Last updated: 10 February 2001

From: sherrodc@ipa.net (Clay Sherrod)

TELESCOPE PERFORMANCE III - How Far Can I See? The Myths and Moderations of Magnification

In my series to help you understand better the major functions of a telescope, I have stressed three categories that are important responsibilities of your telescope:

1) LIGHT GRASP - the telescope's ability to enlarge the capability of your own eye, collecting more light so that you can see fainter objects more clearly, this increasing your "Limiting Magnitude," or how faint you can see as well as increasing your....
2) RESOLUTION - the ability of your scope to discern very close and delicate markings, also a result of the size of the telescope's lens or mirror, but also limited by such things as the steadiness of the air and your own visual acuity;
3) MAGNIFICATION - the last and least important (YES, I said "...the least important!") of ALL functions of any astronomical telescope.

This is NOT to say that magnification isn't important to you....certainly it is. It is the gift we have been given to magnify Saturn to be able to see the rings clearly, to make out the phases of Venus, to resolve a fuzzy glow in Hercules into a ball of hundreds of thousands of stars.

Of course "magnification" is important....just not THAT important.

Let's say, for the sake of starting, that the human eye has a magnification of "1"; that means that if your friend standing next to you is 6-feet tall, you "see" him as six feet tall, a ratio of 1:1.

By contrast we will equate that to common 35mm cameras. A "single lens reflex" camera (the kind you look through the lens, not a viewfinder) typically comes equipped with a 50 millimeter (50mm) lens which provides an image scale on the film of 1:1 - remember that ratio? It is the same ratio as the human eye, so the standard camera lens provides a real-life portrayal of image scale on the film.

Are you with me so far? If so, then let's add a telephoto lens to the same camera of, say 150mm, or 3-times (50 divided into 150 = 3x) the magnification over the standard 1:1 ratio of the regular lens.

So you're closer to the object you are photographing, or so it seems, with the telephoto lens...that's great! Yep, but there are some drawbacks, just like there are in your telescope:

1) when you increase magnification, the BRIGHTNESS of the object you are observing diminishes in direct proportion to the increase;
2) magnification DECREASES CONTRAST after a point and thus provides a more "washed-out" appearance of your object, and consequently begins to lose the saturation of pure color out of the object;
3) not only does "magnification" increase the size of your object (if you observing, say Jupiter at 150 power, you are in essence bringing the image to where it APPEARS 150 closer to you than with the naked eye), but you are magnifying EVERYTHING ELSE as well. This means that you are increasing the effect of tiny vibrations by 150x, you are increasing unsteady air moving between you and what you're looking at by 150x and you are increasing your eye fatigue.

But....all that being said....magnification is not a "bad" thing. It's like many other vices in life. It's okay - and even beneficial when used in moderation. It's when you overdo it, that the effect can be detrimental. And most amateur astronomers new to the wonderful hobby do, indeed, try to overdo it.

It's not your fault! We see advertisements for little bitty telescopes that are "454 Power!" To prove that it is "454 Power" there are images all over the boxes and ads for these scopes (taken by large telescopes by a guy with a long beard and government subsidies) showing EXACTLY what "454 Power" looks like.

Oh, come on. The modern consumer is too smart to fall for that anymore.

So....how much magnification is TOO MUCH magnification?

Well, we hear a lot of people - reputable telescope manufacturers, educators, other amateur astronomers - who all suggest that (and I can almost quote): "...how much power you can use depends on how big your telescope is..."

Well part right, like being half-crazy. There's a LOT more to magnification than simply how BIG the telescope is. If we were on the moon, for example, free of moisture, dust, pollution, haze and AIR, observing with an ETX 90 could reveal as much as the largest telescopes could on this Earth with its heavy blanket of air. That is but one limitation among many:

1) the atmosphere (the steadier the night, the higher magnification you can use) - See my discussion concerning Seeing and Transparency on this web site.
2) the telescope size;
3) the telescope quality;
4) the type of eyepiece used;
5) the object you are looking at.

First, remember the "bad seeing rule." If the stars twinkle overhead, then restrict your observing to either low power or deep sky objects at a maximum of around 100 power REGARDLESS of what telescope you use; indeed, smaller telescopes CAN USE MORE POWER PER INCH than large ones in times of bad seeing without noticing the detrimental effects of "bad seeing."

When the air is very, very steady (like stagnant polluted air in summer), you can use 50 x to even 80x per inch aperture (175x on a 90mm scope or 250x on a 125mm scope). The transparency (clearness) of the sky has NOTHING to do with your ability to use magnification, nor does moonlight or streetlights or barking dogs. Only your telescope (aperture and quality) and the atmosphere steadiness.

Here is the golden rule of magnification (maximum values given PER INCH APERTURE):

1) No right stars twinkle rapidly (naked eye) near the horizons nor anywhere across the sky - Great conditions: up to 75x to 90x per inch aperture;
2) Bright stars twinkle on horizon, but no higher in the sky - Good conditions, 50x to 75x per inch aperture;
3) Bright stars twinkle from the horizon to nearly half-way to the zenith (directly overhead) - Average night, 25x to 50x tops per inch aperture;
4) Bright stars twinkle to the naked eye all across the sky (usually when it is deep clear) - Very poor conditions, limit your observing to 100x TOTAL power or less; a good night for deep sky observing.

CONCERNING EYEPIECES - There are many "super-duper wide field" high power eyepieces on the market today which are very expensive, and indeed very good. But I have yet to find one as good as a medium power eyepiece coupled with a top-quality 2x barlow or telenegative lens. High power observation is just that - looking at something CLOSE. Wide fields are nice, but not necessary. It has been my experience that the wide field short focal length eyepieces (for high power) suffer from light loss and detailed resolution. Using a lower power and more simple (i.e., I love Plossls and even the older Orthoscopics) eyepiece has the advantage of reducing eye fatigue and giving you some extension AWAY from pressing your face against the telescope!

OBJECT YOU ARE LOOKING AT - This is an important, and often overlooked, aspect of selection of magnification. Deep sky objects are "extended diffuse" objects, meaning that their brightness is scattered over an (sometimes great) area. Using very high magnification results in losing contrast, and thereby some of the finer detail of faint extended objects; in addition many of the deep sky objects are larger than can be accommodated in short focus eyepieces. ALSO...remember I mentioned that the field of view becomes DARKER when one increases magnification? Very true with deep sky objects. Up to about 100x or 150x, regardless of telescope used, the increased power actually INCREASES the object contrast by slightly darkening the sky around it; however, after that is the point of diminishing return.

Of course, the planets and moon are also "extended objects," but they suffer from no lack of brightness, so the 75x to 90x per inch on a steady night is great for such observing.

In short, never over-rate "power." It is the least important thing that your telescope does for you. Indeed, I pity the poor youngster, so excited about astronomy and his or her first telescope. They pull out the 4mm eyepiece and the triple power barlow (454x) and aim the darned thing at the sky, simply awaiting their first glimpse of the beauty of God's creation.

What do they see? Nothing.

But try convincing the same novice to DROP the magnification down to 25x or 40x.....and the thrill is gone.

I mean, if you can MAGNIFY SOMETHING, what the heck do I even want a TELESCOPE for?

Good luck, and keep it down....please.

P. Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain

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