M45 - Circular Artefacts

[IanL]

Here is a first very rough cut of M45 from Sat December 1st; first light with my new Evostar 80ED rig. Capture details:

Evostar 80ED + Skywatcher 0.85x FR/Flattner, Unmodified EOS500D, ST80 / QHY 5 / PHD Guider.

14 x 600s Lights, ISO400, 13c Sensor Temp according to Exif

30 x 1/4000s Bias, ISO400

10 x 600s Darks, ISO400, 26c Sensor Temp according to Exif (had to do on Sunday, so not a good match, will try to get some better ones next weekend if cold)

30 x 1/800s Flats, ISO400 (T-shirt flats, going to build a light box as not ideal)

I have processed this in PixInsight using the BatchPreprocessing script, completely on default settings through to integration. (I first tried a complete manual calibration and integration process but it was a dismal failure, don't know why yet as it should have been identical to to the BPP process but clearly wasn't).

Then I did a bit of basic processing to get it to a point where I could see what I had. The main issue is the full moon on Saturday night, so there was a very strong gradient from bottom (dark) to top (very bright) of the frame. I used DBE to kill this off and it did a fairly good job, but you can see there is still a bit of gradient left at the top since I couldn't push the sample acceptance/rejection parameters and further to include it.

Did a small amount of Atrous Wavelet noise reduction (has made the stars too soft, but no point in pursing that until some of the other issues sorted), followed by a bit of ACDNR and SCNR. In all cases I have used some fairly conservative settings just to get a reasonable idea of what is in the image. Finally did a histogram stretch and a bit of curves.

The main question I have for the experts is what are the odd circular artefacts scattered across the image. They are about the same size as the largest star haloes. They are especially noticeable across the bottom half of the image, and there are both dark circles and bright ones which appear to be in diagonal lines running from top left to bottom right.

I'd have said either they are dust bunnies that haven't been taken out by my (admittedly poor) flats, but I'd have expected only dark artefacts in that case. Alternatively maybe some kind of reflection artefacts from the optical surfaces in the scope/FR set up, but wouldn't they generally be bright artefacts (maybe not if reflections from one surface are 'shadowing' another one?) Also, I seem to recall seeing similar circular (dark) artefacts on some of my SCT images when processed in PI, but I haven't got any particularly good ones due to guiding issues so never pursued it.

Any thoughts? Also any other pointers on improving this image? (Yes I know, better lights with no moon, better matched darks, better flats!)

Cheers

[rfdesigner]

ok. let's say it's dust bunnies.. but that they changed between lights and flats. That way you could be correcting for dust that's in your flat but isn't in the image and so end up with bright circles.

Do you think it's possible that the dust could have moved between lights and flats?

Derek

[IanL]

Actually you could well be right there, I hadn't thought it all through but that makes perfect sense. The 500D has the sensor cleaning process turned on by default and I need to add a step to my workflow to turn it off since that would nicely migrate the dust around if I am taking flats later. Suspect this is mystery solved, but will have a look at my flats in detail tonight to confirm.

[martin_h]

Yep, looks like dust to me.