Just One Hour

The August, 2024 issue of Sky & Telescope has an article by me about shooting star trail images. It’s too bad I didn’t have this shot before because it’s my favorite star trails image to date. I setup my camera behind an equatorial mount (I was shooting M106 at the time), and set my canon EOS Ra to take 30 second exposures repeatedly. Then, as I describe in my afore mentioned article, I loaded all the images as layers in Photoshop and set the blend mode to “Lighten”. I did paint out a few airplane trails in the individual layers, but otherwise the image is accurate. There are even a few short meteor bursts if you look carefully for them.

Star trails over a telescope
One hours worth of star trails show just how much the stars appear to move in such a short amount of time.

This image shows why we use an equatorial mount for long exposure astrophotography. As the Earth spins, the stars move considerably, and in this case you can see exactly how much in only an hours time. An equatorial mount works by aligning it’s axis of rotation with the Earth’s, and then rotating in the opposite direction of the Earth’s motion. It’s like being on a merry-go-round, and you have to turn your head to keep looking at someone or something off in the distance. Without an equatorial mount, objects zip by in the eyepiece or camera pretty quickly. There are also alt-az mounts that track objects in the sky without having to do much alignment work, however, because of the way they move, objects in the camera will rotate in place as you track them across the sky. This makes long exposures a bit more challenging 😉

For context, I’ve added just a single 30 second frame as well. As a stand alone image, it’s not too bad either IMHO.


A telescope against a starry sky
Here’s just one of the 120 frames used to create the star trails image.

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