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Artifacts with dark frame subtraction
Posted: Sat Aug 11, 2018 4:52 pm
by RickEvans10473
I'm seeing the artifacts next to stars when acquiring DSO images with dark frame subtraction. I'm using a recently acquired ZWO ASI294MC-Pro.
During my last imaging session I imaged without dark frame subtraction pre-processing then used the SharpCap dark frames along with flat frames using Autostakkert. The artifacts disappeared in the Autostakkert images even when stretched.
I was forced to upgrade to SharpCap 3.1 from SharpCap 2.9 since 2.9 does not seem to recognize the ASI294MC.
[ZWO ASI294MC Pro]
Pan=516
Tilt=352
Output Format=PNG files (*.png)
Binning=2
Capture Area=2072x1410
Colour Space=RGB24
High Speed Mode=Off
Turbo USB=80(Auto)
Flip=None
Frame Rate Limit=Maximum
Gain=300
Exposure=8
Timestamp Frames=Off
White Bal (B)=55
White Bal (R)=55
Brightness=20
Temperature=-0.3
Cooler Power=11
Target Temperature=0
Cooler=On
Auto Exp Max Gain=285
Auto Exp Max Exp M S=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Apply Flat=None
Subtract Dark=None
#Black Point
Display Black Point=0.08203125
#MidTone Point
Display MidTone Point=0.106839328022272
#White Point
Display White Point=0.8671875
TimeStamp=2018-08-10T05:23:38.4910370Z
SharpCapVersion=3.1.5193.0
Re: Artifacts with dark frame subtraction
Posted: Sun Aug 12, 2018 3:01 pm
by oopfan
Hi Rick,
Could you upload the Autostakkert image so that we can compare and contrast?
I am "old school" so I do dark frame subtraction offline. I noticed that you have dark frame subtraction turned off in SharpCap:
Perhaps that explains the hot blue pixel in the center of your image.
Brian
Re: Artifacts with dark frame subtraction
Posted: Mon Aug 13, 2018 1:42 am
by RickEvans10473
Brian,
Unfortunately, I don't have an Autostakkert image for the image in the original posting. Actually there should have been two images in the original but for some reason the second image was not posted.
Below is an image of M15 stacked images dark frame subtracted by SharpCap 3.1.
For the M22 image the dark frames were subtracted by Autostakkert.
The red line artifacts are evident in the M15 stars but absent in the M22 stars.
M15 - Dark frames subtracted by SharpCap
- M15_Cropped_00_19_g3_ap73.jpg (283.18 KiB) Viewed 4871 times
Re: Artifacts with dark frame subtraction
Posted: Mon Aug 13, 2018 1:45 am
by RickEvans10473
M22 Image deleted in last post
- M22_cropped_23_04_g3_ap154.jpg (384.08 KiB) Viewed 4871 times
Re: Artifacts with dark frame subtraction
Posted: Mon Aug 13, 2018 2:13 am
by oopfan
Rick,
Yes I see the artifacts. I am sure that Robin will solve.
Regarding the forum losing an attachment, I noticed the same in one of my posts last week.
Brian
Re: Artifacts with dark frame subtraction
Posted: Mon Aug 13, 2018 3:00 pm
by RickEvans10473
"Yes I see the artifacts. I am sure that Robin will solve."
Certainly, hope so. The only reasons for me "upgrading" to the paid license of 3.1 is 2.9 would not recognize my ASI294MC Pro and I liked the convenience of real time dark frame subtraction. I hope the flat frame subtraction, which I have yet to try, as like you I do offline, works.
Re: Artifacts with dark frame subtraction
Posted: Tue Aug 14, 2018 4:33 pm
by admin
Hi,
do you have a copy of the dark frame that you were using and an unsubtracted light frame that you can share?
The artifacts you are seeing look like they may be caused by SharpCap's hot pixel detection/removal code going a bit crazy. When a pixel in the dark frame is flagged as 'hot' then SharpCap will steal the pixel value from the pixel immediately to the left for the final image. Normally this should be no problem as less than 1 in 5000 pixels will be flagged as hot, so the chance of 2 or more in a row being hot is really very small indeed. Here it looks like there are many hot pixels in a line perhaps?
Anyway, if you can share copies of the unprocessed frame and dark with me I can investigate - they will probably be too big to upload here as attachments, so best to upload them to dropbox or similar and send me the links.
cheers,
Robin
Re: Artifacts with dark frame subtraction
Posted: Tue Aug 14, 2018 4:45 pm
by oopfan
Robin,
It appears that the artifacts are only happening on the brightest stars. Perhaps the camera's pixel scale and the telescope's focal length have led to severe undersampling. SC could be misinterpreting stars as hot pixels. The pixel size is 4.63 um so if the telescope is relatively short this could explain it. Is there a setting in the dialog to turn off hot pixel detection?
Brian
Re: Artifacts with dark frame subtraction
Posted: Wed Aug 15, 2018 3:07 am
by RickEvans10473
Hi Robin
You asked: "do you have a copy of the dark frame that you were using and an unsubtracted light frame that you can share?"
Dark Frame
- dark_10_frames_0.0C_2018-08-09T00_23_04_5555.png (867.83 KiB) Viewed 4809 times
Re: Artifacts with dark frame subtraction
Posted: Wed Aug 15, 2018 3:10 am
by RickEvans10473
Un-subtracted light frame
- M092_00001.png (519.13 KiB) Viewed 4809 times