"Gary Nunn" <[EMAIL PROTECTED]> asked
From the two options below, which is the better telescope, and
why?
Option #1
diameter =114mm, 1000mm focal length, f/8.8
5x24 viewfinder with bracket; three eyepieces (.965") -
H25mm, H12.5mm and SR4mm for magnifications of 40X, 80X,
250X; 3X Barlow lens
Option #2
diameter =70mm, F=350mm, f/5
Includes two eyepieces (1.25") - H25mm and MA9mm for
magnifications of 14X and 39X.
It all depends on the quality of the optics and your purpose.
The 114 mm (4.5 inch) gathers more than two and a half times as much
light as the 70 mm (a little less than 3 inch). With the larger
telescope, you can see a magnitude dimmer. This is significant if you
plan to look at dim, low contrast objects at night.
On the other hand, the 114 mm telescope uses .965 inch diameter
eyepieces. As a general rule, this indicates a lower quality
telescope than one with 1.25 inch diameter eyepieces. You can get
away with using the smaller size -- for a given low price, modern
eyepieces are becoming much better because of computertized shaping.
But I would reject this telescope for this reason. On the other hand,
you should think about other telescopes with at least a 100 mm
aperture.
Nowadays, most inhabited areas -- even India -- suffer terrible light
pollution. For example, I am half way between two cities each ~40
miles (~50-60 km) away. Their light crosses over my head roughly a
mile (1600 m) up and illuminates any aerosols or other haze particles
that might be there. Over the past few decades, aerosol density has
increased. This means less contrast. With the exception of the moon
and double stars, astronomical objects have intrinsically low
contrast. Because of the loss in contrast, I have lost several
magnitudes over the past three decades. From an astronomical point of
view, I no longer live in a `rural' area.
In my experience in the north east of the US, the only thing you can
look at with a 70 mm telescope at night is the moon. Forget Mars or
Jupiter or anything outside the solar system except for bright
doubles, like Albereo. (A friend of mine jokes that the closest good
seeing for us is Baffin Island in northern Canada.)
Roger Clark wrote a very interesting book about the physiology of
seeing dim, low contrast objects. (`Visual Astronomy of the Deep
Sky', Roger N. Clark, Cam. Univ. Press and Sky and Telescope, 1990,
ISBN 0-521-36155-9, ISBN 0-933-346-54-9)
With the 114 mm telescope (presuming its optics are OK), you can see
and recognize a few smudges on Mars and zones on Jupiter. This is
exciting if you know what you are looking at. Depending on the
darkness of the sky, you may be able to see some nebulae, too. I used
a 114 mm telescope for quite some time.
On the other hand, if your purpose is to look at birds during the day,
the 70 mm is easier to carry. This is very important. You are more
likely to use a telescope that is lighter and easier to carry.
In either case, do not forget the whole optical train: not only do
you need a good objective, but also good eyepieces. Consider spending
US$150 on a good eyepiece. Remember to get your collimation right,
too. That makes a big difference. Please bear in mind: the figure
that counts is what you see, not just what the objective produces.
When you talk about `better than 1/4 wave' or whatever, in the case of
a reflector, that means that the combined errors of the main mirror,
the secondary, and the eyepieces all total up to less than 1/4 wave.
Fortunately, modern optical manufacturing technology has advanced
remarkably. Nowadays, optics are much better than they were.
Often nowadays, the weak point is in the mount. A good mount is
critical. Otherwise, whatever you look at will vibrate or be hard to
center.
By the way, do not consider a viewfinder of less than 50 mm. I can
tell you from sad experience that 25 mm or 30 mm viewfinders are
useless. You might as well aim the telescope tube in the general
direction and then use a low power.
Or get a Telrad `1x' viewer that puts an illuminated circle at
infinite distance in your field of vision. (If made bigger, we could
call the Telrad a `heads-up display'.) You will be able to line up
the lighted circle near whatever you are looking at and then, with a
low enough power, jump to it (or hop through a variety of intermediate
stars).
Incidentally, `fast' telescope optics (low f numbers) give more
distortion. This is intrinsic. Telescopes with an f number less than
6.5 or 7 are dangerous. You may need compensating lenses in your
optical path. An f ratio of 10 is roughly spherical, and a good many
such telescopes have spherical rather than parabolic main mirrors.
--
Robert J. Chassell
[EMAIL PROTECTED] GnuPG Key ID: 004B4AC8
http://www.rattlesnake.com http://www.teak.cc
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