Flat frames creating a radial pattern! Not sure how to fix this : (

[pipnina]

I got my first light with my RisingCam IMX 571 (aka. alternative ASI 2600mm) mono cam, and although the image below is very impressive for only 20 minutes of data, the flat frames are causing this horrific banding (and to be honest my vignetting is rather harsh, 32000 peak and 14000 minimum when taking flats in my 16-bit capture, owch)

I have put a coat over my prinary mirror cell to stop light ingress, and my optical setup is: a very wide, diffuse, and bright bulb (3300 lumen 5500k colour temp). Followed by two sheets of 80gsm printer paper right up against the telescope aperture, then my Skywatcher 130-PDS, which goes into a Baader MPCC MK3 coma corrector, through an ASI 36mm 7-slot filter wheel (no filters yet) and finally into my RisingCam 571. This effect occured when my histogram peaked at 95% saturation and when it peaked at 50%, both the same pattern and intensity of banding in both versions. I assume it must be something in my optical system since this is the second camera I have seen this effect on : (

My Nikon D3200 had the same effect and I'm pulling my hair out trying to guess why it's doing this! I'm using Deep Sky Stacker 4.2.6, flats set to median mode, lights set to Median Kappa-Sigma, bias frames set to median.

Any help greatly appreciated!

Thanks

[ollypenrice]

Is it like this a) in the flats themselves and b) in the stacked lights if you don't apply the flats?

Olly

[vlaiv]

Indeed, it would be good to see both raw light frames and raw calibration frames (one of each - whatever you are using, flats, darks, bias, etc ...).

Image you presented has some clipping so things can't be seen clearly - but it looks like you have some mismatch in your flats - maybe something moved between lights and flats, or you changed focus position.

Here is suspect part of the image

Background appears to have "emboss" effect on it - this usually happens on camera rotation for example or shift of some sorts.

[pipnina]

5 minutes ago, ollypenrice said:

Is it like this a) in the flats themselves and b) in the stacked lights if you don't apply the flats?

Olly

Here's my stretched flat frame (it's off center, I need to work out my collimation a bit more... it's all tilted correctly, but the secondary is too far into the tube)

Stretched sub

Stretched stacked image without any calibration

I also did a stack which included bias frames but no flats, and it looked the same as the one that used no calibration files at all : /

 

[pipnina]

4 minutes ago, vlaiv said:

Indeed, it would be good to see both raw light frames and raw calibration frames (one of each - whatever you are using, flats, darks, bias, etc ...).

Image you presented has some clipping so things can't be seen clearly - but it looks like you have some mismatch in your flats - maybe something moved between lights and flats, or you changed focus position.

Here is suspect part of the image

Background appears to have "emboss" effect on it - this usually happens on camera rotation for example or shift of some sorts.

M_81_Light_010.fits

 

Also yea I am struggling to stop my camera rotating at the moment- the grub screws holding my baader MPCC into the focuser are obscured by the filter wheel. Next time I'll be bringing out the pliers to tighten them much more completely!

I've attached some raw files here if you'd like to have a look yourself (ignore the odd file names, I made mistakes when taking them in Ekos, my flats and bias don't have a 60s exposure time haha)

minus5-60second_Flat_001.fits minus5-60second_Bias_001.fits

Edited March 19, 2022 by pipnina

[vlaiv]

3 minutes ago, pipnina said:

I've attached some raw files here if you'd like to have a look yourself

Here is what I would recommend.

Flats look ok but you are missing darks. You need matching darks for lights in order to get good calibration - as is, you have over correction as dark signal is not removed from image.

When I do regular calibration this happens:

Second thing that I would recommend is to simply skip median stacking and use regular average for everything.

Just for to diagnose things, do following:

- get matching darks for your light exposure. Luckily - you can do this at any time as long as you match conditions on the night of shooting (gain, offset, temperature).

- turn of background normalization

- use simple average for all stacking (flats, darks, bias all of it)

See what sort of result you get.

 

[vlaiv]

Actually - hold on a second.

Did you use same bias you shot for flats - for your lights instead of darks?

If you do that - this happens:

Now, I'm not sure if it is down to flats or if it has something to do with bias, but something strange is going on here.

It is not concentric pattern you are getting - but it looks like it might be after stacking - that every sub has some of it and then after stacking multiple concentric rings form instead of just one dark ring.

Ok, so yes - do try dedicated darks so we can see if that is the problem.

After that - I would look into

58 minutes ago, pipnina said:

I have put a coat over my prinary mirror cell to stop light ingress,

that coat and how well does it really shield the scope from light leak.

[pipnina]

3 minutes ago, vlaiv said:

Actually - hold on a second.

Did you use same bias you shot for flats - for your lights instead of darks?

If you do that - this happens:

 

Now, I'm not sure if it is down to flats or if it has something to do with bias, but something strange is going on here.

It is not concentric pattern you are getting - but it looks like it might be after stacking - that every sub has some of it and then after stacking multiple concentric rings form instead of just one dark ring.

Ok, so yes - do try dedicated darks so we can see if that is the problem.

My setup in DSS was:
Lights

Bias (600 at the same gain as flats and light)

Flat (50 at same gain as bias and light)

I also have a library of 500 darks at the same exposure as my lights, same gain as all 3 above, same temp on TEC cooler used for all of these frames -5c)

I am stacking the image now with only flats and darks, am I supposed to use all four types of images in one?

[vlaiv]

23 minutes ago, pipnina said:

I am stacking the image now with only flats and darks, am I supposed to use all four types of images in one?

Ideally you would want:

lights and matching darks (which you already have) and flats and matching flat darks.

You don't seem to have flat darks - but from what I've seen - your bias subs are in fact of the same exposure as flats, right? That would make them flat darks.

Even if they are not fully matched in exposure length - it is better to use bias as flat darks then to skip them completely.

[pipnina]

3 hours ago, vlaiv said:

Ideally you would want:

lights and matching darks (which you already have) and flats and matching flat darks.

You don't seem to have flat darks - but from what I've seen - your bias subs are in fact of the same exposure as flats, right? That would make them flat darks.

Even if they are not fully matched in exposure length - it is better to use bias as flat darks then to skip them completely.

I thought about how you mentioned the stacking process may be cause, and thought about the settings I could change with regards to it.

Turns out, DSS will butcher flats if I use the median kappa-sigma setting, perhaps it was the normal kappa-sigma I had good results with before and I simply picked the wrong one.

On average stacking for the lights I don't get the banding issue any more, but if I select certain combinations of bias/flatbias or dark frames in my stcaking the flats stop working all-together.

I also need to re-take my flats outside I think. none of the artificial sources I've tried are producing flats that are particularly even.

Thanks for your help!