Long Exposure Noise (Hot Pixels)

I've searched the forums but come up with no real answers to this.

Shooting long exposures I'm seeing a lot of hot pixels which isn't like the normal noise you get but a lot of multi coloured dots all over the image. Longer the exposure, higher the density. Is it possible to stop these from appearing? If not, how to get rid of them and how are long exposures done without such noise.

Below is an example of a 4 minute (one shot) exposure using a 10 stop ND and 4 stop GND. Inset is a 100% crop showing the noise.

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What's your ISO (100 I presume given you're using a GND). Do you have long exposure NR turned on in your custom functions section of your camera's menu) ? If you're waiting 2 minutes for one minute long exposures then it's on. If not, it's off.

Chroma noise doesn't pose too much of a difficulty though in post, particularly with images without a lot of changes in color detail throughout the scene. Lightroom does a great job at this, but obviously there are a million ways to go about it.

I think I may have NR turned off in camera coz it doesn't take long at all to save my images (will give this a check). I was under the impression LR is good at clearing things up, and it does indeed, but most pixels remain.

Will give some more long exposures a try over the next few days with NR turned on (if it was off). In the meanwhile what are some of the ways I could go about trying to clean this up?

Welcome to the joys of digital photography! You've just discovered hot pixels - something 'they' forgot to mention when hyping the 'digital revolution.'

There are several third-party RAW processing utilities that offer Dark Frame subtraction.
Try one of these:
http://ufraw.sourcefor​ge.net/
http://rawtherapee.com​/

You basically save a series of images that are taken with the lens cap on, and are exposed at various shutter speeds. When processing your 'real' image, point the program to the dark frame that most closely matches your exposure time. The program will subtract out the vast majority of the noise and nasties.

Hot pixels tend to accumulate with age, so you may need to make new dark frame images every few months.

wow! That looks very interesting, thank you very much, will give it a try.

I thought hot pixels were completely random on every single frame.