How Many Flats Frames is Good enough?

[sreilly]

I'm not really happy with the panel flats I've been getting so I need to try using sky flats and compare. At home I use 15 flats for each master while for the SRO setup I use 5 each. More, less. What's the magic number. The setup are both 12.5" RCs, one with a STL-11002 and the other STX-16803.

Steve

[mschuster]

The SNR of target structures in a calibrated frame can never exceed the SNR of the master flat.

An SNR of several hundred is typically "good enough" for the brightest structures.

So to be conservative, pick say 500 as master flat SNR. This requires 500^2 = 250,000 e- total in the master. For your detector, one flat exposure at 50% is 30,000 e-. So 250,000 / 30,000 = 8 flat exposures are required.

Regards,
Mike

PS: Some high-end folks like 1000 SNR for the master flat. Seems a bit overkill, would require ~32 exposures.

[sreilly]

Thank you Mike, 8 is my new target.

Steve