View Single Post
Old 13-12-2011, 08:35 AM
irwjager's Avatar
irwjager (Ivo)
Registered User

irwjager is offline
Join Date: Apr 2010
Location: Melbourne
Posts: 494
Originally Posted by CDKPhil View Post
It would be the same if you were photographing the sun, with a normal SLR CCD camera and a 50mm lens ( not that it is recommended )
You might use a shutter speed of 1/64000th of a second and get a bright round circle, but use 1/2000th of a second and get your whole frame over saturated.
The only difference between the sun and the stars is the time it takes to expose them.
If you want to do a simple experiment. (not with the Sun ) get your SLR mount it on a tripod set up a LED (point Source light) in a dark room and expose it at different shutter speeds. You will see the exact same effect as what is being discussed.
Unfortunately for astronomers there are huge contrast ratios between the stars and faint fuzzy objects. So Airy disks, bloated stars what ever you want to call them, will be an issue. As CCD technology advances I am sure someone will come up with adaptive pixels, where the user could control the amount of saturation for individual pixels. It would be like the old darkroom technique of dodging and burning but on a far more advanced level.
That'd be great. And there are indeed some techniques we can use in the interim. SkyViking (Rolf Wahl Olson) used one such technique to cancel out much of Beta Pictoris diffraction, revealing the dust disc.
Another one is using Deconvolution (and knowing the exact PSF) but that only works on non-saturated stars. The future is a type of video astronomy where many short exposures are stacked to create a very high bit-depth image with a huge dynamic range (and no overexposed pixels). The only thing holding us back right now is the various types of noise that become worse (such as read noise) when using many frames.
There also the good-old HDR compositing trick we could use today (as used for M42 for example), but alas there is no catch-all formula for smoothly blending the various frames.
Reply With Quote