QHY8L OSC artifact problem

[MarkS]

Thinking about this again, it appears your master dark has already had the master bias subtracted from it, since 2600 - 2300 = 300.  That might be right or it might be wrong - it depends on what your stacking program expects.

What program or programs are you using to create the masters and perform the calibration and stacking?
Do you process using the masters or the original bias, darks and flats?

[Carole]

I used DSS Mark.  I don't use masters to stack always the individual fits files.  

Sorry, I didn't realise there were master stacks in the folders, it could be due to the person I previously shared the files with uploading them to their stacking software direct from Dropbox, in which case ignore those and just use the individual files as the masters are not the result of my stacking.  

Carole

[The Thing]

I used DSS Mark.  I don't use masters to stack always the individual fits files. 

Sorry, I didn't realise there were master stacks in the folders, it could be due to the person I previously shared the files with uploading them to their stacking software direct from Dropbox, in which case ignore those and just use the individual files as the masters are not the result of my stacking. 

Carole

With DSS you should stack with all your flats and darks and bias frames for the first pass only. Next time you deselect those files (make sure you save a file list) and select the Masters that are always created by DSS (and which are always TIF files) to use instead. It makes the subsequent stacking runs where you are tweaking kappa-sigma etc. much much faster as all the calibration preprocessing has been done.

[Carole]

I rarely need to stack more than once Duncan and speed doesn't bother me.  I am gradually going over to AstroArt now in any case. 

Carole

[MarkS]

Carole,

I put your files into DSS and it all worked fine - no artifacts.  So I wonder what the difference is?

Mark

[Fay]

Gosh Carole all that dirt in your imaging train, i think you need a hoover on it!!!

[Carole]

I put your files into DSS and it all worked fine - no artifacts.  So I wonder what the difference is?

I have no idea Mark, unless you used different settings to me. 

I know you don't normally use DSS, but do you know what settings you used, what recommended settings and what stacking parameters you used?

I have had this problem consistently using this camera and there has to be something different if we're using the same data and same software.  I have been told to use GBRG for the generic settings for this camera.  Also what version of DSS did you use?

Carole

[MarkS]

I was using version 3.0.1 and just treated them as mono images.  But that is sufficient to check your calibration files are OK.

I'll download a more recent version and try again.

[Carole]

I've got version 3.3.2.  I just did a mono stack, it's hard to see the artifacts in Mono but they are still there.  I accidentally (for the first time ever) loaded the darks as dark flats.

I am going to try to make some new flats with the camera today (complicated to set up as everything is indoors for the storm) and see if the clean I had of the IR glass in front of the chip makes any difference.  It would be lovely if I have removed the offending particles, but still doesn't solve the mystery.

Carole

[Carole]

Also stacked in AstroArt and the artifacts are still there.

Carole

[MarkS]

I downloaded DSS 3.3.2 and it gives complete rubbish (after multiple attempts) with severely clipped and posterised results as soon as I try to calibrate one frame (using a single flat, single dark and single bias).
However, GBRG is definitely the correct colour balance - I checked this by not using the calibration frames.

So I checked in more detail using another program and an odd thing I have noticed about your files is that the lights and flats have a dark stripe down the left side but the darks and bias don't have these stripes.  The fact that some files have stripes and others don't, causes negative numbers at some points during the calibration of frames and maybe this is what upsets DSS.  It might also upset other software but IRIS copes OK.

It would be interesting to know why some frames have the dark stripe and others don't.  It might be a question for the manufacturers.  It makes no sense to me.  I'm sure this is what is causing your calibration issues.

Mark

[Carole]

severely clipped and posterised results as soon as I try to calibrate one frame

That's very interesting Mark as I have been recently having this problem myself, and indeed if you may recall when I was in France I was unable to process some of my Ha images because of posterisation using DSS (v 3.3.0) and Olly had to stack them for me in AstroArt.  Since I came back I have had similar experiences with some of the filters with my desktop version of DSS V 3.3.2, had to use AstroArt, but this is with a mono Atik camera and not the OSC.  Yet I have never had this problem before, and I am not prone to updating my software (I always think if it works don't risk messing things up), so I wonder why this version has suddenly start producing posterisation problems, though I in the past used to have problems with DSS not subtracting flats from my DSLR images.

Regarding the stripe down the side, this is a feature of the QHY8L it's called overscan, I just checked the fits files for darks and Bias and I think it is there but less obvious because they are black.  

I wonder whether I should update DSS but the fact that I still get the artifacts left behind in Astroart doesn't suggest this particular issue is a DSS problem.

I've tried flats again after cleaning the IR glass but the artifacts are still present, so I took the bull by the horns and cleaned the CCD chip using the QHY instructions.  I haven;t yet re-tested it.

Carole

[Carole]

Hmmm, it seems DSS v 3.3.2 is the latest version and no previous versions on offer.

Carole

[MarkS]

I don't understand why the lights and flats suffer from obvious overscan but the darks and bias don't.

My suspicion is that DSS 3.2.2 is not coping with the overscan region.  I'll run some tests to test this hypothesis.  IRIS has no trouble and nor does PixInsight and if I do it fully manually there is no problem either.

[MarkS]

I cropped the FITS files in PixInsight to remove all the overscanned regions and ran them through DSS again.

I got exactly the same clipping and posterisation problems again.  I've tried setting both "Generic GBRG" and "QHY8" as the camera type for the FITS files but it made no difference. 

I've run out of ideas and patience with DSS now!

Mark