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Back focus question - I hate asking dumb questions but..............

Astrophotography Refractor Filters Equipment
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#1 jmurra31

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Posted 29 June 2022 - 01:04 PM

So it didn't take long really........ 8 months using a mirrorless camera and lens and I will shortly be ordering a OSC and refractor. My question is this..........I understand the principles of back focus but I plan to use either no filters or a tri band filter etc via either a filter drawer or wheel (not yet decided) but do I have have to remove/add spacers to correct back focus whenever I switch between no filter/filter or vice versa to compensate for the thickness of the filter?............If so is there a way to avoid this?, many thanks 



#2 bobzeq25

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Posted 29 June 2022 - 01:14 PM

So it didn't take long really........ 8 months using a mirrorless camera and lens and I will shortly be ordering a OSC and refractor. My question is this..........I understand the principles of back focus but I plan to use either no filters or a tri band filter etc via either a filter drawer or wheel (not yet decided) but do I have have to remove/add spacers to correct back focus whenever I switch between no filter/filter or vice versa to compensate for the thickness of the filter?............If so is there a way to avoid this?, many thanks 

When you stick a filter into the light path you inevitably alter reducer spacing.  But it's like 1/2-1/3 the filter thickness which is a very small change.  It may be unnoticeable.  It may actually help.

 

Best way to find out is to try.



#3 jmurra31

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Posted 29 June 2022 - 01:25 PM

Thankyou I will, not sure when I'll see any of it though nearly everything out of stock. Based on what you say then it would be best to base my back spacing on no filter rather than with filter?



#4 barbarosa

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Posted 29 June 2022 - 02:02 PM

One "trick" is to always have a filter in place and that filter can be an IR or UV/IR blocking filter. Generally though I think that you can default to the spacing specified in the manual. It is easier to add a mm than to subtract one.


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#5 ntph

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Posted 29 June 2022 - 02:23 PM

Not so much of a trick as almost a necessity to have a UV-IR cut filter for most cameras--improved focus metrics and performance and minimizing bloating stars. Make your spacing adjustments with that filter in place. Then if or when you add a dual-band filter, as an example, the difference in the light path will almost certainly be negligible. And as one wag aptly noted, the best light pollution filter for astrophotography  is gasoline (put it in your car and take your stuff somewhere darker than home!) but now I guess we should add a charged battery.wink.gif Of course, you can always just shoot more subframes too. 


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#6 kathyastro

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Posted 29 June 2022 - 02:54 PM

If you don't have a reducer or flattener, it is not an issue.  Just adjust the focuser until the filtered image is focused.  The adjustment will be small: 1mm or less.

 

If you do have a reducer or flattener, it could be an issue, but a small one.  As noted above, it may improve the spacing (or not).  Try it and see how it works before tearing your hair out fiddling with it.



#7 jmurra31

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Posted 29 June 2022 - 03:23 PM

One "trick" is to always have a filter in place and that filter can be an IR or UV/IR blocking filter. Generally though I think that you can default to the spacing specified in the manual. It is easier to add a mm than to subtract one.

Thanks for reply, I did wonder if that might be a possible solution. I do have access to pretty good skies (bortle 4) for a lot of the time so I can see me often shooting just with a bare sensor some of the time



#8 jmurra31

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Posted 29 June 2022 - 03:29 PM

If you don't have a reducer or flattener, it is not an issue.  Just adjust the focuser until the filtered image is focused.  The adjustment will be small: 1mm or less.

 

If you do have a reducer or flattener, it could be an issue, but a small one.  As noted above, it may improve the spacing (or not).  Try it and see how it works before tearing your hair out fiddling with it.

It will be with a flattener but not a Reducer (at least initially), thanks for your thoughts....my question only came about because I saw plenty of info about adding filters will alter back focus but not much about what to do about it.....I guess do my spacing, wait and see but have a couple of .5 or 1mm spacers on hand :)


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