Multiple long exposures for star trails


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Jul 3, 2004
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Here N There
I am not sure if this is a beginner or expert question. I love to take long exposure photos for star trails. The longest I've done so far is 35min. I use a 35mm traditional SLR and I typically have my negatives scanned(just bought a Epson V770 scanner).

So the question is, I heard about combining multiple shorter exposures, such as 3x10mins instead of one 30min exposure, but how would I combine the photos digitally?

I use photoshop all the time for work, but it's more for illustrations. I may not be as informed about photo editing tools in PS. Any tip will be very helpful. Thanks in advance!
Not in photoshop. Photoshop lacks both an "add" and a "max" function in layer blending. You need another program and there are tons dedicated to this as they are rather simple to code.

Image Stacker is the one I use but there are other freeware ones available too. Just do a good search for image stacking.
Actually, Garbz is slightly outdated in his information. CS3 introduced the ability to stack multiple layers in a smart object and then apply some very basic image arithmetic (add, mean, median, standard deviation, min, max, and I think skew and kurtosis without looking at the software). If you do a Google search for "Photoshop CS3" (or CS4), "smart object" and "stack" or "stacking," you will likely find many basic tutorials.

Or, there's other software as Garbz mentioned.
!!!!!!!!!!!!!! There is to.

As cool as this sounds I can't for the life of me get my combined smart object to allow me to select a stacking mode. That option is greyed out. Any ideas?
try loading the images in through bridge. Im running cs3 extended, and I have noticed that some tasks are easer this way. Select the images click tools/photoshop see if you can find what you need in there.
I did get past that stage. I got all the way to having every image as a new layer in the current image, selecting all the layers and successfully combining them into a smart object as per the instructions on Adobe's website. However I can't for the life of me get the stacking option to ungrey :( when I select my multilayered smart object. Every other smartobject manipulation seems to work just fine.
Garbz - I've seen this happen before, though never on my computer. I was doing some consulting once and the guy did all those steps, and the option to set the stack to something like median was all greyed out. It was very weird because what we did next was he saved it as a PSD file, I copied it to my computer, and the options were not greyed out. I don't think we ever figured out why ... but I just had an idea: Do you have the Extended version of PS?
CS4 version 11.0 I think I'll grab the point update that came out a few weeks ago and try again.

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