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Thread: Flats at low and high ISO - a comparison

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    Default Flats at low and high ISO - a comparison

    Most of the directions on taking flats state that flats should be taken at the lowest ISO. alsetalokin raised questions about WHY flats should be taken at the lowest ISO when lights were taken at high ISO. I'd never read a reason why, so at Al's urging, I decided to do a test.

    Shot 4.5 hours of NGC 891. In the morning, right before sunrise, I shot flats at low and high ISO. Flats were taken with the OTA pointing high in the northern sky. The sun was still below the horizon, and with a 6" ~1500mm FL scope (i.e. narrow FOV), it was pointing at a pretty evenly illuminated sky. I stretched a white t-shirt over the OTA (and held it stretched to avoid any wrinkles or folds).

    Shot 50 flats at ISO100 and 50 flats at ISO1600. The ISO1600 flats were shot first (camera had been turned off for about 3 hours until then). Ambient temperature was 36F.

    Stacked all in DSS in two batches, using the same lights, darks, and offsets (i.e. only difference were the flats).

    I then brought both images into Photoshop and applied 2 levels and 3 curves adjustments (up through the 6th screenshot in this workflow). The exact same adjustments were applied to both images (I actually applied the adjustments to one image and then copied all the adjustment layers over to the other one).

    Both flats corrected overall vignetting to the same level. The difference was in noise.

    I took a crop of the center of each image, converted into GIF to create an animated comparison. The image below is a 100% crop of the full frame:

    To me, this test suggests that with everything else being equal, using ISO1600 flats produce more noise than using ISO100 flats.
    Last edited by Fuzz; 09-18-2010 at 04:57 AM.
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  2. The Following 10 Users Say Thank You to Fuzz For This Useful Post:

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    Fuzz, that's great work, well done, a true experiment from which conclusions may be drawn. It certainly looks like the ISO 1600 flats introduced more noise, or the ISO100 flats removed more, or something. You can really tell the difference in the background noise.

    But...I actually think the galaxy itself looks better _with_ the noise.

    I know sometimes one adds noise deliberately during processing; is this a case where the noise in the subject itself actually improves the view? Or is it just my imagination?

    Fuzz, the Master of Layers... it possible to make an image using the ISO100 flats for the background and the ISO1600 flats for the galaxy, and then do the blink-compare with that? I mean, if you aren't doing something more important, like sleeping or eating, that is...

    Thanks for your hard work, I am convinced and will be shooting a new set of flats as soon as I am able.


    (I don't see a "Thanks" button on Fuzz's post !! )

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    Default Re: Flats at low and high ISO - a comparison

    Thanks for this!
    Thanks!, Condensermike
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