Calculating whether I can insert a filter wheel between my QHY10 camera and TRF-2008 field flattener

I would like to use a light pollution filter with my QHY10 camera for nebulae as my location is badly light polluted and the situation is getting constantly worse due to new estates and industrial units being built around Lichfield. However such filters are not always ideal for all targets such as galaxies and I would therefore like to be able to move the filter in and out of the front of the camera without affecting the orientation of camera and telescope/field flattener. Doing this would mean that I can take series of flat frames with filter and without filter and then rotate it in or out as needed.

In my previous post, I have attached the instruction manual for the Tele Vue TRF-2008 field flattener that I use with the QHY10 camera.

Here it is again (PDF):

Tele Vue TRF-2008 Instruction Manual

This manual states that the optimal distance from flattener to camera sensor is 56mm.

Teleskop Service do a manual filter wheel for 2 inch filters.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p9358_TS-Optics-manuelles-Filterrad-fuer-5x-2–Filter.html

The specifications of the filter wheel are as follows:

These show that the filter wheel requires just under 30mm from M48 thread to M48 thread.

However this now means I would need 2 x M48 to T2 adapters, one each side of the filter wheel.

First Light Optics sell such adapters:

So if I used the First Light Optics adapters above then I would add 2 x 10mm into the optical path giving a total of 30mm for filter wheel and 20mm for adapters = 50mm.

Although this appears to be small enough to fit within the optimal 56mm distance from field flattener to camera of 56mm, the page reproduced from the QHY10 camera instruction manual below shows that the QHY10 adds a back-focus requirement of 20mm meaning that the total back-focus requirement would be 70mm when optimal distance is 56mm.

QHY10 camera instruction manual PDF version accessed October 2019

However, if I can find a filter wheel with similar front-back width as the one above buy with T2 threads rather than M48 ones so that I don’t need adapters then it should work OK.

Andy

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