"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.
All good points.
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.
I disagree, particularly on a telescope in the size range you are considering. As long as it is a good one. The 6x30 finder that came on my RV-6 Dynascope back in 1970 is a good one. The one on the Meade 6" which the community college where I teach purchased for use by classes is a piece of pure crap which I have never been able to adjust to get a clear image. I suspect it may take some machine shop work or perhaps just junking and replacing it. I would consider _adding_ something on the order of a 10x50 finder, if it is possible to do so without throwing the scope out of balance.
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).
Note that if you plan to use the telescope from a less-than-ideal location in terms of light pollution, a Telrad may be almost useless. Also, they are subject to dewing up badly on humid nights.
Incidentally, `fast' telescope optics (low f numbers) give more distortion. This is intrinsic.
Because to produce a short focal length, the mirror must be more extremely curved (deeper in the center wrt the edge).
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.
-- Ronn! :)
Ronn Blankenship Instructor of Astronomy/Planetary Science University of Montevallo Montevallo, AL
Disclaimer: Unless specifically stated otherwise, any opinions contained herein are the personal opinions of the author and do not represent the official position of the University of Montevallo.
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