Long exposure star trail photo and NR

blackstar

Senior Member
Hi,

I just wonder if NR (noise reduction) is worthy of a star trail photo taken by a long exposure, e.g., 30 min. It'll take another 30 min to finish NR after shutter is closed. Is the NR effect equivalent to the long waiting time? (I guess that after shutter is closed, the camera, etc. can be moved away from the scene while NR is still on.) Would anyone share opinions and examples from experience? Thanks for your response.
 

blackstar

Senior Member
Thanks, Fred.

Ok, NR is recommended being turned on when a single long exp shot is taken, but not when multi-interval shots are done (avoid time gap in between shots). Now the question turns to why Nikon cameras need to take the same long time to make the "dark frame"? As someone pointed out that one can take a dark frame self once at the temperature being shooting. So once done shooting or during a lull, simply take a 3 minute or so shot with the lens cap on and maybe a sleeve to ensure no light gets in. This becomes a dark frame that can then later applied for dark frame subtraction in PS by simply putting the two frames as layers with the dark frame on top and then using the Difference blending mode and adjusting the opacity to eliminate or reduce the hot spots. I'm not sure this out-camera way of NR can perform similar effectiveness. If it does (or better), then the question is: why can't Nikon cameras take less time to shoot dark frame? (Oh, well, guess this will become a dumb question... )

And I may have to take back my guess that after long-exp shot and starting NR process, camera, etc. can be moved away from the scene. Because shooting dark frame should be done under the condition at the scene.
 

Fred Kingston

Senior Member
You raise several questions, each requiring a serious response. Bear in mind, sensors "change" over time... One might produce more/less noise for the same duration of time at different time intervals. I don't want to use the word degrade because that implies a certain failure but the noise evident in a single shot may move or become more/less intense from one shot to the next. There are just too many variables to say that a single saved shot will have the same noise as one taken 10 minutes later... Hence, if you're going to use the NR shot as a comparison between frames... it needs to be taken under as much of the same conditions as the shot being compared to... Storing/saving/using black shots in-camera for that purpose would take a bit more resources within the scope of all the other things a camera's memory and processor has to do... Camera designers are always balancing features against available resources, against expected revenues... and frankly, the big guys, Nikon and Canon are doing a great job just staying alive in today's camera space...

I haven't looked recently at the 3rd party Star Stacking/Trail software, but I recall at least one of them used a black shot for comparison as you mentioned in PS... so it is at least somewhat common to do that process in post processing rather than in-camera.

I believe the article I referenced earlier made mention of the fact that the camera could be moved during the "Processing NR" comparison message... Although, I don't imagine a reason to do that during a session of shooting Star Trails...
 

blackstar

Senior Member
Thanks again, Fred.

I found something interesting (and somewhat even more puzzling) in an article of Professor Morison's Astronomy Digest: " It is often not well noted that a single dark frame is actually quite noisy and will add noise into an image..." And so the author offers a second option to the in-camera NR: multiple dark frames for post-process (out-camera). The theory (and practicality) is that the averaged stacking multi-dark frame will vastly reduce noise in individual dark frames. However, it also has timing and technical cost on controlling and stabilizing sensor temp. Hence, there is the 3rd option: no dark frame used; NR off. The experiments demonstrated in the article are some deep space and the Milky Way imagings (not really single long exp star trail shot as in 30 min). The results of 2nd (multi-DF+PS) and 3rd options interestingly invite your verdict:

withoutDarkFrame.jpg

withdarkframe.jpg
 
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