Learning about exposure with NGC6888 Crescent Nebula

Posted by XTSee on 5th October , 2012

Here is another image I took while at the Kelling Heath Star Party 2012. Sorry! It’s a bit disappointing. I chose NGC 6888 The Crescent Nebula as a possible try with my Canon EOS 600d DSLR after reading the September issue of Sky at Night magazine, where one of the observing articles showed this as a likely candidate.

NGC6888 The Crescent Nebula

NGC6888 The Crescent Nebula

If you read my previous post you’ll know that on the same night I had lost M57 behind the trees, and so NGC6888 was next on my list. In the image to the right you can only just make out a faint wisp of the brightest part of the nebula (click the image to open it full size, then hit Back when done). You would have to compare it with one of the many other successful images of The Crescent in Google Images to see how it should look.

So what went wrong?

Obvious really - bugger all exposure time. I set up to take 20 x 30 second exposures at ISO6400. Out of those only 7 were useful for stacking due to trailing caused by wind or periodic errors in the worm gear of my mount. While the 7 images will have helped reduce the amount of noise in the image (I also combined in Darks+Bias in the pre-processing), 30 seconds just wasn’t sufficient to bring out this faint nebula when post-processed in Nebulosity 3 and Photoshop. I think something more like 5 minute exposures is required reduced to ISO800, but for that I will need to use autoguiding - something I don’t have the right equipment for just yet (although I’m close). Also my EOS600D has not been modded to remove the infra-red filter, and so I’m only likely to bring out the brighter parts of the nebula anyway.

So while this image is a bit pants, well apart from the fact its quite a nice starfield, I try to take something positive from it, and hope that in due course I’ll take a much better one which does The Crescent proud, and will put this one to shame!

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