Subtracting darls /flats in live Stacking

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
carlwagle
Posts: 66
Joined: Thu Mar 07, 2019 6:58 am

Re: Subtracting darls /flats in live Stacking

Post by carlwagle » Sun May 05, 2019 3:09 am

I don't think you will ever see me "give up" but I might seem to take some steps backwards at times. Carl Wagle
Here is the camera settings:


[ZWO ASI178MC]
Debayer Preview=On
Pan=0
Tilt=0
Output Format=PNG files (*.png)
Binning=1
Capture Area=3096x2080
Colour Space=RAW16
Temperature=26.2
Hardware Binning=Off
High Speed Mode=Off
Turbo USB=40
Flip=None
Frame Rate Limit=120 fps
Gain=195
Exposure=2.014811
Timestamp Frames=Off
White Bal (B)=48
White Bal (R)=50
Brightness=50
Auto Exp Max Gain=130
Auto Exp Max Exp M S=13631
Auto Exp Target Brightness=106
Mono Bin=Off
Banding Threshold=35
Banding Suppression=0
Apply Flat=None
Subtract Dark=C:\Users\CHWag\Desktop\SharpCap Captures\2019-04-30\Capture\23_37_29\Capture_00001.png
#Black Point
Display Black Point=0
#MidTone Point
Display MidTone Point=0.5
#White Point
Display White Point=1
TimeStamp=2019-05-03T03:41:50.1279396Z
SharpCapVersion=3.2.5986.0
TotalExposure(s)=40.29622
StackedFrames=20

User avatar
turfpit
Posts: 869
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Subtracting darls /flats in live Stacking

Post by turfpit » Sun May 05, 2019 10:11 am

Carl

I can't see any point dwelling on the bad dark frame. You managed to work out why it was bad - that is progress and one less mistake to make in the future. The camera settings file looks OK. Maybe best to capture a new master dark, capture a live stack and work with that.

Consider deleting the bad master dark then it can never bother you again.

Dave

CraigG
Posts: 8
Joined: Thu Nov 29, 2018 11:37 pm

Re: Subtracting darls /flats in live Stacking

Post by CraigG » Sat Oct 26, 2019 10:49 am

Hello!
As a new SC Pro licensee, one of the features that I'm incorporating into my work flow is dark frame subtraction on the fly while stacking. What I'm finding is that when I pre-process with my saved dark file...same sub exposure time, sensor temperature cooling setting, etc., ...each new frame shows the histogram peak moving slightly to the left in the main live stacking box. I have to readjust the black and midpoint setting lines to compensate for this move after each frame is stacked. Eventually, the histogram peak has moved off screen somewhere to the left and I can no longer properly compensate for it. The image is still visible but is darker than it should be because the dark line setting position is pushed to the right farther than normal and can't go any farther left. I use the logarithmic histogram setting in the live stack box.

The saved images in my FIT files are also affected and are so dark that I cannot properly process them in Nebulosity.

Camera is an Atik Horizon mono running in SC via ASCOM drivers. My saved SC dark file represents 15 subs of 60 second duration and was used in a one hour live stack.

Any thoughts about what might be causing this? I think my saved dark frames are good but with some limited functionality due to the ASCOM driver dependence, I wonder if it's something else.

Thanks so much!
Craig

User avatar
admin
Site Admin
Posts: 3186
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Subtracting darls /flats in live Stacking

Post by admin » Sat Oct 26, 2019 7:52 pm

Hi Craig,

Sometimes you can get extra peaks appearing to the left of the main peak due to drift of the image – basically if the image drifts then some areas will no longer be receiving new data and will get darker as a result, which either creates a new histogram peak to the left or widens the existing peak slightly to the left. You can check out whether you are having significant drift by looking at the drift graph tab in the live stacking area.

If that doesn't turn out to be the case then I guess my question would be : if you don't try to adjust the controls for the movement of the histogram peak then does the image gradually get dimmer as stacking progresses?

Another thing you could do is share at least one light frame and the dark frame that you are using with me for further investigation – they will be too big to upload to the forums that you could put them somewhere like dropbox or Google Drive and then share a link here

Cheers, Robin

CraigG
Posts: 8
Joined: Thu Nov 29, 2018 11:37 pm

Re: Subtracting darls /flats in live Stacking

Post by CraigG » Sat Oct 26, 2019 8:47 pm

Thanks Robin!

I'll inspect the drift during my next session but I don't recall it being significant. I am guiding using PHD and have a reasonable sine wave pattern.

Yes, if I don't adjust the mid and black levels, the image darkens noticably on each frame. The black level adjustment "maxes out" fairly early in the stacking process. I use the auto stretch function to make the best of the image until the stack is complete. The histogram exiting "stage left" only happens when pre-processing with darks and the histogram peak is normal and stable when stacking without dark subtraction.

Just thought about something...do the Live Stacking Digital Gain settings (2x, 4x etc) have to be the same in both the Dark library file and the light frames being currently stacked? Does it matter?

I love a challenge! Thanks for your good counsel and I'll see what file infomation I can send you. If you have an email address that you can share, I've had good success with using WeTransfer to send large files to Atik Support, etc. Please let me know if that's an option.

Thanks Robin!
Craig

User avatar
admin
Site Admin
Posts: 3186
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Subtracting darls /flats in live Stacking

Post by admin » Sun Oct 27, 2019 9:20 pm

Hi,

The digital gain in the live stacking panel is only used for the star detection and doesn't affect the dark frames or the image frames or the final stacked image in live stacking at all, so at least we can rule that one out.

Cheers, Robin

CraigG
Posts: 8
Joined: Thu Nov 29, 2018 11:37 pm

Re: Subtracting darls /flats in live Stacking

Post by CraigG » Mon Oct 28, 2019 11:30 am

Hello Robin-

Thanks for the explanation! Well, I'm happy to say that I was able to spend some time under the stars last night and had great success with imaging the Pleiades while using Dark Subtraction. The histogram peak did move slightly left with each frame but still had plenty of room for black level adjustment throughout the live stacking session.

The difference was that I was using:
-30 second subs, not 60 second
-Baader UV/IR cut filter rather than my Ha filter.

I have no idea if either one or both of those would make the difference. I ran out of time to retest with 60 second subs but I'll report back. Anyway, I'm thrilled to have it work as advertised for now!

Thanks again Robin!
-Craig

User avatar
admin
Site Admin
Posts: 3186
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Subtracting darls /flats in live Stacking

Post by admin » Mon Oct 28, 2019 7:34 pm

Hi,

I wonder if some reason in your initial attempts the dark frames were over correcting for the background brightness (a setting that was not quite the same?). If the dark frame was actually brighter than the background of the image frame then each added frame would give you a negative signal. The results would certainly be weird although I'm not quite sure if they'd match what you are seeing.

Cheers, Robin

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest