Saturday, August 17, 2013

Perseid Meteor Shower - 2013

On the evening of August 12, I watched the Perseid Meteor shower. Visually, I only saw a handful of meteors. I saw more last week at the Canyon of Eagles dark site.

To catch some meteors on camera, I set up the DSLR on a fixed tripod pointing to the northeast. I exposed a continuous series of 700 frames, each a 15sec exposure at ISO-1600. This was the maximum exposure length I could get on an 18mm lens without the stars trailing. I used Astrophotography Tool to program the capture sequence and to help focus the camera lens on a bright star. The download time between images was about 10sec, so the camera had a 60% chance of catching any given meteor, assuming that the meteor was in the camera's field of view. Just after setting up, I visually saw one large bolide that unfortunately was not in the part of the sky at which the camera was looking ... worse, it was where I had just moved the camera away from in order to get less shots of planes landing at the Georgetown airport.

The next morning, after carefully combing through the images, I found 10 frames with meteor streaks and a lot more with airplanes and satellites. It's simple to tell the difference, as the real meteors never appear on consecutive frames, which would require them to be in the sky for more than 10 seconds.

On the computer, I attempted, and failed, to make a large mosaic stack using all of the images. This was because none of the software I tried were capable of correctly distorting the images to compensate for the projection of the spherical sky onto the camera focal plane. I gave up that approach and, instead followed this workflow:
  • I selected three sets of 16 frames from the beginning, middle, and end of the imaging session. Selecting more frames turned out to be a disaster as the distortion-induced misalignment of stars caused the sigma-clipping stacking algorithm to remove all stars from the periphery of the resulting frame.
  • Since the star-field was slowly drifting up, I cropped the bottom part of each frame to remove the trees.
  • Using Deep-Sky Stacker, I stacked each of the sets using the "standard" alignment mode.  
  • I saved the three stacked images as as 32bit TIFF and then imported each as layers into Photoshop.
  • Using the GradientXTerminator plug-in, I removed the sky-glow gradient from each layer. Trying to piece together a set of frames with mismatched sky-glow would have made it impossible to get seamless transitions.
  • I converted to 16bit mode using the gamma/exposure mode of the HDR tool.
  • I rotated and scaled each frame to best align the stars. Due to the relative distortions of the frames, this could not be done perfectly.
  • I imported a single frame containing the trees and horizon from near the end of the imaging session, aligning with the previous three.
  • I used the Auto-Blend Layers tool to create a smooth mosaic across all four layers.
  • Next, I found each of the individual frames containing meteor trails and imported these as additional layers, aligning each with the overall mosaic.
  • I applied a layer mask to each meteor layer in order to select just the meteor trails.
  • Using adjustment layers, I applied levels, curves, color-balance, and contrast changes to enhance the image.
  • Finally, I added an adjustment layer to re-introduce the sky-glow gradient across the full mosaic which I had removed earlier on each of the individual stacks. 

The final image clearly shows the radiant point in the western portion of the constellation Perseus. I was surprised how precisely the meteors seem to emanate from a single point. This is never quite the impression I get when catching them visually.

Perseid Meteor Shower 2013 - Georgetown, TX - August 12-13

No comments:

Post a Comment

Relevant comments and questions are welcome but submissions with spam-links will not be published.