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Fine Tuning Field Flattener Back Focus

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#1 BenKolt

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Posted 04 July 2015 - 03:58 PM

Greetings!

 

I've been spending the past several nights fine tuning the spacing between my QSI 683wsg-8 camera and an Explore Scientific 3" Field Flattener .7X Focal Reducer.  I've been imaging with an Explore Scientific 127mm Apo Refractor.  Somewhere between 10 and 15mm spacing I expect the stars in the corner of the image to be nice and circular.  At 10mm they are oblong pointing towards the center, and at 15mm they are slightly oblong perpendicular to the direction to center.  I may need, say, 13 or 14mm.  Unfortunately my set of spacing rings doesn't allow me to fine tune any further than this.

 

Can somebody recommend spacers that allow for the kind finer of adjustment that I need?  For example, rings of sizes not confined to 5mm steps, variable, smaller than 5mm, etc.?  Thanks for any tips on how to make this happen.

 

Best Regards,

Ben

 

 



#2 turnerjs085

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Posted 06 July 2015 - 08:13 AM

Short of having an adapter made, you are probably going to have to use washers between the threaded connections to get a few extra mm of length. If your flattener has a t-thread connection on the camera side, ACE hardware sells a ~1mm thick plastic washer (look in plumbing parts) that will fit over male t-threads and give you some space. If your threads are long enough you can stack more than one washer. Worst case with short threads, you can put one between the flattener and 10mm spacer and one between the spacer and camera. Once you get it dialed in, have precise parts make an adapter of the proper length if you don't want to use the washers permanently.

Hope that helps,
Jeremy

#3 jeffweiss9

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Posted 06 July 2015 - 08:44 AM

I've used this for 1.6mm spacing for confirming backfocus; not permanently:

 

http://scopestuff.com/ss_sttw.htm

 

Clear skies,

-Jeff


Edited by jeffweiss9, 06 July 2015 - 07:18 PM.


#4 andysea

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Posted 06 July 2015 - 09:22 AM

I would get a custom adapter from Preciseparts. They do an excellent job.



#5 rainycityastro

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Posted 06 July 2015 - 10:06 AM

Agena Astro sells these spacers from Baader

http://agenaastro.co...r-ring-set.html

 

These can help you get the spacing right within about 0.5 mm. Please note that once these Delrin rings go on, they are nearly impossible to take out without marring your other t2 adapters. So be sure before you put them on.



#6 BenKolt

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Posted 06 July 2015 - 10:42 AM

Thanks for all the great tips.  Sounds like the best thing to do it use the washers to dial in the optimum spacing and then have a permanent spacer custom made as you suggest.

 

Ben



#7 WesC

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Posted 06 July 2015 - 07:02 PM

I would get a custom adapter from Preciseparts. They do an excellent job.

 

Yep, this is what I just did. Not only that but they helped me confirm that my calculations were correct for the adapters I was ordering. :waytogo:



#8 WesC

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Posted 06 July 2015 - 07:07 PM

Does your flattener have a back working distance number listed in its documentation? If you have that number, and the depth of your filters you can easily calculate exactly what spacer size you need.

 

For example, in my case I have what Takahashi calls "back metal distance" for my reducer of 62.5mm. I added 1mm for the offset of my 3mm thick Astrodon filters and then I subtracted the "optical path length" of the QSI-683wsg8, which is 50.17mm.... 62.5+1-50.17= 13.33.

 

So I ordered a 13.3mm adapter from Precise Parts. :)



#9 WesC

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Posted 06 July 2015 - 07:20 PM

I read on the OPT website that the back focal distance is 55mm, which is pretty common for connecting to a DSLR...

 

So, for example, if you use my formula... it would be 55mm(reducer)+1mm(for your filters?)-50.17mm(QSI683)=5.83mm or about a 6mm spacer. The Precise Parts website adapter builder can only go as short as 7mm for connecting these two devices... but that's still pretty close.



#10 BenKolt

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Posted 06 July 2015 - 08:01 PM

I was unable to find an advertised back focus for this particular FF/FR.  I began by assuming that it was 55mm and determined to just do trial and error to dial it in.  In the end this is always the most accurate method anyway because of variabilities.  The QSI documentation says my camera takes up 50.17mm of back focus and that I should add 1 mm to that due to the Astrodon filters.  That's 51.17mm.  If it were truly 55mm total, then I would expect something more like 4mm to optimize a flattened FOV, however I'm finding that I need something between 10 and 15mm, probably closer to 15mm.  The 55mm total back focus was likely an incorrect assumption on my part.

 

I ended up ordering an Orion variable spacer that can adjust from 12 to 17mm.  I have no idea about its quality, but I'm willing to try it out.  Once I'm satisfied that I've optimized, I'll probably order a custom-sized one.

 

Thanks again for the good ideas.

 

 



#11 CharlesW

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Posted 06 July 2015 - 08:03 PM

What did you calculate the back focus of the field flatner as? I have nearly the same setup and would like to contact Precise Parts to get an adapter made. You must have posted that about a second before I asked. I'll watch your thread. 


Edited by CharlesW, 06 July 2015 - 08:04 PM.


#12 andysea

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Posted 06 July 2015 - 08:15 PM

I think the back focus is reduced by the glass thickness of the filter divided by 1/3, that is what I read on the QSI back focus document. 

With astrodon filters the back focus of your camera becomes equivalent to 49.17mm. 

So assuming 55mm backfocus:

55mm - 49.17mm = 5.83mm spacer needed.

 

If you want to add the 1mm you need to add that to the thickness of the spacer, in this case the math would be:

(55+1)-50.17=5.83.

 

The filter increases the optical back focus of the camera but not the actual mechanical back focus so you need to make up for the increased back focus with the spacer.

 

I hope this makes sense.



#13 josh smith

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Posted 06 July 2015 - 08:52 PM

I think the back focus is reduced by the glass thickness of the filter divided by 1/3, that is what I read on the QSI back focus document.
With astrodon filters the back focus of your camera becomes equivalent to 49.17mm.
So assuming 55mm backfocus:
55mm - 49.17mm = 5.83mm spacer needed.

If you want to add the 1mm you need to add that to the thickness of the spacer, in this case the math would be:
(55+1)-50.17=5.83.

The filter increases the optical back focus of the camera but not the actual mechanical back focus so you need to make up for the increased back focus with the spacer.

I hope this makes sense.

Exactly. I used a 6mm spacer when I had ff attached to the camera requiring 55mm of spacing with Astrodon filters. It provided perfect stars to the corner.

Edited by josh smith, 06 July 2015 - 08:52 PM.


#14 BenKolt

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Posted 07 July 2015 - 12:10 AM

andysea:

 

You're right - I meant to say that I needed to subtract 1 mm due to the filter in order to get the math right.  I didn't see the minus sign on the QSI page.  My main point, however, is that my field results don't match up with the math.

 

CharlesW:

 

I hesitate to advise you to go get a part made based on my experience, because it sounds like others are telling me the back focus for this particular FF/FR is 55mm.  I was unable to find that information myself and cannot verify.  All I can say is that my experience is telling me to use a spacer of something just less than 15mm.  At 10mm the corner stars are slightly elongated, pointed radially towards the center of the image.  At 15mm the stars are slightly elongated in a direction perpendicular to the radial direction.  This implies they should be circular somewhere in between.  I seem to need more than the 5mm of spacing indicated by the math if the 55mm number is correct.  I started with 5mm and my radial elongation was even more pronounced.  If you do go off and order something made and it turns out to be incorrect, please don't blame me for it!  I advise you to dial it in first with your own spacers and come to your own conclusion.

 

Best Regards,

Ben



#15 josh smith

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Posted 07 July 2015 - 02:54 AM

Ben, you accidentally posted this twice. Did you read the response in the other posting regarding variable spacers to find the correct distance before having a pp made?

#16 Nicola

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Posted 07 July 2015 - 04:18 AM

I used the PreciseParts calculator for the adapter from my FSQ85 focal reducer to the qsi583wsg: in theory I'd need a 22mm adapter to reach the correct distance, but the PP calculator gives 10.7. Hmmm..I'm wondering who is right...



#17 BenKolt

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Posted 07 July 2015 - 09:42 AM

Josh:

 

Whoops - did I post twice?  The topic must really be on my mind!  I was responding to somebody else and then repeated myself.

 

Yes, I read all the good advice in the responses and I concur with the recommendations.  Thanks again, all!

 

Best Regards,

Ben



#18 WesC

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Posted 07 July 2015 - 05:23 PM

There's some confusion about how to calculate the offset for filters. I know I was certainly confused!

Some of this stems from how difference manufacturers spec the math. Do you subtract it or do you add it? Well, either, depending on what you are talking about.

 

But the bottom line is this... a filter's glass inserted in the light path moves the focus rearward by a distance of t(n-1)/n, where t = the thickness of the plate and n is the refractive index.  For most glasses with an index of about 1.5, that means that the distance is roughly t/3.

 

So you can add it to the back focal distance from the reducer to the image plane, or you can subtract it from the optical path length of the camera. Either works, and should arrive at the same amount, so long as you actually understand what is happening.

 

John Hayes kindly sent me a PDF with a ray trace in it that really helped solidify what is going on. Take a look. :)

Attached File  Focus Shift.pdf   257.21KB   222 downloads



#19 WesC

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Posted 07 July 2015 - 05:27 PM

Ben, if your variable adapter works out, awesome! If not I would just call up Explore Scientific and ask them for the true back working distance of the reducer and exactly where on the reducer that is measured from. Some manufacturers will include or exclude threads, or included spacers into their numbers. You need to know all of that to get to the exact number for your particular application.G

 

Good luck!




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