Wednesday, February 09, 2011

Northern Star Trails

It's been quite some time since I've last imaged anything astronomical, so to get back into the swing of things I thought I'd tackle something relatively simple .... or so I thought.

Capturing star trails is a great was of seeing the night sky in motion. It requires a fairly simple set up, and can be done with most modern digital camera pretty easily. I set my 20D up in the shelter of my daughters Wendy house, mounted on a Gorilla tripod, pointed it up to the side of Polaris, and left it to it overnight capturing 30 second frames continuously.

In the image below, Alpha Ursa Minor (Polaris) is the tight curl at the top left, Beta Ursa Minor is the next bright reddish trail in the middle of the frame. The next bright red trail 3/4 of the way across the frame is Alpha Ursa Major (Dubhe) with other members of Ursa Major bunching up to the right of the frame. You can also see one remaining airplane tracking across to the top right, though there were 4 other trails on the original data set (removed in the processing) which is quite impressive considering the small area of sky this image covers. If you watch the movie (here
on YouTube - make sure you watch at 360p resolution) you should also see the wonderful double of Zeta Ursa Major (Mizar) slide around the view.

You need a nice wide angle for star trails. This was taken with a 28mm lens (which translates to a 42mm lens on a full frame camera) so is rather on the tight side. I chose this lens over my 10mm wide angle as I wanted to use the Astronomix CLS light pollution filter - the view to the North is directly over a set of street lights - but the 2" filter would not fit on the wide angle lens. Powered by an external lead acid battery and a battery adaptor, I knew I could set it on continuous shooting mode at 30s exposures and not run out of juice half way though the night. To fit all the frames onto my SD card though I needed to shoot medium size JPG's - the images would be downsized anyway so I did not need the higher resolution, but a shame to lose the dynamic range of a RAW image, but that was the price I had to pay for the size SD card I had. The camera was then left to its own devices over night whilst I got some sleep.

It was a cold night and by the morning, although in a sheltered spot, the lens had quite a bit of condensation on it as well as spots of frost. A quick examination of the frames showed that the quality did deteriorate around 2am
(seen as a fading of the stars trail as they rotate clockwise) but at least the stars were still there. My next project will be to build a lens warmer (dew heater) out of an old toaster and a few bits of fabric :)

I took dark, bias, and flat frames as usual, except these were also recorded in jpg format rather than my regular raw format. The unfortunate offshoot of this is the DeepSkyStacker would not pre-process the frames to calibrate them before stacking (DSS will correct colour balance via flats which is very advantageous when using a CLS filter). Without the use of DSS I turned to Iris - a very powerful, but not so user friendly package. It still did a pretty good job in calibration, but would not automatically colour balance the results and took a lot more manual labour for the same results I would have got in DSS with minor intervention. I also used Iris to composite the final trail shot as well as producing doubly binned (so the processed frames were 1/4 original size) frames for an animation. The animation frames were further treated by running them through a photoshop action to apply a maximise filter (posterise the stars a bit so they are not lost in the compression process), resize, and get the correct aspect ratio for YouTube to be happy. Even then, the compression really kills the definition of the stars, but you get the idea. The original quality wmv movie is soooo much better!

I'm fairly pleased with the final result - it nicely shows how Polaris is just off the Earths axis of rotation as well as capturing Ursa Minor and a part of Ursa Major. There was some movement of the tripod early in the exposure, shown up as a kink in the trail most noticeable at Polaris, and the bathroom light caused the lens flare across the frame - red because the CLS filter has removed the yellow colour from the flare. I did learn that something simple like this can prove to be more complex than originally thought. Most of the pain came from the use of the CLS filter and shooting JPG's, but from my location not using the filter is not an option, though I shall invest in a larger CS card and shoot in RAW next time :)


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6:00 am  

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