Star Trails - Help needed with understanding noice

[soren.martensen]

Hi All,

Have been looking into star trails of late and have seen some fantastic photos of the long exposure type. The photos I have seen have all had a nice blue sky, no noice what so ever and looks like they have been taken during daytime.

I had a go the other night for the fun of it, so did a try with long exposure, 53minutes in total with another 53minutes of in camera noice reduction. There was no moon out at the time.

(don't worry about the composition, missed the Southern Cross, or the sharpness (I messed that one up as well)

Exif file:
Exposure Time = 3219"
F Number = F5.6
Exposure Program = Manual
ISO Speed Ratings = 200
Date Time Original = 2009-04-26 03:20:12
Focal Length = 17mm

My question is more about the noice i am getting. see attached 100% crop.

Is this amount of noice normal, i know you get less noice by stacking 100 or so images in startrails for example, but I have seen a lot of photos from long exposures that look totally free of noice and are looking like almost daytime (see http://www.naturescapes.net/112006/ej1106.htm ) or have they been done with a lot of PP afterwards cloning out the noice?

Or is it the moon less night that is causing a problem, giving me the noice and the brownish sky? Sorry for the longwinded question. Hopefully some of you guys can stear me in the right direction.

[soren.martensen]

Sorry - thought i would attach the full image as well. No PP has been done yet, just resized and saved as jpg.

[A.S.I.G.N. Observatory]

I would take a number of darks, say 10 to 20. Each dark should be taken at exactly the same exposure length as the lights.

Subtracting the noise averaged out from a number of shots, is way better than the in-camera noise reduction.

Is that dirt on your sensor? Some flats would definitely fix this if that is so.

Baz.

[jeff2231]

Hi Soren,

I'm having exactly the same problem with noise. I have a Canon 40D and even when I use in camera NR and then use the canon software to take it out I'm still left with a heap of ugly noise.

How nice would it be just to take the photo then not have to play around with any PP :P

[soren.martensen]

Hi Jeff,
Yeah, I would have thought, in this day and age with the way technology goes forward in leaps and bounds, that they would have worked out a better way to reduce the noise you get with long exposures.:confused:

I have found, by doing it the way Baz is saying by taking a few dark frames and average them out, "startrail" does get rid of a lot more of the noice when you ad the dark frame to the startrail than the camera does. It is still not perfect though (especially on the single long exposures) and certainly nowhere near as good as some of the shots that I have seen. Or maybe people have painstakingly gone over the images and cloned out the noise. Huge job in some instances :(

I'm still trying a few different approaches, lately though we have had a full moon or it has been overcast so haven't had much of a chance to do more trials.

Combining lots of 30 seconds exposures with dark frames in startrail does seem to give a better result with less noise, although your foreground doesn't come out well. I might have to try some "light painting", maybe with the flash, to get the foreground to stand out better. Or combine the startrail with a good picture of foreground interest.

Will post more pictures when I get the chance.