Hint of the Horsehead

A tale of woe, perseverance, surprise and hope. With a cold front bring low humidity to reduce Austin's sky glow, I was hoping for some relatively dark skies in the time between when bright evening lights are out and the last quarter Moon rising. I wanted to shake down my relatively heavy 200 mm lens with my Vixen Polarie star tracker to see how well it tracked with the extra weight. After a good alignment on the North Star with Vixens easy to use sighting scope, I set the switch to star tracking speed and was disappointed to see that my batteries were dead. After plugging in my external USB power pack, the switch indicator lit up and I thought I was good to go.

I quickly found noticeable star trails on my images. After spending a lot of time debugging the issue, I found that it was cured by turning the Polarie tracker OFF and ON. It seems that applying the power with the switch already ON was a bad idea and the tracker had been in a funky state. Unfortunately this ate up my time before moonrise. I decided to proceed anyway.

The sky glow from the moonlight kept my exposures to 30 seconds. I shot just over 20 minutes of exposure of Orion's belt and 4 minutes of darks at ISO 1250 with my Sony a6300. The f3.5 Vivitar 200 mm lens was stopped down to f4.5. There was some chromatic aberration at the sides with this lens, but tracking seemed rock solid with 30 second light frames.

The Horsehead Nebula is a famous dark nebula just below the left star in Orion's belt, beautiful and difficult to image. Also known as Barnard 33, it was discovered by Williamina Flemming in 1888. The silhouette of the dark horse head is framed by a dim deep read Hydrogen alpha emission nebula. Production cameras have infrared filters in them for terrestrial images that filter out most Hα light. Images of the Horsehead Nebula require long exposures and cameras specially modified to remove the IR filter. At first was no hint of the Horsehead Nebula in my light frames. After stacking and stretching the images, I began to see a hint in the full size image.

The image below is a 1:1 pixel rotated crop of the Horsehead with some additional enhancement:

I'm pleased to have a recognizable image with only 20 minutes of exposure, under an urban light dome, with a bright last quarter Moon in the sky and a stock camera. I plan to revisit the Horsehead Nebula under the right dark sky conditions and a several hours of exposure some time in the future.

Content created: 2016-11-20




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