Using layers in Star Trails???

[banjobob]

I made 10 pics, each 30s exposures, of the stars. I did this in an attept to avoid a really long single exposure. The problem is that I can not figure how to layer them in photoshop. Do I adjust the opacity of each one, if so to what? Any help or better ideas would be a great help. Thanks

[PacAce]

If you adjust the opacity of the layer, you will, in effect, make the stars semi-transparent and darken them at the same time. You might want to try selecting the black background of each of the layers except the bottom-most one with the magic wand tool and then deleting the black background. Set the tolerance to 1 and check the Contiguous option. You may need to use some feathering (1 or 2 pixels) if the edges come out rough. Hopefully it'll come out the way you want.

[Mad_Martian]

There was an article on this topic in Sky and Telescope for March 2004.

A quick re-read of it this morning indicates that they used the Lighten blend mode. According to the article, when the layers being blended are essentially the same, the blend will select the brighter pixels from each layer. Foreground objects don't get summed together and neither does sky glow. But since the stars are brighter than the background sky, the star trail from each exposure should get selected.

The article had some really nice images, with one characteristic feature: the star trails have small but visible gaps representing the time between exposures.

I haven't tried this myself, but the article was a good read. I couldn't find the article on their web site, though, so you'll have to find a print copy.

[PhotosGuy]

Take a look at this, Then try the different layer modes 'till you find the one that gives the best result.
http://photography-on-the.net/forum/showthread.php?t=63368