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Darks Problem or, more likely, beginner error?


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Hi all,

On Wednesday I decided to to have a second go at DSO astrophotography using what equipment I currently have, Celestron Nexstar 102 slt on a Skywatcher EQ3 pro mount, Canon EOS450D unmodded connected to laptop running Backyard EOS, unguided as unable to get that working, yet, work in progress on that.

I targeted M101 and took 40 x 180s exposures at iso 800. I also took 10 darks at same settings, no other calibration frames, as I will add more things as I get a better understanding as I go forward. My problem is with the results, stacked in DSS.

This image is the autosave file produced after stacking the lights and darks (converted to jpg for posting here).



This is the autosave after stacking with no darks...


.......which I was able to produce this image, using GIMP.



So any ideas on what I am doing wrong???

Would other calibration frames help??


Thanks for any advice





Edited by TerryJ
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DSLR darks are problematic as the temperature profile may not match the light frames. I've given up with them myself, I find a sigma-clip when stacking dithered frames does a good job of removing DSLR noise patterns. These days in DSS I stack with dark bias, a copy of the master dark bias as a master dark and flats (when I can get them to work, they often seem to over-correct).

Hope that's some help. The above works fine for me but I usually work with fast camera lenses, so my signal-to-noise ratios are high.

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On 17/04/2020 at 14:52, TerryJ said:

40 x 180s exposures at iso 800. I also took 10 darks at same settings, no other calibration frames


Excellent effort. Well done.

I found that the 450 responds well to dark frames only if you use calibration which includes a dark optimization algorithm. siril's optimisation works well to reduce noise and banding. Just remember that for the optimization to work, you'll need to subtract the bias from the dark frames before creating a master dark. Take around 50 bias frames at 1/4000s in the dark and stack them to produce a master bias. Take dark frames at the same exposure as the light frames. 

I'd also recommend taking flat frames. These will improve your image a lot.

The image looks fine except that the background is really dark; the galaxy lost a lot of fainter stuff.




Edited by alacant
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