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Distance from reducer to sensor

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#1 Jesus Munoz

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Posted 01 December 2019 - 01:57 PM

Hello,

 

I have been evaluating the images taken with my Celestron 9.25 XLT with a 0.63X reducer/corrector, a Celestron OAG and a ZWO ASI1600MM-C (I still don't have a filter wheel). Analyzing it in MaximDL with the Astrometry function I get that I have a focal length of 1399 mm when it should be 1480 mm. To increase the focal length, does the reducer/corrector distance must be lowered or moved away?

 

Thank you.

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#2 Jesus Munoz

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Posted 01 December 2019 - 02:20 PM

My distance from the reducer (last part of the cristal reducer on the camera side) to the plane of the sensor is around 105 mm.

 

 

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#3 roelb

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Posted 01 December 2019 - 07:05 PM

Reducing the back focus distance between reducer and sensor will decrease the reduction and as such increase the actual scope focal length.



#4 mclewis1

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Posted 05 December 2019 - 09:16 AM

Jesus,

 

Your setup looks very good. The reducer is mounted directly on the rear cell and you have 105mm back to the sensor.

 

Why do believe you have to have a focal length of 1480mm? The actual focal length of your C9.25 may not be exactly 2350mm to start with (although it should be pretty close to that).

How are you measuring the "reduced" focal length of 1399mm? Could there be some inaccuracy in that number?

The reducer itself might also be slightly out of spec. 

 

What is your optical quality like with the reducer (good quality stars in the corners)? If you are happy with the quality I would simply accept that your setup is working at approximately f6 instead of f6.3. If the optical quality isn't what you expected then you may be able to shorten the camera's mounting to the OAG using different T thread adapters.




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