Flat frames not subtracting properly from lights

[peterbrack]

I have taken an SHO image of tarantula
Have created master bias, dark, and flats for each filter.
I have calibrated SII, Ha OIII, with the bias, dark and flat masters, then used star alignment, and then integration
All goes okay. When i went to combine the 3 images in ChannelCombination I then really noticed the dust motes standing out
I then tried to do the whole process with batch preprocessing, which produced the same results
I had a look at each Master Flat of SII, Ha, and OIII and they all look pretty much the same, similar dust mote distribution/pattern.
So they don't look like Image Calibration is working properly for the flats- seems to be fine with the dark and bias, as I see the reduction in noise there.
Haven't noticed this issue previously
Can see the dust motes just , on each Master Light, but they do stand out more when combined
Any ideas about why this might be occurring?
Have added drop box links to - master flat Ha; master Light Ha, and a rough SHO with channel combination showing the dust motes

Peter

https://www.dropbox.com/s/3ojbvfe7le8vcog/MasterFlatHa.xisf?dl=0

https://www.dropbox.com/s/j80qrtg16kologd/MasterLightHa.xisf?dl=0

https://www.dropbox.com/s/o286ymb7fo9dsvd/Image81_DBE.xisf?dl=0

[sharkmelley]

Are you are really subtracting flat frames from the lights?

That will cause all sorts of problems.  Light frame calibration by a flat is a divide operation, not a subtract operation.

Mark

[peterbrack]

Yes I meant divide
I just do what the ImageCalibration script does

[dld]

Peter,

How did you obtain your Flats? I can see some strange square structures after running an ABE on your flat (helps to see things better). Also the bright horizontal band at the top of your light isn't well represented from your flat.

[peterbrack]

I obtained the flats through ACP. Took 10 flats per filter. Then calibrated and integrated them in PixInsight.

The dark band at the top is in the light frames- I don't know exactly what this is. Have had it for a while now. But yes it is not on the flats, just comes up on the light frames for some reason