Hi Joel,
Wow! A titan! Thanks, I've never been called a titan of anything.
Probably for good reason too :-)
Anyhow, I'm many days late getting through the digests and spotted your
note below. I may be much too late already but if you'd still like some
advice just say so. However, the abbreviated form is:
Some have already advised about getting binoculars. I would second that
and pay attention to the binocs either having a tripod mount or being
able to be equipped with same. Being able to mount the binoculars on
your tripod will be a big boost to good observing. There are small
tripod adapters sold which usually screw onto the centerpost of the
binocs. I have one which I found by searching the web. Cost about
$10.00
The binoc advice mentioned getting 7x50 but didn't say why. A 7x50 is
referred to as a "night glass". In telescopes and binoculars there is a
physical beam of light (the exit pupil) that emerges from the eyepiece
whose diameter is determined by the aperture (50mm in this case) divided
by the magnification (7x). So, 7x50 binocs have an exit pupil that is
7.14mm in diameter.
This is not by accident. The reason is that the diameter of the pupil
of a fully dark adapted human eye is approximately 7mm. So, the exit
pupil of 7x50 binocs is sized so that all of the light beam collected by
the 50mm objective lens will (just barely) fit into your eyeball. If we
increased the magnification as in the 10x50 binocular then the exit
pupil will be 5mm in diameter and will more easily fit into your eye.
However, the actual field of view of the binocular (or telescope) is
determined by the >apparent< field of view of the eyepiece (typically
40-50 degrees) divided by the magnification. So, increasing the
magnification shrinks the diameter of the exit pupil but it also shrinks
the field of view. Increasing the magnification also decreases the
apparent brightness of objects by spreading the available light over a
larger area (except for individual stars which are a separate case
altogether).
Telescopes (or binoculars) which produce this 7mm exit pupil are
referred to as "RFT's" or Richest Field Telescopes. That is, for a
given size of objective lens, they produce the largest possible field of
view with the brightest possible image. Also implied here is that
telescopes have a minimum practical magnification. If the magnification
is less than the RFT magnification then the exit pupil is larger than
7mm and cannot all fit into your eye. In other words, the full light
gathering capacity of the telescope is being wasted.
This can also happen with a 7x50 binocular if you do not have a dark
observing site where your eye can be fully dark adapted and your pupil
fully dilated. If you don't have dark conditions you might as well
choose a higher magnfication or a smaller binocular. 8x40 or 7x35 or
10x50 all produce a 5mm exit pupil. These also make a good birding
binocular since they allow for your partially dilated pupil in early
morning or early evening light when birds are most active. If all you
wanted was a full sun binocular then a 7x21 will do fine when your pupil
is closed down to 3mm. In full sun, anything larger than a 7x21 is just
excess weight because the full light beam of the larger binocular is
simply unable to get into your eye. Kind of like having a Zuiko 180mm
f/2.8 that you can only use at f/16.
On to telescopes. The department store Meade 4-1/2 reflectors that you
can buy for about $225 aren't too bad as far as the scope and the mount
are concerned. The eyepieces, however, are pure junk. Typically they
are cheap Ramsden or Huygens designs in 0.965" (std. microscope)
diameter tubes. They also have very short (unusable) focal lengths
designed to achieve high (unusable) magnifications. What you need are
Kellner, Plossl or orthoscopic eyepieces in 1-1/4" (std. telescope)
diameter tubes. Fortunately, the Meade eyepiece adapter can be changed
over by changing the threaded adapter on the focusing tube. Meade will
probably give you the correct part for free if you call them. Decent
eyepieces can be had for about $30 from many places (including Adorama
in their "ProOptic" line)
The Meade 4-1/2" reflector has a 114mm diameter and 910mm focal length.
114mm divided by 7 power yields a minimum (RFT) magnification of 16.3x.
Then, 910mm divided by 16.3x says that the longest usable focal length
eyepiece on this telescope is about 56mm. Such a long focal length is
about as common as hen's teeth so you'll have to settle for something
more common, typically 32 or 40mm. 32mm would yield (910/32) 28.4x and
40mm would yield (910/40) 22.8x. With a good Plossl eyepiece having a
50 degree apparent field of view these low power views would have an
actual field of view of approx 1-3/4 degrees (50/28.4) for the 32mm
eyepiece and 2-1/4 degrees (50/22.8) for the 40mm. For comparison, note
that the full moon has a diameter of approx 1/2 degree. (The sun does
too but don't try looking at that!). On this f/8 telescope these low
power and (relatively) wide field eyepieces would exhibit some
vignetting since the diameter of the secondary mirror would be too small
to catch the full beam necesary to fully illuminate the edge of the
field of view. You may well find that you will end up using the
telescope at its minimum magnification to observe wide field "deep sky"
objects such as nebulae and galaxies much more frequently than observing
the moon and planets at high magnification.
On to shortest focal length eyepieces. Maximum >practical<
magnification for any telescope is about 50 (some say 60) power per inch
of aperture. That says a 4-1/2" scope should get you up to 225x.
However, unless you have extraordinary dark skies and ideal observing
conditions without atmospheric turbulence a more practical limit might
be half of that or, say, 100-125x. (My 8" Celestron is typically used
at 50-125x). 8mm is a common focal length which would yield 114x on the
Meade. So a good set of eyepieces might be 32, 16 and 8mm and give you
a doubling of the image size with each change to a shorter eyepiece.
Kind of like having 100, 50 and 24mm Zuikos in your kit. For
comparison, 30x is sufficient to show you the rings of Saturn... and a
good view of Jupiter... and almost nothing will give you a good view of
Mars.
As to photography through this scope. Best to fogedaboudit except for a
shot of the moon. Better would be to mount the camera piggyback on the
scope, use the scope for guiding a longish, wide angle time exposure
taken through some of your Zuikos. This is not meant to imply anything
about the optical quality of the Meade mirror but rather point out the
difficulty of trying to take a long exposure of a moving object with a
moving, relatively unbalanced, undamped telescope at 910mm focal length.
Now, after having said all this stuff about the 4-1/2" Meade reflector,
would I actually recommend that you buy one? I dunno, that depends.
There are plenty more options depending on how much money you have and
what you want to do. As a first scope, though, it's probably an OK
investment if you realize that you just gotta throw away those junk
eyepieces. A Dobsonian telescope is something that should be considered
as well but you might have to own the small Meade for awhile before you
can appreciate the low power, low cost, large diameter, wide field
Dobsonian. And you might have to own the Dobsonian for awhile before
you fully appreciated the even lower cost, even wider field, even
brighter binoculars we started this discussion on. Well, I said a lot
more than I intended and think I'll post to the list also just in case
there are some other folks who could benefit from all this basic
telescope/binocular stuff. It took me a long time to learn it all 35
years ago.
ps: The Meade 4-1/2" does meet one very critical requirement I have for
a Newtonian telescope on an equatorial mount... it has a rotatable
tube. Without a rotatable tube, on an equatorial mount the eyepiece can
get into very awkward positions such that you have to be a contortionist
to get into a proper observing position. I can think of nothing worse
to discourage use of the telescope than an aching back or craned neck or
wet knees from kneeling on the ground.
Chuck Norcutt
Woburn, Massachusetts, USA
------------------------------------------------------
I buzzed through the archives for info about telescopes and adapters for
OM
bodies and got lots of great info from OM list titans Paul Schings and
Chuck
Norcutt. Thanks guys (well after the fact).
I'm not likely to become an astronomer. I'm not likely to use a
telescope to
photograph anything, but we were thinking about a telescope as a "family
holiday gift," and it is possible that my son may conceive an interest
in
nocturnal sky lore and want to try to photograph something someday.
Or maybe not. Hence my caution about a large investment.
Are the various Meade brand scopes one sees at Walmart etc. OK? Are
they good
enough to encourage (rather than discourage) interest? Are they worth
getting
adapters for at least for starters if the boy wants to try a shot of the
moon,
etc.?
Joel W.
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