Since the Mars hype is fading, I’m seeing sales on telescopes here and there. Probably left-over stock from merchants stocking up for the big astronomical event (or increased awareness on my part but it doesn’t really matter).
Anyway, I see long, skinny telescopes and then the short, large-diameter variety. I realize it’s probably the difference between refractive and reflective types but is there a benefit or penalty to either (other than what fits in the car’s trunk)?
A larger aperture means greater light-gathering ability as well as higher resolving power. Long focal length combined with a wide aperture means a higher f: ratio which means brighter images. Ignore the magnification hype. Above a certain value, increased magnification does not buy you greater resolving power. No small telescope is going to go past about 200x useful magnification. After that, you’ll be making fuzzy details into larger fuzzy details. Tube length is not a good indicator of focal length, since some types, called Cassegrains reflect the light the full length of the tube twice. These have the eyepiece at the rear of the tube. Newtonian reflectors have the eyepiece at the front end.
Mostly yes because it’s more expensive to make big lenses than big mirrors. Refracting telescopes have to contend with that old devil chromatic aberration. That is that light refracts at different angles based on it’s wavelength/color. This is why cheap binoculars show red and blue fringes for things off the center axis. You can correct this with multi element lenses that use different types of glass in each element but that dramatically drives up price. Mirrors have no chromatic aberration and that’s why Sir Isaac whositz invented his namesake Newtonian telescope.
Do a Web search on “buying your first telescope.”
Diameter - “Aperture,” as it’s known - is the primary measurement of a telescope. The things in the sky aren’t necessarily too SMALL to see, they’re too DIM. The apparent size of the Andromeda Galaxy is three times wider than the Moon, and it’s right up in the sky right now, so why haven’t you noticed a great big spiral galaxy three times bigger than the moon? Because it’s too dim to see very much of it. The more aperture you get, the more light you gather. With a six-inch aperture, the Andromeda galaxy is very apparent, and looks like a central point of light with a fuzzy oval around it. Add more aperture and you start to see its spiral arms and structure. Less aperture and you would see less and less of it.
Long skinny telescopes are of course refractors. Because lenses are expensive, they tend to be much more expensive for the amount of aperture you get. A six-inch Newtonian reflector on a cheap Dobson mount will cost you three or four hundred bucks, but a six-inch refractor would cost more like $1200-$1500 for a simple model.
Refractors do have the advantage of providing more contrast - more precise detail can be seen in them - because there’s no obstruction. If you look inside a reflector, you can see the secondary mirror is smack dab in the way. You might wonder why you don’t see it when using the telescope; it just gets focused out of view. But the obstruction does slightly reduce contrast. For that reason, some people like to use refractors when they look at planets (which are very bright and don’t need much light gathering, but have lots of little details) and use reflectors when looking outside our solar system, at objects that are dim and need lots of light gathering.
Basically you have three main kinds of telescopes:
Refractors, long skinny guys. Advantages: Good contrast, tend to be a little easier to fit into your car. Image appears right side up, whereas in the other models it’s upside down. Disadvantages: Less aperture for your money, chromatic aberration.
Newtonians - simple reflectors. There’s a mirror at the bottom of an open tube and it bounces the light up into a secondary mirror at the top and it bounces the light through the eyepeice and into your eye. Advantages: Maximum aperture for your money, you can get 10" models for cheap. Simple and easy to use. Disadvantages: Tend to be bulky, require occasional collimation (Adjustment of mirrors to ensure they’re aligned right.)
Cassegrains, of which there are several kinds. These are reflectors that look shorter than Newtonians. Light comes in, bounces off a curved mirror, straight into the secondary, and back down through a hole in the bottom, out the eyepeice, into your eye. Advantages: More compact than Newtonians, with all the advantages that come with that. Many come with built in goto computers. Disadvantage: Mondo pricey.
The magnification of a scope is determined by the eyepeice and the scope’s focal length. Focal length/eyepeice focal length = magnification. A standard beginner’s newtonian might have a 1200mm focal length, so if you put a 10mm eyepeice on it, bam, 120X magnification. As pointed out, however, there’s a practical limit to this; you can’t just slap a 3mm eyepeice and expect 400x viewing. Rule of thumb is that the absolute max you can resolve is fifty times the scope’s aperture in inches. And that’s if things are PERFECT.
Having said that, you don’t really need a lot of magnification. You’ll usually use your low-to-mid power magnifications; a lot of deep sky objects look better than way and it’s easier to find your way around.
The other thing to bear in mind is a mount. Mounts come in all sorts and if you get a shitty one you’ll regret it; a wobbly mount ruins the fun. A big telescope, however, requires a big mount, and these suckers get heavy. A bigass 10" Meade Cassegrain with a properly sturdy mount and a computer will weigh a hundred pounds and up; of course you can break it down a bit but we aren’t talking about something you just swing over your shoulder. A beginner should strongly consider a Dobsonian mount for a Newtonian scope, which is basically just a wooden swivel/pivot assembly. It’s cheap and stable. Useless for photography, but an amateur should just learn the sky.
Of course, you should buy a pair of binoculars first. You’d be amazed what cool stuff you can see just with that.