Blue flats no good; all others ok ??

Acquisition Calibration Deep Sky Troubleshooting
11 replies•491 views
AstroRBA avatar
AstroRBA avatar
Good Day All - I have a strange problem with flats:

I collect L, R, G, B, S, H, O flats all during the same session, all at about 30K, using the exact same illumination flat panel with no changes between flats acquisition (other than time duration in order to stay around 30K) -  but the blue flat (even after several repeat attempts) always still shows dust motes when applied ?? All others apply perfectly. Oh, and all are applied with the exact same WBPP configuration settings. Light frame durations are all 300, 100, NB are all 600, 100 and Darks, Bias are all perfectly matched in temperature, offset etc. Flat also match temperature, offset, etc. 

Any ideas would be greatly appreciated!

Thanks !
Helpful Respectful Engaging
andrea tasselli avatar
andrea tasselli avatar
Are they under corrected or over corrected? And what sensor?
AstroRBA avatar
AstroRBA avatar
andrea tasselli:
Are they under corrected or over corrected? And what sensor?

Hi Andrea,

It's an ASI6200MM - not too sure which way the correcion is wrong (not sure how to determine that?) but the mystery is why all of the other flats apply perfectly?

It's a 7P ZWO FW and all others lock into place fine; can't see why the blue would not do the same?

All conditions are identical except for the filter (and the slight exposure time  adjustment to keep the value around 30K as with the others)..

Maybe the filter has some huge moisture stain that can't be fixed ? Although there are several and they appear exactly as dust motes do; I can examine the individual motes on the flat and the lights which don't appear out of line or anything.

I'm stumped

Pete
andrea tasselli avatar
andrea tasselli avatar
Just post a processed image in B and I think I can tell. Usually this stuff is due to wrong bias or just having the bias counted twice so I do without, just flat-dark and that is it.
AstroRBA avatar
AstroRBA avatar
andrea tasselli:
Just post a processed image in B and I think I can tell. Usually this stuff is due to wrong bias or just having the bias counted twice so I do without, just flat-dark and that is it.

Thanks for the info - Wouldn't the others suffer from the same ailment in this case (bias counted twice?) - I have bias, dark and flats (and lights of course) - not sure how to generate a flat-dark or how to apply it in WBPP? I've read about them but never produced / applied them.
andrea tasselli avatar
andrea tasselli avatar
Thanks for the info - Wouldn't the others suffer from the same ailment in this case (bias counted twice?) - I have bias, dark and flats (and lights of course) - not sure how to generate a flat-dark or how to apply it in WBPP? I've read about them but never produced / applied them.


If you use NINA than it is very easy to make them, they are just darks of the same length of the flats their master will be applied to (that is subtracted from). Which is is what you do. Not subtracting bias from flats or bias from darks but just flat-darks and, conversely, master darks without subtracting bias. I spurned this WBPP thing long long time ago and I do everything one step at the time so that I have full and unbiased control of every pre-processing step. But the only way to know why most of them work and one doesn't is exactly that.

The reason from not subtracting bias from darks or flats or lights is that some sensors, notably modern CMOS ones, have variable amplitude biases or no way to establish one unequivocally, as used to be and still does for CCDs. The only catch is that you should never ever scale darks (and I'd also check whether that is enabled in the WBPP settings) with CMOS. Given that different filters have different exposures for flats at a fixed amplitude (itself something I'm not really keen on) you may unwittingly fallen in this catch.
AstroRBA avatar
AstroRBA avatar
andrea tasselli:
The only catch is that you should never ever scale darks (and I'd also check whether that is enabled in the WBPP settings) with CMOS. Given that different filters have different exposures for flats at a fixed amplitude (itself something I'm not really keen on) you may unwittingly fallen in this catch.


I guess I could grab some dark flats easiliy enough and learn how to use them and I'll check the dark scale setting in WBPP - just an aside, I even tried DSS with some blue frames and had the same result. Funny thing is that I have considerable data from recent sessions (with older flats) and all was well on those occasions. I'm pretty sure I'm not doing anything different (but I guess I could redo some of them to see if it now happens on those too?)

Anyway, thanks for the info!

Pete
Dr. Dmitrii Paniukov avatar
Dr. Dmitrii Paniukov avatar
I'm having this issue with ZWO L filter. Sometimes it gives good ftats, something it doesn't. Recently I started seeing some weird lights from it, I guess it's just the filter. In my case I'll remove it and use the whole in the filter wheel. Other filters are just fine, never had a single problem with them.
Stephen Duffy avatar
Stephen Duffy avatar
I have had this problem as well, and it occurred when I tried to re-use flats from a prior session. So I've begun generating flats each session (not every night, but at least once during a 3-day session). This is the only way I can guarantee that all flats are taken under the exact same conditions. In CMOS cameras there seem to be more variables than in CCD cameras. I reuse bias frames, and have stopped using dark frames because in a CMOS the bias frame will generate ~500 ADU and the dark frame will generate ~575 ADU. So it's easy to over-subtract as someone else mentioned. If you dither, darks are not so important.
Helpful Insightful Respectful Concise
AstroRBA avatar
AstroRBA avatar
Stephen Duffy:
I have had this problem as well, and it occurred when I tried to re-use flats from a prior session. So I've begun generating flats each session (not every night, but at least once during a 3-day session). This is the only way I can guarantee that all flats are taken under the exact same conditions. In CMOS cameras there seem to be more variables than in CCD cameras. I reuse bias frames, and have stopped using dark frames because in a CMOS the bias frame will generate ~500 ADU and the dark frame will generate ~575 ADU. So it's easy to over-subtract as someone else mentioned. If you dither, darks are not so important.

I do take fresh flats regulary (and darks and bias every few months) but dark flats still confuse me (I'm fairly early into AP; just over a year, but I've put many hundreds of hours into PI to date)  - I do see that bias and darks are very similar, as you say, and I've read somewhere that dark flats should all maintain the same exposure time? (varying gain I suppose to keep a constant value between filters?) 

I was dithering but stopped (at least temporarily) because I had to throw out too many frames, I haven't figured out settle time or something I guess?

Anyway, no matter how frustrating it can be, this AP pursuit is still way better than watching garbage on TV ! Ha Ha !
Howard Richard avatar
Howard Richard avatar
I had trouble using a flat panel light source for NB flats. I now use natural light (sky flats) and get workable flats.
Well Written
James Ross avatar
James Ross avatar
Stephen Duffy:
I have had this problem as well, and it occurred when I tried to re-use flats from a prior session. So I've begun generating flats each session (not every night, but at least once during a 3-day session). This is the only way I can guarantee that all flats are taken under the exact same conditions. In CMOS cameras there seem to be more variables than in CCD cameras. I reuse bias frames, and have stopped using dark frames because in a CMOS the bias frame will generate ~500 ADU and the dark frame will generate ~575 ADU. So it's easy to over-subtract as someone else mentioned. If you dither, darks are not so important.

I do take fresh flats regulary (and darks and bias every few months) but dark flats still confuse me (I'm fairly early into AP; just over a year, but I've put many hundreds of hours into PI to date)  - I do see that bias and darks are very similar, as you say, and I've read somewhere that dark flats should all maintain the same exposure time? (varying gain I suppose to keep a constant value between filters?) 

I was dithering but stopped (at least temporarily) because I had to throw out too many frames, I haven't figured out settle time or something I guess?

Anyway, no matter how frustrating it can be, this AP pursuit is still way better than watching garbage on TV ! Ha Ha !

*Dark flats are just your normal flats calibrated with matching darks. So if your oiii filter needs a 10s flat then calibration with a 10s dark makes it a dark flat. The logic is the same as why light frames should be calibrated with matching darks - you want to remove effects like amp glow that scale with exposure time from your flat frames. Each filter can have different exposure time and different matching darks, no need to vary gain.

I find dark calibration is much less necessary for L R G B flats as these flat exposure times are typically very short.

It is definitely worth getting your dithering working as it can essentially remove some artefact that are otherwise incredibly difficult to postprocess out.
Helpful Insightful Respectful Concise
Related discussions
Flat Darks prep and use in PI
Recently, I switched from the ASI 2600MC to the ASI2600 MM Pro with LRGB filter wheel. It's been a complicated journey for sure. Im trying to figure out dust motes, flats etc and was really stumped. I often find numerous dust motes in my my linea...
Blue filter flat issues may relate to flat darks preparation differences.
May 25, 2025
Issue in frames calibration?
Hi everyone, this is a long post, but it’s needed to provide proper context. Thanks in advance to anyone willing to read through it and help me figure things out! I’ve been struggling for days with what seems to be a calibration issue (or possibly a ...
Frame calibration problems could explain blue flat dust mote persistence.
Jul 8, 2025
What would cause flats to not correct properly?
Pix Insight WBPPQHY600m – CDK14 + ReducerFilters are 50mm with filter masks, the Astonomiks even have a built-in mask (plus additional plastic masks).Chroma filters have the same patternSpike-a-flat panelThere are no light leaks - daytime darks = nig...
Blue flats issue; author troubleshooting filter-specific flat calibration problems.
May 23, 2022
Flats undercorrecting vignetting
Hello all! I'm facing issues trying to calibrate my light frames with flats, they seem to be undercorrecting the vignetting on the frame: The equipment I'm using is the following: - Modificated Sky-Watcher Quattro 250P OTA - Sharpstar 1x GPU ...
Vignetting correction issues might relate to blue flat application problems.
Aug 6, 2025
WBPP Dark Calibration
Ran into something odd today and could use some help figuring it out. I’ve been using PixInsight and the WBPP script for a while now without issues, but this time my darks seem to be overcorrecting my lights. After integration, there’s a noticeable d...
WBPP dark calibration settings could affect blue flat performance inconsistently.
Apr 26, 2025