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Old 23-05-2019, 08:15 PM
Stefan Buda
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QSI683wsg adapter length

I need to make a camera adapter to mount a QSI683 on a CDK250 but the information I have does not make sense to me.
Can someone please explain to me how a filter or a window makes the required adapter shorter? Please read the small print about the back focus being reduced by the thickness of the glass/3.
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Old 23-05-2019, 09:21 PM
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Quote:
Originally Posted by Stefan Buda View Post
I need to make a camera adapter to mount a QSI683 on a CDK250 but the information I have does not make sense to me.
Can someone please explain to me how a filter or a window makes the required adapter shorter? Please read the small print about the back focus being reduced by the thickness of the glass/3.
Hey Stefan. It's because the focal plane of a converging light cone falls short by the stated amount with a plano element in front of a sensor.

The thicker the filter, the more the converging cone is refracted.

Yes, the refractive index of the filter also makes a difference, but likely an order of magnitude smaller.

Hope that sheds some light
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Old 23-05-2019, 10:06 PM
Stefan Buda
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Originally Posted by Peter Ward View Post
Hey Stefan. It's because the focal plane of a converging light cone falls short by the stated amount with a plano element in front of a sensor.

The thicker the filter, the more the converging cone is refracted.

Yes, the refractive index of the filter also makes a difference, but likely an order of magnitude smaller.

Hope that sheds some light
Peter,

That would only be possible if the refractive index of the filter/window was less than 1. I never heard of such optical glass. As a rule of thumb, a plane parallel window, made of common optical glass, will EXTEND the focal plane by 1/3 of its thickness.

So I'm still stumped about what they are trying to say.
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Old 24-05-2019, 04:38 PM
Stefan Buda
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Anybody here uses a QSI683? I would like to hear from you how you worked out the required spacing for your camera.
I'm getting very frustrated by this. For the life of me I can't understand how a filter, according to the QSI document, can make the imaging train shorter. I must be misinterpreting something but I haven't got a clue what.
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Old 24-05-2019, 06:08 PM
HenryNZ
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I read from CN that QSI has a backward definition of “back focus” which often causes confusion. The adjustments in the table is meant to be made to the optical length of the camera. Therefore, if you add a filter you make a negative adjustment to the optical length, which in effect means you need a longer adaptor to make up for it. I can’t find the thread on CN now but it seems to be a recurring question posted on CN from time to time.
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Old 24-05-2019, 06:09 PM
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I just followed the instructions. If you have Astrodon filters (3mm thick), take 1mm off the recommended length of the adapter.

DT
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Old 24-05-2019, 06:48 PM
Stefan Buda
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Originally Posted by HenryNZ View Post
I read from CN that QSI has a backward definition of “back focus” which often causes confusion. The adjustments in the table is meant to be made to the optical length of the camera. Therefore, if you add a filter you make a negative adjustment to the optical length, which in effect means you need a longer adaptor to make up for it. I can’t find the thread on CN now but it seems to be a recurring question posted on CN from time to time.
What a relief to hear that. Thanks, you saved my sanity. It is a very screwed up reasoning because the overall optical train gets longer by one third of the filters thickness.

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I just followed the instructions. If you have Astrodon filters (3mm thick), take 1mm off the recommended length of the adapter.

DT
In that case you went in the wrong direction with the length of your adapter.
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Old 24-05-2019, 07:22 PM
phomer (Paul)
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Quote:
Originally Posted by Stefan Buda View Post
I need to make a camera adapter to mount a QSI683 on a CDK250 but the information I have does not make sense to me.
Can someone please explain to me how a filter or a window makes the required adapter shorter? Please read the small print about the back focus being reduced by the thickness of the glass/3.

Stefan,


I recall getting totally lost on this when I needed an adaptor. A glass filter will extend the focus point 1/3 of the filter width further away from the objective or mirror. It effectively extends the focal length, so you would need to increase the adapter length.


Paul
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Old 24-05-2019, 09:34 PM
Stefan Buda
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Stefan,


I recall getting totally lost on this when I needed an adaptor. A glass filter will extend the focus point 1/3 of the filter width further away from the objective or mirror. It effectively extends the focal length, so you would need to increase the adapter length.


Paul
Paul,

Thanks for chirping in and please believe me that I'm completely at peace with Snell's law. My problem is purely with the QSI document I posted.
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Old 25-05-2019, 07:30 AM
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Hi Stefan, I have had three QSI's from about 2009 onward. I have found the table daunting too but I believe this problem is caused from the idea that the filters are within the casing of the camera and not in an external filter wheel.

So for example my GPU corrector is 55mm back focus. The Camera has a back focus with the mount I have of 50.17mm. If I subtract 1mm from 50.17 I end up with 49.17 and that subtracted away from the 55mm back focus is 5.83. This corresponds really well with the actual imaging for curvature.

Or looking at it another way, you add 1mm to the 55mm back focus of the corrector and then subtract the 50.17 from the 55 and you get a sum of 5.83. ie 55+1-50.17=5.83 (apply BODMAS). In this example you need to add the filter to the back focus of the corrector for refraction and then subtract the actual back focus of the camera.

Hope that helps.
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Old 25-05-2019, 07:59 AM
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Paul explained it very well.

I have QSI 690 and increasing spacing between Riccardi reducer and the camera by 1mm to account for Astrodon filters gave the best results at f/4.5. I also tried decreasing the spacing by 1mm and star correction got significantly worse. Didn't see as much difference with both scenarios with a dedicated flattener at f/6.
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Old 25-05-2019, 08:30 AM
Stefan Buda
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Quote:
Originally Posted by Paul Haese View Post

I believe this problem is caused from the idea that the filters are within the casing of the camera and not in an external filter wheel.

So for example my GPU corrector is 55mm back focus. The Camera has a back focus with the mount I have of 50.17mm. If I subtract 1mm from 50.17 I end up with 49.17 and that subtracted away from the 55mm back focus is 5.83. This corresponds really well with the actual imaging for curvature.

Hope that helps.
Perfectly explained. I knew it had something to do with the way of looking at it.

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Originally Posted by Slawomir View Post
Paul explained it very well.

I have QSI 690 and increasing spacing between Riccardi reducer and the camera by 1mm to account for Astrodon filters gave the best results at f/4.5. I also tried decreasing the spacing by 1mm and star correction got significantly worse. Didn't see as much difference with both scenarios with a dedicated flattener at f/6.
Indeed.

Thanks everyone. Case closed.
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Old 25-05-2019, 10:23 PM
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Hey Stefan. It's because the (camera) focal plane (from the) converging light cone falls short by the stated amount with a plano element in front of a sensor.

The thicker the filter, the more the converging cone is refracted.

Yes, the refractive index of the filter also makes a difference, but likely an order of magnitude smaller.

Hope that sheds some light
Sorry I expressed myself very poorly ....yes you are totally correct...I was talking about the camera's position...anyway.. Problem solved!
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