Past, Present and Future of the SCT

There is one case when baffling is very important in
planetary observation. During twilight, dawn and daytime
observations (some of the best seeing is during these
transitional periods)

Very true. In that case the daylight sky would interfere if the baffling was
completely missing. However, some amateurs spend a great deal of time trying to
reduce the internal reflections of an SCT baffle tube, and then claim that it
has enhanced their planetary contrast. In most cases I would suspect a placebo
effect.

Roland Christen

hello

"Chris1011" a écrit dans le message news:
...
For planetary imaging, light grasp and focal length are of prime concern.
This
allows shorter exposures to freeze the seeing without the image being lost
in
the base level detector noise. If you go the AP web site gallery section,
click
on the last Mars image, you will see what a 10" can do under less than
ideal
conditions at a latitude similar to Montreal:
http://www.astro-physics.com/

These kind of images are now commonplace where the person combines
thousands of
individual frames to build up contrast. No great trick. All it needs is
focal
length and light grast - the bigger the scope, the better. It does not
work
quite the same way visually. You would not guess the dramatic difference
until
you actually used scopes side by side.

I do disagree that there is a huge difference between
visual observation and imaging, for several reasons :

- light grasp is indeed important for imaging in order to fight against
turbulence, but it is important in visual observation too!
To cleary see details on the planets (specially dimmer planets like
Saturn), you have to enlarge sufficiently the image, to the detriment
of brightness, and in low light it is more difficult for the eye to see
low contrast and small details. Anything being equal, the Crepe ring
is easier with more light!

- the idea that image processing can compensate for degradations
is almost completely a myth. Of course contrast can be manipulated
by processing, but take two sets of raw images, a good one and a
poor one: the modification of contrast that could be applied on the
poor one could also be applied on the good one, and the second one
will stay far ahead! There is something very important that
anyone who seriously practices image processing understands:
in the real world, image manipulations present strong limitations.
Processing cannot be pushed indefinitely because it generates
artifacts and leads to unnatural contrasts and shapes (we often
see these effects in overprocessed amateur images). Above all,
there is noise, and a detail that is lost in the noise cannot
be extracted just by image processing. Again, the better
raw image will show more details because they emerge
from noise, and there is no way for image processing
to make a poor image looking as detailed and natural.
Image combination helps to increase the signal to noise ratio
(not to increase the contrast) and to extract details from the noise
in individual images, but again, combine the same number of
good and poor images and you'll see which one gives the
best result after processing! Just look at the planetary images
of the HST before and after installation of its corrective optics:
even with huge processing (that cleary show noise) made with
all the knowledge of NASA engineers and computers, the early
images cannot compete at all with the new ones that are
less processed but much more detailed and free of noise,
thanks to better optical quality.

- moreover, if image processing could really compensate for loss
of contrast (due for example to obstruction or optical defects), it
could also compensate for misalignment that has quite similar
effects on contrast and image quality, and experience shows
that is it absolutely not the case and that (unfortunately) even a
hint of misalignment has visible effects on the image that cannot
be cancelled by processing. Processing just helps to take the
good things from images...if there are good things inside it. Just
like photo laboratory, it cannot make good a poor image.

So is it as simple (simplistic? ;-) ) as "The bigger the scope the
better"? Certainly not!!!

regards

has enhanced their planetary contrast. In most cases I would suspect a
placebo
effect.

Hi Roland:

I dunno. Lots of people swear by flocking, etc. I've never thought about doing
it, though, over 30 years. ;-)

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

I dunno. Lots of people swear by flocking, etc. I've never thought about
doing
it, though, over 30 years. ;-)

I do. Flocking doesn't change the sharpness of the optics, does not eliminate
any optical aberrations, does not smooth out rough surface finish, does not
reduce the central obstruction, etc. These are the primary things that affect
planetary contrast. Flocking does affect the ability to see faint deep sky
objects in light polluted skies, but again does not help any in really dark
skies. Planets are so bright with respect to the background sky, that it makes
virtually no difference if baffles are there or not. The light from the planet
is blocked from coming directly into the eyepiece by the secondary mirror, and
any light that does come in on the side from the background sky does not have
enough brightness to register with respect to the planet itself.

Roland Christen

naw, not worth it.. sct collimation takes at most 5 minutes, and you spend
3 minutes looking for the screwdriver.

On 20 Jul 2003 13:08:24 GMT, ospam (Jmpngtiger) wrote:

Micro-servo motors built into the secondary holder with a push button remote to
make collimation easier. One wouldn't have to reach around the tube while
looking through the eyepiece, no risk of dropping the allen wrench, and no risk
of accidently touching the corrector plate.

Herm
Astropics http://home.att.net/~hermperez