I've read about people who take -10 minute exposures of star trails, and then overlay them on photoshop to merge all of the exposures into one single picture. The reasons for doing this, I suppose, is to eliminate the overexposed sky if the moon is out.

Pretty much what I want to do is, for example, take these two pictures of star trails below, and merge them into one single picture where the star trails are continuous, if you know what I mean.


http://www.flickr.com/photos/37565608@N06/3522546568/
http://www.flickr.com/photos/37565608@N06/3521733431/

 

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Never tried this before, but I had a quick look, pop them both in to the same PSD file, I put the lighter one as the bottom layer, then set the top layer to "screen". This gives you the continuous effect, though in this example, there was a tiny bit of shift between one photo and the next, so they don't line up perfectly.

I'd use a tripod and a remote, set it up for whatever exposure you want (I liked the 5 mins one from your two) then just sit there and click the remote until you get all the shots you want.

 

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No, the reason for the multiple exposures is because the camera generates a tremendous amount of noise the longer the shutter is opened. On analog film there is no noise so the shutter can be left open for however long you wish.

For me I wouldn't go longer than 2 mins per frame or 5 mins on one of those 6 grand camera bodies because the noise level would eat you alive.

edit: What camera are you trying to use?

 

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Canon XSi

 

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For that camera I would do at most 3 mins then do 10 shots to make 30 mins of exposure.

 

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Ghost is correct. I use a 30D, and even as low as 2 mins the noise at 100 ISO (As low as I can go), is pretty noticeable.

 

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Yep, on my old Canon the noise was horrible at 30 secs. Basically you need to find your camera's sweet spot and work it out from there. I doubt the XSI will do more than 90 secs to 2 mins before the noise will eat you alive. The thing is the noise will be taken care of better on the newer cameras (it does this right before the image is saved) so you might get 3 mins but if it were me I would be doing 2 min shots or 3 on the outside if the darkness is so low it demands that long of a shot but expect some noise (it isn't a film camera afterall).

 

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Here is the result of several 2,3, and 5 minute exposures from last night.

The more I overlayed in photoshop the pictures, the brighter each one got...would that be the noise? or just the way it is.

http://www.flickr.com/photos/37565608@N06/3527952032/

 

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No,
noise in digital is the same thing as "Grain" in film, and it comes from ISO and or long exposures.
Basically its those lil flecks of color you see at the bottom of the photo, and I suspect its in the sky as well, but the photo is to small to really tell.

 

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Spot on and the sky has the noise as well.

 

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