Unknown Stacking Issue

[Snoani]

I wonder if anybody is able to provide me with some advice to resolve and unexpected stacking issue.  I was out last night and spend the time imaging two targets, NGC 281 (image that I have attached) and M45.  

Both targets were being imaged using the same equipment but using different ISO and exposure lengths and so I took separate calibration frames.  At the beginning of the night I took flats and bias for each target and I took the darks at about 4am after taking the light frames.  There was therefore no difference in how I took the calibrate frames for each target, apart from changing the settings. 

I stacked M45 today and have a nice output image and was expecting the same for the Pacman Nebula, however, to my surprise the stacked image came out as below.  

I have checked the lights and they all look good, so I don't anticipate a problem with them.  I haven't stacked them on their own yet but will try this later.  I am expecting this to be a problem with the calibration frames somewhere, but I am not experienced enough yet to know what the issue is.  I have stretched the master flat and can't see an issue, so think it may be the darks or bias?

I'm using DSS to stack and haven't changed any settings that I normally use, and have never had this issue before.  Again, I am also conscious that my other target of the night stacked well.  I also tried restacking from scratch but with the same result.  

Any guidance would be most appreciated because from the light frames, I was expecting a decent final image.  

Thank you

[Snoani]

As an update I have stacked the light frames without any of the calibration, which has produced the image below, the as thought the lights appear to be ok and so it is to do with the calibration frames/ stacking process.  

**Update** I have since stacked with each calibration separately and can confirm that it is an issue with the flat files.  I can't see anything obviously wrong with them, and the flats for M45 were taken immediately afterwards and these stacked without issue, so I am still at  loss.  

 

Edited September 21, 2020 by Snoani

[alacant]

2 hours ago, Snoani said:

beginning of the night I took flats and bias for each target

 

52 minutes ago, Snoani said:

the flats for M45 were taken immediately afterwards

Which is it?

Anyway, use the flat frames which worked. Unless you moved the camera relative the telescope between targets, if they worked for one set of lights they'll work for the other.

If you're with a DSLR, lose the dark frames and dither instead. 

HTH

 

[Snoani]

31 minutes ago, alacant said:

 

Which is it?

Anyway, use the flat frames which worked. Unless you moved the camera relative the telescope between targets, if they worked for one set of lights they'll work for the other.

If you're with a DSLR, lose the dark frames and dither instead. 

HTH

 

Sorry for the confusion I mistyped, and thanks for your reply.  Flats and bias were taken at dusk and the darks were taken immediately after the lights.

The flats that I took for the other target were using a different ISO, and going with the DSS manual, should be taken at the same ISO than the lights.  Is it an issue to use flats taken at a different ISO?

Thanks for the tip on dithering.  

[alacant]

46 minutes ago, Snoani said:

flats taken at a different ISO?

Not really. Flats correct optical irregularities. So long as the camera hasn't been moved and the light path to the same is identical, the iso is irrelevant, especially in this case where we're trying to rescue a set of light frames. Keep it the same if you can of course...

Cheers

Edited September 21, 2020 by alacant

[Snoani]

Thanks for your guidance, much appreciated.  I'll give the other flats a try in this case.