DARKS removal leaving black spots

[Russe]

Anyone having the same trouble?

When I apply my darks, then instead of hot pixels I get black spots in their stead in my (noisy) grey background?

What to do with them?

Any idea?

[rfdesigner]

Sensor Temperature difference between lights and darks. If darks were taken with a hotter sensor, then the hot pixels will have higher values and so when you take away you get dark pixels.

I did an experiment to see if I could use hot pixels as a temperature indicator to scale dark frames.. sadly it doesn't work as the laws governing the currents in hot pixels are basically random, hot pixels form from a variety of silicon defects and each one will have subtly different reasons and temperature profiles. Some hot pixels can almost step in current at certain temperatures.. but they all follow the basic rule of cooler is better.

I'd make a 'hot pixel map' from the dark frame and use that to correct the worst offenders.. you MAY not need to apply the dark frame then.

EDIT: can also happen if you have saturated hot pixels.. i.e. light frame hot pixel = 65535 dark frame hot pixel = 65535.. light - dark = 0. fix is to use a 'hot pixel map'

Derek

[Russe]

Thanks. Googling now "how to".

I guess similar to layer masks when controlling stars.

[Russe]

I'm currently running a trial of different stacking results in DSS. It's actually quite remarkable what a difference one can achieve by ticking one or the other option. In particular the hot pixel remover shows a great variability in its results.

[Russe]

Now I'm trying bias used as dark, and hot pixel removal as I have no BPM + FLATS...

[Russe]

The hot pixel removal in DSS is definitely a powerful tool. The settings change from exposure to exposure, so are quite different. I've tried out several LIGHTS only stacks first b4 moving towards including FLATS n BIAS.

[ollypenrice]

Using Astro Art's instructions I made a Hot Pixel Map (AKA Defect Map or Bad Pixel Map) by clipping 2000 ADU off the black point of an appropriate master dark. AA has a box for Defect Maps. I put a master bias where a normal dark would go. This works better than darks for my noisy 11 meg camera.

I know nowt about DSS though, I'm afraid.

Olly

[Psychobilly]

I stopped using darks with a DSLR years ago... mis-matched darks can produce worse results...

Sometimes the cosmetic hot pixel removal in DSS can also remove the center pixel of stars..

Peter..

[ollypenrice]

Another good post-fix is to use the colour select tool in Ps to grab the offending pixels and then hit them with the Median filter.

Olly