OnTheRopes said:
Is this a common problem? In a word Yes every d800 has this issue!

Yes, and you can safely expand this to: Every digital camera has this issue.

There are two major noise contributions - read out noise and thermal noise. As the exposures time increases so does the thermal noise, but the read out noise stays the same.

The read out noise dominate in the short exposures and the thermal noise dominate in the long exposures (rough generalisation). The thermal noise will also increase with the ambient temperature.

I have had communication with Nikon about this and nothing can be done apart from using in camera LENR which of course doubles your exposure time and is hardly useable for multiple long exposures.
You can shoot a dark frame often done by astronomical photographers and then subtract from this in photoshop but you will need to read up on the technique.

In camera LENR automates dark frame subtraction by taking a single dark frame of equal exposure time and subtracting it from the picture. If you don't want to loose the extra time used for automatic dark frame exposure and subtraction, you can just do this manually after the shoot.

Take your pictures with a fixed exposure time without LENR. When you're done and while you're packing up the equipment, let the camera take some dark frames with the lense cap on and the same exposure time.

When you get back you can subtract the dark frame from all the pictures. If you want to increase the quality of the dark frame you can take a number of dark frames (perhabs 16 or something similar) and average these before subtraction. This will reduce random noise from the dark frames.

Best regards
Soren

satellites said:
I think he is saying that this photo is the finished product after he eliminated the hot pixels...interesting that the d800 doesn't offer the pixel mapping featuring considering even the nikon v1 has this!

Yes this is the one after processing. I didn't have LE NR on. I have done a similar length shot on my 5D mkIII again without LE NR on without anywhere near as many hot pixels on it. Another shot that is a little longer has even more hot pixels

I think he is saying that this photo is the finished product after he eliminated the hot pixels...interesting that the d800 doesn't offer the pixel mapping featuring considering even the nikon v1 has this!

Ironheart said:
Do you have long exposure noise reduction on or off? Long exposures will create hot spots on the sensor and the LENR will create a dark frame to subtract from your image. Also many folks use a series of short exposures and stack them with software to create star trails. 8 minutes is a very long exposure for a digital sensor. How exactly did you map out hot pixels on your Pentax?

The K-5 has a pixel mapping menu function. It only takes a few seconds.. I just tried it on my new one.

Is this a common problem? In a word Yes every d800 has this issue!
This is a major issue with the d800, msmoto is right you need to post the full size image or at least a 100% crop to see what you mean, but if you look on the last few pages of the D4 D800 issues thread you will see my post on this subject.

Try reading this blog http://scottreither.com/blogwp/2012/07/01/nikon-d800-e-long-exposure-issues-problems-2/#comment-988 which explains the problem.
I have had communication with Nikon about this and nothing can be done apart from using in camera LENR which of course doubles your exposure time and is hardly useable for multiple long exposures.
You can shoot a dark frame often done by astronomical photographers and then subtract from this in photoshop but you will need to read up on the technique.

below is my example of the problem on a 3 minute exposure, this problem raises its ugly head on any exposure of around 60 seconds or longer.

http://s51.photobucket.com/albums/f396/Hannabella/?action=view&current=testshot-1.jpg

If you really want to show folks hot pixels on the photo, upload the full size image to Flickr so we can go there and see it.

Do you have long exposure noise reduction on or off? Long exposures will create hot spots on the sensor and the LENR will create a dark frame to subtract from your image. Also many folks use a series of short exposures and stack them with software to create star trails. 8 minutes is a very long exposure for a digital sensor. How exactly did you map out hot pixels on your Pentax?

I shot an 8 minutes exposure yesterday: this one:

and it took me ages to process as I had hundreds and hundreds of hot pixels. Is this a common problem? I was intending to use the D800 for long exposure ie star trails of over 30 minutes and this will be a major issue. Have done this with my Pentax 645D with only a few stray hot pixels. Pentax enable you to remap the sensor from the menu. Apparently with Nikon you have to send the camera away. I don't want to have to send a camera away an wonder when I will get it back when the likes of 645D and K5 you can do it in camera.