Wednesday, December 19, 2007

An Unexpected Journey - Comet 17P-Holmes

This was an unexpected journey for both me and the astronomical community. Comet Holmes burst onto the scene in October of this year to the delight of many an astronomer around the world. Telescopes were trained on Perseus and this receding visitor as it spread away from Earth on its long journey around the Solar System. I too wanted to get my telescope trained on this unusual beast, but my newly arrived daughter (giving me much, much more delight than a comet could ever do) meant all astronomical activities were put on hold. More on that later, but first the nitty-gritty technical details :)

This image is the result of 2279 8 second frames - giving a total of 5 hours, 3 minutes, and 52 seconds total integration time - taken between 28th October and 12th December 2007 with my trusty Canon 10D, 50mm f1.8 lens (operating at f2.0), stuck on a regular field tripod. This mosaic shows only 12 of the days between these dates to give a good spread without overlap. One can also see on this object the wonderfully large open cluster Mel20 in the heart of Perseus (the trapezoid above centre just below Holmes) as well as open cluster M34 towards the bottom right of the image.


With my daughter taking up most of my time at home, I quickly realised that the best I could muster was to stick my camera on a tripod in the garden, point it at Persues, and lock the shutter down in continuous shutter mode and hope for the best. The light pollution around my house is incredible, but this actually worked out OK. 8 seconds is as long as one can go with a 50mm lens on a tripod at that declination without trailing. Even at this exposure at f2.0 and ISO400, the background was easily saturating so longer exposures wouldn't have done a lot anyway.

It became an addictive exercise :) I would get in from work, set the camera up, and leave it running whilst I had my dinner and spend time with my daughter. Each night that was clear I was filling up my 1Gb card and rapidly filling up my hard drive!

I used Deep Sky Stacker to produce daily frame stacks (incorporating darks, flats, and bias frames). These were then aligned with respect to a single "master" frame using PixInsight's dynamic alignment tool. Each night the sky quality was very variable - light pollution reflecting of various densities of fog caused problems calibrating the frames correctly. I used the Automatic Background Extraction and Auto Histogram tools in PixInsight to remove the LP gradients and to get a common mid-tone for all the frames. These were then all bought into Photoshop, aligned in layers, and the white and black points around the comet equalised with the master reference frame.

As it turned out, I could not use all the frames on the mosaic as Holmes did not traverse its own distance across the sky within 24 hours, but I was blessed with good skies and so had fairly good coverage.

In total, I took 2279 frames, each of 8 seconds duration. This gives a total "integration" time of 5 hours, 3 minutes, and 52 seconds. Bar far the longest exposure I've ever synthesised! What is really interesting is that I managed to
(just) capture down to magnitude 11.5 stars around Mel20 with just a 50mm lens and 8 second subs ... I would not have believed it possible, even without my light pollution!

All in all a rather enjoyable project :) Now ... what's next on the list ......

2 Comments:

Anonymous 2 back yard amateur star gazers said...

two amateur back yard astronomers in Mount Dora, FL thank you for your great photo work and accompanying description of how you did it! Your daughter has an exciting world ahead with a dad as "far out" as you, here's to looking up! Jim & Andrea

12:56 am  
Anonymous Anonymous said...

This is a great montage! Very good indeed! Rick, Bristol.

6:14 pm  

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