"Chris L Peterson" wrote in message
...
On Sat, 07 Jan 2006 16:16:24 GMT, "Frank Johnson"
wrote:
While it's true that webcams are doing wonders for certain types of
seeing,
in many areas of the US during the Wintertime, the jetstream is
constantly
overhead and even a webcam cannot undo this blurring. I keep reading
about
these new adaptive optics scopes, like the new pro scope used for solar
imaging, which also incorporate speckle imaging and reconstruction- why
nothing for amateurs? Couldn't blurring of frames (caused by high
jetstreams) be deblurred or "reconstructed" so blurring is minimized.
With
computers and the power they possess these days, I'm surprised there
aren't
programs that can do this or maybe jetstream effects still can't be truly
nullified.
It simply isn't possible to do much along these lines with amateur
systems. Adaptive optics requires a reference object very close to the
target, and in almost all cases that object (if one exists at all) is
going to be dim. For this reference to be useful, you need to collect
enough photons to get out of the noise. Since detectors are already
nearly 100% efficient, the only way to get enough light is with a large
aperture. Adaptive optics starts becoming practical as apertures get in
the range of a meter, and larger is better. Obviously, this is well
outside what most would consider amateur optics.
The lack of available reference stars is another problem for amateurs.
Professional observatories can address this by making their own, using
lasers to excite atoms high in the atmosphere. But aside from cost,
shining such lasers at the sky requires special permits that pretty much
preclude their use by amateurs.
Finally, adaptive optics wouldn't provide much benefit to most amateurs
because it can only correct the field over a few arcseconds- much
smaller than typical observing or imaging fields. It is more a
scientific tool for examining very small zones at high resolution than
something useful for improving the aesthetics of an image or view.
Adaptive optics would really only be useful for most amateurs for
observing (or imaging) very small (or small areas) and very bright
objects- the Sun, Moon, and brighter planets. Right now, adaptive optics
are expensive enough to produce that there probably isn't enough market
for so limited a device to justify development. That might change in the
future, but don't expect to see anything useful for deep sky observing-
ever.
What may come around, assuming that low readout noise cameras become
generally available, is software that selects high quality short frames
and stacks them automatically. This is done already with planets, of
course, but doesn't work with dim objects. Unfortunately, as the size of
the field increases, the number of useable high resolution frames
decreases. It might take several hours to collect a few minutes worth of
data.
Deconvolution techniques can help to reconstruct distorted images, but
there is no certain way to determine just where any given photon came
from. Deconvolution is a sort of educated guess, but it frequently
produces invalid data- and there is no getting around that problem.
Hi, yes, I already pretty much understand the current techniques involving
adaptive optics. I myself have experimented on Jupiter by using one of its
moons as a PSF. Then I tried to apply max ent to Jupiter only to end up
with a very noisy and artifact prone result. As you said, there simply
might have not been enough signal to work with.
There *is* a phenomenon I've noticed with my setup on nights when seeing is
plagued with an overhead jetream: because my tracking isn't perfect, I make
small drive corrections during capture to keep a chosen planet centered in
the field. Sometimes when I've adjusted the tracking, there will be moments
while the drive corrector is engaged where the planet I'm imaging suddenly
becomes clear. When I let go of the corrector, the blurs caused by the
overhead jet return but for a few moments there were clear frames. One
might suspect a sort of vibration as the reason, but I have noticed this
with several different mount/ scope combinations and I think it has
something to do with maybe "catching up with" or "matching" jetstream winds
for a moment (a second) while the planet is being moved back to the center
of field.
I still search for a CCD camera with very high sensitivity, rapid frame
captures and very low noise. The webcam is ok, but falls short in the noise
and sensitivity department. Commercial CCD cams, although much less noisy,
can't capture nearly as many frames per time as a webcam and are much more
expensive. I recently read here about meteor CCD cameras possibly offering
the best signal to noise ratios and sensitivity, but it's tough finding
anyone currently using those.
Frank
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com