Background levels, raw image vs stack

[rockenrock]

Hi Robin,
I like how the background level drops dramatically when SharpCap stacks raw images. In my light polluted location my typical background is .12 for the darkest night.
As SharpCap captures and stacks 20 raw images, the background level drops to roughly .012. Sharpcap is not doing any processing, such as applying flats/darks. Can you help me understand what is happening?

Thanks,
Roger

[admin]

Hi Roger,

there are a few things that could give this effect - for instance there is a 'background subtraction' processing tool in SharpCap now that will subtract either a uniform or gradient background, but you would need to have turned that on.

Perhaps the best thing is to post the capture settings file from adjacent to a saved image (saved stack) that shows the effect - this will include all the camera and stacking settings and allow me to work out the likely causes.

cheers,

Robin

[rockenrock]

Hi Robin,
I can see the background subtraction is turned off...
Any other reason the processing would reduce the background level?
Thanks, Roger
+++++++++++++++++
[ZWO ASI2600MC Pro]
FrameType=Light
Debayer Preview=Off
Pan=0
Tilt=0
Output Format=FITS files (*.fits)
Binning=1
Capture Area=6248x4176
Colour Space=RAW16
Hardware Binning=Off
High Speed Mode=Off
Turbo USB=73
Flip=None
Frame Rate Limit=Maximum
Gain=1
Exposure=30.000s
Timestamp Frames=Off
White Bal (B)=50
White Bal (R)=50
Brightness=50
Window Heater Power=Off
Cooler Power=42
Temperature=0
Target Temperature=0
Cooler=On
Auto Exp Max Gain=350
Auto Exp Max Exp M S=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Trail Width=3
Minimum Trail Length=100
Trail Detection Sensitivity=9
Remove Satellite Trails=Off
Background Subtraction=Off
Planet/Disk Stabilization=Off
Banding Threshold=10
Banding Suppression=0
Apply Flat=None
Hot Pixel Sensitivity=5
Subtract Dark=None
NegativeDisplay=0
Display Black Point=0
Display MidTone Point=0.5
Display White Point=1
Notes=
iOptron CEM60,iEQ45Pro/30Pro,CEM25,SmartEQ Pro+,AZ Mount Pro,Cube II Mount=RA=00:44:10,Dec=+41:26:05 (JNOW)
TimeStamp=2022-10-26T10:54:36.2767799Z
SharpCapVersion=4.0.9268.0
TotalExposure(s)=600
StackedFrames=20
LiveStack.SaveRawFrames=None
LiveStack.AlignFrames=False
LiveStack.Derotate=True
LiveStack.StarsForAlignment=15
LiveStack.StarDetection.Sensitivity=75
LiveStack.StarDetection.SuppressHotPixels=True
LiveStack.AlignAutoDisabled=True
LiveStack.ReduceNoiseAmount=0.5
LiveStack.BrightnessLimit=92.70253
LiveStack.FilterBrightness=True
LiveStack.AutoBrightnessLimit=True
LiveStack.FWHMLimit=4
LiveStack.FilterFWHM=True
LiveStack.WarningThreshold=5
LiveStack.StackingMode=0
LiveStack.SigmaClipInitialCount=5
LiveSack.SigmaClipThreshold=3
LiveStack.SigmaClipLowLimit=1
LiveStack.AutoSaveReset=False
LiveStack.AutoSaveResetMinutes=5
LiveStack.LogarithmicHistogramHorizontalAxis=False
LiveStack.AutoSave=True
LiveStack.BlackLevel=0
LiveStack.WhiteLevel=100
LiveStack.MidLevel=25
LiveStack.WBB.db=0
LiveStack.WBG.db=0
LiveStack.WBR.db=0
LiveStack.Saturation=1
AutoStretch.Strength=2
LiveStack.StarDetection.FaintStarsOptimization=False

[admin]

Hi,

nothing obvious in those settings - the only thing is that there is a default stretch applied in the live stacking settings which will brighten up the faint nebulosity - this might have some apparent effect on the background, making it less noticeable, but it's a bit dubious.

What sort of measurements did you take of the background and in what capture files?

cheers,

Robin