captured FITS not saturated, apply flats saturated!

[troypiggo]

Capturing and calibrating images from my QSI583 with Nebulosity. The captured light frames have a max pixel value of 64000, which I assume is not saturated as it's less than 65535.

It seems a very nice, round number that 64000 and it's consistent among all light frames. Could be stars, or since this image is uncalibrated, it could be noise in the form of hot pixels I suppose.

When I apply darks to it, the max value drops to, say, 63636. This makes sense as darks are subtracted.

Now if I apply my flats to it, this is where I'm puzzled. I have an image that I think is unsaturated. I've ensured my flats have their histogram around 1/3 to 1/2 of max, and there's no blackpoints or whitepoints reached in them. Flats are divided into the light frames. After this process, somehow my calibrated image has a max pixel value of 65535 and a min of 0! I've tried variations with blurring the flats in case there's hot pixels or black pixels in there, same result though.

Anyone know/understand what's going on here? Do you reckon it's Nebulosity specific or is it common/normal? Am I worrying over nothing?

[Terry B]

A few points
You probably do have saturated images if you have pixels at 64000. I assume that the camera has antiblooming and probably this is stopping the pixels reaching ~65000.
Flat framing will change these levels and can make some increase. A flat is divided into the light frame pixel by pixel.
For the median value of your flat, those pixels will be divided by 1 resulting in no change. In dimmer areas of the flat the pixels will be divided by a number less than 1 resulting in an increase in the count of those pixels in the light frame.
The reverse occurs in the highlit areas on your flat.
Think about what happens when there is dust on your CCD. The counts under the dust mote will be lower than expected resulting in a dark dust bunny on your image. You take a flat frame and this shows up the dust bunny. When you apply flat it brings the count of the pixels under the dust bunny up to the same as the surrounding region therefore making the dust bunny disappear.
As to saturated pixels. Does it really matter? For me it does if I am doing photometry but for pretty pics it is pretty hard to not saturate bright stars and still get enough signal in extended objects to see them.

[peter_4059]

Troy,

Are you using bias frames with the flats?

[troypiggo]

Not using bias frames with the flats. They're only 5s exposures. I am using a master flat dark.

I checked some of the stars, and even though the pixel value is 64k, they do appear saturated (flat top on the stellar profile).