Full-frame CMOS issues at f3 using an FSQ106 - Help appreciated!

[SyedT]

Hi all,

Chasing after large mosaics, I decided to take the plunge and have got a full-frame sensor (Altair Astro 61MFX). I already have a Tak FSQ106-ED and its matched 0.6x reducer which brings the scope down to an insane f3. I was previously using a QHY268M with this combination, and corner stars were not perfect, but not that bad either.  Binning was causing a headache with star shapes and colour combination, however,

I definitely wasn't expecting perfect stars with the new combination, but despite accurate backfocus (56.2 mm) there's significant star distortion all around the image, which makes me think this is related to field curvature (despite Tak advertising a 44 mm corrected imaging circle at f3). I removed the reducer and hey presto, the distortion is gone apart from one corner (which is definitely tilt as Tak focusers leave a lot to be desired).

I've uploaded some images on Google Drive, would appreciate any advice: https://drive.google.com/drive/folders/1K2318n9blCVL9acrAl0ybandzZ-HgB_f?usp=drive_link

If this reducer doesn't work, I may end up trying the 0.73x reducer or just using the FSQ at its native f5 speed.

Cheers!

[Fegato]

I don't have any experience with fast refractors, just the Hyperstar and now RASA. But once you get down to F/3, your CFZ is obviously very tight, just handfuls of microns. So it might not be a problem with the reducer per se - could just be that your backfocus distance is a tiny bit out (plus any minor tilt). The shape of those stars might be some sort of optical issue, but they are all pointing in to the centre, so you might just need a tiny bit more distance to the sensor? 

My experience at F/2 has been that I need a precise tilt / backspace tool (in my case, Octopi) as part of the image train in order to do the various tiny adjustments necessary to get a nice flat field. The flat field is there to be had, but requires patience and the right tool to find it. If there's a tilter on the camera you could try tiny adjustments with that perhaps? (although those tend to be a bit rough and ready)

 

[kirkster501]

I assume you are using autofocus?  The fact that the central stars are good suggest your focus is good but, still, it is worth pointing out just as Robin has done, that focus distance changes in the CFZ at F3 are minute.  Any issues with the chip not being 100% orthogonal with the illuminated field of the telescope will cause these issues you discuss.  Are you 100% sure you have no sag whatsoever in the imaging train?  Is everything screw-in in the imaging train?  There is no slippage in the focuser when pointed at high angles?  I had that issue on my FSQ85 focuser and had to tighten it up since I too had this issue when using the reducer.  Is the telescope itself collimated and producing a flat field?  These questions are not easy to answer, I concede.

Edited November 14, 2023 by kirkster501