Archive for September 17th, 2009


Reverse aperture fever: the allure of small telescopes

September 17, 2009
The 30-inch Obsession--never be insecure again!

The 30-inch Obsession--never be insecure again!

One of the mantras of amateur astronomy is that “Aperture rules”. There is a lot of truth to this. Set up a telescope on the sidewalk and the first two things passersby will ask are “How much does something like this cost?” (answer: less than you probably think) and “How much can it magnify?” (ditto). Telescopes are unavoidably bound up with magnification in the popular imagination.

Astronomers, amateur and professional alike, are usually much more concerned about a telescope’s light gathering ability and its ability to resolve fine details, both of which depend on aperture. Resolution scales directly with aperture, so a 6-inch scope has twice the resolution of a 3-inch scope–important for teasing out details like the cloud bands of Jupiter, the Cassini division in Saturn’s rings, and for resolving globular clusters from patches of fuzz into clouds of stars. Light gathering ability is a function of area, and thus scales with the square of aperture. A 6-inch scope has four times the light gathering ability of a 3-inch scope, so it will pick out many more faint galaxies and nebulae than the smaller instrument.

There are a couple of caveats, or rather one caveat that cuts a couple of ways. How much you can see with a telescope depends heavily on the sky. If the light pollution is bad enough to swamp out the signal from those faint galaxies and nebulas, the advantage of a 6-inch scope over a 3-inch scope is somewhat reduced. To a certain extent you can fight light pollution with more aperture, but as many or more people choose to fight it by buying more portable scopes than can be easily transported to dark sky sites. I really have seen more with my 3.5-inch scope under dark Oklahoma skies than I have with my 6-inch scope here in the light-polluted swamp of LA county.

The other limitation imposed by the sky is seeing, or atmospheric turbulence, which manifests at the eyepiece as a roiling blur over whatever you’re trying to see. The less atmosphere you have to look through, the better, which is why observatory telescopes are on mountaintops, and why seasoned stargazers try to catch their targets as far from the horizon as possible.

Seeing is the main reason why astronomers are rarely worried about the magnification potential of a particular telescope. The general rule of thumb is that a decent quality telescope can magnify 50x per inch of aperture before the image starts to break down. The limits, then, for my 3.5-inch and 6-inch scopes are 175x and 300x, respectively. But most nights in most places seeing will ruin the image before the inherent limitations of the optics. In other words, the sky gives out before the scope.

So on the telescope ledger we have light-gathering and resolution on the “bigger is better” side, balanced against cost and transportability. Any scope that hits the sweet spot in the middle is bound to sell like hotcakes–6- to 8-inch Dobsonian reflectors and Schmidt-Cassegrains are probably the most popular “serious” telescopes in the world, because they gather quite a bit of light but are not too much, cost- or weight-wise, for average folks.

My 3.5-inch scope set up for birding.

My 3.5-inch scope set up for birding.

And yet…and yet. There is something about small telescopes that many people find hard to define and equally hard to resist. My fellow astro-blogger Treehopper recently wrote a great post about small telescopes, and an e-quaintance in Singapore has dedicated his whole blog to the subject. I suspect that part of the appeal is the ability to really take a small scope to the edge of the envelope. I often sit down behind a little scope with the attitude, “C’mon, little guy, show me what you can do!” I find that with small scopes I am often pleasantly surprised at how good the views are, and with big scopes I am often disappointed that the sky isn’t better. Not the fault of the big scopes, but a factor nonetheless.

My reverse aperture fever is also grounded in the perhaps irrational conviction that stargazing should be inexpensive, accessible, portable, and fun. Yeah, a 12-inch truss tube Dob would show me roughly a zillion times more than my 3.5-inch scope, but I get tired just thinking about wrassling one of those things around. Being able to grab my little scope on its tripod in one hand, a folding chair in the other, and to start soaking up the light of long ago and far away about 30 seconds after I get the urge–I like the idea of doing that, and I like actually doing it even better. Nothing unique here–lots of amateur astronomers have “grab-n-go” setups–it’s just that for me, grab-n-go is where it’s at. Since I got a decent mount for my little scope, my Dob has only gotten used once or twice. And who cares? As Uncle Rod says, “There’s no wrong way to do amateur astronomy.” (My personal emendation: “except to not do it at all”.)

Another factor–and a particularly appropriate one this year–is the knowledge of how much our forebears achieved with modest instruments. Haven’t you ever wanted to be Galileo, who pointed his ‘military instrument’ to the moon out of nothing more than simple curiosity, and ended up discovering how the heavens go? From Turn Left at Orion (p. 202), one of the best books for beginning telescope users:

Galileo discovered the four major moons of Jupiter (forever after called the “Galilean satellites” in his honor); he was the first to see the phases of Venus and the rings of Saturn; he saw nebulae and clusters through a telescope for the first time. In fact, a careful checking of his observations indicates that he even observed, and recorded, the position of Neptune almost 200 years before anyone realized it was a planet. He did all this with a 1″ aperture telescope.

Charles Messier, who found the hundred deep sky objects in the catalog that bears his name, started out with a 7″ reflector with metal mirrors so poor that, according to one account, it was not much better than a modern 3″ telescope. His later instruments were, in fact, 3″ refractors.

The point is this: there are no bad telescopes. No matter how inexpensive or unimpressive your instrument is, it is almost certainly better than what Galileo had to work with. It should be treated well. Don’t belittle it; don’t apologize for it; don’t think it doesn’t deserve a decent amount of care.

Telescope vs beverages

My little scope with beverage containers for scale.

The counterpoint to “Aperture rules” is “The best telescope for you is the one that shows you the most”. I spent my first year and a half in amateur astronomy on a quest (to be detailed in future posts) to find that perfect scope. That it turned out to be a little thing the size of a 2-liter soda bottle is both a happy accident and welcome confirmation of my conviction: stargazing should be inexpensive, accessible, portable, and fun. As Tony Darnell concluded in one of the best pieces ever written about telescopes, “If you’re outside looking at the stars without a big smile on your face or a feeling of awe in your heart, you’re not doing it right.”

The moon through my "humble" 3.5-inch scope.

The moon through my humble 3.5-inch scope, photographed with a humble point-and-shoot digital camera humbly handheld up to the humble eyepiece. Tremble before my awesome humility!