Can I stack overexposed subs?

[Leonid]

Hello everyone!

 

Let me explain my workflow and thought process and then ask for advice.

 

So this winter February 2023 I tried wide-field astrophotography. I did 60 subs of 30s exposure at ISO1600 on a clear Bortle class 3-4 sky imaging the Canis + Orion + Taurus region with terrain in the foreground. Equipment is Canon 2000D (full spectrum mod with Astronomik IR-cut clip-in filter), Samyang/Rokinon 14mm lens, SkyGuider Pro mount, intervalometer, and lens heater. At first, I was a bit disappointed with the result, the stars in the corners of the image were elongated and the Ha regions of Orion were barely visible after aggressive stretching in post. However, as I understand it is just a fact of life – wide field lenses DO distort the corners, and even with full-spectrum modified DSLR a significant imaging time is still required to start seeing Ha in the final image. The resulting image is attached. However, the captured subs and the resulting stacked image were editable. I could stretch them, play with the image, and learn something new about DSS and post-processing. And this was the first time I captured that many subs with 30s exposure (my personal record), so all in all I was very happy with the result.

 

Now, this May a few days ago I decided to find a darker sky (Bortle 2), and image the Ha regions in Cassiopea for a total of 100subs 30s exposure each. I was expecting a significantly better result, and I was better prepared for the session (took my time to prepare the gear in advance, study the weather, the location, the framing of the target, etc etc). This time I used Sigma 18-35 lens @26mm, f/3.2. The result was disappointing. Now, I want to understand why.

This time of year Cassiopea at my latitude ~59N is low in the NNE sky, and there are only a few hours of astronomical twilight during the night. The Cassiopea is in the general direction of the sky where the sun is below the horizon during astronomical twilight. I automatically used the same ISO and exposure settings as I did during my winter session (since I was left happy with the result at that time). The resulting subs felt on the overexposed side (the lower left portion of the image captured a lot of twilight, but no clipping), but I assumed that this can be significantly reduced in post. However, when I stacked the ~80 subs 30s exposure in DSS, the resulting image is useless in post. The stars are washed out, only the brightest stars still resemble the star shape (each individual sub is showing a nice sharp and focused star, even the faintest one), and the overexposed area of the image is unusable in post. None of the Ha regions are showing any color.  Images attached.

 

 stacked edited winter session (60x30s exposure, ISO1600, Samyang 14mm f/2.8)

 

individual sub May session (30s exposure, ISO1600, Sigma 18-35 @26mm f/3.2). Cassiopeia on the lower right of the image

 

stacked ~80subs 30s exposure 

 

zoomed-in portion of the individual sub (NGC 869 h Persei cluster). Stars are sharp.

 

zoomed-in portion of the stacked image (NGC 869 h Persei cluster). Stars are just blobs of color.

 

So these are the questions I have:

1)      Why are the stars washed out in the stacked image, while individual subs have sharp stars?

2)      Is there a limitation on the amount of exposure/histogram for the individual subs to be used in the stacking? (Histograms attached)

3)      The individual subs have a much wider range of levels and curves adjustment than the resulting stacked image. Does this happen due to overexposure of the individual subs? (Histograms attached)

4)      If I were to use lower exposure time (eg 10 seconds), lower ISO (eg ISO400) and made more subs, would the result be better? 

5)      Sigma 18-35 lens is advertised as a sharp zoom lens. It is quite sharp in daylight photos, but at night stars are still distorted in the corners. Does this mean that with current equipment in the market, wide-field astrophotography will always suffer from distorted stars in the corners? Is mosaic the only option?

 

Thank you for your time and help!

Edited May 16, 2023 by Leonid

[mcrowle]

Hopefully I can help with a couple of these.

Re. 1) is it possible that you had the 'Fix Remaining Hot Pixels' setting on the Cosmetic tab enabled when DSS stacked? (I think that's what the setting is called, but I don't have DSS on this machine to check). It is supposed to remove hot pixels left after the dark frames were applied, but I've found it will at least partially remove stars, as in your example image. I only have it enabled if my stars are too bloated to look like hot pixels 😃.

Re. 5) stars are a very severe test of lens performance, as their pinpoint nature mean any distortion is obvious. Most, if not all, lenses will have issues around the edges, but this can be improved by stopping down the aperture. That's not ideal unless you are using a fast lens though.

Regards, Mike

 

[Leonid]

Thanks Mike,

The fix with the cosmetic tab worked indeed. 

As for the elongated stars, I will try stopping down next time to compare. Although I understand that they will still be elongated. 

I think that my subs are overexposed to see any faint Ha nebulosity. I guess the northern summer night sky is pretty much worthless for wide-field astrophotography. Need to increase focal lengths to avoid twilight skyglow. However, when I was actually out there in the field, the sky felt quite dark. But the interesting parts of the sky with Ha nebulosity (Cassiopeia, Cygnus etc) tend to lie close to the twilight glow of the northern summer "night", which becomes apparent on the 30s exposure ISO1600. Maybe will reduce the ISO and shorten the exposure next time.

Regards, Leonid

Edited May 16, 2023 by Leonid

[happy-kat]

in addition to the tips above could also after register select the light with the highest score as your reference frame and review any very low scored lights and deselect them and when stacking use sigma clipping which might help your star shapes, hope that helps

 

[Leonid]

12 hours ago, happy-kat said:

in addition to the tips above could also after register select the light with the highest score as your reference frame and review any very low scored lights and deselect them and when stacking use sigma clipping which might help your star shapes, hope that helps

 

Thanks, I know this part  There is also a score for the sky brightness in the DSS, so I picked the frames with least amount of skyglow.