Stacking images in PS for stars trail

[sai]

Hi everyone! I'm taking long exposures for stars trail but have herad that if

you take several short exposures images you get less noise in the images. Also

by staking them togheter and as noise is random Noise will cancel itself with

the image below. But how do you actually do it? Add images as layers and then

what? opacity?

thanks

simon =)

[colinsouthern]

Simon,

 

I haven't been able to test this for myself since our conversation on the Canon EOS forum (we've had a week of mostly rain here).

 

If you'd like to eMail me, say, 3 shots I'll have a crack at merging them for you.

 

Thinking about it, I'm pretty sure we're going to have to experiment with the blending modes otherwise the black sky will cover the white star of the layer(s) below it.

 

I'm hoping that noise won't be a big issue - after all, the noise and stars are (hopefully) towards opposite ends of the histogram - I'm hoping that raising the black point (shadow clipping) will take care of that.

 

Cheers,

 

Colin

[sai]

Colin,

thanks a lot for the offering =) it's been pouring here as well for the last week so I haven't had the chance to take the pictures either. crazy weather, hey? Anyhow whenever I do take them I'll let you know.

thanks a million again

simon

[colinsouthern]

Know worries!

[wayne_fox]

My guess is you will be using either the lighten or the screen blending mode.

 

Noise isn't as random as you might think. Noise is electrical interference being generated

by the electronics of the sensor itself. The longer your exposure, the lower your signal to

noise ratio thus more noise. The chip tends to generate noise in a somewhat consistent

manner. Many things affect this, including the chip heating up during heavy use or long

exposures.

 

Many DSLR's actually take a "dark" exposure once your shutter speed gets slower than a

certain level, which is basically an exposure of the same length of time without opening

the cameras shutter ... basically noise. If you take a 15 second exposure, you might think

the camera has locked up, because it sits there for the same amount of time appearing to

do nothing. It will then use this exposure to "map" out noise. Obviously this can only work

with a shutter speed that you can set on the camera, so for manual exposures it doesn't

happen (who would want to wait for a 6 hour dark frame after shooting a star trail any

way).

 

You might find some help by searching some astrophotography sites, I think stacking

exposures is a pretty common techique. I'm not sure they do it for noise, but use it to

"multiply" the layers to create density in light areas.

 

Good luck.

[obakesan]

Hi

<P>

I just tried a quick n dirty with two images from a coolpix some time ago (its been no good for photogaphy at night here either) and found that making one layer a 'difference' layer in the blending really cleared up the image ... one image was taken with the lens cap on to capture noise. Perhaps the technique might be to successively add another image and flatten

<P><div></div>

[obakesan]

and here's the original file<div></div>

[obakesan]

I suppose this is well known

<P>

http://www.astropix.com/HTML/J_DIGIT/COMPEXP1.HTM

[sai]

WOW that's some change between the two images! thanks a lot =)

So how did you do it?