Sony A7S Concentric Banding

[MarkS]

Hi Carole,

Those aren't daft questions, nor are they daft suggestions.  Everything is worth considering when someone has reached the point of ditching a camera.

I know it's a daft question but what does the image look like without flats applied, I know it will have dust bunnies and vignetting, but would be interesting to see the difference.

It's impossible to do.  I'm stretching the data by a factor of 200 or more to reveal the IFN.  Without flats applied, the vignetting means that only a tiny area of the image would be visible, with the rest either saturated or black.

Also for arguments sake, have you tried applying the flats and no bias subs:
I know you technical people will shudder at this suggestion, but I have in the past stacked an image and it's come out really awful, perhaps something on the lines of what you are getting or worse.  Then some-one suggested to me to leave out the bias as it can sometimes muck it up.  I did exactly that and the image came out just fine.  This has happened to me on 2 or 3 occasions over the years though I must confess not on every image.  Just thinking "anything might be worth a try" if only for the sake of elimination. 

Good suggestion and I'm already doing that.  The colour banding does not exist in the bias or darks so I've eliminated them from the ongoing inquiry and use a synthetic offset instead of the bias frame.

Any chance of letting us have some unstacked files and calibration files so we can "have a go" at them. 

No chance.  I'm not precious with my data but you need hundreds of subs to show this problem.  They wouldn't even fit on a DVD backup!

I've wasted most of my day on this but I think I've found something very surprising.  I'm waiting for dusk when I can take various sets of sky flats to either confirm or refute a very surprising hypothesis. 

Mark

[Carole]

Waiting with baited breath.

Carole

[MarkS]

Last night's test results were inconclusive.  I'll take another batch tonight.  But I did get an image of the moon and Jupiter in a cloud gap. 

Oh dear, I've just noticed how much a brand new Nikon D810A actually costs!

Mark

[MarkS]

Last night's batch confirmed my worst fears: I can't even calibrate one master flat with another master flat of the same ISO when there is a brightness difference between them i.e. the back of camera histogram peak is in a different place.  Both master flats were taken in the same twilight session but the result leaves coloured vertical swathes through the data.

I'll admit that I'm applying a huge stretch to the data but it's no different to the stretch I would apply to an image of M81 and M82 to bring out the Integrated Flux Nebulosity.

Mark

[Carole]

So if l am understanding this correctly, since sky flats could vary, what about an artificial light source which will be consistent. 

Presume using a single flat will not work.   Otherwise  could you just duplicate the single flat to produce a master. 

Dreadful scenario.

Carole

[MarkS]

Yes an artificial light would definitely be constant.  But it would be white light, so the relative positions of red green and blue histograms would not match the relative positions of red green and blue histograms of the orange sodium light pollution in my light frames.  This is the actual problem - my flats don't calibrate my lights - I get purple and green vertical swathes through the data.

I'm just calibrating master flat against master flat to see if I can narrow down the problem in a simple test environment.  If I can't even do that successfully then there is absolutely no hope of calibrating the lights.

Mark

[Carole]

Many people use EL panels for their flats, including OSC, but maybe they are not stretching the images so much. 

I do know I had some very strange results with my QHY8L which was a OSC camera and could never work out what was going on there. I was not stretching to the extent that you do.  It makes me wonder now whether that might have had anything to do with the flats, though not the same problem as you are faced with.

In the end I abandoned OSC and just stayed with my Mono camera.

This is one of the images I got, I don't think I saved the other one.

[Carole]

Another thought, though it might be a stupid one.  What would happen if you changed your flats/master flat to mono before calibration?

It's only supposed to be dust and vignetting you are trying to eradicate? 

Carole

[MarkS]

Another thought, though it might be a stupid one.  What would happen if you changed your flats/master flat to mono before calibration?

It's only supposed to be dust and vignetting you are trying to eradicate? 

That's my back up plan - I already tried it and it works pretty well.  It's not an ideal solution because these stripes are encoded in both the lights and the flats.  If they are not in the same place in both then they are not corrected in the final image.  Similarly, a flat converted to mono cannot correct the stripes in the lights but it does reduce the problem by half.

I love that image you posted - it looks like the background cosmic signature from the big bang!

Mark

[Carole]

So my idea of changing flats to mono wasn't so daft then.   Glad it seems to work better than the colour version.

I love that image you posted - it looks like the background cosmic signature from the big bang!

I got that on several images and thought at first I had buggered up the sensor when I cleaned it with Alcohol as it had some very stubborn bits of dirt.  Solving the problem was beyond me and all the help I got on one of the forums, so I sent it to some-one who was used to the camera and the software I could not get on with and he managed to get it working OK, so I just sold it.  I think it might have been something to do with debayering and the software getting confused (at least that was the analysis of one of the people trying to help me), but far too complex for me to sort out.  I never had that problem when I used APT, but for some reason APT kept crashing, so it was when I was trying to use other capture software that I ran into this problem.   Mono is much more straight forward albeit a bit of a learning curve to combine the data. 

Well hopefully you are making some progress.

Carole

[Carole]

Would be interesting to see the results of an image calibrated with a coloured flat and then the same image calibrated with a Mono flat, if you already have "one you made earlier".

Carole

[MarkS]

Would be interesting to see the results of an image calibrated with a coloured flat and then the same image calibrated with a Mono flat, if you already have "one you made earlier".

I'm doing a whole batch of test runs but each run takes a few hours because there are hundreds of light frames.  The idea of a mono flat doesn't actually work in practice because it doesn't correct the coloured horizontal banding on the left hand side of the split sensor.

Mark

[Carole]

it doesn't correct the coloured horizontal banding on the left hand side of the split sensor.

Bummer, forgot about that problem.

Carole

[MarkS]

it doesn't correct the coloured horizontal banding on the left hand side of the split sensor.

Bummer, forgot about that problem.

So had I 

Mark