Comet Lovejoy
Taken by Roger Clark on January 15, 2015 @ Mauna Kea, Hawaii
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Comet Lovejoy C/2014 Q2 shows an intricate ion tail On January 12, 2015. Image from Mauna Kea, Hawaii at the 9,200 foot level. This image was obtained with a Canon 7D Mark II 20-megapixel digital camera and 300 mm f/2.8 L IS II lens at f/2.8 and ISO 1600. No dark frame subtraction, no flat fields. Tracking with an Astrotrac and no guiding. The 10.5 minutes total exposure (twenty one 30-second exposures). During the approximately 16 minutes to make the exposures, the comet moved significantly. The 21 exposures were aligned in two ways: align on the comet head so the combined image had stars trailing, then align on the stars with the comet trailing. For the align on the comet image, the stars were deleted, and for the align on the stars image, the comet was deleted, then the two images combined. To delete stars and not destroy detail in the comet's tail, I found that methods on the internet were poor, so I deleted the stars using the healing brush in photoshop in a long and tedious process. Removing the comet was easier: use the comet only image, blur it and subtract it from the image aligned on the stars. Modern DSLRs like the 7D Mark II include on sensor dark current suppression and low fixed pattern noise at ISOs around 1600 and higher, making no need for dark frame subtraction. Modern raw converters correct for light fall-off and also correct for hot/dead/stuck pixels. This makes processing low light images easy: simply align and average.
Photographer's website:
http://www.clarkvision.com/
Comments
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Roger

Absolutely stunning image of Comet Lovejoy!, and your shared info is so much appreciated. You have taught us all something today, as useful information about post processing that is so generous of you to share it with all.

Can you indicate any advice for say the use of a Sony NEX5 Mirrorless APSC sensor camera? I have tracked it considerably well in the piggy-back fashion secured atop a Celestron Nexstar 5i Cassegrain for up to several minutes time exposure and with old rare glass Mamiya 55mm and Pentax 200mm lenses, but still researching the best post-processing for a limited newer cross-over grade digital camera such as mine. Thank you for any further info. >
http://www.dpreview.com/galleries/1579463287

Best regards,

Mark Seibold, Artist-Astronomer, Portland Oregon
Posted by markseibold 2015-01-19 16:44:41
Hi Mark,
The Sony sensors have a good reputation, but at least on some sony cameras, sony does lossy low bit encoding of raw files. Youll just need to try yours and see how well it works for astro. I do have a series of articles on post processing astrophotos in case that helps:
http://www.clarkvision.com/articles/night.photography.image.processing/
and the next article in the sequence.

Roger
Posted by RogerNClark 2015-01-19 23:44:57
 
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