Neowise processing, what is causing this?

jlodge

Active member
Nov 5, 2018
29
0
Nikon D750, Tamron 90mm f2.8, ISO 800, 50 x 30s tracked.

Bias, darks, flats, processed in PI, I have applied DBE and get this result, is it light pollution or something else (Neowise.jpg)?

The histogram when exposing was towards the top of the midtones because the sky was still a bit bright, it doesn't get properly dark here in the north of England yet. Taken from a Bortle 4/5 where there isn't much LP in the direction of the comet.

The flats and bias were not taken at the same time as the lights and the darks (which were taken together).

I've never tried AP with my 90mm macro lens before so have no basis for comparison, I have used my D750 extensively for astro and not come across this.

I posted on CN and a member suggested to use ABE with function degree 1 and subtraction (ABE.jpg). It looks smoother but the red is still there, I tried another pass of ABE and it looked more like my initial attempt. I have also attached my master flat which has the boosted STF.

I have also run BPP a few times, the first is without bias frames (only darks and flats), the second is darks only and the third is with bias and flats.

Neowise.jpgABE.jpgABEBackground.jpgMasterFlatBoosted.jpgNoBiasABE.jpgNoBiasNoFlatABE.jpgNoDarkABE.jpg
 

dld

Well-known member
Sep 24, 2017
161
14
Hello,

I don't have a definite answer but if you wish, I can take a look if you could supply a light, a dark, a flat and a bias frame.

If you haven't removed the lens from the camera and you have some spare time, you may try to refocus (if needed) and take some new flats. Luckily you are using a prime lens and you can't be affected by zoom creep :). Just in case, a set of new bias files wouldn't hurt at all :)!
 
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jlodge

Active member
Nov 5, 2018
29
0
Hello,

I don't have a definite answer but if you wish, I can take a look if you could supply a light, a dark, a flat and a bias frame.

If you haven't removed the lens from the camera and you have some spare time, you may try to refocus (if needed) and take some new flats. Luckily you are using a prime lens and you can't be affected by zoom creep :). Just in case, a set of new bias files wouldn't hurt at all :)!
Thank you, that is very kind! Would you like masters or just single frames?

Yes that's why I wasn't too worried about taking the flats after the imaging session, when I took them I was focused around infinity and hadn't touched the focus ring, it had only been in my bag so I'm fairly certain that it would be more or less in the same place, even so I read somewhere that there is leeway with focus when taking flats?
 

dld

Well-known member
Sep 24, 2017
161
14
You're welcome!

Single frames should suffice and will take less time to upload!

I think so, but to be sure, it shouldn't hurt taking some flats with the lens as it is, check the focus, and take new flats if you find that the lens was de-focused.
 
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jlodge

Active member
Nov 5, 2018
29
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You're welcome!

Single frames should suffice and will take less time to upload!

I think so, but to be sure, it shouldn't hurt taking some flats with the lens as it is, check the focus, and take new flats if you find that the lens was de-focused.

Here is the We Transfer link, I noticed that I took the darks with a different lens on (a telephoto which I also took images with, although upon inspection the focus was slightly off annoyingly), which in theory should not affect things as the lens cap and the eyepiece were covered, plus it was dark outside.

I will try to take some new flats tomorrow.
 

dld

Well-known member
Sep 24, 2017
161
14
Hello again,

I've tried calibrating your light with various combinations of settings using ImageCalibration and the only thing I can say is your problem is a combination of sky transparency and flats of low exposure time. For the first, I guess you image the comet at a low altitude and you have suffered (as myself and probably many other comet hunters) from variable sky transparency.

Now regarding flats, I've seen this behavior when the flats exposure time is short (1/200s in your case). I don't have a good explanation why this happens. In your case there is room for more exposure time. Since this is a DSLR camera (14bit), a saturated frame will show a peak in the HistogramTrasform graph (set at 16bit) at x=0.25. Your debayered flat has median values R=0.0152, G=0.0236 and B=0.0216 (obtained with Image > Statistics). Experiment with your flat exposure times in order to obtain a histogram with peaks around halfway that number (0.125). Given your flat medians, 1/50s must be a safe choice.

Another tip is to take flats with a diffuser placed at random positions between exposures. Any inhomogeneities of the diffuser will be suppressed when you integrate your flats.
 
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jlodge

Active member
Nov 5, 2018
29
0
Hello again,

I've tried calibrating your light with various combinations of settings using ImageCalibration and the only thing I can say is your problem is a combination of sky transparency and flats of low exposure time. For the first, I guess you image the comet at a low altitude and you have suffered (as myself and probably many other comet hunters) from variable sky transparency.

Now regarding flats, I've seen this behavior when the flats exposure time is short (1/200s in your case). I don't have a good explanation why this happens. In your case there is room for more exposure time. Since this is a DSLR camera (14bit), a saturated frame will show a peak in the HistogramTrasform graph (set at 16bit) at x=0.25. Your debayered flat has median values R=0.0152, G=0.0236 and B=0.0216 (obtained with Image > Statistics). Experiment with your flat exposure times in order to obtain a histogram with peaks around halfway that number (0.125). Given your flat medians, 1/50s must be a safe choice.

Another tip is to take flats with a diffuser placed at random positions between exposures. Any inhomogeneities of the diffuser will be suppressed when you integrate your flats.
Thank you again dld!

For the sky transparency, would using fewer subs in the stack help do you think? Perhaps trying groups from around the same time? My total integration time was 35 minutes, so maybe a concurrent block of 5 minutes from different points?

I will give the new flats a go, I am using a tablet set to maximum brightness, white screen with a sheet of white paper over it. This is a new thing for me because at the society observatory where I have imaged for the past two years we have an EL panel for the telescopes and when I've done DSLR widefield I've never taken flats.
 

dld

Well-known member
Sep 24, 2017
161
14
Regarding the sky transparency issue, I would suggest you to first resolve the flats issue and then inspect the individual frames in order to reject the worse of them. But that depends if you want to go for a simple image with a more "natural result" meaning no gradient removal and just an auto STF strech with channels unlinked, or you have the time and patience to remove gradients with DBE and obtain an even and black background.

If you have the newer Tamron lens which does not change size with focusing, you may point the camera/lens at zenith, place the paper/tablet over the lens (without the lens hood) at a slightly different place each time and take flats. Try to increase the exposure time by using a thicker paper.

A handy method to inspect your flats (besides the FlatContourPlot script) is to perform an ABE at your debayered master flat. Use a high number for the Global Rejection Deviation setting, and Division for the Correction.

I hope this helps. Unfortunately the faster and widest a lens is, the tougher to obtain good flats. The problem gets worse if the lens has zoom and/or focus creep, i.e., changes length. Any flat irregularities will appear in the master light. Even with perfect flats, the wider the lens, the wilder the light pollution gradients are, making them harder to remove.

Good luck!