Red and blue specks on long exposures

molested_cow · Oct 4, 2011

The image is at 200% of a long exposure star trail shot. I couldn't do NR on the camera because I was doing continuous shots for stacking. Is there a good way to get rid of these color specks in photoshop? I've tried desaturate the color channel but it looks weird, unless there's a proper way of doing it. Thanks!

Destin · Oct 4, 2011

molested_cow said:
The image is at 200% of a long exposure star trail shot. I couldn't do NR on the camera because I was doing continuous shots for stacking. Is there a good way to get rid of these color specks in photoshop? I've tried desaturate the color channel but it looks weird, unless there's a proper way of doing it. Thanks!

The generally accepted method would be to use the clone stamp tool, or healing brush.

andrewleephoto · Oct 4, 2011

That would be noise... Try post processing it out in lightroom.

Big Mike · Oct 4, 2011

There is some 'noise' visible in that image, but the red & blue specs are, I think, what we would call hot/stuck pixels. If I recall correctly, they can be caused by heat build up on the sensor, which would certainly be more of an issue with long exposures.

If there aren't too many of them, the healing tool or clone stamp would probably be the best way to get rid of them. If they are in the same spots on all the images, you might be able to automate some of it. You might even be able to use a dust mapping application to fix them. Check your camera's included software package.

If they are more random, you might try the patch tool in Photoshop (under the healing tool). Although, that may or may not smooth out your texture. Worth a try anyway.

molested_cow · Oct 4, 2011

Yes I am talking about the red and blue specks. They are all over the place, so the healing or patch tool will be pretty useless, especially when I want to preserve the star trails.

So what can I do to reduce the specks from photo shooting point of view? Let's say I want a total of 30min exposure, right now I am doing 3x10min. If I do 6x5min, will it reduce the presence of the specks?

Destin · Oct 4, 2011

molested_cow said:
Yes I am talking about the red and blue specks. They are all over the place, so the healing or patch tool will be pretty useless, especially when I want to preserve the star trails.

So what can I do to reduce the specks from photo shooting point of view? Let's say I want a total of 30min exposure, right now I am doing 3x10min. If I do 6x5min, will it reduce the presence of the specks?

Yes, shorter exposures should reduce them... To a point. They are basically caused by your sensor heating up during long exposures. So my guess would be that even if you did 30x1 min. Exposures, the sensor is still going to heat up, and they are still going to appear, at least in the later exposures.

Destin · Oct 4, 2011

Also.. What camera body are you using?

KmH · Oct 4, 2011

Quit looking at photos enlarged to 200%. :lol:

At 3x10 min, what ISO were you using?

You also have to be aware that dark portions of an image have very little signal, so it doesn't take much noise for it to be visible.

Drake · Oct 6, 2011

Were you shooting RAW? Most of the spec should disappear once you load your RAW shots in ACR.

Destin · Oct 6, 2011

Drake said:
Were you shooting RAW? Most of the spec should disappear once you load your RAW shots in ACR.

That's not true at all. They'll show up more in raw because there is no in camera attempt to cover them up.

molested_cow · Oct 6, 2011

D700, RAW. So how do those "perfect" shots do it? If the shot requires at least 30min of exposure, the sensor will heat up no matter how many shots I break it up into.

Also, if I shoot it with shorter shutter speed, means I have to bump my ISO up way high to get the same exposure, there will be too much noise. Currently it's at 400 if I remember correctly.

Destin · Oct 6, 2011

molested_cow said:
D700, RAW. So how do those "perfect" shots do it? If the shot requires at least 30min of exposure, the sensor will heat up no matter how many shots I break it up into.

Also, if I shoot it with shorter shutter speed, means I have to bump my ISO up way high to get the same exposure, there will be too much noise. Currently it's at 400 if I remember correctly.

This is on a d700?!?! What iso was it taken at? I believe higher isos can cause more hot pixels.

Also, could you post the full image? I'm interested to see how much they really show up

Garbz · Oct 6, 2011

molested_cow said:
D700, RAW. So how do those "perfect" shots do it? If the shot requires at least 30min of exposure, the sensor will heat up no matter how many shots I break it up into.

Also, if I shoot it with shorter shutter speed, means I have to bump my ISO up way high to get the same exposure, there will be too much noise. Currently it's at 400 if I remember correctly.

You're missing the breaking up bit. Also the fact that hot pixels are a function of "on" time. So if you take 2x 15min shots rather than 1x 30min shot you'll have fewer hot pixels. This is why image stacking is the preferred method of astrophotography in general, lower noise.

Also If you put a small gap in your shots then your sensor won't heat up near as much. Running the camera 30min continuously produces far more heat than running it 25seconds on, 5seconds off, for 30minutes. Plus you'll never see a hot pixel on a 25second exposure

.

Failing that there's always the option of freezing your camera which is also something astrophotographers tend to do. Seen some pretty neat camera cooling rigs on the internet.

molested_cow · Oct 6, 2011

ISO400, stack of three photos each about 10min.

I don't know how to post full size pic, so here's the link that you can download from. The file will be available for a week.

http://www.yousendit.com/download/T2djb241bWdVVGxqQThUQw

It's 8.7mb.

Destin · Oct 6, 2011

molested_cow said:
ISO400, stack of three photos each about 10min.

I don't know how to post full size pic, so here's the link that you can download from. The file will be available for a week.

http://www.yousendit.com/download/T2djb241bWdVVGxqQThUQw

It's 8.7mb.

Wow, you weren't kidding. They are everywhere. I would definitely try more shorter exposures, as it should keep the sensor at least a little cooler. My guess is you'll still start getting hot/stuck pixels in the later exposures.

As for you original question, as to how the super clean ones that you see in..say, nat geo are done... I'm sure it's doable on digital. But Film would sure make it alot easier becuase well, there are no pixels to heat up and get stuck.