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Sunday, 29 November 2009

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Maybe you already know this, but the best sharpness enhancing technique in astro photography is "stacking" multiple exposures together.

Free stacking software like Registax is available to produce a final picture that has greatly reduced noise and improved sharpness. It works considerably better than trying to sharpen single images using the usual photographic software.

Because you shot a burst of frames of the sun, you should already have a number of pictures that would work. Just edit out the obvious dogs, and feed the rest to Registax.

I assume stacking techniques would also work with any subject in which there is no relative motion (i.e. landscapes on a still day).

Oh, now I have to get one of these things. Its your fault!

Have you tried stacking images in Registax or something similar?

Dear W & B,

Yes indeed, stacking is a promising technique for photographs such as these, but I haven't had the time to play with it yet.

Not only would it be valuable for improving sharpness, but I am fighting noise. As I said, the light one gets thru a scope such as this is not very bright, and I don't dare use very long exposures because of the atmospheric turbulence problem. So, even when I get a very sharp frame, low contrast detail (of which there is plenty on the surface of sun and in prominences) gets lost in the noise.

I will be giving it a try at some point. Probably will wait for a nice combination of good seeing and attractive solar features to inspire me.


~ pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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-- Digital Restorations http://photo-repair.com 
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@William, I'm not an astronomer, but the surface features of the sun move. Can't stack images of subjects that are moving relative to one another and changing shape.

Dear Mark,

Yes, indeed! These nicely illustrate variations in seeing. The top transit is not terribly sharp at all-- probably a limiting resolution in the same range as my dinky little scope (3 arc-sec) can achieve.

The middle transit nicely shows the pernicious nature of turbulence. The underlying granularity on the surface of the sun is pretty uniform, in reality. The smeary bands overlaying the enlarged photo illustrate how the seeing can very from point to point even in a single frame.

The bottom transit is wonderfully sharp right where the shuttle is-- definitely sub-arc-sec resolution. But if you pull up the full-size JPEG, you can see how lucky Thierry was; in that frame, the image of the shuttle fell on a tack-sharp zone right between two big smeary bands of turbulence.

Luck favors the prepared... but solar astronomy involves LOTS of luck.

pax / Ctein

Janne...

See this page for some stacked images of solar prominences...

http://www.astronomie.be/registax/ericspage.html

The sun's features change slowly enough for the technique to be useful.

"Focus the telescope so the image looks sharp to you"

If the camera lens is set to infinity but your vision is nearsighted, like mine, wouldn't the eyepiece be focused too far in for the camera lens to focus correctly?

Dear John,

I'm very nearsighted, and I do the 'scope focusing without my glasses. Works fine.

pax / Ctein

This kind of photography involves either an aesthetic subtlety or scientific peculiarity that I don't comprehend; or perhaps "grok" would be the better word. Since resolution seems to be key, it would appear that the overall graphic-ness of an image isn't important (I rather like the top image in fig. 2, as a graphic, that might go on (perhaps) a bottle of Japanese mineral water.)

And since the photographed features are much better done elsewhere, no particular scientific curiosity is being satisified. I conclude, then, that one gets from this the same engineering/quasi-scientific pleasure derived from shooting off amateur rockets to see if you can recover an intact egg from the nose cone...

Understand, I don't object to any of this, as it seems far less harmful than, say, an interest in methamphetamine or under-wired bras, but, I just don't understand. On the other hand, I have a good friend who breeds hostas, perhaps the most inane plant on earth. I don't understand that, either, yet he remains a good friend.

JC

Perhaps something like http://research.microsoft.com/en-us/um/people/jiansun/papers/Deblurring_SIGGRAPH07.pdf>this would be an alternative to stacking?

(Admittedly the blur kernel due to turbulence is very different from that due to camera motion.)

Dear John,

Yeah, think of it like model rocketry. Just something that some people find cool to do for onesself. It does not actually accomplish anything of note. Rather like making tourist snapshots of famous scenes while on vacation. People know they can buy better ones. Some people do, even.

I thought it was pretty cool how easy it turned out to be to make photos through this telescope. Then, of course, I got wrapped up in the geekery of wondering just how good I could make those photos come out. Saner people would stop at Step 1.

pax / Ctein

EP,

No, this isn't going to work.

The reason is that the method used in the SIGGRAPH paper requires a very sharp, albeit noisy from underexposure, image to provide the reference data for sharpening up the properly exposed photo. No such reference photo would exist for solar photos.

The problem is that there are two aspects to atmospheric blurring. One is, indeed, dynamic-- constantly moving air that makes the image shimmer and shake. Exposures longer than fraction of a second will get blurred by this kind of 'motion blur.'

Once you get down into the 1/60th-1/250th of a second range, you're freezing out that motion. What you get is kind of like photographing through a sheet of rippled glass. The photo is nice and sharp... but the image of what's behind the glass isn't.

pax / Ctein

Pictures of the sun scare me..so do kerosene heaters.

Cool

@Ctein: Ah, I see, thanks for the clarification. I had assumed that the blur was entirely dynamic and hadn't considered that turbulence would give a distorted/blurred picture even at very short exposures.

I purchased a 40mm Coronado PST in August 2008. Since then I have captured hundreds of photo's of prominences, flares and occasional sunspot groups.

I do not use a fancy digital camera, just a humble SPC900NC webcam which I purchased for £35 from my local Maplin store.

Video capture is easy using HandyAvi (shareware $30). A short 200 frame movie is made to capture the prominences and then another exposed for the disk detail. The individual frames of both movies are then stacked using Registax5 (freeware) and the two separate images are composited with Photoshop Elements 4.

Below are links to my PST albums on Facebook and Flickr -

Facebook:
http://www.facebook.com/album.php?aid=2015736&id=1242938734&l=a88243b836

Flickr:
http://www.flickr.com/photos/astronomer_dave

Looking forward to following your PST imaging and updates on here.

Kindest regards,

Dave

'@Astronomer_Dave' on twitter

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