Hot pixels. How to remove blue and green spots.

[galaxy1307]

The enlarged cutting of a photo taken of the stars with parts of a mountain at the right side shows white, red and blue spots. The exposure time is 18 minutes at 200 ISO taken in warm weather.



The white spots at the folowing picture are removed by a program which removes Hot Pixels. But the blue and red spots are not removed. So how is it possible to remove these spots?

 

[Tinderbox (UK)]

Did you use high iso noise reduction, the camera takes the shot and then takes another shot with the shutter closed, and then subtracts the hot pixels from the first shot, in your case two 18 min exposures, nevr tried it myself but you can read up about it.

John.

 

[480sparky]

Looks more like high ISO noise.

 

[Josh66]

You can take what is known as a "dark frame" and use that to subtract the noise. Take a picture with identical settings, except leave the lens cap on. That photo will have the high ISO and long exposure noise in it, in the same place. You can use that to subtract the noise from this photo.

edit
It's basically the same thing that long exposure noise reduction is doing.

 

[KmH]

You use noise reduction software, or the in-camera Long Exposure noise reduction.
As Josh alludes to, the camera makes a second photo with the shutter curtains closed that is the same exposure time as the original.
So if you make a 10 minute exposure, the entire process takes 20 minutes

Noise reduction tends to soften the focus on an image. So some sharpening is also usually called for.

Photoshop, Elements, Lightroom all use Adobe Camera Raw's noise reduction software.