Re: DSS set black point to zero now automatic
Posted: Wed Nov 01, 2023 9:33 am
@Startrek Hi Martin, I'm not sure about your last post. Maybe something went wrong?
The official StarTools forum
https://forum.startools.org/
Yeah, that's it, Mike. I agree with all this.Mike in Rancho wrote: ↑Tue Oct 31, 2023 8:23 am as well as googling Pixinsight and "no black point correction" which is their version of the exact same issue.
It's all about the extraction from RAW, by way of the options that can be fed into DCRAW or whatever. But DSS' terminology lends itself to confusion, IMO.
PI explains that you want the "no black point correction" option (and I believe it is a default setting) in order to get your pure RAW data. Or at least as pure as one can get out of a DSLR. If one does choose to use black point correction, then DCRAW will subtract off some form of the bias level, using a value from the DSLR's data (if so recorded and written to the RAW file). One does not normally want that. Instead, you want to do a full calibration yourself using more precise sets of bias, darks, etc.
In DSS, SBPTZ is the same as PI no black point correction. Zero = "no." Meaning zero (nothing) gets subtracted off of your RAW files by DCRAW.
So, if using DSLR, I imagine you would almost never want to uncheck SBPTZ.
I'm still not sure about this, too. I remember I read about this, but I will have to check and rethink this.Mike in Rancho wrote: ↑Tue Oct 31, 2023 8:23 am I suppose you could uncheck it and try to do a lights - flats only calibration, as DCRAW will then subtract off the camera's estimated bias level?
Again agreed - and this is almost what I wrote ealier in this thread.Mike in Rancho wrote: ↑Tue Oct 31, 2023 8:23 am I can also see now why you might not want this to be fully automated. Defaulted to checked, yes. But, especially if I was trying to do some kind of unique calibration for a particular circumstance, I would be wary of DSS unchecking SBPTZ behind the scenes based on an algorithm (i.e. trying to out-think me) and having things go haywire.
Both true. So what do we do now? Most people here are using dedicated astrocams these days, not DSLRs. So it's kind of irrelevant from that point of view. Besides, it's still not "completely clear" as you wrote, and I'm afraid that's not enough to get into a discussion with the developers of DSS One option might be to wait until someone here encounters a problem. Having an example would make it much easier to open a thread in the DSS forum.Mike in Rancho wrote: ↑Tue Oct 31, 2023 4:43 pm Well I still haven't found a totally clear, nuts and bolts explanation of things. [...] But this is a DSLR RAW issue only, so nothing that needs to be accounted for with astrocam FITS.
I think we can now be pretty sure that this is not the case. (Please see my post above).
I stumbled across exactly the same thing when I switched to ASTAP a year ago or so. DSS is a bit smarter in this regard as it automatically uses Bias to get the calibration equation right in case of missing Darks. I think.Startrek wrote: ↑Tue Oct 31, 2023 10:59 am From my experimentation with Mono data sets in ASTAP so far , you must use the full suite ie Darks ( or Bias ) Flats and Flat Darks ( or Bias ) otherwise your Flats don’t to the job , I ended up with an inverse Vignetting when I left the Darks tab empty. Han confirmed with me on the ASTAP forum that I should use my Bias in both Darks tab and Flat Darks tab, which I do now.
I'm still not sure. This might be a different story or at least only a part of it? But I still haven't got the point when one would intentionally switch this box off and use camera black subtraction. Maybe that's the missing key for us. I will have to read and think about it again.
I wouldn't go quite this far. It's maybe more of an accident, or just the way DSS goes about things that you can set up a bias-only (to be used on both sides) calibration with ease.
I reused an old Bias master and DSS crashed It drove me nuts. I will have to take new Bias frames first.Mike in Rancho wrote: ↑Fri Nov 03, 2023 5:40 am or it can also go terribly wrong, [...] - especially if reusing old masters.
Cool, Dietmar!
Agreed. I was still being frustrated about the crashes and haven't analysed that yet this morning. Just wanted to tell.Mike in Rancho wrote: ↑Fri Nov 03, 2023 4:57 pm You have calibrated light-flat without any correction of dark level on either side, thus reverse vignetting.
Yeah, that's a poor man's calibration. But why not.Mike in Rancho wrote: ↑Fri Nov 03, 2023 4:57 pm So at least in the case of this particular DSLR, it would seem that unchecking SBPTZ causes synthetic dark/bias (black level) subtraction, and would let you get away with making a stack even without separate darks, dark flats, or bias. The result would not be a perfect and precise calibration, but it's in the ballpark and probably corrects vignetting and dust spots to some degree.
I forgot to mention: I did that yesterday as well and it worked fine for me, too.Mike in Rancho wrote: ↑Fri Nov 03, 2023 4:57 pm nd continued the test by unchecking SBPTZ for a comparison, thus using the "synthetic" camera dark level as subtraction from the RAW files, in this case the light and the flat.