Craine wrote:neufer wrote:Craine wrote:I take it "Charge Bleed" is what is causing the circles around the stars? Perhaps you have a link where I can read up on that?
Geck is referring to the over exposed Hubble picture.
The small circles around the APOD stars are primary mirror
Airy disks.
The large circles around the stars are secondary mirror
Airy disks.
Hah! I am learning new stuff.
Thanx!
Tho...with modern technology can't it be compensated for? From what I read it is something that can be anticipated, and so it may be possible to let a computer reduce that effect?
The charge bleeds are those bright white lines coming off the stars. Each pixel of the CCD is like a bucket which can only hold so many electrons. Once that bucket is saturated, they start to overflow into adjacent pixels in the row. In this case the stars were overexposed because astronomers were looking for planets rather than at the stars themselves. I guess they were hoping to find a very large planet orbiting quite distant from the star since the star's
point spread function and the charge bleed artifacts cover up closer ones. They didn't find any planets that time, but that was 20 years ago. It was only in that same year that the very
first exoplanet discovery was confirmed. Exoplanet observational strategies were being formed and evolving rapidly during that time and soon after we saw a boom in exoplanet discoveries. These days so many are discovered that they can barely make the news.
To add on to what Chris already answered (while I was writing this post!): Airy disks, diffraction spikes, and all that stuff around the star are caused by the nature of light. We can't get around it any more than we can avoid anything else in nature, like gravity. As Art said, the stars are overexposed; a shorter exposure will avoid the charge bleeds. Another useful tool is a coronograph which blocks much of the incoming light of the star from reaching the detector. Often, the point spread function is subtracted from the image. For Hubble at least, its point spread function is very stable over time so it is possible to do this. Here is an example of combined use of a coronagraph and point spread function subtraction, allowing us to view circumstellar disks around several stars:
http://hubblesite.org/newscenter/archiv ... web_print/
The James Webb Space Telescope will feature some entirely new technology invented specially for it occluding the light of bright sources. They're called
microshutters and they're amazing. Astronomers will be able to use them like coronagraphs to selectively block out points of light they don't want to see, but with much greater precision and with as many sources as can fit on the detector instead of just one.
Oh, if you are confused about what a point spread function is, just think of it as everything you see when looking at a star. The star itself is a point smaller than a single pixel but its light spreads out from there because of light's wavelike behavior and how it interacts with a telescope's parts or even your eyes. When you see glare from a bright light, some of that is your own eye's unique point spread function.
At the Limit of Diffraction
I've only ever seen one of those once, and also had the chance to look through the telescope -- in the store. 
