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Still a bit confused...


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Nope - long exposure means the shutter is held open for several mins while photons gather on the chip - typically for very faint dso's. In short exposure the shutter is opened and closed very quickly - usually for brighter objects like planets. If you held the shutter open for several mins on a planet all you'd get is a big white blur.

Hope that helps :)

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Is it possible to use lots of short exposures to create a long exposure or am I misunderstanding?

It just depends upon what you are photographing. You can take numerous short exposures, digitally combine them into the equivalent of one single long exposure equal to the total time of the short exposures using a process called stacking. Ten 10 second exposures stacked are the same as one 100 second exposure.

Several computer programs are available to do this... one of the better being freeware ... Deep Sky Stacker. There is a limit to the lower duration of the short exposures as too short may be insufficiently exposed for the computer program to register the shot. These computer programs can also remove noise, adjust for the earth rotation, and other attributes.

This process, multiple short exposures stacked into one long exposure, allows owners of azimuth mounts to photograph deep space objects as exposure times with an azimuth mount are around 30 seconds before field rotation becomes a factor. It even allows people with inexpensive mounts and tripods to capture deep space objects as they can discard the numerous exposures that are not up to par and only use the exposures that are good.

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Short exposures are used for planets - you taken them with a webcam and get as many as you can. These are usually captures using something like a webcam.

These are then stacked in piece of software like Registax.

For Long exposure imaging - for galaxies, nebula etc... you take an exposure of 30s or as long as you can. These frames are called sub frames (subs), you then take as many as you can and stack them using a bit of software like DSS or Maxim.

The stacking process increases the signal to noise ratio and enables you bring out fainter details that would have been lost in the noise without the stacking.

Obviously with DSO imaging the longer the sub the deeper/fainter you'll get and there is no substitute for longer exposures - but you can get some cracking results at the shorter end of the scale.

I won POW a few years back with an image of M31 taken with lots of 30 second subs...

Hope that helps.

Cheers

Ant

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I think everyone has covered the qestion, but there is one advantage that hasn't been mentioned.

Post imaging selection, which is another advantage of taking many images. For the planets with hundreds of very short images or DSOs with dozens of 10+ minute images what you have is the opportunity to pick the best or dipose of the worst images. This means if something goes wrong you don't lose all your imaging.

So for the planets if you stack your 1% best images you can get something very very sharp.

For DSOs you can do something called sigma clipping, which means instead of just adding each image together you look at the images statistically. Before you add each pixel up you strip off any value with a vastly different brightness to the same position in the other images. Result: satellite/aeroplane streaks don't appear in the final image.

You also get the advantage of taking a much longer exposure (more data) without saturating any part of the image.

hope this helps

Derek

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