Flat frame - seeking advice

[astropixel]

Hi. I would really appreciate feedback on these images. I'm getting closer to getting all of this under control, but still have some concerns. Following advice elsewhere, histogram distribution is about 50% for both images, but I suspect it should be around 30 - 35% - on the bright side.

The top image is a sky flat, the bottom created with a luminance screen, the lens interposed with paper.

DSLR 1000D, Canon 200mm prime lens - f/3.2, iso 800, 1/30th to 1/40th second shutter speeds.

Both have been processed according to Vicent's Process Icon instructions (bias calibrated only). Should they also be dark calibrated?

[Nigel Ball]

Hi astropixel

I used to have a Canon 1000D

The A/D converter is a 12 bit device so the maximum adu is 4095. I used to aim for 30-50% of this value

I've attached one of my flats so you can compare. This a CR2 file loaded in and stretched and screen grabbed

HTH

Nigel

[astropixel]

Starting again with my reply.

I created a completely new master bias and dark frames to calibrate the new flats.

Using the Statistics tool, I was able to check the Median value of different exposures - the camera histogram is not a reliable indicator. By converting raw files with AHD and converting to greyscale (as you would to debayer), the K value approximated RGB values. I was just a matter of refining the camera settings - trial and error.

I came up with these settings - so regardless of the method of obtaining the frames, future attempts should be consistent.

At ISO 800 (the setting I generally use for AP), f/3.2, 1/30th second, EV +1 - approximates 60 - 65% on the camera histogram, and gives an ADU of approx 2200 - a little high, but the next settings down increased the error on the lower side of 2048 - on my camera anyway.

Feedback on this effort would be appreciated. I can't be certain of all aspects of the flat frame, though I have taken a lot of care and effort in reaching this stage.

[astropixel]

In relation to PI, what iso setting should be used for flats? Possibly it doesn't matter. DSS recommends the same setting as lights, but my reading leads me to believe that flats are better taken at a lower iso - say 100.

[Nocturnal]

Ugh, DSLRs and ISO. Glad I don't have to deal with that. One gain, one offset for everything 

Assuming for a moment that ISO is a bit like gain you simply have to make sure that your flats and dark flats are taken with the same ISO. Once normalized as a master flat it can be used with lights taken with any ISO.

[astropixel]

Exactly, iso is a misnomer - thanks Sander - understood