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Back focus? Optical path length?

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#1 Micah Sittig

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Posted 20 February 2020 - 03:50 AM

I'm struggling to understand back focus/optical path length and it's making me nervous about ordering pieces of equipment that may end up being the wrong size, so I'm here to ask for your expert advice.

 

My setup is an Astro-tech AT80EDT refractor on an Orion Atlas EQ-G mount + QHY PoleMaster polar scope. I also ordered the Astro-tech 0.8x Reducer/Field Flattener specific to the AT80EDT (on back-order from Astronomics). I'm trying to have the set-up working for both DSLR and CMOS imaging.

 

For the DSLR prime-focus photography I ordered the appropriate T-ring (three actually, since I'll be working with my students and their cameras). Besides this I'll need a T-mount to 2-inch adapter, correct? Also, according to Astronomics the reducer/flattener can screw straight onto the OTA and has 42mm T-threads to connect to the T-ring on the camera. The reducer/flattener is also listed as having a back focus of 55mm to the image plane, and Astronomics assures me that this is standard for DSLR camera bodies. So I should be good to image directly or through the focal reducer, correct?

 

For the CMOS imaging, I have a QHY MiniCam5F with built-in filter wheel. This has a CS/C-threads, and a little 1.25 inch barrel adapter. My plan is to ditch the barrel adapter and use a C mount to T mount adapter to attach the MiniCam5F to the focal reducer, but I would need to make sure that it is spaced back by 55mm, correct? My biggest doubts are here: what kind of connectors do I need to get the spacing correct? I know the MiniCam itself has an optical path length of 19.95mm from the front face of the built-in filter wheel to the sensor. Does this mean I need 35 more millimeters of spacers to make sure the CMOS sensor is at the correct 55mm distance from the focal reducer? Maybe something like this:

 

https://smile.amazon.../dp/B07R7HG3GB/

 

Or something like this for more precision:

 

https://www.svbony.com/sv109

 

Should I worry about the contribution of the C-mount to T-mount adapter to the optical path length? I don't see this information Amazon, so should I be contacting the manufacturer? 

 

Finally, I realize that I don't have a guide-scope or off-axis guider; those didn't fit in the budget this year but I'm hoping to purchase one for the next schoolyear.

 

Thanks in advance for your help,



#2 sg6

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Posted 20 February 2020 - 04:26 AM

Back focus is a slightly poor term, however there seems no real alternative.

It is a distance from some defined face to where the image should be.

 

If no optics in the path then the image is where the prime focus is: Say a 600mm focal length lens, That is easy as it is 600mm from the objective.

 

Now say the manufacturer makes 2 tubes for it, one of 300mm length and one of 400mm length - just an aluminium tube. Now your "back focus" oddly changes.

 

One is 300mm from the end of the OTA, the other is 200mm from the end of the OTA. Nothing has really changed in the optics however. So "back focus" is a poor term.

 

For the DSLR prime-focus photography I ordered the appropriate T-ring (three actually, since I'll be working with my students and their cameras). Besides this I'll need a T-mount to 2-inch adapter, correct? Also, according to Astronomics the reducer/flattener can screw straight onto the OTA and has 42mm T-threads to connect to the T-ring on the camera. The reducer/flattener is also listed as having a back focus of 55mm to the image plane, and Astronomics assures me that this is standard for DSLR camera bodies. So I should be good to image directly or through the focal reducer, correct?

Yes 55mm is a sort of standard. Unfortunately DSLR's are not "standard" so the answer to the last sentence is Maybe.

 

Which DSLR are you using, that is required and is essential.

 

Lets say you use a Canon DSLR and the flange distance is 42mm.

So the DSLR has in a way 42mm distance already build in.

You need 55mm so another 13 from somewhere.

The T-Ring is likely 8mm, maybe 10mm. Going to guess at 8mm simply because I read one the other day that said 8mm. So 42 in the DSLR and 8 from the T-Ring, thats 50. You need 55mm Somewhere you need an extra 5mm spacer.

 

Next "problem" some T-Rings are machined such that the metal surface sits back by 8mm, so it is 8mm thick but is sort of "recessed" by 8mm, so adds nothing to the separation.

 

Big question is the DSLR, Canons are often 42mm, Nikons F mounts are 46.5mm, my Sony is 44.5mm. And that is just a small selection. E.G. Canon FD/FL = 44, Canon EF = 42, Canon RF = 20, Canon EF-M = 18 and that is just Canons.

 

List of flange distances:

 

I have no idea if an adaptor exists and if so what is required but WO do adjustable flatteners, simply a fine screw thread to alter the separation. I have one for a ZS 61 and it makes life so very easy. I simply get to around 10mm of the position and adjust the reducer to suit the best distance. Remember that many flattener/reducers will say something like 55mm +/-10%, which is 5mm. Means even with 55mm exactly, you may need to have small changes from that.

 

In back focus it seems most values are "approximate".


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#3 mistateo

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Posted 20 February 2020 - 04:28 AM

For DSLR imaging, you thread the t-ring directly onto the rear threads of the reducer and connect the t-ring straight onto the camera and boom, 55mm back focus (most manufacturers will make the t ring the right thickness to achieve 55mm from the front face of the t-ring to the sensor, but double check to be sure).  For the other camera, it will be a little more difficult, but still doable.  First you need to determine the distance (in mm) from the camera sensor itself to the threaded connection on the front of the filter wheel.  On ONE site I saw that the "flange distance" was 20mm, not sure if it was accurate or not, but I'll go with it for this example.  Then you need to figure out the optical length of that c to t2 adapter you listed.  It looks to be about 5mm or so (again you will have to verify these).  So 20mm + 5mm = 25mm, and the recommended back focus is 55mm.  This means you would want to go from scope -> reducer -> 30mm t2 spacer -> 5mm t2/c mount adapter -> 20mm filter wheel/camera to sensor.  With a sensor this size you can get close and not have to obsess over 1-2mm error.  When you get larger sensors, the back spacing becomes critical to the .5-1mm range.


Edited by mistateo, 20 February 2020 - 04:29 AM.

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#4 kathyastro

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Posted 20 February 2020 - 07:28 AM

Yes 55mm is a sort of standard. Unfortunately DSLR's are not "standard" so the answer to the last sentence is Maybe.

No, no, no!  While DSLRs themselves have various body depths, A DSLR with its T-ring attached is always exactly 55mm deep.  This is a defined standard, and all T-ring manufacturers adhere to it.  If you have the right T-ring for your camera, the depth is always 55mm.


Edited by kathyastro, 20 February 2020 - 07:29 AM.

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#5 james7ca

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Posted 20 February 2020 - 07:41 AM

No, no, no!  While DSLRs themselves have various body depths, A DSLR with its T-ring attached is always exactly 55mm deep.  This is a defined standard, and all T-ring manufacturers adhere to it.  If you have the right T-ring for your camera, the depth is always 55mm.

I believe that Kathy is correct (as usual), however, it is possible to get so-called "short" or "zero length" T-rings if you want the T-ring to use as little of your backfocus as possible. But, you have to look for these specifically since all "normal" T-rings provide 55mm of extension given the camera mount they are designed to work for.

 

By the way, I'm pretty sure the 55mm spacing comes from the T-mount lens specification which was also 55mm (for the lens flange to film spacing). You can even find lenses that use the screwthread (M42x0.75) T-mount which is pretty useful if you want to use the lens for astrophotography.


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#6 kathyastro

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Posted 20 February 2020 - 07:56 AM

I believe that Kathy is correct (as usual), however, it is possible to get so-called "short" or "zero length" T-rings if you want the T-ring to use as little of your backfocus as possible. But, you have to look for these specifically since all "normal" T-rings provide 55mm of extension given the camera mount they are designed to work for.

 

By the way, I'm pretty sure the 55mm spacing comes from the T-mount lens specification which was also 55mm (for the lens flange to film spacing). You can even find lenses that use the screwthread (M42x0.75) T-mount which is pretty useful if you want to use the lens for astrophotography.

Yes.  The "T" stands for Tamron, a lens manufacturer who came up with this standard, so that one lens could be used on several different brands of SLR cameras.  Obviously, camera bodies had to be made parfocal for a single lens to work on all of them, hence the T-rings.

 

Folks with Newtonians sometimes think that using a mirrorless camera, with its shallow body, will solve the notorious back-focus problem.  They are chagrined to find that the standard T-ring for their camera extends the depth to the standard 55mm, same as every other camera.  Those folks need something called a T-minus ring, which is a non-standard ring that has the appropriate threads but is substantially shorter than a standard ring.
 


Edited by kathyastro, 20 February 2020 - 07:57 AM.



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