FITS header- FILTER keyword for FLAT-DARKS?

Can you add the “FILTER” keyword to the FITS header for Flat-Dark frames?

I’m exploring using new features of PixInight’s WBPP script and need to be able to tell it how to differentiate my flat-darks when I use the same exposure for two of the filters. The script can use any FITS keyword to group files, so it would be nice if, when doing flat-dark frames, the FILTER keyword was present. We already select the filter for flat-darks in order to induce SGP to use the correct exposure as determined by the flat calibration wizard, though I don’t recall if SGP actually sets that filter since it is a dark frame…

OK, I messed the process up a bit. When setting up an even for flat-darks, I first select Type=Flat, then choose my filter so that the exposure will be populate from the values determined from the Flats Calibration Wizard and stored in the profile. Then I change the Type to Dark. I’ve been doing that for years. Is that the correct process or am I doing it wrong? I just tested with 4.1.803 and this method of creating a Flat-Dark event does not result in a FILTER keyword in the fits file.

Thanks - Shane

I mean… it’s not an “incorrect process”, but the problem is that dark frames are filter agnostic and don’t emit that information. Ideally, there would just be a type for flat dark. I’ll think about this for a minute…

Hmmm. So what is the recommend process for creating a flat-dark event? Am I missing something obvious? (Wouldn’t be the first time…)

  • Shane

That’s what I’m saying… There is no process for it so anything you do isn’t wrong. It’s just “not supported”. This is the part I am thinking about. Trying to understand how much work it is to add and it can be a minor release or needs to go through a beta period.

OK. I just figured out a work-around for my specific case by changing my filename pattern to encode the filter in a way that the WBPP script can use. Since no one has had a problem with this for … 6 years? … and there are no other comments on this thread about it, maybe it is not a big deal (or even a little deal).

I can get by without it. I expect you have much important things to work on! The thing that would help would be to remove the trailing spaces from the filter name in light frame FITS files–unless that, too, is something that I somehow introduced!

Thanks - Shane

We do already have a partial implementation for dark-flats. Just looking at why we never released it.

Shane i’m just seeing this thread now. There really isn’t a ‘problem’ to solve here.

Flat-darks are just darks. you take them as darks and add them into wbpp as you would any dark. WBPP will know how to apply flat-dark to your flats based on exposure time (e.g., 5 second flats will be matched with 5 second darks).

there is only one flat-dark that’s applied to all your flats, and as Ken said, it’s filter agnostic


Yeah, Brian. As I’m working through WBPP with my setup, I’m figuring out how to make it select the correct masters. My stuff is a bit weird as the data set spans 4 years and two telescopes. Since I already had the filter encoded in the file name, I just wrote a Python script to add it to the fits header. I got all of the master flat-darks done (14 different sets) and they are correct. I have already deleted my FILTER grouping word for the processing the lights now.

And I’d already told Ken that I can get by without it a couple of messages ago. As I get more familiar with WBPP, I probably won’t need the filter even for my master flat-darks. I think my problem was the “exposure tolerance”, which was (somehow–I don’t remember setting it and I don’t know why I would have" set to 0.5, which ended up combing some of the flat-darks for different filters. I’ve since set that exposure tolerance to 0 and will try running it again next time I need to generate master dark-flats.

Anyway, I agree with you that there is nothing that needs to be fixed. I even plan on making (at some point) a flat panel that will allow me to shoot all flats at 5 seconds. That may be a stretch getting a brightness difference to get luminance and O3 to the same exposure!

  • Shane

cool Shane

exposure tolerance can be tricky.

I think the WBPP authors realized it could get a little weird when it came to auto-selecting, which is why they implemented keywords. so you can make sub-selections of files in WBPP and assign them keywords, and then use that to apply to various lights, subs, etc.

I looked at what we had before and the reason we didn’t release it because its onlt difference from a “regular” dark is that it attempts to auto match gain / ISO of the “paired” flat. That sub-system of pairing cal frames in order to ensure synchronized settings is not ready to go and, because of this, we never released it.