Is this a software bug or processing issue?

It looks like the dark edges are amp glow or dark noise that is not well removed so that the light pollution tool cannot compensate for that. Try cropping the image to remove the edges and then see how the like pollution tool works. There's a video by Mabula for using the tool.

Posted by: @astrogee

It looks like the dark edges are amp glow or dark noise that is not well removed so that the light pollution tool cannot compensate for that. Try cropping the image to remove the edges and then see how the like pollution tool works. There's a video by Mabula for using the tool.

Thank you Astrogee.

I am wondering whether my flats are bad as the corners look pretty bad.  I have been experimenting a little with them.  Lately they have been looking darker.  I will attach an example of a flat that I took.

I have been moving the histogram away from the right a little, but not really a whole lot.  In my opinion though this is way too dark.

Would this be the culprit to my bad results?

Actually your flats look very flat. Probably no need for them - you do not seem to have vignetting nor a dirty sensor.

It doesn't really matter how bright they are, nor the colour because the stacking software will normalize them. You only have to make sure that all colours (RGB) in the histogram are away from the left and right edges of the histogram.

Try stacking without calibration files, then see what you get. Maybe your calibration frames are not matched for your setup - i.e. flats require the same optical setup including focus, and darks should match the same ISO, exposure time and temperature, bias should match the same ISO.

Posted by: @astrogee

Actually your flats look very flat. Probably no need for them - you do not seem to have vignetting nor a dirty sensor.

It doesn't really matter how bright they are, nor the colour because the stacking software will normalize them. You only have to make sure that all colours (RGB) in the histogram are away from the left and right edges of the histogram.

Try stacking without calibration files, then see what you get. Maybe your calibration frames are not matched for your setup - i.e. flats require the same optical setup including focus, and darks should match the same ISO, exposure time and temperature, bias should match the same ISO.

That is the one thing that I shared between my ISO400 and ISO800 sessions.  I will shoot a set of ISO400 bias and try that change by itself.  I don't know if this will prove to be what is unhinging my results but it would be a good single difference factor to prove out.

Yesterday I processed the same images with a bias that includes a match to the ISO400 frames.  So now every session is truly independent of each other.

The results were not perfect but were much better than my prior effort.

Another oddity that I noticed with the old file was that the size was enormous. (120Gigs of only APP data).

The improvement either had to do with the Bias needing to match or there was some other undiscovered corruption.

The good thing is that it appears it is not a bug with version 1.080.

Excellent! The flats don’t look right actually, almost every setup has a bit of vignetting and a tiny dust speckle (unless the sensor is cleaned extremely well each time). So that might be worth examining a bit further.

Posted by: @vincent-mod

Excellent! The flats don’t look right actually, almost every setup has a bit of vignetting and a tiny dust speckle (unless the sensor is cleaned extremely well each time). So that might be worth examining a bit further.

Vincent; I'd like to be certain as well that the flat's are functional.

I will say that there are a couple darker spots on the flats so they do have that pattern to remove from the lights.

Vignetting on the other hand; I don't know.  I do know that in the past I've seen clear evidence of vignetting with my flats when I did the flats by eye.

Lately I've been trying to get the histogram a little left of the right edge of the histogram.  When I viewed my previous flats from my photo editing software they appeared to be straddling the right side of the histogram.

In another post I notice that you can load and view the histogram with loaded flats.  When I view the flat loaded in the APP software it doesn't appear favored at all to the right side.  Am I reading this correctly?

What this seems to me is that I might need to take a slightly longer frame and have the photo lighten up a bit.  This is contrary to my original thought.

Coloration wise I have seen others with almost the same shade of grey, but with vignetting more prevalent. 

Any ideas on whether I should be adding or subtracting exposure time to get the preferred flats?

What do you see when you switch off DDP (checkmark on the right side). The view you see now is stretched and a flat should have its signal in about the middle of the histogram, without having to stretch it. I think part of the issue here is underexposure indeed. Where are you seeing your histogram when taking flats? If it's on a non-linear scale (like at the back of DSLR's), it's very difficult to judge what the correct exposure should be. APP shows the linear histogram, the real range of your sensor.

Posted by: @vincent-mod

What do you see when you switch off DDP (checkmark on the right side). The view you see now is stretched and a flat should have its signal in about the middle of the histogram, without having to stretch it. I think part of the issue here is underexposure indeed. Where are you seeing your histogram when taking flats? If it's on a non-linear scale (like at the back of DSLR's), it's very difficult to judge what the correct exposure should be. APP shows the linear histogram, the real range of your sensor.

Vincent,

When I switch off DDP the result is similar to when it was checked.  The histogram still favors to the left.

If indeed APP is showing the real range of the sensor then it appears I will need to increase the exposure time slightly to get a better flat.

When I brought this into Luminar which is my photo editing software it appears that the histogram was to the right, however it's positioning on the screen can make what I see a bit misleading.

It does seem however that I might have been seeing the same indication within Nikon's display.  I will take a look at this.

Knowing now that I can verify flats in APP I likely will trust it's reading.

It appears that my initial method was better than what I tried to fix.

There is a possibility that my view frame may have shifted (not intentionally) but I might like shooting some flats over again and reapply them to my most recent results.

Astrogee and Vincent, thank you for picking out a flaw in my procedure.  I'm anxious to see if it makes a difference.

I worked on tweaking the flats exposure time.  I am using the method of placing T-shirt over lens.  The day is quite sunny but I am aiming towards the woods.

I did a whole series of different timings between 1/1200 and 1/100.  At 1/1200 the exposure looked about midway on the camera but it was all the way left in APP.  I finally chose an exposure of 1/250 as it does appear visibly better on the APP histogram without in my opinion over exposing.

The end result on APP looks "considerably" different in APP than the frame that I shot.  The picture itself is white with a pinkish hue.  The picture in APP is bluish green.

You can now easily see the 2 blemishes on the frame.

At this point would the flat frame look "normal" in APP?

I did notice that there "appears" to be 4 places (circled in photo) where the stretched photo could be toggled on and off.  I believe nothing would be stretched.

I do think the coloration in APP is still strange but I have a feeling that these frames will yield better results when I start processing them with my original project.

Let me know if this still is out of the normal.

The color is not a problem, flats are used for the dust and the vignetting only. You are taking the flats with too high shutter speeds though, normally you would try to get those to about a second. This does require being able to dim the light falling in. I am seeing a bit of a dust spot if I’m not mistaken? On the top right of the image? I think it still is not in the correct exposure, the peaks in the histogram should be shifted more to the middle.

Posted by: @vincent-mod

The color is not a problem, flats are used for the dust and the vignetting only. You are taking the flats with too high shutter speeds though, normally you would try to get those to about a second. This does require being able to dim the light falling in. I am seeing a bit of a dust spot if I’m not mistaken? On the top right of the image? I think it still is not in the correct exposure, the peaks in the histogram should be shifted more to the middle.

Vincent: Thanks again for pointing out an issue with my process.  I had no idea that flats would need to be that long of an exposure.  Prior to a couple weeks ago I was only trying to achieve a mostly white image with discoloration around the corners.  In doing so the exposures were still very short.

Lately I had been trying to get the histogram peak off the right edge of which I was taking some very short exposures and the end results were considerably darker, and as noted in this thread the flats were very flat.

This is such a detail driven hobby, yet I don't know how I missed the details pertaining exposure time for flats.  Perhaps partly due to the fact that many people have a program or the camera do auto flats the timing hasn't been discussed a lot.

Since reading your post I have found several places where flat exposure times were discussed.  It appears that there isn't a hard and fast rule but the goal as you have mentioned is to get mid way on the histogram.

I might have to make some adjustments and get another set of flats.

Thankfully I have not rotated my camera, yet I'm sure the focus has changed somewhat.  Still, I'm hoping to get better results with my original project due to a truer flat.