On Mon, 23 Feb 2004 20:43:11 GMT, Chris L Peterson
wrote:

On 23 Feb 2004 11:09:37 -0800, (Lurking Luser) wrote:

OK, here is another one of my harebrained ideas that is probably
flawed in some obvious way that you fine people will point out to me.
But here goes.

It seems to me, with the advent of digital cameras and computer
graphics, you could build a single objective lens refractor and fix
chromatic aberration with PhotoShop. You could take three pictures.
The first focused in the blue part of the spectrum, the second in the
mid frequencies, and the last in the red zone.

One problem is that color sensors have crappy filters in front of them. A result
of this is that lots of red makes it through the blue and green, etc (or similar
for CMY). This is one reason that color sensors don't do a very good job with
most astronomical targets. For terrestrial imaging, there is lots of image
processing going on to reconstruct something like accurate color. But anything
that makes it through the wrong filter in your case is just going to contribute
to blurring, which isn't really fixable.

People using conventional three filter imaging with B&W sensors or film get very
good results without needing an apochromat by doing just what you suggest- but
their filters are much higher quality. Normally, you use an achromat since the
more complex design minimizes other aberrations as well. A single element
objective that is well enough corrected for good imaging would need to be
aspheric, and would probably end up costing more and performing worse than a
basic achromat.

_______________________________________________ __

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Wouldn't a single element lens need to be refocused for each RGB image
and thus be of a difference image scale eg blue image 'smaller' than
red image? CYM wouldn't work as each filter transmits two primary
colours - each coming to a different focus.