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Interesting.
I use the T shirt also but aim the scope at a white sheet draped on the wall in front of the scope. I then aim a manual flash at the wall and take a 1/60s exposure. The histogram attached shows that the response to each colour isn't even. The colour balance is set to daylight in the histogram. If I set it to "flash" then the red channel moves further to the right.
On a different not, I think it is probably better not to "defocus" for flats. This will slightly change the distance between the focal plane and any optical surface that is between the T shirt and the focal plane. This will make dust motes slightly incorrect in size & not divide out as well.
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