Sky glow... Leave it of delete?

[Larry 1969]

Hello all!
I spent all night on M106 and going through my subs I see the first 200 min or so the frames are quite a bit brighter than the last 270 min...
My question is: Should I toss all of that data or try to process out the sky glow?
I was too excited and got on target super early... Guess it wasn't quite "dark"...

Thanks!

Larry

[Star Dad]

I have a similar problem due to a neighbor's "on all night light" whenever my scope is pointed near it. I have taken to tossing those images when the delta noise level is quite visible. I tried using and and not using them and concluded I was better off with fewer subs - even though Nebulosity can normalize the images. The difference was quite striking. Your results may differ.

[Larry 1969]

I may try it both ways and see for myself...
It's a modded DSLR with no filters and by "sky glow" I mean daylight. Not artificial light. Like it just wasn't quite dark enough...

Larry

[dritter]

I've run into that same thing when I start imaging before full astronomical darkness sets in. The initial images are certainly brighter than those taken later on.

As far as using those frames goes, I usually try processing both with them and without them. And there is no clear answer, or at least none that I could find. Sometimes the final product looks better without them, sometimes it's better with them.

The one thing that seems to help is getting more total time on target. If I get a few nights worth of data, then the final version seems to be about the same regardless of whether the brighter frames are included or not. It's when there is limited data, the results are less predictable.

[UlteriorModem]

I run into this sort of thing quite frequently especially after shooting an object over several nights, moon phases, and time changes.

I flick through the frames using the 'Blink' module of PixInsight and cull out the ones with the washed out backgrounds. Then I will further cull the frames using PI's subframe selector script. Even if it only leaves me with only 5 or 6 frames.

The old saying "Garbage in, Garbage out" applies here.

[Larry 1969]

So, I went through the subs and tossed planes, satellites and other weirdness and ended up with 129 X 3 min subs.
All loaded into DSS with darks, flats and bias frames. I selected stack 100% of the frames but DSS decided to only stack 89...
Is there a way to know why so many frames were not stacked? It chose 89 and that includes frames from both sides of the meridian...

Thanks!

Larry

[UlteriorModem]

Maybe it could not find a star match in a couple?

[Larry 1969]

Maybe it could not find a star match in a couple?

Maybe... But that's 40 X 3 min frames dropped. Didn't look bad as I was going through them.
I even lowered the star detection threshold to 20% which showed over 200 stars...

Is there a way to know why certain frames were dropped?

Larry

[UlteriorModem]

You will have to wait for someone more familiar with DSS to help answer your questions. I am sorry.

[JayTee]

Hi Larry,

What was your Star Detection Threshold set at? You may want to decrease your current number. So just setting the use amount to 100% doesn't mean it will actually use 100%. Yeah, I know that sounds weird!

From the DSS manual:

Star detection: For each picture, DeepSkyStacker will attempt to automatically detect the stars. In simple terms, DeepSkyStacker considers that a star is a round object whose luminance decreases regularly is every direction, and whose radius is no more than 50 pixels. Note that DeepSkyStacker will reject elongated star images which might occur if your mount isn't tracking correctly. Once the star is detected its exact center is computed by fitting a Gaussian curve to the luminance.

DeepSkyStacker will only stack images that contain at least eight stars that are common between all light frames. In practice, this means that you should set the Star Detection Threshold in the Settings.../Register Settings or Register checked pictures/Register Settings/Advanced dialogue so that DeepSkyStacker detects 20 or more stars to stand a good chance of finding eight stars in common between all light frames.

Additionally:

You can modify this threshold in the Advanced tab of the "Register checked pictures/Register Settings" dialog, or by selecting Settings.../Register Settings.
Reducing the Star Detection Threshold will result in DeepSkyStacker finding more (fainter) stars, on the other hand, if you increase the threshold, then only brighter stars will be detected and so this will reduce the number found.

Setting the threshold so low that many hundreds of stars are found will be counter-productive as there will be much more data to process for star registration and if too many stars are detected there is a greater chance of mis-registration. You should probably aim for over 20-25 stars and no more than a couple of hundred or so.

If you have set the detection threshold low and DeepSkyStacker is still not finding enough stars because the image is underexposed, you can increase the image brightness by using the "Brightness" adjustment in the Raw/FITS DDP Settings.

If your images are noisy (as a result (e.g.) of Light Pollution) you may need to enable the "Reduce the noise by using a Median Filter" option.

To help you find the best threshold for your light frames you can compute the number of stars that will be detected. To do this, DeepSkyStacker uses the first checked light frame and temporarily activates hot pixel detection.

You should note that this number is only a guide and that the real number of detected stars may vary if you have checked dark, offset and flat frames.

Cheers,
JT

[bobharmony]

Checking the detection threshold is a good place to look when all the subs aren't included in the stack.

On the subject of light backgrounds, I can only offer my experience. I got better results from my recent M81/M82 image when I dropped the first 10 subs of the night from the stack. I did start a little early, and the rest of the dropped subs were light as a result of Ursa Major climbing out of the light dome of Hartford. Every situation is different and you should experiment with different combinations, in this case it was worth dropping a few of the early subs to get better results.

Bob

[Larry 1969]

JayTee: Thanks! I normally set the star detection threshold so that it detects somewhere around 100 stars. I'll try lowering it and see what happens.
I used to get this problem when imaging with my dob because of the star trailing but these are 3 min guided subs...

Bob: Thanks! I plan to experiment a bit once I get the stacking sorted. It's dropping 40 frames and I can't tell which ones. I can't change anything until I know what's actually being stacked.

Larry

[bobharmony]

Larry - if you scroll to the right in the file list in DSS, there is a column that lists the number of stars detected in each light sub. Take a look and see if any of those numbers are 0. Lowering the detection threshold will increase the number of stars detected overall.

Bob

[Larry 1969]

Thanks again Bob!
I didn't realize that but I went ahead and lowered the SD threshold to 12% and it detected 320 stars and all 129 images stacked.
Now I can play with the results!

Larry