## Artifacts with dark frame subtraction

Discussion of using SharpCap for Deep Sky Imaging
RickEvans10473
Posts: 11
Joined: Mon Mar 26, 2018 1:35 pm

### Artifacts with dark frame subtraction

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
Attachments
cropped_Bloom_test_ngc6791_00001.png (103.75 KiB) Viewed 2339 times
oopfan
Posts: 769
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

### Re: Artifacts with dark frame subtraction

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:

Code: Select all

Subtract Dark=None
Perhaps that explains the hot blue pixel in the center of your image.

Brian
RickEvans10473
Posts: 11
Joined: Mon Mar 26, 2018 1:35 pm

### Re: Artifacts with dark frame subtraction

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 2315 times
Last edited by RickEvans10473 on Mon Aug 13, 2018 1:48 am, edited 3 times in total.
RickEvans10473
Posts: 11
Joined: Mon Mar 26, 2018 1:35 pm

### Re: Artifacts with dark frame subtraction

M22 Image deleted in last post
M22_cropped_23_04_g3_ap154.jpg (384.08 KiB) Viewed 2315 times
oopfan
Posts: 769
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

### Re: Artifacts with dark frame subtraction

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
RickEvans10473
Posts: 11
Joined: Mon Mar 26, 2018 1:35 pm

### Re: Artifacts with dark frame subtraction

"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.
Posts: 5226
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

### Re: Artifacts with dark frame subtraction

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
oopfan
Posts: 769
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

### Re: Artifacts with dark frame subtraction

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
RickEvans10473
Posts: 11
Joined: Mon Mar 26, 2018 1:35 pm

### Re: Artifacts with dark frame subtraction

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 2253 times
RickEvans10473
Posts: 11
Joined: Mon Mar 26, 2018 1:35 pm

### Re: Artifacts with dark frame subtraction

Un-subtracted light frame
M092_00001.png (519.13 KiB) Viewed 2253 times