Tips for Telescope Buyers
Derrick Pitts, Hon.D
Interested in buying a telescope? Making the right purchase need not be a daunting task if you're armed with a few simple guidelines. There are several different types of telescopes on the market and the first place to start in choosing one is to decide what you want to do and how much you want to spend.
Are you a beginner? A casual observer? Or are you an experienced sky observer? If you're a beginner or a casual observer, this guide's for you. You'd probably like to see the moon up close, observe the planets or get a good look at a passing comet, but you're not ready or interested in spending the time or money to do astrophotography just yet. You might also like the telescope to have some portability so you can take it to dark sky locations or with you on vacation without much trouble.
Getting Started: Binoculars
The best way for a complete novice to start is with a pair of binoculars. Binoculars have several advantages. They're lightweight, portable, easy to aim, have low powers of magnification and a wide field of view. Just in case you lose interest in astronomy, they can be used for other observing pursuits. Also, they tend to be less expensive than a telescope and you might have a pair already.
Binoculars are rated by their power of magnification and the size or aperture of the front lens. This information is contained in those cryptic numbers on the eyepiece end of the binoculars. They read as 7x35, 7x50 or some combination of numbers on either side of an 'x'. To define the numbers, let's start with the 'x'. It stands for the binoculars' power of magnification. So in 7x35 we have 7-power magnification. Typically, binocular power is usually 7, 8 or 10. The higher numbers like, 10, 15 and the occasional 25-power are rarer and usually quite expensive. They're also impractical for casual observing because you'll need a tripod to hold them without jiggling at 10-power and higher.
The other number is the diameter measurement of the front lenses in millimeters. This number can range from 35 to 70 and higher. As with the magnifications though, the higher the number, the more expensive and more difficult they are to handle. This brings us to our first tenet of buying binoculars or telescopes: Always buy the biggest aperture you can afford. Reason? With bigger lenses or mirrors in your optics, you can gather more light. The primary misunderstanding about telescopes and binoculars is that magnification is your ultimate goal. While magnification is nice, you can't magnify what you can't see. In other words, if you aren't gathering enough light from an object to make it visible, what's the use of magnification? With binoculars purchased for bird-watching or enhancing the view of the game from the nosebleed section, you can also do some casual skygazing. Ordinary binocs will give excellent views of the moon and greatly improved views of Jupiter, Saturn, and comets. Good binocs start at around $65 and go up from there. Check to make sure they're built well and test them by looking at something with lots of vertical lines. You want to make sure that the lines don't curve near the edges of the field and that they have a hard, sharp focus. Did you know that most comet discoveries are made by amateurs with binoculars? Or that those old binocs you have in the drawer at home are more powerful than Galileo's first telescope? Try them. They might just be the best way for you to begin.
Telescopes come in two basic types: refractors and reflectors. Refractors use lenses to gather light from dim objects while reflectors use a mirror. Refractors are the long narrow tubes with the eyepiece at the back end. Reflectors are usually shorter, fatter and open at the top. In most styles of reflector, the eyepiece is placed near the top. Refractors provide good, sharp images and are good for observing pinpoint light sources such as stars and planets, but tend to be on the expensive side. Reflectors are good for observing more diffuse light sources such as galaxies and star clouds. They also provide good, sharp images but they are much less expensive because reflectors reflect the gathered starlight from just one surface. In refractors, the light has to pass through two lenses and the surfaces of both (four altogether) have to be polished as close to perfectly as possible. This is both time-consuming and labor-intensive. Going by our first tenet of "always buy the biggest aperture you can afford," you can almost always purchase a much larger reflecting telescope for what you'd spend on a smaller refractor.
In many stores telescopes are sold by their power of magnification. It actually has very little importance in purchasing a telescope because of the second tenet, "you can't magnify what you can't see." As magnification increases, resolution decreases. The rule of thumb for magnification limit in telescopes is 50 power per 25 millimeters (25.4 mm = 1 inch). It works like this: the main lens or mirror of a telescope gathers light from an object and forms a small image inside the tube near the eyepiece. The eyepiece magnifies the small image. Different eyepieces have different magnifications. The numbers on the eyepiece refer to their focal length. If you divide the focal length of the telescope’s main lens by the focal length of the eyepiece, the resulting number is the magnification you'll get with that particular eyepiece. For example:
900 mm focal length (telescope) divided by:
25 mm focal length(eyepiece)
= 36 power magnification
The focal length of the telescope’s main lens or mirror is shown either as a raw number like this:
ƒ = 900mm
Or like this:
This is the ratio of the diameter of the lens or mirror to its focal length. Don’t worry about the numbers, it just says the focal length of the lens is 5.6 times the diameter of the lens!
The lower the eyepiece numbers, the higher the magnification. Remember: the rule of magnification is 50 power per 25 mm of aperture. In a 60 mm scope, the best you can do is 100 power. In a 100 mm scope, 200 power and so on. If the advertised magnification is 500 power, the scope size HAS to be 250 mm(10 inches!) or more of aperture, otherwise the loss in resolution will degrade the image dramatically. It's also just plain better to begin with low powers anyway. Telescopes have high magnification and a small field of view - just the opposite of binoculars. Using lower powers of magnification in a telescope make it easier to find and keep track of the objects in view.
Your telescope must have a finder scope, a smaller telescope mounted adjacent to the eyepiece. The finder, with much lower power and much wider field makes object location much easier.
For casual observing, motors to drive the telescope for tracking objects aren't really necessary. After just a little practice you'll find that guiding a telescope by hand as you observe is quite easy, especially at low powers of magnification. Motors are important if you're planning to do astrophotography or perhaps if you plan to show an object to a multitude of people in a short amount of time. For beginners, complication of operation and price are dramatically reduced if you don't have a motor drive - save it for your next, larger telescope.
Computerized "Robotic" Telescopes
The wonders of the digital age have now made it possible for anyone to own what used to be professional observatory-grade optics and highly accurate tracking systems. Many telescopes now have a computerized guiding system that uses an on-board database of hundreds of thousands of objects. The database provides the locations of the objects and the microprocessor control systems run the telescope across the sky to the desired object at the push of a button. Ah, bliss! No longer does the operator have to know where anything is in the sky, the computer will do it all for you, right? Wrong! Even with GPS positioning capability, the operator still has to be able to know the night sky well enough to realize whether or not the guidance system knows what its doing. So you the operator DOES have to know what’s up there to confirm that the scope is pointing accurately - especially at start-up. Otherwise, the scope will just point to where it thinks the object is – oblivious to whether or not an object actually is there. Remember: it’s just a machine, and not a very smart one at that. So don’t expect one of these scopes to do it all for you. They do have advantages but those only become obvious once you really learn how to use the instrument.
The mount or tripod of the telescope should be firm and steady. The more massive the tripod is, the less prone the telescope will be to fall over or to wind vibration. The simplest telescope to set up and use is the Dobsonian reflector. The optic tube just sits in a giant turntable-type mount. It's very easy to operate and could be the best value for your dollar. You'll find them advertised in the magazines listed below.
To make this all worthwhile, you'll need instructions on how to set up and use your telescope. Some of the best books to teach you how to use your telescope are actually the old 8.5x11" black-type-on-white-paper books published by Edmund with titles like How to Use Your Telescope, The Edmund Sky Guide and the MAG 5 Star Atlas. Don't be fooled by their mundane appearance. They are well written, clearly illustrated and give step-by-step directions on how to get the best from whatever instrument you purchase. You can still find them online and in used bookstores.
How to Buy A Telescope: Resources
Astro magazines are getting scarce! They’re still available at your local bookstore, but they all have websites now. Be sure to see their special editions.
• Sky and Telescope: a more technical periodical for a more advanced amateur observer.
• Astronomy: the magazine for all the rest of us. Great articles, prettier pictures.
Basic Telescope Formulae:
- Mag = fL(objective)/ fL(eyepiece)
- fL = distance from objective to focal point
- Pitts’ Power Law - 50x/in(aperture)
- Price x Weight = Immovability
- Use(scope) is inversely proportional to Mass
Derrick H. Pitts is the Chief Astronomer and Planetarium Programs Director of the Fels Planetarium at The Franklin. He’s been a ‘telescope jockey’ for 40 years.
December 5, 2019, 10:57am
Buying an instrument: lens testing, evaluating stands and accessories
First and foremost, the type of microscope you buy will be determined by your particular interest or chosen field of study. Those wishing to study opaque mineral ores may want nothing more than a simple stereomicroscope and a reflected-light stand. Others, interested in pond life may want an inverted transmitted-light microscope; field workers may wish to find a portable microscope. The majority of individuals wishing to find and purchase a light-microscope will probably be interested in an upright transmitted-light microscope for examining thin sections of tissue, cells, crystals or rock sections, for example. In practice, stereomicroscopes, portable microscopes and inverted transmitted-light microscopes are comparatively rare and far less easy to come by than reflected-light or transmitted-light stands. I assume, in this article, that you are not looking to buy a microscope for a child or school. Most toy microscopes sold in shopping centers or home shopping catalogues are too small for any serious use, and have inherently poor mechanics and optical systems. Do not be deceived by the high magnification claimed by these primitive microscopes: not only are toy microscopes useless at 500–1,000× overall magnification for, as explained earlier in this series (part 2), resolution of fine detail is more important than magnification alone.
Microscopists are often also interested in photography, yet both the new and the second-hand camera markets are vast compared to the much smaller new or second-hand market in microscopes. Nevertheless, the second-hand camera market will often have microscopes for sale. By and large, unless you wish to pay several thousands of pounds for a modern microscope the choice comes down to looking for a second-hand stand. Nevertheless, despite the smaller market, suitable microscopes can be found. The microscopy web page Microscopy-UK is useful for advice. If you prefer the idea of owning a new instrument, it is a good idea to have a very definite idea of what features you want in your new instrument to fulfill the job it is intended to do. It is also advisable to talk to other people who hold the same interests or work in the same field of study to ascertain what equipment they find essential, and what other items are merely useful.
Whenever you enter a field that is new to you, you have to build up a certain knowledge base to enable you to select and purchase equipment wisely, and it pays dividends to talk in depth, to listen to others and glean their experience. Do not be tempted to adapt your requirements to the instruments on offer, particularly where the market choice is small. Listen, take advice, draw up your list of requirements and stick to them. That applies whether you decide to buy a new instrument or an older second-hand one. The advice given by Arthur Barron nearly forty years ago in 1966 is still relevant, and I quote from his fourth chapter in Using the Microscope, which I recommend as further reading, particularly if you are considering purchasing a stand built in the first half of the twentieth century, or earlier:
“It is a waste of money and effort to purchase an instrument far beyond one’s probable requirements, when simpler equipment will do all that is required just as efficiently and much more easily, but it is also a great mistake to attempt difficult work with an indifferent instrument bought perhaps because of an initial underestimation of one’s future needs. The various components must of course bear some relation to each other. It is useless to have an elaborate stand for use with only low power lenses, and it is equally useless to purchase a wide-aperture objective of high quality without a correspondingly high-class illuminator, or to use components on a cheap stand designed solely for low power work.”
Without wishing to dwell on the issue, I mention these points because in the past I have had people asking for advice once they have realized that the first microscope, they bought was either not adequate to the task expected of it, or found to lack a useful feature. For example, it may not be possible to set up full Köhler illumination because of a fixed diffuser in the light path, or the condenser is absent (a mirror stand), it may be impossible to center the condenser, or it may be fixed and thus unable to be exchanged for another type such as a low power, dark-ground or phase contrast model.
General mechanical considerations
Most modern (e.g. made post-war) new or second-hand microscopes will be modular. That is, various accessories can be added to them, such as a reflected-light illuminator for epi-illumination, or a trinocular head with a phototube can replace the monocular or binocular head. These models may be more expensive, but they possess obvious advantages over those microscopes whose function cannot be altered because their design is fixed. So, I suggest that you glean information, draw up a detailed yet realistic ‘wish-list’, and opt, if at all possible, for a design of microscope that can be expanded at a later stage as your hobby or studies develop. Whatever the final list, I recommend that it should be possible to fix a binocular head to the microscope. The only disadvantage of a binocular head is that it will not allow the means of altering the tube length by either optical or mechanical means to correct for spherical aberration. Whilst a monocular head will do this easily by means of a drawtube, it can be very fatiguing to use. Furthermore, an angled binocular head allows the specimen (particularly important with aqueous specimens) to rest horizontally on the microscope stage.
For a microscope stand to be really versatile, not only must it be modular in its design, and capable of accepting accessories and extra equipment, it should preferably have a self-contained built-in illuminator which removes the need for an external lamp source to be aligned on the optical axis each time it is used. Convenient, but not essential, is a built-in transformer, generally rated up to 12 volts 15 Watts. This confers a degree of independence and portability to the stand: an extra (heavy) transformer is not needed, but the trade-off is the inability to change and upgrade the light to a 100-Watt source if the extra illumination is needed, say for dark-ground contrast enhancement. Careful selection of the substage mechanism which carries the condenser will permit a range of contrast techniques to be employed. Ideally the condenser should be able to be changed from the simple Abbe condenser to be able to do this. A good-quality rack and pinion that does not slip, a bracket that will accept various condensers and centering screws on the condenser are essential.
Plain stages with stage clips will suffice in an elementary model, but an attachable mechanical stage is preferred since it allows precise and reproducible positioning of the slide. If stage clips are fitted, ensure that they do not foul the front lens of high-power objectives with a short working distance. Rotating stages are extremely useful for work with polarized light, where the specimen will almost certainly need to be set at a certain angle with respect to the crossed polars. Rotating stages are also useful for photomicrography so that the subject can be fitted into the film format without needing to rotate the camera housing (in some integral photomicrographic stands this cannot be done). Mechanical stages can also be used to grip a thin plastic carton for wet work, such as pond-life studies, if an inverted stand cannot be found. Most condensers will be able to focus through the thin plastic found in food cartons, and the optical quality of the plastic container will not interfere with work at low magnification.
Check all mechanical movements of the focusing controls (I should say here that I find coaxial controls more convenient than separate coarse and fine focus knobs) and the mechanical stage; they should be precise with little or no backlash. Often you will have to decide what tolerances are acceptable. This is particularly true if the equipment has been well-used. You may, for example, decide that you can live with a good-quality rotating mechanical stage that is very good in all respects save that one of the coaxial stage movements is slack. What cannot be tolerated is a stage that is not perfectly smooth and square throughout all its area to the optical axis. A stage out-of-true will necessitate constant refocusing, and the image may well be partly blurred over the field of view. Ensure that the limb (if the microscope design focuses on the limb) does not fall down under its own weight, or that of an attached camera, and drive the objective down on the specimen. Check that the stand is solidly-built yet not top-heavy, and will support extra equipment without suffering from instability or vibration. Finally, as with anything else you buy, the overall appearance of the microscope stand will tell you how it has been used and cared for. Avoid an instrument with corroded parts, enamel chips that have been filled with a different lacquer, dents, excessively worn and loose rackwork and screws with torn or gashed slots.
The various optical components have been described in previous articles. The most important components are the objectives, and these in particular should be examined closely to ensure that they are optically perfect when bought. The objective is so crucial to the proper working of the microscope that really it is the microscope. This is why two of this series of articles (part 4 and part 7) are devoted to the objective. The central importance of the objective also explains why, in many cases, a microscope stand may be sold with less than its full complement of objectives, the more desirable items having been retained or sold separately. When evaluating an objective, there are some simple tests that you can systematically carry out.
First a visual inspection, in the same way as with the stand, will tell you how well used or well looked after the objective is. Has the objective been fitted with an RMS thread, or is it intended for one of the more recent microscopes, where manufacturers have decided upon their own brand convention? Hold the lens up to a strong light and examine it closely, as well as looking through it at each end. Has the inscription around the barrel and the colored identification ring been scratched or disfigured? Are there chips or dents around the mounting of the front lens element? On lower power objectives it may be possible to see if the anti-reflection coating on the front lens has been scratched. With a dry objective, look at the black matt area around the front lens element: is it intact and free from blemishes or scratches, which might otherwise have also affected the lens. Solvents such as acetone or alkalis will also damage this coating.
With oil-immersion objectives, ensure that no oil has remained dried onto the front lens (inspect it from the rear to ensure that all the lens elements in the barrel are clear). Also check that no oil has seeped into the barrel of the objective and lodged in the mounting holding the lens elements. Traces of oil films when checking the spring-loaded barrel runs freely will usually indicate this. Check that any internal diaphragms turn freely, and are not damaged. Often an older second-hand objective will seemingly have dirt on the back lens element. It may, of course, be fungus growing between the elements as a result of poor storage in hot humid climates. Other objectives will exhibit separation of the lens elements. Newer lenses are cemented with a synthetic ultraviolet adhesive, older lenses used Canada balsam cement. The cement can break down over time, creating a gap between the lenses. This usually starts from the outside edge and works inwards. A lens that is separating, where the balsam has broken down, will have either a murky appearance, or show interference fringes looking similar to oil or petrol floating on water. The images from such objectives lack both definition ‘crispness’ and contrast. Inspect the back focal plane of the objective critically with a (phase) centring telescope.
The best test of an objective is to evaluate it using a variety of specimens. Use those that you are familiar with and, preferably, have inherent well-stained fine detail. I use a very thin section of renal glomerulus stained with toluidine blue. Commercial firms use quantitative interferometric testing to evaluate objectives, but using specimens well-known to you in this way can give a good picture of an objective’s worth. Colorless specimens are essential for making a judgement about color aberrations showing up as color fringes around the edge of the object as you focus up and down through the plane of the specimen. I find the diatom Navicula lyra to be useful for this test. Residual color will, of course, be present to some degree; more so in an achromat than an apochromat objective. In the absence of serious defects, the difference between a good and a poor objective will come down to how crisply the image contrast is presented to the eye. An Abbe test plate can be used to evaluate spherical, off-axis and color aberrations, but this is rare, and not absolutely necessary. The star test will suffice, and it is possible to make your own test slide easily; for the details, see Fletcher (1988) or Oldfield (1994). To truly evaluate the resolving power of an objective, a resolution test plate fabricated by electron beam lithography should be used, but historically diatom frustules of species such as Amphipleura pellucida and Pleurosigma angulatum will suffice. Again, it is better to avoid natural variation by using the same selected specimen in a custom-picked test plate. Suitable diatom slides for objective testing are available from Klaus Kemp.
These remarks concerning the objective also hold true for the eyepieces and condensers, where applicable. All glass should look clear, colorless and bright. Finally, ensure that the prisms of the binocular head are properly ‘collimated’ or in alignment. If the two fields of view are out of synchrony, then it is likely that the binocular head has been dropped at some time in its life, or is very poorly constructed. Either way, it may by impossible to view or fuse the two images properly into one, and this will cause eyestrain.
Maintenance of your microscope
I recommend the two articles by M. I. ‘Spike’ Walker (Bulletin of The Quekett Microscopical Club No. 26, pp. 14–16 , Autumn 1995 and No. 27, pp. 18–24, Spring 1996) for further detailed information on maintaining the light microscope.
Barron, A. L. E. (1966) Using the Microscope. Chapman & Hall, London.
Fletcher, J. R. (1988) The star test for microscope optics. Microscopy 36/2: 153–159.
Oldfield, R. J. (1994) Light Microscopy: an illustrated guide. Wolfe Publishing, London. ISBN 0-723-41876-4
© Jeremy Sanderson, Oxford, 2010