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by garik

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I've created a few photos of the Moon using a Maksutov-Cassegrain telescope and a DSLR. I've tried different ISO settings, shutter speed, bracketing to find the best settings.

Here I show a (reduced size) example photo (ISO 200, 1/250 sec, digital processing: +2.5 EV)

Moon

Here is a smaller part of the Moon: alt text

What is the best way to digitally postprocess the images? I've read several articles mentioning noise reduction and image stacking but without any details. For instance there are several noise reduction techniques, which one is the best here? How many photos should be taken for image stacking? Is it the best to take the photos with the same settings...

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2 Answers 2

up vote 15 down vote accepted

I would read this article for information on stacking and how to properly stack photos: http://www.naturescapes.net/docs/index.php/category-technical/145-long-exposure-astrophotography. The relevant information is farther down in the article.

Looking at your shot, it appears that there is a fair amount blur, I'm guessing due to incorrect auto-tracking? Before doing any stacking, since this is the moon, and exposing the moon is a LOT easier than exposing stars...I would make sure your auto-tracking is correct. Its difficult to do, but once you have your mount set properly, you should be able to get MUCH sharper photos with a lower ISO setting, an not need to do any stacking.

If you are interested in photographing the sky or stars, then stacking is a much more useful tool, as getting a good, clear, bright exposure with low ISO of the sky requires either one very long exposure, or several long exposures stacked. Stacking definitely helps manage noise.

For more on Astrophotography, see this post: How do I get started in Astrophotography?

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Thanks for the answer, I'll check the links. I do not have auto-tracking. I know that it would improve the pictures but currently I have no money for it. –  asalamon74 Aug 26 '10 at 12:40
    
If you don't have tracking, then I would use a higher ISO. The noise is manageable in post-processing, either through stacking, or just basic noise reduction tools. Blurring, however, is something you can't eliminate. Most of my astrophotography is at ISO800 or ISO1600, and if I had a better camera like the 5DMkII, I would use ISO3200. Lightroom 3 is pretty good at removing chroma noise, and does an excellent job at removing luminance noise. –  jrista Aug 26 '10 at 15:53

The Moon has a lot of great contrast, unfortunately this makes a single shutter speed and a single exposure non-optimal. You will want to take as many frames as you can...start with around 24, taking a couple of hundred is not unheard of...or crazy talk...more is better. For your shutter speed take some test images at various speeds until you have three speeds: under exposed, just about right and over exposed, think of it as an HDR subject in gray scale. Your going to mix these in post with the under exposed images will provide detail of bright ejecta and crater rims and the over exposed will provide detail for dark or shadowed areas. Add shutter speeds if you have black shadows or blown out ejecta. You might also consider taking some dark frames every 30 minutes or so...lens cap on, shutter speed at 2x the slowest you are using and click...subtracting these from your images will cancel any hot pixel defects (although at fast shutter speeds they may not be necessary).

Now the fun begins... pre-process, stack, post-process...Don't be surprised if you end up doing this more than once. It takes me around 18 hours to process a 24 image galaxy stack...nebulae can take significantly longer.

A bit of blur is hard to avoid, mostly it comes from shooting through the atmosphere and some nights are better than others. This is another advantage of having multiple exposures; you can toss out the worst offenders...like the one where you bumped the scope or caught a contrail. The cool part is that with the right software package and a process called deconvolution you can correct most atmospheric blurring and get a nice sharp image.

Processing astro images is a lot of work...but the finished product can be one of the most spectacular images you will ever see...worth the effort in spades.

Things your going to need/want for a start...

  • The Handbook of Astronomical Image Processing. The Bible. Comes with a really great software package for processing images. Noise reduction, stacking, deconvolution, wavelet filters, color mapping, kitchen sink, etc. Excellent.

  • RegiStax Free image stacking software, very powerful, one of the very best for planetary images. User interface is a bit odd increasing the learning curve...but hey... did I mention it was free ?

  • PixInsight The Photoshop of astrophotography. Amazing feature set with excellent implementations of the high level functions: wavelet processing, star masks, deconvolution, etc. It is in a league of its own. The package price is very reasonable and the do have a trial.

The images you posted look like a great start...just be careful. If you don't want to get hooked on this hobby (read: obsession). I would advise you: Stop now.

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