Horizontal Lines in Flat Capture

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jdaliix
Posts: 17
Joined: Mon Jun 11, 2018 2:01 am

Horizontal Lines in Flat Capture

#1

Post by jdaliix »

All my cameras are ASIs. When capturing flats, all of them without fail have horizontal lines running across the viewer pane of SharpCap, and this happens on three different laptops, over years, and 5-6 different cameras. Sometimes I get lucky and, even though the lines are in the SC viewer, they don't wind up showing up on the PNG. I don't know why that is, but it has made it so that if I screw around with it long enough, I can get lucky. The first shot is of SC during the setup, obviously, and the rest are flat captures in the saturation OK zone. This was all done with an ASI2400, I so far I'm unable to get one to capture without the lines.

Any ideas? And if not, does anybody have any suggestions on some software I can use to capture flats?

Thanks,

-- Jason

PS, the image insertion tag didn't work, so I edited to show just the URL's, which are apparently clickable.

http://spacestreams.com/images/sharpcap-screen.jpg
http://spacestreams.com/images/sc-flat1.jpg
http://spacestreams.com/images/sc-flat2.jpg
http://spacestreams.com/images/sc-flat3.jpg
http://spacestreams.com/images/sc-flat4.jpg
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Re: Horizontal Lines in Flat Capture

#2

Post by admin »

Hi Jason,

it looks like the sort of banding you get an an image due to the illumination flickering at a high frequency. Most CMOS astro cameras are 'progressive shutter' which means that the exposure does not happen for all pixels at the same time - runs down the image (or up maybe). If the illumination is varying in brightness while this happens, you get banding.

Options to deal with this include

* use a natural light source for flats, which will not flicker
* try to find an artificial light source that flickers less (trial and error)
* use longer exposures (0.5s, 1s, etc) along with lower gain and reduced illumination to reduce the effect.

cheers,

Robin

PS. One exception to the 'progressive shutter' rule are cameras based on the IMX174 sensor - they are 'global shutter' - the whole image is taken at exactly the same time.
jdaliix
Posts: 17
Joined: Mon Jun 11, 2018 2:01 am

Re: Horizontal Lines in Flat Capture

#3

Post by jdaliix »

Hi, Robin.

I'll try those suggestions. My light panel is built just for making flats, but I did figure out last night that if I had it turned almost all the way down, so that I could get the exposure up to about 500 ms, the banding was still there, but the flat itself didn't have it. I was pretty happy about that and I got some of my best EAA shots ever.

Thanks,

-- Jason
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