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How To Take Flats

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Tony Gilkerson edited this page Dec 3, 2017 · 5 revisions

Important things to remember!

  • It is very important that everything in the imaging train is exactly as it was when you shot your light frames. Filters, dew shields, correctors, dust, etc. should be in place and not touched or cleaned.
  • Do not remove the camera from the telescope, it needs to be in the same orientation as the light images.
  • Do not move the focus position. Lock it down when you are finished shooting light frames (or even better, before).

Procedure

Summary: Stretch t-shirt over end of scope, point at sky, expose to half saturation and take a bunch of images at ISO 400 and stack them.

T-shirt flats for my DSLR 700D-T5i:

  • Drape a layer or two of a clean T-shirt over your objective and with a rubber band make sure there are no wrinkles
  • Set camera to MANUAL mode and the ISO to 400
  • Start with an exposure duration of 1/120 shown as Tv120s
    • Or run this plan and watch the histogram (C:\aegmnt\astrobagel\media_library\BackyardEOS\Plans\FLAT-8INASTROGRAPH-SkyGlow.txt)
  • Adjust the exposure level to hit the half saturation point.
    • Use the Pixel Stats in Nebulosity and compare the MAX to the Mean for the entire image. So that Mean = MAX/2
    • Alternate method: adjust exposure so the histogram values are well off the left and right side. Leave at least a quarter or so of the histogram blank on each side, ideally with it roughly centered. If you’re going to err on one side, have the peak more on the left than the right.
  • Capture enough images to stack to a clean master, about 31

Footnotes

Flats should be located next to the lights given that a new set is needed for each session. For example:

C:\aegmnt\astrobagel\media_library\2017\M33_2017_11_10
├───FLAT
└───LIGHT
    └───M33

The following BackyardEOS plan and be used to find the right exposure. Under bright skys the TV=1/20 looks good in Nebulosity histogram. Note the BackyardEOS histogram did not agree with Nebulosity, it was more to the left.

C:\aegmnt\astrobagel\media_library\BackyardEOS\Plans\FLAT-8INASTROGRAPH-SkyGlow.txt

References:

  • https://photographingspace.com/how-to-create-dslr-and-ccd-flat-frames-for-astrophotography/
  • http://www.alberts-astro.com/AP_Blog/2009/12/238/
    • My favorite… T-shirt flats: With your camera at focus and in the same position of your Lights, drape a layer or two of a clean T-shirt over your objective. with a rubber band or other elastic, make sure there are no wrinkles. Point the scope to clear sky away from the sun. Adjust the exposure level to hit the half saturation point and then capture enough images to stack to a clean master, 8 to 20 frames should be enough. I like to adjust the light getting to the camera (by where I point the scope, or number of cloth layers) so I can set the exposure time somewhere between 0.5 to 2 seconds.
    • If I can’t wait ’til dawn: I use a laptop screen positioned close to the scope objective. I just get the laptop to display a white screen (open a text editor for example), point the scope at it, adjust the exposure and shoot. It works well. Some folks also drape a T-shirt over the objective to help diffuse the light more.
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