When comparing telescopes, how do you tell which one can see the farthest?

Answer 1

If you are new to using a telescope, understand this general rule of thumb: Highest power does not always equate with "farthest seeing."

The relative size of the lens or mirror can indicate the approximate power (about 50 X magnification for every usable inch of aperture). The amount of magnification is just an indicator of how small (or far away) the telescope can "see."

The difference between a very inexpensive 500 X scope and a very expensive one is that the cheap one will probably have both a fuzziness and a color separation (not all colors focused in the exact same place). These problems (called aberrations) are corrected by the coatings and grinding precision which add to the cost of the telescope.

Check out the sites below for additional details:

Answer 2

By far the most important factor is aperture, that is, the diameter of the main lens or mirror. Larger aperture means more light gathered, means fainter and more distant objects are visible. Aperture also determines how much magnification you can reasonably use.

Don't be fooled by little telescopes promising big magnifications. The only astronomical telescopes that are sold based on magnification are "department store trash" scopes. The fact is, with the right eyepieces and barlow lenses you can get as much magnification as you want out of any telescope. More than 50x per inch (2x per mm) is generally useless, though.

The other thing you'll need to see the farthest is dark skies. Those distant galaxies are very faint and you can't see them from a light-polluted city no matter how big a scope you have, because the city sky is brighter than they are. There *are* lots of things you can see from the city, but most amateur astronomers travel to dark sky sites at least occasionally.

Answer 3

Two factors affect this. The first is its focal length. A longer focal length will give you a narrower field of view, essentially a higher degree of magnification. The other is how big a mirror it uses, which is directly related to how much light it can gather. For example, you can have a telescope with very high magnification, but still not make very dim objects visible because of a small reflector. Virtually all of the ones sold have this balance optimised already, so a good general rule is, the bigger the mirror, the more you will be able to see.

Answer 4

Lens size and body tube size (the length of the telescope). Usually the telescopes that have the right combination of large lens and long body tube are the best for seeing things like the moons around Jupiter.

Answer 5

Seeing the "farthest" basically means being able to gather a lot of light over a long period of time. That's how Hubble does it. To gather more light basically means having a bigger lens or mirror, but there are other things to consider.

Check http://www.telescope.com/content/learningcenter/contentmain.jsp?iCategoryID=20&CCNavIDs=19,20 for the basics of choosing a telescope.

JMB

Answer 6

injanier is right. More distant objects are dimmer, so you need a larger diameter mirror to collect more light to see those dimmer objects. That is why professional telescopes have become so large.

Answer 7

The field of amateur telescope making is considered an offshoot of the amateur astronomy community. Amateur telescope makers (sometimes called ATM’s), as their name implies, are not paid professionals. They build their telescopes for the enjoyment of the hobby, or so they can make a personal contribution to the field of astronomy.

Contents [hide]
1 The beginnings of a “hobby”
2 ”The Poor Man's Telescope”
2.1 Mirror making
2.1.1 Foucault test
2.1.2 Aluminizing or "silvering" the mirror
3 Telescope design
3.1 The Dobsonian: A case in point
4 Telescope making books and other published information
5 See also
6 External links



[edit]
The beginnings of a “hobby”
Ever since Galileo took a Dutch invention and adapted it to astronomical use, telescope making has been an evolving discipline. Many astronomers after the time of Galileo built their own telescopes out of necessity, but the advent of amateurs in the field building telescopes for their own enjoyment and education seems to have come into prominence in the 20th century.

There were many published works that sparked interest in building telescopes, including Rev. William F. A. Ellison's 1920 book "The Amateur's Telescope" and several articles in Popular Astronomy by Russell W. Porter, including one appearing in March, 1923 about the Telescope Makers of Springfield. In 1925 Albert G. Ingalls featured Porter and the Springfield Telescope Makers. in an article he wrote for Scientific American magazine. There was so much public interest, a whole series of articles were written by Ingalls on the subject. Those articles (and later three books titled “Amateur Telescope Making” Vol. 1-3) helped people around the world take on the task of constructing their own instruments. In the US, the ready supply of surplus optical components after World War 2 and later Sputnik and the space race greatly expanded the hobby.

[edit]
”The Poor Man's Telescope”

A 6” Newtonian reflector built by a school student on display at StellafaneThe types of telescopes that amateurs build vary widely. They range from the very simple to complicated designs including Refractors, Schmidt Cassegrains and Maksutovs. The most popular telescope design is the Newtonian reflector, described by Russell W. Porter as “The Poor Man's Telescope”. The Newtonian has the advantage of being a simple design that allows for maximum size for the minimum expense. And since the design employs a single front surface mirror as its objective it means (for the person creating their own optical elements) that there is only one surface that has to be figured, as apposed to three for the Maksutov and four for the refractor and the Schmidt-Cassegrain. Typically a Newtonian Telescope of 6” aperture is considered a good starter project.

[edit]
Mirror making
Since the Newtonian Reflector is the most common telescope built by amateur telescope makers, large sections of the literature on the subject are devoted to fabrication of the primary mirror. The mirror has to be carefully "figured" to an extremely accurate paraboloid shape. The tools used to achieve this shape are surprisingly simple, consisting of a of similarly sized glass "tool", a series of finer abrasives, and a polishing "pitch lap" made from a type of tree sap. Through a whole series of random strokes the “mirror” becomes a perfect spherical section. At that point it only takes a slight variation in polishing strokes to create the desired paraboliod shape.

[edit]
Foucault test
The equipment most amateurs use to test the shape of the mirrors, a Foucault tester, is, like the tools used to create the surface, simple to fabricate. At its most basic it consists of a light bulb, a piece of tinfoil with a pinhole in it, and a razorblade.


Foucault test setup to measure a mirror
Shadowing on the surface of the mirror as it appears during Foucault testing, depending on knife-edge positionAfter the mirror is polished out it is placed vertically in a stand. The Foucault tester is set up at a distance close to the mirror's radius of curvature. The tester is adjusted so that the returning beam from the pinhole light source is interrupted by the knife edge. Viewing the mirror from behind the knife edge shows a pattern on the mirror surface. If the mirror surface is a perfect sphere, the mirror appears evenly lit across the entire surface. If the surface is parabolic, the mirror looks like a donut or lozenge. It is possible to calculate how closely the mirror surface resembles a perfect paraboloid by placing a special mask over the mirror and taking a series of measurement with the tester. This data is then reduced and graphed against an ideal parabolic curve.

[edit]
Aluminizing or "silvering" the mirror
Once the mirror surface has the correct shape a very thin coating of a highly reflective material is added to the front surface.

Historically this coating was silver. Silvering was put on the mirror chemically. This was then polished. Silvering was typically done by the mirror maker.

Since the 1950s most mirror makers have the coating applied by a firm specializing in the work. Modern coatings usually contain Aluminum and other compounds.

The mirror is aluminized by placing it in a vacuum chamber with electrically-heated nichrome coils that can sublime aluminum. In a vacuum, the hot aluminum atoms travel in straight lines. When they hit the surface of the mirror, they cool and stick. Some mirror coating shops evaporate a layer of quartz on the mirror, others expose it to pure oxygen or air in an oven so that it will form a tough, clear layer of aluminum oxide.

[edit]
Telescope design

A large fork mounted telescope and several other amateur built telescopes on display at StellafaneThe telescopes amateur telescope makers build range from back yard variety all the way up to sophisticated instruments that make meaningful contributions to the field of astronomy. Instruments built by amateurs have been employed in planetary study, astrometry, photometry, comet and asteroid discovery to name just a few. Even the “hobbyist” end of the field can break down into several distinct categories such as: observing deep sky objects, observing the planets, solar observing, lunar observation, and astrophotography of all those class of objects. Therefore the design, size, and construction of the telescopes vary as well. Some amateur telescope makers build instruments that, while looking crude, are wholly suited to the purpose they are designed for. Others may strive for a more aesthetic look with high levels of mechanical “finish”.

The difficulty of construction can be a factor in an amateur’s choice of project. For a given design the difficulty of construction grows roughly as the square of the diameter of the objective. For example a Newtonian telescope of 4 inch (100 mm) aperture is a moderately easy science fair project. A 6 inch (150) to 8 inch (200 mm) Newtonian is considered a good compromise size since construction is not difficult and results in an instrument that would be expensive to purchase commercially. A 12 inch (300 mm) to 16 inch (400mm) reflecting telescope is difficult, but still within the ability of the average amateur who has had experience building smaller instruments. Amateurs have constructed telescopes as large as 1 meter across (39 inches), but small groups or astronomy clubs usually take on such projects.

[edit]
The Dobsonian: A case in point
Main article: Dobsonian telescope

An amateur built Dobsonian telescope of moderate sizeSince the overall design of a telescope tends to be an exercise in creative problem solving the types of instruments amateurs come up with can be unique to their field. A case in point is the Dobsonian telescope. John Dobson, the originator of the design, wanted an instrument fully optimized for deep sky observing. Since Dobson was a monk living in a monastery at the time the telescopes he built had to meet the following criteria:

it had to be as large as possible
it had to be easy to build
it had to be constructed with commonly available materials
it had to be inexpensive
it had to be durable
it had to be portable (to get to dark sky locations)
His solution was not only a unique design, it also consequently caused a quantum leap in the size of instruments that amateurs build.

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Telescope making books and other published information
Albert G. Ingalls (ed.), Amateur Telescope Making (Vols. 1-3). Orig. edition: Scientific American; new rearranged edition: Willmann-Bell Inc.
Allyn J. Thompson, Making Your Own Telescope, 1947, Sky Publishing, ISBN 0-933346-12-3.
Jean Texereau, How to Make a Telescope, Willmann-Bell, ISBN 0-943396-04-2
David Kriege, Richard Berry, The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes, 1997, Willmann-Bell, ISBN 0-943396-55-7
[edit]
See also
Amateur astronomy
Group 70 — Attempt at making the world's largest amateur-built telescope
Optical telescope