Since I live in an area of high light pollution, this is entirely academic, but for long time I’ve enjoyed astronomical pictures. Now, the bigger the telescope, the better the image. But how much better?
Let’s say that I’m interested in the solar system. What would I see with an 8" telescope? A 10" telescope? A 12" telescope? And so on up to 20" (which seems to be the limit of transportability, depending on the type).
Equally, let’s say that I’m interested in the stars. What could I see at each size?
Why are telescopes so long? Can’t you just increase the curvature of the mirrors?
And why are telescopes so expensive? How much more exact are they than spectacle glasses?
What you can see with a telescope depends mainly on its light gathering capacity. The light gathered is proportional to the surface areas of the diameters you mention. Generally, with scopes of those sizes, you are looking at a reflecting model (has the concave mirror at the bottom end, and a small angle mirror that passes the image to an eyepiece near the open end and perpendicular to the tube.
The magnification is (IIRC) the focal length of the eyepiece divided into the focal length of the mirror, and does not depend on the mirror size at all, theoretically. However, a larger mirror allows you to use higher magnification more effectively than you could with a small one. Given the same magnification, say 7x (typical of binoculars) the larger mirrors would reveal more of the faint stars. Many of the beautiful photos you’ve probably seen were taken at low magnification, but with large lenses or mirrors.
With an 8-inch, you could see Mars, Jupiter, and Saturn very well. Under good viewing conditions you would easily see the rings of Saturn and the Cassini division, the Martian icecap and color variations, and the four large moons of Jupiter. For Jupiter itself, you should be able to clearly discern the horizontal banding.
The moon would be awesome. I used to have a 4 1/2 inch, and could effectively use 300x on the moon. This gets you to where you can see objects a few miles in diameter. During the time of the new moon, if you look at the terminator between night and day, you can see mountain shadows reaching across craters, like something from the cover of an old Arthur C. Clarke novel. Damn, I gotta get back into this! but I live in one of the worst light pollution areas anywhere.
The two advantages to a telescope over the naked eye are, you can see fainter things, and you can see more detail (resolution). These are determined by two properties of the telescope, the aperature and the focal length. A larger aperature helps with both: More surface area means you can grab more photons, and more width will theoretically improve your resolution. The only limitations on aperature size should be portability and budget (the price increases very quickly with aperature, after a certain point).
Focal length, on the other hand, is a trade-off. You’ll get better resolution with a longer focal length, but you can see dimmer objects with a short focal length. Which is more important depends on what you want to look at, but for most things that amateurs like to see, resolution is better. There are plenty of objects with interesting details that are bright enough to see easily through a small telescope, but all you’ll get from seeing dimmer objects is bragging rights at being the first person in your astronomy club to see a particular faint fuzzball.
Resolution is inherently limited by the atmosphere, as well. I think most telescopes are designed so that the resolution limit of the scope is about the same as that of the atmosphere, since there’s no point making it any better. Of course, this does depend on local conditions, and some places are better than others.
Above all, never buy any telescope that makes a big deal out of its magnification. Magnification without the resolution to back it up is useless: It’ll just turn a small fuzzy blob into a big fuzzy blob. And you can get whatever magnification you want on any telescope you want, by swapping in different eyepieces.
This may be deja vu to those of you with good memories since I’ve said it here several times before:
They’re easy to use, help you to become familiar with the sky in an easy way (an essential, IMO, before using a scope), more than portable and convenient, and can be used for, say, bird watching, if you ever get tired of astronomy.
I have an 8" scope but I still wind up leaving it idle an awful lot while I take a peek through my 16 X 70 binoculars. Just spectacular!
Most of this has been covered, but I’d like to emphasuze a couple of things.
The large size of the primary mirror on a reflector (or the front lens on a refractor) is used to gather light, but is also necessary for good resolution. About 175 years ago, an English clergyman whose hobby was astronomy noticed that he was able to resolve two close-together stars with a large diameter (but otherwise not-good-qualitry) lens, but not with an excellent lens which was of excellent quality. The bigger the lens/mirror, the smaller the image and the finer the resoultion.
If you’re jusy neginning, that’s probably not your highest prioority, but it can make a difference if you want to observe double stars, or fine features on the moon, or something. But my hand-held binoculars will resolve the Galilean satellites of Jupiter.
Magnification of the ratio of the focal lengths, so you could make a very high magnification, very short telescope, but it won’t have very good resolution, as Chronos points out – you’ll get a big fuzzy blob instea of a small one.
The very first astronomical telescopes were of enormous length, because they had a hard time making really good optics. A good mirror should be parabolic, but the natural process of telescop grinfing produces spherical, not parabolic, surfaces. Parabolas and spheres are almost the same over a small region (relative to the radius of curvature), so they made really long radius mirrors in order to get really large mirror diameters that were indistinguishable from a parabola. The next stage in telescope evolution was to hand-grind corrected mirrors that were more accurately parabolas. Today, it’s easy to turn out parabolas (or even more exotic aspheric forms), and you can buy a very good quality compact telescope from places like Edmunds.
there’s probably an aateur astronomy group near you – look them up via the internet and attend one of their sessions. I’ve attended some at the McAuliffe planetarium in Concord, NH, and I’m impressed with the size and quality of the home-built telescopes.
I haven’t seen any prices thrown around here. So, if I wanted to see the rings around Uranus (heh, heh, i always wanted to say that) what type of money would I have to drop for a good quality telescope?
Simple telescopes (Newtonian reflectors, refractors) have secondary off axis aberations (coma* most notably) that become much worse as focal length is decreased. Grinding the deeper mirror is more expensive, as more material must be removed, and testing and figuing become more critical, as more accurate measurements must be made so that measurement errors scale with the shorter focal length. Also, the secondary mirror in a newtonian must be quite a bit larger when the FL is reduced, this slightly reduces light gathering ability, but puts a bigger"hole" and increase diffraction. A shorter tube is also harder to baffle to eliminate glare.
As a result, most telescopes are designed with the longest focal length that will accomidate the desired field of view. In a Newtonian, this is usually driven by how much comatic abberation one is willing to tolerate at the field edges.
*Comatic aberation causes stars not located precisly in the center of the field of view to become comet shaped. It becomes worse and worse as the edge of the field is approached.
To see Uranus? A few hundred will do just fine, for an 8-inch Dobsonian. You can further save money by just buying the optics and building it yourself.
But you’re not going to see the rings with anything from Earth. I don’t think they were even discovered until the Voyager flyby.
The rings of Saturn, on the other hand, I can see through an old pair of binoculars I got for five bucks at a garage sale. Not very clearly, but you can tell they’re there.
Sadly, the article does not mention the size of Herschel’s telescope. It could have been anything from 6" to 1.3 meters; with a mirror made of copper-tin alloy.
Just one question I promise! Do any Dopers have any experience / care to comment on how well (or not) the computerized hobby telescopes work? I’ve always been interested in these things - as they claim to be able to go right to a star/planet/etc from their database and then track it through the sky.
Lots of good advice here so far, but to really answer the questions you raised would take a book.
Therefore, what you really need before plunging in and buying a telescope…is a book! Hie thee to a library and check out a few good books on beginning astronomy. One I highly recommend is A Backyard Astronomer’s Guide which will give you loads of information on telescopes, how to observe and everything else you need to know.
Contact a local astronomy club if possible, and go to one
of their star parties where members set up scopes. You can look through them and get a better idea of the pros and cons of each type (and costs).
Without getting some prior knowledge of a complex subject, it would be like asking, “I want a car. What should I get?”
I second the suggestion to start with binoculars. You will be amazed at what you can see with a good pair of, oh say, 10X glasses. A hint: when you first start, get to a dark site, put out a patio lounge and lay down on it. Sitting or standing and observing with binoculars will give you a crick in the neck.
Or, if the binoculars you get have a tripod scoket, and they should, then use a good sturdy camera tripod.
Start by looking at the terminator line on the moon which will amaze you. Then look a Jupiter and you may see 3 or 4 of her moons. Then check with your book or guide and find a nice star cluster or two. Then take a gander at the Milky Way. Look for the Andromeda Galaxy. All this stuff will be wonderful in binoculars.
Learn the sky first, study the books and then pick a telescope. Oh yeah, don’t hesitate to look for used scopes, many great bargains.
You have to set it at the start of the night (or at least, when you mount it: You could in principle keep it mounted forever), so it knows what it’s doing, but once you do that, it’s just as it says: Pick any object in its database, and it’ll take you there. The main difficulty is if an object isn’t in its catalog, but you can probably enter new objects and coordinates, or aim it manually and tell it to track that point.
To set it at the start of the night, the usual procedure is to point it manually at three known stars. I’ve never actually done this myself, but I imagine the procedure is something like the computer telling you to point at Vega, then when you’ve got Vega centered in your field of view, you hit “OK”, and then it tells you to point at Deneb. It may put you in the right neighborhood to start by guessing based on assuming where it is, that it’s on level ground, that its internal clock is right, etc.
OK, here’s a further question to confuse the issue. (BTW, I like the OP question).
What kind of bucks are you talking to get a telescope that will let you see a nebulia with distinguishable colors? You know, the cool stuff you see in the astronomy photos. Let’s just say that the dollar amount isn’t as important as not wanting to waste money on a POS that just gives you a little clearer view than the old binoculars. Are we talking, $1000, $10,000, $1 million? Is it just something that should be left to the certified astronomers or can an amateur spend a few bucks and see some really cool stuff?
The local observatory has a 14 inch Cassegrain, in which the ring nebula (M57) appears as a hazy white ring. No color is visible with this ~$5,000 scope. The great nebua in Orion (M42) is nice, but typically, you need to photograph nebula to get much color out of them. Star clusters, galaxies and planets are spectacular in this scope.
I’ve just watched a programme about the Voyager ermm, voyage, and they were interviewing an astronomer who had been convinced by observations that the rings (or, actually, arcs, not full rings) were there, and she had some difficulty persuading the rest of the team, and trying to get them to program in some observations. Since the flyby was so short, this was a risk, but they went with her suggestions and, sure enough, Voyager observed the arcs.
IIRC, the rings of at least one of the gas giants was first detected by ocultations of stars it was passing in front of – but that may have been Jupiter.
