Not sure how to explain this column defect

[macnmotion]

Hi. Take a look at the following image. Top row Left to Right: Master L Flat, Master Dark (5 minutes), Super Bias. Bottom row: 5 Minute L sub.

Note the column defect in the flat. This appears on all flat subs, all filters, different nights. But no such defect on the L sub or on the dark/bias. Why would this only show on the Flat?

When I calibrate, the flat "removes" the defect from the raw subs, then I have to use cosmetic correction to fix the column in the calibrated subs.

Does anyone have any idea why this might happen only on the flats? Seems like it's causing me extra work in cosmetic correction, as well as damaging clean data.

Thanks. Andy

[bulrichl]

Hi Andy,

image is missing.

Bernd

[macnmotion]

Sorry Bernd, human error :-) Hopefully it's visible now in the original post. Andy

[bulrichl]

Is it possible that the defect developed new currently (if so, the flats must have been taken after the L subs)?

In order to process the current frames, it might be preferable to correct the MasterFlat, NOT the calibrated L subs.

Bernd

[macnmotion]

Hi Bernd, this has been going on all season, I just never looked carefully enough to see that I was introducing the defect with the master flats. I'm wondering if it's showing on the flat because the flat is illuminated while the light frames are not? I did an extreme stetch on a Lum sub, absolutely no column issue there. It's only on the flat subs, every flat sub all season.

I can try fixing the master flats, that would probably work. I can use cosmetic correction to remove that partial column defect from each master flat.

[bulrichl]

What I meant is this: A new defect can arise spontaneously (potentially by a cosmic ray hit?). Then frames that were taken before this event will not show the defect whereas frames that were taken thereafter will show.

Take one new light frame and one new flat frame successively and compare. It is not plausible at all that the defect is on the light frame but not on the flat frame. The camera doesn't know whether you are taking flats or lights.

Bernd

[macnmotion]

Hi Bernd,

I have calibration subs and light subs going back the entire season. The column defect is in the flat subs only, not any other subs. For the entire season, the defect appears on the flat frames only, not darks, bias or lights.

So for example flats taken In February have the defect, as do flats taken two different dates in March, and again this past weekend. Lights taken over that entire period do not have the defect. My master dark/bias were taken in January, before the first flats were taken, they do not have the defect but I understand your comment that the defect could have occurred between that date and the date of the first flats. However I would expect to see this defect on any light image taken since February, and the defect simply isn't there.

The camera is a PL16803, and has been on that scope since December 2017.

[RickS]

I wonder if it's some sort of non-linearity that only happens when you get to a significant proportion of full well.  Have you looked for traces of it when that column coincides with a bright feature in a light?

Cheers,
Rick.

[bulrichl]

Hi Andy,

until now I thought that a column defect either does occur on all frames or not at all, but probably Rick is right and there are defects that arise under certain circumstances (e.g. definite illumination) only.

I would ask the manufacturer, FLI.

Bernd