Selecting and Using Binoculars and Telescopes
The best aid to astronomical observation is a clear, dark sky, away from city lights, and your eyes. I see more with my naked eyes while holidaying at Eagle Bay than I do with binoculars from light-polluted Perth suburbs.
Good binoculars increase SIGNIFICANTLY what you can see. Good binoculars are MUCH BETTER than a cheap (junk) telescope!
When choosing binoculars, you're better off with the 50 mm diameter lenses (rather than the standard 35 mm): i.e., "7 x 50" instead of "7 x 35". The light gathering ability increases at the SQUARE of the lens diameter, so 50 mm lenses gather twice as much light as 35 mm.
If you purchase much larger and/or higher power binoculars, you may want / need to clamp them onto a tripod -- their magnification also magnifies how much we "wobble".
Of those pictured above, the one on the left is new (Bushnell 7 x 50: A$ 170 tax-ex); that on the right is my 28 year old 7 x 35.
Good links:
Binoculars: Halfway to a Telescope (Alan MacRobert, Sky and Telescope magazine)
Choosing Binoculars (Alan MacRobert, Sky and Telescope magazine)
BEWARE the cheap telescopes sold in department stores, some camera shops, and by mail-order.
A recent ad from "Inspirations" shouts:
Magnify up to 525 times! - astronomical reflecting telescope -- A$199
The scope, on a wobbly altitude - azimuth mount, has a 60 mm objective lens. The maximum usable power is about 50 x per inch (20 x per cm) of lens or mirror, so that telescope's MAXIMUM useful magnification (power) is about 120 ! Not 525 !! The extra magnification simply magnifies the fuzzy image so that it is useless. [Most of the time, telescope users will use about 10 - 20 power per inch.]
Magnification is much less important than light gathering power, which is simply a function of the lens - mirror diameter.
Analogy: When searching for a small object in a dark room, which is more useful, a magnifying glass or a torch? It's the same thing in the sky -- we are usually looking for FAINT objects...
So, if you're prepared to spend no more than $ 200 or so, invest in binoculars, not a small telescope.
On the other hand, for $ 700 - 800 you CAN get a decent telescope! Here is mine:
It is a Tacso Newtonian reflector, with a 114 mm (4 1/2 mirror) and a very good equatorial mount (with manual slow-motion control); it cost A$ 560 tax-ex at Australian Geographic.
Good points: Very good primary optics, very good mount.
Bad points: The tiny 5 x 20 finder scope provided is useless, and the eyepieces provided weren't much better.
In the photo above, you see a new 6 x 30 finder telescope (A$ 70) and one of three new eyepieces (each A$ 110). With these additions, it is an excellent little unit.
Eyepieces
L to R: 40 mm f/l Plossl (gives 23 x with this telescope),
25 mm f/l Orthoscopic (gives 36 x), 12.5 mm f/l Orthoscopic
(gives 72 x), and Barlow lens (doubles the power of each eyepiece).
I use the two lower-power ones about 95 % of the time!
The 4 1/2 inch Newtonian available from York Optical (Perth) comes with comparable main optics and equatorial mount, the better finder telescope, and two good eyepieces. At A$ 580 tax-ex, it's a better deal. [You need to spend at least A$ 180 to get the Tasco model up to scratch.]
Larger Newtonian reflectors on Dobsonian (NOT equatorial mounts) are an alternative: cost is about A$ 680 tax-ex for a 6 inch mirror, and A$ 850 with an 8 inch mirror (discussed below). The very fine Celestron 8 inch Cassegrain "go to" (computer-driven) telescope is a marvel, but you're looking at $ 5500 tax-ex.
If "Cassegrain telescopes" and "Dobsonian mounts" mean nothing to you, read on...
There are three basic optical systems in astronomical telescopes:
refractors use a complex objective lens to collect the light;
classic or "Newtonian" reflectors use mirrors to reflect and focus light on a converged point; and
catadrioptics, the most common type of which is the Schmitt Cassegrain, which combines a refractor and a reflector in one package. They are compact and portable.
Image Copyright Celestron Corp.
Here are some good illustrations of how they work (from Australian Geographic Guide to Space Watching, a book that I recommend very highly)
Cost for size or mirror or lens: Newtonian reflector (cheapest), Cassegrain reflector (intermediate), and refractor (most expensive).
There are two basic types of mounts: altitude-azimuth and equatorial. There are also variations on each.
An altitude-azimuth (or "alt-az") mouth provides, not surprisingly, two axes: altitude (up and down) and azimuth (left and right). The telescope can be moved independently on either axis. These are the cheapest mounts available. However, since the stars appear to move due to the earth's rotation, they make tracking astronomical objects very difficult (and since the telescope's magnification reduces the field of view, objects "drift" out of the field of view very quickly).
Tasco refractor on altitude- azimuth mouth
A newer style of alt-az mount is the "Dobsonian": a classic "push it into position" mount. Its main advantage it is low cost (and +/- ease of manual tracking); its main disadvantage is its weight. The mount is quite stable because it is low to the ground. It is also very smooth in both motions which allows the observer to track celestial objects easily. The small size of the mount also allows easy transport of these telescopes compared to an equatorially mounted Newtonian Reflector. But like any (non-computer controlled) alt-az mount, they are not suitable for astrophotography (where precise tracking is required). More on Dobsonian mounts.
Celestron Dobsonian-mounted Newtonian reflectors of various sizes
An equatorial mount also has two axes, but one is adjusted so that it is parallel to the earth's axis. This allows easy tracking by moving the telescope on one axis only (which can be automated with a small "clock drive" motor). An equatorial mount also allows locating objects by their co-ordinates (discussed in another section) alone. Unfortunately, equatorial mounts are more complex and more expensive than simple alt-az mounts.
Equatorial mounts come in two basic styles: the traditional (or German) equatorial mount (suited to long telescopes), and the fork-style equatorial (suited to the compact Cassegrain telescopes).
Tasco Newtonian 4.5 inch reflector on German equatorial mount
(mine before I replaced finder telescope and eyepieces)
(from Australian Geographic Guide to Space Watching)
Celestron 8 inch Cessegrain on fork-type equatorial mount and motor drive
Quite recently, computer-controlled "go to" telescopes have become available on the "amateur" market. A good example is the "Ultima" or "go to" version of the Celestron 8 inch (20 cm) Cassegrain. The computer controls both axes of the alt-az mount to make it function +/- as an equatorial mount. After aligning it on any two bright start, it "knows where it is" and tracks automatically (9 volt motor drive). More impressively, one can select any of 10,000+ celestial objects in its database (or type in the co-ordinates of any object), Voila -- the telescope finds it !! Expensive ? Yes ! Worth the money? YES !!
I purchased one of these in April 2000. It is BRILLIANT, exceeding my high expectations. Here is a review.
Celestron 8 inch Cassegrain "Ultima" computer-driven "go to" telescope
Meade 8 inch Cassegrain computer-driven "go to" telescope
Clearly is must be strong and stable: it supports the telescope and mount. The tripod is better on uneven ground than is the pedestal.
This is the small telescope attached to the main one; it helps you locate objects in the main 'scope. Why do we need it? As noted above, the main telescope has a SMALL field of view (typically one degree or less), while a finder telescope typically has a field of view of six degrees or more. Centre the object on the crosshairs in the finder telescope, and it should be seen in the main telescope.
I noted above than many telescopes in the lower price ranges (A$400 - 700) come with very poor eyepieces; that is, they have a narrow field of view, poor eye relief (you have to get too close, especially if you wear glasses), or just plain mediocre optics. Why does this happen? Because they are CHEAP!
Less expensive telescopes often come with eyepiece diameters of 0.965 inch, rather than the "standard" of 1.25 (or 2 inch when you get into the multi-thousand dollar telescope range). Ensure that the telescope WILL ACCEPT 1.25 inch eyepieces (because most of the good ones come in this size only).
The combination of eyepiece focal length and telescope focal length determine the magnification: thus you will want two or three different eyepieces. For example, my 4.5 inch Newtonian reflector has a focal length of 900 mm. Thus:
40 mm f/l eyepiece gives 22.5 x magnification (900 / 40 = 22.5)
25 mm eyepiece gives 36 x
12.5 mm eyepiece gives 72 x, etc.
For more magnification, I need a shorter focal length eyepiece, and they tend to have poor (short) eye relief. An easier and cheaper option is to use a "Barlow lens". It is inserted into the eyepiece holder, and the eyepiece is inserted into it. Barlows typically increase the eyepiece's magnification by 2 or 3 times. Thus, when I use my 12.5 mm eyepiece with a 2x Barlow, I get 144 x (instead of 72 x); rarely will I ever want to use more magnification with a 4 1/2 inch telescope.
There are also many different types of eyepieces depending upon their optical configuration. Hygens and Ramsden styles are the cheapest, poorest, and to be avoided. Good Orthoscopic, Kellner, Nagler and Plossl eyepieces start at about A$ 110 each and are good value for money. [It is possible to spend A$ 700 or more on a top-of-the-range Plossl or Nagler!]
(from Australian Geographic Guide to Space Watching)
That sounds confusing: so what should I buy?
York Optical (Perth) 4.5 inch Newtonian on equatorial mount, good accessories: A$ 580 (tax-ex: quoted 31-1-2000). [Motor drive A$ 180 extra approx.] Excellent value for money!!
Bintel (Sydney) 4.5 inch Newtonian on equatorial mount, good accessories: A$ 635 (cheaper tax-ex) plus shipping (but possibly not as good a deal as the York telescope, above).
York Optical (Perth) Dobsonian Newtonian reflector telescopes (similar to Celestron pictured earlier, but in my opinion, better-made):
6 inch Dob - Newtonian A$ 680 (tax-ex: quoted 31-1-2000)
8 inch DOB - Newtonian A$ 850 (tax-ex: quoted 31-1-2000)
[These are VERY GOOD prices !]
The $ 64 Question... Do I spend on "optics" or "mount"???
As you can see, it's about the same price for a 6 inch Newtonian reflector on a Dobsonian mount, as is it for a 4.5 inch Newtonian on an equatorial mouth. Which is better value?
If you ONLY want to look at easily-seen objects (moon, bright planets, a few of the bright "deep sky" objects [such as the Orion Nebula]), I'd probably go for the Dobsonian reflectors. However, you students will NOT learn to locate and track objects -- this requires an equatorial mount. That's why I bought the 4.5 Newtonian with equatorial mouth. I'm happy to discuss this with you...
And if you can afford it...
The Celestron 8 inch Cassegrain with motor-driven fork equatorial mouth is a fantastic telescope -- it has double the light gathering power of a 6 inch and four times the light gathering power of a 4 inch (approx.). It provides for the "proper" location of deep sky objects, and then tracks automatically. You can do serious amateur astronomy with this instrument. About A$ 2100 tax-ex.
While the Celestron Ultima "go to" won't teach students how to locate objects manually, it is a teacher's dream: set it up on two bright stars, click the button, and find the galaxy you want!! Fantastic!! About A$ 5500 tax-ex. My review.
[both telescopes are available from both York Optical and Bintel -- check the prices on the day...]
Again, if you're thinking about purchasing a telescope, I'm happy to discuss with you. I now have the 4.5 inch Newtonian, the 8 inch Celestron "Ultima", and three pairs of binoculars, when I visit schools.
Good links:
I don't agree with all they say... (and remember, prices are US$ in the US)..., but...
Buying the Best Telescope (Alan Dyer, Sky and Telescope magazine)
How to Choose a Quality Telescope for Under $500.00 [US$ = A$ 1000] (David Lent)
Or, you could build your own -- a GREAT class project, and a way to save money:
Good links on amateur telescope building
How to Build a Dobsonian Telescope
Reviews of two excellent telescope building books
William Herschel's telescope (circa 1860)
© Anglo-Australian Observatory, Photograph by David Malin, text © David Malin
Another good source on telescope basics and using telescopes from Celestron
Written by Stephen R. Kessell
Updated 5 October, 2000
