

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Posted 12 August 2020 - 11:28 PM

The basics.

 

It's not the filter wheel that makes a difference, only the filter itself.

 

Some people find they get better results with a little more or less spacing rather than 55.0.

 

You can easily compensate for a filter (or just tweak backfocus without one), by using these.

 

https://www.highpoin...ring-set-t2ring


Edited by bobzeq25, 12 August 2020 - 11:29 PM.

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#3 Craig_

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Posted 12 August 2020 - 11:46 PM

The basics.

 

It's not the filter wheel that makes a difference, only the filter itself.

 

Some people find they get better results with a little more or less spacing rather than 55.0.

 

You can easily compensate for a filter (or just tweak backfocus without one), by using these.

 

https://www.highpoin...ring-set-t2ring

Oh, I totally get the filter is what makes the difference. That's what my question really is, ZWO give the steps to hit 55mm backfocus using the drawer but 56mm using the EFW. Both could use the same filter, so why the difference in backfocus?



#4 bobzeq25

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Posted 12 August 2020 - 11:47 PM

Oh, I totally get the filter is what makes the difference. That's what my question really is, ZWO give the steps to hit 55mm backfocus using the drawer but 56mm using the EFW. Both could use the same filter, so why the difference in backfocus?

I'd guess someone made a mistake.



#5 Craig_

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Posted 12 August 2020 - 11:55 PM

I'd guess someone made a mistake.

Hmmm that doesn't inspire confidence.

 

I know 55mm back focus even with a 1.85mm filter in my optical path currently works for me, but looking at other filter mounting options (screwing it into an adapter ring inside the optical train is cumbersome) and it seems the drawer would be risk free (no change to current backfocus) whilst the EFW may not be, as it will add 1mm of backfocus I do not currently have, at least not without swapping out adapters vs what it ships with.


Edited by Craig_, 12 August 2020 - 11:56 PM.


#6 GrafikDihzahyn

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Posted 14 August 2020 - 12:01 PM

The back focus is just a starting point. You'll need to fine tune the adjustment. Astro-Physics has a good writeup on getting the distance correct. My flattener with filters and everything ended up having a backfocus of 53.25mm (Stellarvue flattener stated 55mm). I needed to get a couple of .5mm, 1mm, and adjustable spacer. The thicker the filter, the more backfocus. Astrodon's 3mm filter will move it 1mm, Astronomik's 1mm filter moves it .3mm.

 

Far too many variables for there to be a definite answer. You just have to play with it. You may be satisfied with what you see in your image, but OCD made me get mine as perfect as possible. You can download CCD inspector for a 30 day trial. I'd recommend purchasing a set of calipers.




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