Dark frames and noise reduction

[andylast]

As a newcomer to digital imaging, can someone answer a question I have? I have read about Dark Frames and wondered what they do that post processing features like De-speckle and Noise Reduction can't do? I say this as before I bought a License, I tried (for the first time) to capture an image of the Ring Nebula on a planetary camera. It worked fine but I noticed all the coloured hot pixels covering the image. As I didn't have the Dark Frame feature available, I used GIMP and the Noise Reduction feature. It got rid of them all. There must be something the Dark Frames feature does that removes noise or artifacts, that post processing can't quite do. What am I missing?
Thanks
Andy

[admin]

Hi,

the difference is that with a dark frame you effectively measure for each pixel the brightness it produces with no light over your exposure time. By subtracting those values away from the images, you remove that noise fairly exactly. Dark frames also help removal any spurious glow in the image (often called amp glow) which can affect some cameras in longer exposures.

Noise reduction in something like GIMP or Photoshop works in a different way, to put it simply, it is looking for pixel values that are very different to surrounding ones and correcting the values. This is a 'best guess' process and not as exact.

In either case, the process works best to remove hot pixel noise if done to the individual frames from the camera before stacking. Once stacking has taken place, the noise is reduced by averaging, but that makes it harder to pick out the hot (and cold) pixels.

One thing worth knowing is that SharpCap has a 'Hot Pixel Removal' and (latest 4.1 version) 'Hot and COld Pixel Removal' mode in the 'Subtract dark' control, which helps remove hot/cold pixels from captured frames without the need for dark frames.

cheers,

Robin

[andylast]

Hi Robin

Thanks for that. Its the kind of answer I needed.

Andy