What about long exposure noise?

[dslater]

So as new cameras come out, we see manufacturers, reviewers, and others constantly comment on how much high-ISO performance has improved. However, what about long exposure noise? Has that improved as well? Is the improvement as great as high-ISO?
  I like to use my camera for astrophotgraphy and there's always a question of where's the sweet spot? I can increase the ISO and use a shorter exposure at the cost of high-ISO noise, or I can reduce ISO for a longer exposure at the cost of long exposure noise. So the question is, where's the sweet spot that balances these two sources of noise?
  By long exposure, I'm talking about exposures on the order of 30sec to as long as 60 minutes.
  Any thoughts on this? Is there a relationship between high-ISO performance and long exposure performance?

[Peter Connan]

Interesting question

I tend to shoot 30-second exposures and stack them, simply because that is the longest exposure my camera can count down.

I have shot a few exposures of 4-10 minutes, and can't really see any sign of long-exposure noise, although I am  not sure I would recognize it.

[dslater]

BY default, the camera has long exposure noise reduction turned on which removes much of the noise. However, when photographing stars, you need to turn this off ( at least on my D300 ) or the NR will wipe out most of the stars as well as the noise.

[Matthew Currie]

So here's my ignorant question of the day:

Is there a difference in noise between, say, ten 30-second exposures stacked, and one 5 minute exposure? 

[Ilkka Nissilä]

Is there a difference in noise between, say, ten 30-second exposures stacked, and one 5 minute exposure?

Well, for a 5 min exposure (with a telephoto lens) you should have very good alignment of your tracking mount so that the stars stay in position in the frame, instead of creating trails. I think likely it is less risky to do ten 30-second exposures and combine them. However, I don't know where the noise optimum lies. It would depend on the camera, most likely.

[Bjørn Rørslett]

The stacking of many (statistically) independent images will average out details and accordingly, also reduce noise.

Keeping the recording system perfectly stable and aligned (in tracking)  is of course mandatory whether one does stacking or single, long, exposures.

[Frank Fremerey]

My father uses a tracking head and videographs for stacking to cancel out atmospheric movement.

see http://astro-vr.de the website is in German.... the pictures not 

He also builds his own refractors which should be great telephoto lenses of 3000 mm
If I find some time I'd like to take some long distance wild life shots with these....

[Matthew Currie]

The stacking of many (statistically) independent images will average out details and accordingly, also reduce noise.

Keeping the recording system perfectly stable and aligned (in tracking)  is of course mandatory whether one does stacking or single, long, exposures.

Thanks.  I sort of suspected that, but since I've never gotten into stacking, and don't do astrophotography (I don't think the D3200 is ideal for that purpose, to say the least),  I wasn't sure.

[Bernard Delley]

BY default, the camera has long exposure noise reduction turned on which removes much of the noise. However, when photographing stars, you need to turn this off ( at least on my D300 ) or the NR will wipe out most of the stars as well as the noise.

The long exposure noise reduction is technically an automatic black frame subtraction. This is to compensate dark current related noise which may build up in long exposures. This built in 'long exposure noise reduction' is simple to use and sometimes convenient. It is also inconvenient as for each image it take a dead time while recording the black frame with same exposure time. I heard that with external timer exposing 10 minutes, it would be digesting for another 10 minutes with this setting.  Astrophotographers often like to take black frames under their control and do black frame subtraction in post processing.  The black frame subtraction definitely does not remove stars. However, the built in procedure multiplies noise by square root 2, which does not help faint stars.  This limitation of black frame subtraction can be overcome by black frame stacking, which all needs to be done in post.

The D300 damages stars by its hot pixel suppression (HPS) algorithm which is on for 1/4s and longer exposures. Unfortunately, this feature cannot be turned off.
Recent Nikon cameras have improved a big lot in the low light area. Not only has the D7200 for example much less input referred read noise than D300 (see Bill Claff's web site) it also has higher quantum efficiency and also higher full well;  there is now no objection against the  new HPS which is well designe to distinguish hot pixels from star images.
Except, minor objection,  it still cannot be turned off . Also the sensor is not much in need of dark current compensation, since dark current is so well controlled.