Flat frame contains star trails

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saguaro
Posts: 51
Joined: Tue Feb 14, 2017 9:04 pm

Flat frame contains star trails

Post by saguaro » Tue Mar 20, 2018 8:56 pm

I used the Capture Flat feature for the first time, in version 3.1.5092.0, with an ASI1600MC-Cool camera. I'm using a Win 10 64-bit laptop.

I typically take twilight flats, since they are convenient for me. When I've taken flat frames for processing in DSS, I first turn off tracking for my mount, so that any stars visible in the frame will move from one frame to the next, and thus be calibrated out. I typically take flats this way using 1-sec exposures, and DSS always calibrates out any stars.

To create and use a flat in SharpCap, first I turned off tracking to my mount as I typically have done. I then used the Capture Flat feature and adjusted exposure time until SharpCap said the Histogram was OK. I think I ended up with 4-second exposures. SharpCap then created a flat frame, but the flat had star trails in it.

Here is a link to the flat frame fits file: https://www.dropbox.com/s/sfqp6vcedx9wg ... .fits?dl=0

Did I do something wrong or is this a bug? Thanks.

-Robin

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admin
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Re: Flat frame contains star trails

Post by admin » Tue Mar 20, 2018 10:13 pm

Hi,

I think that basically the sky brightness was too small when you were taking your flats in this way, so that stars were showing. Since with flats you are aiming for even illumination aside from the effects of vignetting/dust/etc in the imaging train, having visible stars is going to be a problem. Even if they didn't trail they'd appear as bright spots in the flat frame and hence as dark spots in flat corrected frames.

If you took flats earlier with a brighter sky then this would be less of a problem, but to be honest you can see bright stars in a telescope at noon if you are aligned and focused, so no guarantee. Perhaps try something like a sheet of frosted perspex which could be put in front of the telescope and would throw the stars out of focus but keep the even illumination?

cheers,

Robin

saguaro
Posts: 51
Joined: Tue Feb 14, 2017 9:04 pm

Re: Flat frame contains star trails

Post by saguaro » Tue Mar 20, 2018 10:35 pm

Thanks for your quick and informative message! I will follow your suggestions for taking sky flats. I do normally take them in a brighter sky, to keep the exposure around 1 second. But I was running late and ran out of twilight :o

I'm really enjoying using the Live Stacking feature!

-Robin

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admin
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Re: Flat frame contains star trails

Post by admin » Wed Mar 21, 2018 6:58 am

Hi,

no problem - nice to hear that you are getting good use out of SharpCap.

cheers,

Robin

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