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Telescope Buyers' FAQ - Part Three

What about accessories?

I have already said most of what you need to know about accessories, which is that (A) aperture wins. If you are planning a telescope budget, and eyepieces, finders, and such account for the lion's share of your funds, sit back and think carefully about what you are about to do - it might be better to get a bigger telescope instead of fancy accessories. A 10-inch telescope with a hand magnifier as an eyepiece will give a better view of most objects than an 8-inch telescope with the finest eyepieces in the world. Why? Because aperture wins!

Yet if you are up against limits of telescope portability, or have lots of money, or like technology, go ahead and buy fancy accessories. I won't tell, provided you remember that aperture wins.

In any case, I will mention some plain-vanilla accessories that you might want to have, and maybe a few chocolate ones, too:


A small number of good ones is better than a large number of bad ones. You will need a low-power, wide-field eyepiece, both for finding things and for low-power views of big, diffuse objects. It might give a magnification equal to five or six times the telescope clear aperture, in inches. On my f/11 Celestron 14, the low-power eyepiece has a 55 mm focal length, and is mounted in a two-inch barrel, so that the front lens - which sets the field diameter - can be as large as possible. (In little f/10 or f/11 telescopes, internal baffles may mean that no light gets to the edges of a two-inch wide eyepiece; if so, don't bother with the extra cost of one.) On my f/5 8-inch Dobson, I use a 20 mm eyepiece, which doesn't need a two-inch barrel.

The next power you will likely reach for is medium to medium high, for a good look at detail in the object in view. Such an eyepiece might give a magnification of 20 to 30 times the telescope clear aperture, in inches. On my C-14 I use a 12.4 mm eyepiece, and on my 8-inch Dobson, a 4 mm. The objects you look at with high power probably won't be very wide (though they might be), so for economy, you might not want a super-wide-field type.

Your next choices will depend on what you like to look at. If you are not sure, hold off buying more eyepieces till you find out.

"Fast" f-numbers, typical in Dobson-mounted Newtonians, require fancy, expensive eyepieces to give good views, because the steeply converging light cones of these instruments are difficult for an eyepiece to cope with, particularly away from the centre of the field. Slower instruments can use simpler eyepiece designs. It is a "Catch-22" of amateur astronomy, that cheap telescopes (fast Dobsons) need expensive eyepieces, but expensive telescopes (most refractors and Schmidt-Cassegrains, with slow f numbers) can use cheap eyepieces.

"Zoom" eyepieces, which change focal length at the twist of a knurled ring, tend not to be very good. Barlow lenses, also called telextenders, multiply the focal length of the telescope with which they are used: It used to be that they generally worked well only with telescopes with large f-numbers, where they were not needed - another "Catch-22". Yet I have heard that there are now Barlow lenses that work with fast telescopes, where they are indeed needed, but I urge a try-before-you-buy approach to selecting one.

For over fifteen years I used an eyepiece set bought in roughly 1980. It featured no fancy designs, just a 55 mm Plossl, 32, 20, and 12.4 mm Erfles, and 7 and 4 mm Orthoscopics. The 55 and 32 mm eyepieces were in 2-inch barrels, the others in 1.25-inch barrels. All were very good quality - the 55 and 32 mm were from University Optics, and the others were Meade Research-Grade. All worked reasonably well even at f/5, and the 68-degree apparent field of the Erfles was enough that I was untempted to buy wider-field types. Besides, a big Erfle is already so heavy that I must rebalance the telescope to use one. I did use the 4 mm eyepiece on the C-14 now and then, but occasions where I want that much power are rare.

In mid 1996 I bought more eyepieces, mostly out of curiosity. I found that decent Plossls are comparable to decent Orthoscopics. I bought several Vixen "Lanthanum" eyepieces, which have built-in matched Barlow lenses to give 20 mm eye relief, even at such short focal lengths as 2.5 mm. I don't need glasses to observe, but even so, long eye relief makes viewing more relaxed - I'm not worrying about bumping the eyepiece. It also facilitates public viewing - I focus with my glasses on, and tell everyone to leave theirs on and not refocus.

Note what high-tech eyepieces can and cannot do. The best give wider fields of view, with fewer eyepiece aberrations near the edges, than older types. The improvement is most noticeable at fast f numbers. If that's important to you, you might want some. But eyepieces are not aperture stretchers. They can neither increase image detail beyond the theoretical limit for the aperture, nor increase the number of photons that make it to the focal plane. If you think otherwise, you are making the same mistake as the clueless beginner who buys a drug-store refractor because it says "Magnifies 400 Times!!" on the box. The best an eyepiece can do is not make things worse. A simple eyepiece, with good coatings and well-polished lenses, will show all the on-axis detail a telescope has and absorb almost no light. That's what counts most for astronomical work.

In 1980, I bought 6, 12 and 25 mm Ramsden eyepieces - an old, simple, design - for about ten dollars each. I use them at star parties without telling what they are. They have only four surfaces, so simple coatings give good throughout, and there are few chances for bad polish to scatter light and ruin contrast. The field of view is narrow, but on axis, at slow f numbers - f/10 or longer - they give up nothing to new designs; images are superb.


What kind of finder you get depends on how you use it. If you plan on looking mostly at fine details in bright objects, then you might buy a big finder, in the hope that most of what you look at in the main telescope will be visible in it, too. But that won't work if you push your telescope to its faint-object limits - you would need a finder as big as the main telescope. You might then consider a finder that will show stars exactly as faint as on your charts. It helps a lot in identifying what you are looking at through the finder, if every star you see is charted, and vice-versa. Once the right pattern of stars is in the finder, you can put the crosshair where the object lies, even if it is too faint to see.

In dark sky, the 10x40 finder on my C-14 shows stars to about magnitude 9.5, which matches my big charts. The 7x35 on my 6-inch Maksutov does almost as well. In suburbia, the 5x24 finder on my 8-inch Dobson goes to about magnitude 6.5 (which would be the naked-eye limit in darker conditions), thus matches many naked-eye star atlases.

Unit-power finders, like the Telrad, let you to stare at the sky with both eyes open and see a dot, circle or crosshair of light where your telescope is pointing. A peep sight, made by taping bits of cardboard to your telescope tube, may work as well, and will be much cheaper, and any magnifying "straight-through" finder (in which you look in the direction the finder is pointing) can be used with both eyes open - let your brain fuse the images, so you can use the finder's crosshair with the other eye. I tried a unit-power finder (Orion's) on my 90 mm refractor, but found it always inferior to the original 6x30 finder. My opinion about unit-power finders is in the minority. Many prefer them to those which magnify. Some folks use the Telrad's circles of known diameter to measure angular distances when finding things.


Preferences vary greatly. What I find useful, in order from simple to complicated, is more or less the following:

A simple planisphere, preferably a plastic one that won't sog out with dew and that may survive being sat upon. It's a fast way to find out whether a particular object is up before I go observing, or to determine how long I have to wait before it is well-placed.

A pocket atlas. I am particularly fond of Ridpath and Tirion's The Night Sky, from Running Press in Philadelphia, PA. It is about three by five inches and half an inch thick, and it is out of print. Write Running Press and complain.

A table atlas, bound as a book that will lie reasonably flat, showing stars to the naked-eye limit and lots of deep-sky objects to boot. I happen to use an old Norton's Star Atlas; there are lots of others.

A deep atlas, such as Uranometria 2000 or the AAVSO atlas, with a stellar magnitude limit of 9 or 9.5 and a vast number of objects. What's important here is to have enough stars charted that there are plenty in every finder field.

A planetarium computer program (Bill Arnett reminded me). If you are a beginning astronomer, I do not suggest you rush out and buy a computer, but if you already own one, you might bear in mind that there are programs that will turn your console into a window onto the simulated heavens, with features for finding, displaying, and identifying things. I happen to have the rather old Voyager 1.2 for my even older Macintosh II; there are plenty more, both for Macs and for the world of MS-DOS and its descendants.

Some folks run such a program on a laptop at the telescope. Please put red cellophane over your console if you do.

I have had limited use for the popular oversize-format charts with lesser magnitude limits, like 7.5 to 8.5; they don't show enough stars to be useful with most of my finders and are too cumbersome. The plastic-laminated versions make good place mats, though. Everyone should use the box of a Dobson as a picnic table at least once.

A red flashlight, so you can read your charts and notes without ruining your night vision, or that of people near you. The kinds that have a red light-emitting diode (LED) instead of a flashlight bulb are particularly good. If other observers scream and throw things, your light is probably too bright.

A logbook. This item is not for everyone, but I find it useful to record my observations, even if I don't do anything other than note that I saw a certain object with a certain telescope and magnification. Logbooks make fun reading when it is cold or cloudy, and often there will be reason to look up something long after the fact. Besides, if you quote frequently from your logbook, you can make your friends think you are an active observer when you really gave it up years ago.

What about observing skills?

Even some experienced amateur astronomers think that seeing things comes free and easy, with no more effort than opening your eyes: but as currently popular slang so evocatively articulates ** NOT **.

Vision is an acquired skill. You must learn it, you must practice, and you must keep learning new things, and practicing them, too. Buying a bigger telescope to see more is like buying a bigger kettle to be a better cook, or buying a bigger computer to be a better programmer. Not that it won't help - it might - but cooking and programming depend far more on knowledge and experience than on artefacts. So does visual astronomy. People with garages full of telescopes (pardon me while I try to close the door to mine) are in great part victims of materialism, marketeering, and hyperbole. Practice is cheaper, and works better. As I said near the beginning of this article, an experienced observer may see things with a small telescope that a beginner will miss with an instrument five times larger, even with objects and sky conditions that favor both equally.

What skills may you hope to cultivate? What techniques should you practice? Not all have names, but here are a few, in what I think is order of importance; what matters most comes first.

It can take a long time to see everything in a field, even if you know exactly what you are looking for.

Eyes, telescope, and sky vary from night to night.

Dark adaptation
Avoid bright lights before observing: It takes your eyes hours to reach their full power of seeing faint objects.

Averted vision
The part of your retina that sees detail best, sees low light worst. Look "off to the side" to find lumps in the dark. Many observers use averted vision on faint objects, but not for faint detail in bright ones. Detecting something doesn't mean you've seen all you can. Don't let the dazzle of a galaxy's lens keep you from tracing spiral arms out beyond the width of the field. How about increasing magnification, and using averted vision to see if you can see more detail in the paler, but larger, image? Averted vision helps with double stars, when one star is much fainter than the other, even if the faint star is bright enough not to need averted vision if it were by itself. That is, averted vision seems to facilitate the detection of low contrasts as well as faint objects.

Stray light avoidance
Even when it's dark, background glow interferes with detecting faint objects. Keep it out of your telescope and out of your eyes. Try eye patches and eye cups for eyepieces. My first view of the Sculptor Dwarf Galaxy was with my jacket collar pulled up over my binocular eyepieces. I looked like a cross between the Headless Horseman and the Guns of Navarone, but I saw the galaxy.

Moving the telescope
The eye sometimes detects motion, or changing levels of brightness, more easily than static images. Jiggle the telescope, or move it back and forth, to make an object "pop out". Try it while using averted vision.

Not moving the telescope
The eye sometimes adds up photons over many seconds; if you can hold your eye still for a long time, faint things may appear. Try it with averted vision.

Clear sky, and enjoy your telescope - Jay Freeman

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Author: Dennis Bishop

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