Help needed with Wipe module on Milky Way test data set

[timoff]

Hi guys,

I need some help with the Wipe module, because I don't understand what it is trying to tell me.

I'm a newbie, but I've been able to process my Andromeda galaxy dataset successfully in the past. So I'm feeling like an idiot, because I'm stuck in the bery beginning of the workflow.

I'm trying to process the Milky Way dataset, which I gathered to test what I can capture/process in the light-polluted area. Trying to do before investing a lot of time in gathering a decent amount of data, to check that I chosen the right settings for capture.

Hardware: Canon 5D mk4, Sigma 14mm 2.8
Capture: No mount, just a tripod, 8 seconds exposure, ISO 3200, 36 shots.
Calibration: darks, flats, bias frames, 20 shots each.
Stacking: DeepSkyStacker, StarTools recommended settings, output is Fits 32 bit integers.
Location: Mauritius, Bortle Class 4 light-polluted area
Other: Some trees captured in the shots.

What I do:
1. Open: Fits from DSS as Linear, from OSC/DSLR...
2. AutoDev: default settings

3. Crop: crop out the trees (I cropped out a lot of the frame, so I suspected all possible problems are just left behind.)

2-crop.jpg (447.26 KiB) Viewed 4499 times

4. Wipe: defaults, instead of removing the light pollution gradient, it amplifies it.

3-wipe.jpg (460.35 KiB) Viewed 4499 times

Any help on which direction is to look at will be greatly appreciated.

[hixx]

Hi timoff,
This seems to be flat overcorrection rather than light pollution, if I am not mistaken.
I am not sure if your flats were done with even illumination. You should take flats using same focus and aperture as the lights
Some thing you want to try out to save thi:
1) Crop out 10-20 px at each of the the borders, not just the trees
2) Increase "Gradient Aggressiveness" to 90%. Values up to 95% might be OK
3) Increase " Dark anomaly Filter" setting to, say 7 or so. If that improves the result, increase further until no more improvement (I use settings around 15 pixels, but this renders pretty long)
4) Under "synthetic Flats", choose the option "Inverse Vignetting (Overcorrection)" and see what it does.
5) Under "Gradient Edge behaviour" try " Bounce Back" setting and introduce a gradient falloff of 60% or so - experiment

Hope this helps,
Clear Skies,
Jochen

[decay]

Hi timoff,

how did you take the flats?

I am not sure if your flats were done with even illumination.

That could be the problem. For me, it was very difficult to make reasonable flats with such a short focal length / large FOV. Since they cannot be photographed against the sky ...

Best regards, Dietmar.

[Stefan B]

If it is overcorrection by flats it may also be caused by bias/dark flats taken with different parameters than the flats. Or no bias/dark flats at all. This has been the case with an overcorrection problem of my own.

In order to see if the flats are actually the problem, the set could be stacked again without flats. Then there should be vignetting instead of overcorrection (which could be counteracted by the synthetic flats option in Wipe).

Regards
Stefan

[timoff]

Thank you Stefan and Dietmar!

Will re-stack without the flats and see what happen. And will re-check darks and biases also.

Yes, the way flats look like was not very good to me, but I didn't know that it will affect the process so badly. I thought that 32 bits are enough to mitigate the impact. Apparently I was wrong.

Regards,
Timofey.

[Mike in Rancho]

Timofey,

Did you do any other cropping besides the trees? From a static tripod I might imagine some field shift that would need to be removed as well. Wipe really doesn't like stacking artifacts. Did you use intersection mode in DSS? Even so, there's usually at least a single row/column of pixels that might need cropping due to debayering artifacts.

I get the feeling you will certainly need flats here. Also, though it may be a minor difference, if you are using the bias as bias (i.e. not in the dark flats tab), with RAW DSLR files DSS usually recommends that you select the Set Black Point to Zero box in digital development settings. Without bias, turn it off.

But yeah, Wipe is hyperstretching your image to show you where flaws are. Might be the edges. Might be flats overcorrection. Or both.

[timoff]

I cropped a lot, really lot of the frame. I mean 10%, and it was after the trees were cropped out.

About flats, I need to understand how to do them with 14mm, because the DOF is really huge, so I rejected the idea of shooting the sky. Tried to shoot the computer monitor from almost zero distance, but the results brought me to the point where I am currently.

About Set Black Point to Zer - I'm almost sure I did it by StarTools recoomendations, but need to re-check.

Yes, I use darks and bias frames as well, because they are the easiest part for me so far

Unfortunately had no time to re-stack without flats, will do soon and post the results here.

[timoff]

Mike,

I forgot to ask - what is field shift?

[Mike in Rancho]

Mike,

I forgot to ask - what is field shift?

Just the sky moving along. Figure 10 seconds to cycle each shot, 36 shots, about 6 minutes - even at wide field I think it shift a bit? Stacking lines the stars up, but the edges will progressively differ. Sounds like you probably did sufficient cropping on all sides though and this is just a vignetting/flats issue.