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Takahashi FSQ-106ED problems

08 Feb, 2020 20:25
Dear astronomy lovers,

I’m having some difficulties with my gear and I hope someone could help. I have a Takahashi FSQ-106ED the older model with the captains wheel. I used this scope with an APS-C sensor camera without any major problems. I have now switched my camera from a Canon EOS 60Da to a Canon EOS R, which is full frame and has 30 megapixels. Allegedly the Takahashi FSQ-106ED has an image circle of 88mm. But I’m already seeing a lot of abnormalities at the corners of the frame.

We tested the scope with a Canon EOS 6D, also full frame but bigger and less pixels, this gave a better result. Also, the issue with metal back. I tried getting close to the suggested 178 mm and that just gave nothing, seems like the number isn’t good, or the metal back is elsewhere. We are assuming the metal back is the last non detachable part from the focuser when screwed all the way in. Maybe someone has the answer?

So, I don’t know if the Takahashi FSQ-106ED is even good with a 30megapix fullframe, or if the new version is somewhat better. Or if it’s just a metalback problem, or the scope has to be recollimated. Or all of the above. 😀

If someone has any clue, any idea, I would be really happy if you could help!
Thanks and Clear Skies!
Edited 08 Feb, 2020 20:26
08 Feb, 2020 22:54
Hi Adrien,
at f/5 the depth of field of the Tak is around 100um,  even smaller if you are using the reducer, so any issue in the optical train, specially with smaller pixels, would show as elongated stars in the corners.
Before blaming collimation, I would suggest to check any sag in the focuser and/or tilt of the sensor. I had a similar issue with my tak fs102+reducer+ASI1600, the culprit was the sensor tilt, that i solved with a CTU tilter.
Backfocus is an issue only if you are using a reducer, in such a case you have to measure the distance from the last metal part of the reducer to the focal plane
hope this helps
09 Feb, 2020 08:16
It will be better if you can show pictures of your problematic images.  Otherwise it's hard for us to tell what's going on.

Unless you use a focal reducer, the back-focus distance doesn't matter at all on FSQ.  None of FSQ's lenses move with the draw tube of the focuser.  What this means is as long as you can reach focus, the back-focus distance is exactly what it has to be.
09 Feb, 2020 17:18
I think it's more accurate if you can check the back focus distance from the end of the flattener or reducer - it should be 56mm. I don't have 106 but I understand the 85 and 106 should have the same back focus requirement.
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