Tried a 1.25 inch filter, is my camera out of line with sensor

[BrendanC]

Hi all,

Next problem...

I recently looked at the possibility of using a 1.25" filter with my ASI533MC and 130PDS, reasoning being that if it worked (which it should, given such a small sensor), I could upgrade to 7nm duoband for less cash.

So, I got me a cheap little Svbony UV/IR 1.25" filter, and tested it last night, just an hour before the clouds came in, on M2.

At first the results looked promising but then I noticed a faint circular outline on the stack, sort of to the right and down from central. On super-stretching it, there it is, but everything else looks central to me.

Wondering if it was a flats issue, I stacked without flats, and got this - looks like the flats are working and again, everything looks sort of reasonably centered.

Then I super-stretched the master flat, and got this. 

It looks to me like my camera isn't quite centrally aligned over the tube, so I can see the edge of the filter and that's what's causing this issue.

I can't quite believe this because I've collimated the 130PDS a bazillion times. I don't know how more accurate I could be. It looks absolutely fine through the Cheshire.

So, is this my problem? If the fix is 'go away and collimate it again' then I'm just going to go back to 2 inch filters and be damned.

Thanks
Brendan

[BrendanC]

OK, got help on this one - turns out I just assumed I could use the 2-to-1.25" adapter and then another adapter I had lying around, to screw the filter into the end of the coma corrector, exactly the same way I use the 2" filter. Wrong. I should have unscrewed the extension tube, screwed the filter in there, in the 2-1.25" adapter, right up close to the sensor, then replaced the extension tubes. Silly me. As they say 'assume makes an ass of u and me'. So, hopefully this will fix the problem next time.

Edited December 12, 2023 by BrendanC

[ONIKKINEN]

This sort of circular outline of the fully illuminated circle being present in an image indicates that the flats do not mechanically match the lights - as in something between the sensor and the mirror moved during the night. There could be many issues that cause this, like focuser slop, secondary slop, primary slop, or the tube itself deforming ever so slightly under gravity in different orientations.

The filter thing you got right, needs to be close to the sensor, but if this circle shows itself again you have more work to do in figuring out which part of the system is not stable.

[BrendanC]

Yes, I'd have thought the flats would still work even if the filter was too far away. Fingers crossed it all just works next time. 

[Clarkey]

Flats should still work. I use a filter on the very front of my FMA180 and it works fine.

FWIW I had a similar donut on my first use of a new Risingcam IMX571. I assumed that it was an issue with the camera - I was wrong. It was caused by dew on the sensor. I gotta new heater for the camera and it's been fine ever since. Just a thought....