K number values and flat fields

[rdryfoos]

The K number at the bottom of the screen has a value of 0 to 1.  Does 1 equal 65,000 ADU?  If I have a camera that has a full well capacity of 25,000, my flats should be around 1/3-1/2 of that so 8,000 to 12,000.

What K value number in PI does this correspond with?  Is it based on the above the number should be 0.114 to 0.19.  But my flats at this level are very, very dark--almost black. 

Can anyone clear this up

Thanks

[pfile]

can't necessarily clear it up but you can switch the readout mode to i16 and the pixels will be interpreted as 0-65535.

if your file is i16, the max possible value should be 65535, so 1.0 = 65535, not 65000.

are the files in question from a DSLR? if so, even though the file is i16, only the bottom 14 bits have any data in them. so a fully overexposed image would read out as 0.25. that may explain why your flats seem dim.

rob

[rdryfoos]

Not a DSLR.  The question is what K values should I shoot for with a camera that has a FWC of 25,000.  I don't care if the number is between 0 and 1 or between 1 and 25,000.  I just need a number.

[pfile]

well if the file is i16 (look at the status bar at the bottom, where it says w: h: n: etc; the 4th number is the bit depth of the file) and you are looking for 1/2 of 25000, then you want 12500/65535 or 0.19.

rob

[rdryfoos]

Thanks--I guess flats are supposed top look very dark before STF huh?  For Ha that is no problem, but I use a Gerd Neuman Flat foil and for me to get .19 for RGB filters, the exposure have to be less than a second.  I already have 3 pieces of paper in teh foil to dim it as the instructions recommend.

[pfile]

what kind of shutter does your camera have? 0.19s flats might show shutter artifacts if it has an iris shutter.

i guess the default gain in my 8300M is set such that full well is much closer to 65535, so my flats don't ever look dark. but i have never really looked into the gain setting nor even characterized the sensor to see where it becomes non-linear. kind of sloppy... but i have been exposing the flats with a mean of about 32k and that's been working fine for me.

rob