Why the photo looks like this after DSS?

[malau]

Hi all,

Was doing comet mode in DSS but just dont understand why it looks like that (the background has many weird pattern). IIRC I saw on the net somewhere once before but cant recall it. Any idea? Many thanks.

[pw]

That looks like noise, I don't know what DSS is, but I'd say excessive increase in exposure in your editor is the cause of the artifacts.

[rcheshire]

This happens if calibration is faulty. The effect can be masked with dithering, but it's best to have a set of darks matched by temperature and exposure time as close to your lights as well.

If you used bias frames, you could try leaving them out and use your RAW darks only - see how it looks. Flats will need to be calibrated with a bias.

[Asterix2020]

I've had a similar problem but only with comet mode - point comet and stars (not trailed). The noise pattern appears in the same direction as the comet's movement. If I stack on the stars and get a trailed comet, the problem goes away. I have not found a solution.

[Octane]

Did you take the image in warmer weather? What camera were you using? What ISO? How long were your exposures?

H

[Asterix2020]

FWIW, my pics with the same result were taken with a cooled DSLR. And the noise is not present on the original RAW images.

[malau]

Thanks everyone. I forgot to mention some tech info:

600D, ISO3200, cant recall how many sec each probably 15-20 sec. Light pullution area. I took it last night.

I may try without BIAS first and see hows going.

Thanks again.

[pw]

It sounds like Rowland is right, some sort of calibration issue is making a pattern with the noise from each frame.

[malau]

apart from DSS, are there any free software I can try ?

[cometcatcher]

I believe Rowland has the answer. I also get noisy streaks in my pics if the darks don't precisely match the temperature of the lights, which can be difficult to do on a night when temps change rapidly. DSS is not the problem, it also happens with Registax.

[malau]

Cool.. thanks.

Btw Love your name :rofl::rofl::rofl::rofl:

[Asterix2020]

I'm not sure it's not DSS. I've attached two images, the resolution's not great due to resizing to under 200 kB but you can still see the mottled background in one. This were the same files stacked in DSS, no darks or flats used, the only difference was one was comet mode with no trailed stars or comet (the mottled one) and one was stack on stars with a trailed comet (to better one).
It may not be visible in the smaller versions, but the direction of the mottling matches the direction of the comet's travel.

[cometcatcher]

In DSS which mode did you use, comet stacking or stars + comet stacking? I do get some weird mottled effects in stars + comet stacking mode but not in the other option.

[Asterix2020]

My first image was "align on stars and comet". I get the mottle background and black holes in the brighter stars.

My second image was "align on stars". That seems to work well, except the comet is trailed.

To get around this, for my final image here I took the comet from the first image and overlayed it in GIMP with the the background of the second image. But it would be nice if DSS could just do what it's meant to.

[cometcatcher]

Okay now I understand. And I have to agree that something not right is going on with DSS in that mode.

[LightningNZ]

Try changing the averaging method. Sigma-clipping is generally good, I've had strange results with the Adaptive-weighted method.
-Cam

[cometcatcher]

Sigma clipping is one of the methods I didn't try, so I'll try that. Actually I'll try them all and report back.

[cometcatcher]

Well I tried all the options under "Stars + Comet Tracking" with all the results looking like a mottled mess, as you can see from the pics. I much prefer Stacking on comet with the default "Average" method which gives nice star trails and still comet.

There's additional options under the Kappa-Sigma_clipping which I didn't try, which would take forever and probably not help anyway.