Star Trails

Seeing as it's the 700th episode of "Sky At Night" tonight, I thought I'd take advantage of the clear sky to take an appropriate blip :-).

This was made up from a lot of 1-minute exposures, triggered by a cheapo intervalometer (automatic remote shutter release with timer function). The photos were then merged together with the startrails program.

The gaps in the star trails are because I had to omit a couple of images that were lit up by a security light triggered by one of our cats! Next time I'll remember to switch that off first!

There are at least 2 airplane trails in it too.

To explain how it was done and why this way: You would think that a single very long exposure would get this effect with the stars drawing trails because of the rotation of the Earth, however the first problem with that is that the foreground is likely to be bleached out if, as in this case, there's street lighting illuminating it. Second problem is that if something goes wrong the entire image would be lost. So the better alternative is to take lots of shots and merge them together. The software I used to do the merging is clever enough to combine shots without making the foreground brighter, whilst still adding together all the individual dots or short lines of stars into very long lines. If the shots encompass a couple of hours or so then you get nice long lines.

My camera doesn't have the option of automatically taking one shot after another, so I bought a remote shutter release that does do that. With the camera set to BULB mode, the remote is set to take a 1-minute exposure, then wait a second (for the camera to save the image) then repeat forever. Or until the camera batteries run flat! I used a battery holder with 2 batteries to get a longer run time.