Kev11
14-11-2020, 09:57 PM
I started a thread on this forum recently headed “Why won’t DSS stack more than one frame?” which attracted more than 40 posts. Unfortunately, by the time I decided to call a halt I was still at square one with my unstackable subs. However, yesterday with a bit of lateral thinking I finally found the setting in DSS I needed to change. I describe the process and the result at the end of this post.
But first I want to clear up two confusions about DSS that came up in the course of the previous thread.
The operation of the threshold level setting on the Advanced tab on the Registration menu. Think of the threshold being like the bar on the high jump stand. Ground level is 0% say 3m is 100%. 2% is easy to jump, (INCLUDES heaps of stars), as the bar moves up 20, 40, 60% .... it gets harder to clear (more and more stars EXCLUDED). The effect can be seen if you Register a set of pictures (Lights) at say 50%. After Registration is finished scroll to the right of the screen to find a column headed Stars, click on the down arrow in the header to bring the light frame with the highest number to the top. Write the number down. Repeat with a lower threshold, say 20%, then at again 80% and you will see the way it works. The confusion comes, I think, because we usually think a higher percentage means “better”.
DSS does not refuse to stack out-of-focus or poorly tracked stars. I have had sequences of frames with more doughnuts than an American coffee counter and more tails than a flock of geese; DSS still stacked them. The final picture was rubbish of course, but DSS didn’t tell me I could not do it.
What DSS default configuration cannot handle is what I call low contrast which in the old film days was the result of serious over- or under-exposure (not that one could over-expose a piece of deep sky). With film one played with the time and temperature of the development tank (assuming one knew beforehand it was likely to be a problem – no second chances with roll film!)
In the digital world an answer lies in settings available in the bottom left-hand section of the DSS screen headed Options, namely Settings... Raw/FITS DDP Settings. On the RAW files tab there is a box headed Colors Adjustment with three adjustments: Brightness, Red Scale and Blue Scale each with a default value of 1.0000. I found by increasing the Brightness to 7.5000 and bringing the Registration threshold down to 5%, DSS could find 200-300 stars in each frame (rather better than 0-4 I was getting originally!) and happily stacked. The attached final picture of NGC55 is a stack of the best 80% from 20 subs of 30s exposure at 1600 ISO. Not a prize winner I freely admit, but a hell of a lot better than the “one frame only will be stacked” result.
Cheers from an old dog learning a few new tricks (slowly).
But first I want to clear up two confusions about DSS that came up in the course of the previous thread.
The operation of the threshold level setting on the Advanced tab on the Registration menu. Think of the threshold being like the bar on the high jump stand. Ground level is 0% say 3m is 100%. 2% is easy to jump, (INCLUDES heaps of stars), as the bar moves up 20, 40, 60% .... it gets harder to clear (more and more stars EXCLUDED). The effect can be seen if you Register a set of pictures (Lights) at say 50%. After Registration is finished scroll to the right of the screen to find a column headed Stars, click on the down arrow in the header to bring the light frame with the highest number to the top. Write the number down. Repeat with a lower threshold, say 20%, then at again 80% and you will see the way it works. The confusion comes, I think, because we usually think a higher percentage means “better”.
DSS does not refuse to stack out-of-focus or poorly tracked stars. I have had sequences of frames with more doughnuts than an American coffee counter and more tails than a flock of geese; DSS still stacked them. The final picture was rubbish of course, but DSS didn’t tell me I could not do it.
What DSS default configuration cannot handle is what I call low contrast which in the old film days was the result of serious over- or under-exposure (not that one could over-expose a piece of deep sky). With film one played with the time and temperature of the development tank (assuming one knew beforehand it was likely to be a problem – no second chances with roll film!)
In the digital world an answer lies in settings available in the bottom left-hand section of the DSS screen headed Options, namely Settings... Raw/FITS DDP Settings. On the RAW files tab there is a box headed Colors Adjustment with three adjustments: Brightness, Red Scale and Blue Scale each with a default value of 1.0000. I found by increasing the Brightness to 7.5000 and bringing the Registration threshold down to 5%, DSS could find 200-300 stars in each frame (rather better than 0-4 I was getting originally!) and happily stacked. The attached final picture of NGC55 is a stack of the best 80% from 20 subs of 30s exposure at 1600 ISO. Not a prize winner I freely admit, but a hell of a lot better than the “one frame only will be stacked” result.
Cheers from an old dog learning a few new tricks (slowly).