Bad Pixel Mapping vs Dark Subtraction

[AlistairW]

Hello,

Normally I take a set of darks at the same time as a set of lights. But recently I have read about creating a Bad Pixel Map instead. This seems to be a temperature independent was of subtraction. Does anyone use this method ? - What are the pros and cons ?

Thanks

Alisatir

[Herra Kuulapaa]

Are you meaning a master dark?

That's the method with Pixinsight and it makes it possible to utilize maximum imaging time. But you need to have the same exposure temperature when creating the master dark image and that's pretty difficult with regular DSLR. You seem to have peltier cooled DSLR so I'd actually recommend you to do so, if just your software supports it.

If you mean incamera/software bad pixel mapping, that only addresses stuck hot pixels, which are always on. Most cameras do that automatically nowadays. Mapping doesn't affect to the noise though.

[AlistairW]

Not sure that is what is meant ..... I was reading the Nebulosity manual, and except is from it ....

Bad Pixel Mapping works differently. You first create a "Bad Pixel Map" (Batch, Bad

Pixels, Make Bad Pixel Map) using a dark frame or stack of dark frames. A slider

appears to let you set a threshold (feel free to use the default). Values in the dark frame

that are above the threshold say "this pixel is bad". Bad pixels, and only bad pixels are

fixed in your light frames by using surrounding good pixels to help fill in what this pixel

should have been.

[Herra Kuulapaa]

Ok, that's only for fixing stuck pixels as far as I know.

You'll get rid of them by dithering anyway and actual darks are often more meant to reduce the background noise. Bad pixel mapping and darks are two slightly different things although in camera/software dark reduction deals usually both noise and hot pixels.

[AlistairW]

I get the impression it is slightly different. It's in the Nebulosity manual (version 3 p29), which it probably copyright so I shan't post any more here. But bad Pixel Mapping is an alternative to using Darks. Darks 'can' introduce noise, where as using a Bad Pixel Map, you only ever subtract. It sounds very interesting, but I have never heard anyone using this method. The only time I have created a Bad Pixel Map is for my guide cam, so that I don't track on a hot pixel, but I think I see how the principle could be applied.

[ollypenrice]

I use the BPM system rather than regular darks, but in AstroArt 5.0. Their system is just a little different from that in Nebulosity. You make the BPM by black clipping 2000 ADU off the normal master dark. In the stacking page this goes in a space dedicated to BPMs. You then put a master bias in the space normally used for a master dark. So the bias takes out the low level noise.

I find this system works better than the subtraction of normal darks. It is less invasive. For reasons unknown my subtraction of regular darks gave a rather inconsistent result, sometimes working well but sometimes not. The Bias/BPM is very consistent and allows me to use the same calibration files for images of all durations, so it saves a lot of time and faffing about. (On a good winter night I can end up with 24 hours' worth of data to reduce.) Despite my chips being the raucus full format Kodaks I get very clean stacks this way, very clean indeed. I do also run an aggressive hot pixel filter. I can't easily dither because ours is a dual scope rig and we tend to run long NB exposures on one and shorter broadband on the other making synchronization impossible.

As ever, you'd be best advised to find what works for you.

Olly

[MartinB]

Bad pixel maps will reduce your hot pixels but not other dark current effects. However, darks can introduce noise and actually degrade your final image. I do use darks but get lots of them, a bare minimum of 30 to create a master dark. When using 30min subs this means 15 hours worth of darks!