Mirror lenses

Check out the sci.astro.amateur newsgroup and the various Meade yahoo
forums as a resource on using a SCT (Schmidt-Cassegrain) design scope
for daytime photography.

These scopes have highly curved fields which will make it virtually
impossible to get focus at the center and edges of the frame, although
this will be less evident with a sub-35mm size digital imaging chip.
Also, as you move away from the optical axis, aberrations such as coma
and astigmatism get worse. The f/6.3 designs are worse than the f/10
designs for off-axis performance. Focus is critical, as is using a very
high shutter speed. The baffling is pretty poor on most of these,
allowing stray light in to reduce contrast. You can improve this area
of performance, however.

You will get better results by using an MCT (Maksutov-Cassegrain) design
scope, but they are more expensive. They have flatter fields, fewer
off-axis optical aberrations, and higher contrast. The Questar is the
best known of this design, but the best values in 6-10 inch apertures
come from Russian optical companies.

Evan Miller



Brian Stirling wrote:

On Thu, 15 Jan 2004 21:40:14 -0700, "Roger N. Clark (change username
to rnclark)" wrote:


Brian Stirling wrote:


Roger, I have been toying with the idea of using a Meade 10" F6.3
1600mm telescope ( 10" LX200GPS-SMT) for a very long telephoto lense
(as well as for astrophotography) and have had mostly unfavorable
responses from virtually everyone I've asked about it. I can see some
potential pitfalls of course, manual focus, fixed aperture being the
most obvious, but I also wonder about the minimum focus distance with
this setup. I have two cameras I plan to use with it: Nikon D100 (6MP
DSLR) and Nikon F100 (35mm film) and I'd need to be sure I can adapt
them to the telescope at the prime focus. I would also wish to be
able to mount my cameras, using my 50mm F1.8 lense to the top of the
tube and take advantage of the tracking system to do longer exposures
without blurring due to the earths rotation. I am a little uncertain
about the long exposure ability of this mount however as it does not
look like an equatorial mount and without that I believe you need a
3-axis tracking system to include rotation of the tube along the long
axis in addition to the other two axis.

So, I don't know if you know much about this Meade telescope but if
you do or know where I can find someone who does, I'd be very
thankful.

I'm confused. The LX200GPS-SMT is a 16-inch f/10 Schmidt-Cassegrain:
http://www.meade.com/catalog/lx/16_lx200gps.html
(Nature photographers: you thought you had it hard carrying around
the 600 mm f/4, see the above!)


The Meade LX200GPS-SMT is a LINE of telescopes and they make two that
are 10 inches. One of the is a 2500mm F/10 and the other is a special
order 1600mm F/6.3.

Do you mean a Meade Schmidt Newtonian? E.g.:
http://www.meade.com/catalog/lxd55/lxd55_series.html
The lxd55 SN-10 is a 10 inch aperture 1016mm focal length f/4
system. The tube assembly is around $1100. I've been
thinking of this one for astro. You can always move
the mirror forward to get the focal plane into position.


One of the advantages of the SCT design is that because focusing is
done by moving the main mirror you can get a very large range of
focus. I do not, however, know for sure just what the minimum focus
distance is.

The problem I see for general photographic work is focusing.
Modern autofocus cameras to not have a good microprism viewfinder
(like older 35mm cameras), so accurate focusing is very
difficult. If your Nikons have interchangeable screens, then
you might do OK. But following action will be extremely
difficult. And if you can't focus accurately, you would
probably be better off with a real autofocus lens, even if
it is shorter focal length, like 300mm f/4.

But for static subjects at long distance, a 1016mm f/4 would
be amazing. It would be great for hawks and eagles in distant
trees, especially with a 1.4 or 2x TC.
I've been waiting to see reviews and photos from
users of the Meade Schmidt Newtonians before I buy. I saw one
comment that they were soft. But was it focus error, or what?



Roger



If a 1016mm f/4 would be sweet with a 1.4 or 2x TC how much sweeter
would a 1600mm f/6.3 be without a TC?

As I said, I don't think I would shell out $3K US for a limited use
telephoto lense (telescope) alone but getting a bright (f/6.3)
telescope of 1600mm FL and computer tracking and alignment as well
might just be the icing on the cake. If only I could be sure of the
minimum focus distance...

Later,

Brian

On Sat, 17 Jan 2004 01:37:17 -0500, Brian Stirling
wrote:

On Thu, 15 Jan 2004 21:40:14 -0700, "Roger N. Clark (change username
to rnclark)" wrote:

Brian Stirling wrote:

Roger, I have been toying with the idea of using a Meade 10" F6.3
1600mm telescope ( 10" LX200GPS-SMT) for a very long telephoto lense
(as well as for astrophotography) and have had mostly unfavorable
responses from virtually everyone I've asked about it. I can see some
potential pitfalls of course, manual focus, fixed aperture being the
most obvious, but I also wonder about the minimum focus distance with
this setup. I have two cameras I plan to use with it: Nikon D100 (6MP
DSLR) and Nikon F100 (35mm film) and I'd need to be sure I can adapt
them to the telescope at the prime focus. I would also wish to be
able to mount my cameras, using my 50mm F1.8 lense to the top of the
tube and take advantage of the tracking system to do longer exposures
without blurring due to the earths rotation. I am a little uncertain
about the long exposure ability of this mount however as it does not
look like an equatorial mount and without that I believe you need a
3-axis tracking system to include rotation of the tube along the long
axis in addition to the other two axis.

So, I don't know if you know much about this Meade telescope but if
you do or know where I can find someone who does, I'd be very
thankful.

I've got an older 8-inch Celestron "Big Eye" that appears to have been
designed as a huge terrestrial scope, then promptly discontinued. It
can be mounted to a tripod like any big tele. While it can take some
impressive photos when conditions are right, that doesn't happen very
often. It is probably the reason why no one has jumped in and made
anything similar since then. With bags of lead shot piled on it and
the mirror locked up, sharp photos are difficult at best, since at the
ranges where it's effective there is usually significant amounts of
haze and atmospheric effects.

Years ago I had a Nikkor 500mm f/5, but it seemed to have low contrast
and poor sharpness. The smaller Nikkor 500mm f/8 has usually shown
better results, image-wise, and is quite tiny. The Nikkor 1000mm f/11
is also a good performer, and it's probably my fault that it doesn't
get taken out more often. The f/11 aperture is limiting except on
sunny days, and we get precious few of the here...

KBob wrote in
:

I've got an older 8-inch Celestron "Big Eye" that appears to have been
designed as a huge terrestrial scope, then promptly discontinued

Most of the Celestron, Meade, Questar etc. telescopes tend to be offered in
a "spotting scope" format, without the astronomical mounting and
accessories, for those who either want them for such purposes, or for those
who have their own ideas about how to mount and just want the OTA with as
little extra cost as possible.

--
Colin J Denman
N 51º 54' 38" W 00º 29' 45" Elev: 125m
email: -- use my first name
home: http://www.cjdenman.freeserve.co.uk