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Crazy question - backfocus - how much wiggle room?

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#1 Scott Mitchell

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Posted 07 July 2020 - 01:24 PM

So, I'm trying to get all the right pieces and parts together to mount Cano compatible lens on my QHY168C. I believe I'm shooting for 55mm, or maybe 44mm is actually the right value for Canon? Let's assume 44 for this discussion (although I'm happy to be corrected there). My real question is how exact does this need to be and which side can I err on (too close or too far)? I assume being off a bit would mean that you'd have to use the lens' focus ring to bring things to focus (coming back from infinity focus on the lens)? Adding to the complexities is the fact that there will be one of a couple of different filters in the imaging train which further tweaks the length of the optical path. It is all just a little... confusing. 

 

Anyone have thoughts on this, and/or been there done that?



#2 SilverLitz

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Posted 07 July 2020 - 01:26 PM

55mm is from the lens mount, and 44mm is from the camera flange.  The 11mm difference is the length of the t-ring/adapter.


Edited by SilverLitz, 07 July 2020 - 02:57 PM.

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#3 Scott Mitchell

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Posted 07 July 2020 - 01:35 PM

Thanks SilverLitz, so sounds like we're talking 44mm from the back of the QHY/Canon adapter. Now the question is how much wiggle room is there in that, and is it better to be short or long?



#4 P_Myers

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Posted 07 July 2020 - 01:41 PM

+/-1mm
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#5 Stelios

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Posted 07 July 2020 - 01:45 PM

A variable spacer like the Baaders  (they make several for different lengths) is your friend.

 

Both long and short will see corner stars get progressively worse.


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#6 Gipht

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Posted 07 July 2020 - 01:51 PM

I would be careful not to get too thick an adapter.   Remember that you may want to put a filter in line with the spacing to the adapter.

 

The adapter I bought for my Canon lenses had a lever that retracted a small metal post which locked the lens into place.  The post was fractionally too long and could not be fully retracted.  I filed it down just a bit and it works fine now.  You might want to check that before you use the adapter.  Having a lens stuck to your adapter is not fun.


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#7 Scott Mitchell

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Posted 07 July 2020 - 02:13 PM

+/-1mm

Wow, not much wiggle room at all. 



#8 Scott Mitchell

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Posted 07 July 2020 - 02:26 PM

I would be careful not to get too thick an adapter.   Remember that you may want to put a filter in line with the spacing to the adapter.

Good reminder Gipht. And help me make sure I get this right. Adding a 1mm thick filter will extend the optical train by that same amount? 



#9 Scott Mitchell

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Posted 07 July 2020 - 02:27 PM

A variable spacer like the Baaders  (they make several for different lengths) is your friend.

 

Both long and short will see corner stars get progressively worse.

Those look really helpful, but unfortunately I won't have 20mm available to fit one into the chain of adapters. It looks like I'll have about 6-7mm to fill in.



#10 Gipht

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Posted 07 July 2020 - 03:02 PM

Correct Scott.  I am new to CMOS cameras and lenses so take this advice with a grain of salt.   If the spacing is too short, you may still be able to bring the camera into focus, though not at the infinity position.   If the spacing is to long, then you are out of luck because you can't get infinity to focus.

 

My experience so far has not shown worse stars on the edges by being a little short on spacing.  My feeling is that you have more then 1mm wiggle room.


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#11 SilverLitz

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Posted 07 July 2020 - 03:03 PM

Thanks SilverLitz, so sounds like we're talking 44mm from the back of the QHY/Canon adapter. Now the question is how much wiggle room is there in that, and is it better to be short or long?

I would expect you will have a little more leeway with a camera lens than that of a field flattner, as lenses can go past infinity.

 

Since lenses are made for short to infinity focus, compared to scopes at infinity only, you have more cushion to the short side.  Think about extension tubes for cameras for macro photography, these add distance to allow closer focus and more magnification.  Extension tubes eliminate the ability to focus long distance.  


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#12 WadeH237

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Posted 07 July 2020 - 03:07 PM

Good reminder Gipht. And help me make sure I get this right. Adding a 1mm thick filter will extend the optical train by that same amount? 

A filter will extend the back focus by 1/3 the thickness of the filter.  So a 3mm thick filter will extend it by 1mm.


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#13 ks__observer

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Posted 07 July 2020 - 03:31 PM

Both long and short will see corner stars get progressively worse.

+1



#14 Scott Mitchell

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Posted 07 July 2020 - 04:23 PM

A filter will extend the back focus by 1/3 the thickness of the filter.  So a 3mm thick filter will extend it by 1mm.

Thanks Wade! Now to figure out the thickness of my filter(s)...




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