Milky Way – 10 Minute exposure – Lessons learnt

I went out to a site about 35km east of Adelaide last night, trying to get a few photos of a dark night sky. I was quite dissappointed by the conditions. Although there were no visible clouds, the seeing was poor. There must have been some thin high altitude clouds and conditions at ground level was quite gusty at times.  I managed to get a few images and tried a “long” exposure of 10 minutes for the first time. The camera was on the iOptron SkyTracker on my normal camera tripod and I aligned the setup to the south celestial pole with a green laser. To legally use one of these lasers in South Australia you need a permit from your local Astronomy Association. This image is a single exposure of 603 seconds with a Tokina 11-16mm lens at f/3.5 at 15mm using ISO-200.

5535 Milky Way 10min 1200

I took a risk with the long exposure as the misalignment of the camera on the tracker would have showed up as star trails which would not have been attractive in this case. The Canon custom function that performs long exposure noise reduction was used to subtract a dark frame from the light frame in the camera. This meant that the camera took 20 minutes before I could see if it worked.

I used f/3.5 rather than the lens’ capability of f/2.8 to reduce chromatic aberrations which tend to smear the stars in the corners into lines. Taking photos of stars (pinpoint light sources) across a wide field is the most difficult test possible for a camera lens. Stopping down the lens by one or two stops will make the image much sharper with less distortion. The image was cropped and rotated a bit to create a banner type image with the milky way centered on it.  (It also got rid of the worst distorted stars in the corners)

ISO-200 is not normally used for deep sky astrophotography but I wanted to get a low noise image and I had enough light with the milky way directly overhead. It worked quite well and there is minimal noise in the image. I did a bit of noise cancellation in Lightroom 4 as well.

Processed with PhotoShop CS2 and Lightroom 4.4. A full resolution image is available at http://www.astrobin.com/full/54564/?mod=none

I still have lots to learn about image processing in Photoshop. It is going to take a few more years before I will feel that I actually know what I am doing ….

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