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Posted (edited)

Hey all, I have been searching for information about flat frames and can't really find what I am looking for. Various sources advised measuring the ADU of the frame or pixel brightness but I can't seem to find this information in PS. I have attached two frames that I took as jpgs as practice. One is how it appears when taken and the other one has had the level changed in PS. This is a new technique I am trying using a tablet and a web page called whitedisplay.com. I tried different display brightness settings but the histogram stayed roughly the same  just the exposure length changed. Are these correct or do I need to add white paper to the screen to dim further? Thxs

large.Capture_00010.JPG.5f4e00210ed1812636ddb849139b4e0e.JPGlarge.stretch_flat.jpg.73779e6016bbed48db597abed0b8279c.jpg

 

Edit: I managed to find some info in PS, I needed to click expanded view in the histogram and select Luminosity. Mean 188, Std Dev 1.83, Median 189, Pixels 393216.

Edited by PH-R
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Yes,  just look at the histogram of the captured flat. It should peak about a third of the way from left to right but exactly where is not critical. 

Olly

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If this is regarding the Canon 10D in your equipment list, simply take flats with the AV setting. 

Michael 

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Thank you. I have taken the flats with the AV setting. Are the flats above suitable? 

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Posted (edited)

There are very scientific methods of taking the flat frames that work out exactly what the ADU should be for your precise set up.  However, the net of these methods is that the ADU setting is between 30% and 45% of the maximum ADU.  So assuming 65535 is the max ADU for a 16-bit camera, somewhere between about 20000 and 30000 is where you need to be.  It does not matter too much exactly where.  I tend to go for 28000 because it is easier to get that level with a longer exposure with a camera with a mechanical shutter.

Your capture program will tell you the ADU for the frame.  If not fits liberator will tell you.  it is the average ADU figure you are looking for.

Edited by kirkster501
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Thanks for the replies. I have just been out and taken some flats and everything is looking good (1.51 exp. peak in centre of histogram). I discovered my error. When I was putting the tablet on top of the telescope even though it looked flush, light was still getting in. I rectified this with blackout over the top of everything.  I set my tablet to the lowest dimness and needed one sheet of white paper between tablet and dew shield.

 

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