Two questions about flats

[CharlesW]

I have a QSI with two download speeds. The fast speed has twice the noise of the quality speed. For the purpose of taking dusk flats, would it be possible to use the fast download speed to get as many flats in as possible, and then use something like TGV Denoise to clean them up? Or, should I just use the quality speed and take fewer flats?

Additionally, is there generally a recommended number of flats to use per filter? I just don't recall seeing a reference for that.

Thanks

[Geoff]

Hi Charles. Don't ever apply noise reduction to flats.  Part of the reason for taking flats is to correct for pixel to pixel differences in quantum efficiency (PRNU).
As to the other question: I imagine the read noise will be completely swamped by the shot noise in the flats but you should do a quantitative evaluation based on your figures.
See Mike Schuster's post in this thread http://pixinsight.com/forum/index.php?topic=7759.msg51474#msg51474 part-way down as to how many flats are needed
Geoff

[Stu]

Hi Charles,
I had a discussion once with Kevin at QSI about the number of flats.  He recommended a total ADU of about 1 million. 

So if you look at this link
http://store.qsimaging.com/kb_results.asp?ID=28

And assuming an average flat ADU of 35,000, that would give about 33 individual exposure.  I usually do 35. 

I never use the fast download speed for anything.  And agree with Geoff.  Don't ever process your flats.