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Tools and Toys of the Amateur Astronomer
There is only one things amateur astronomers love more than gadgets and that's showing them off.
The number of different tools we use just to look at the sky is mind boggling.
I'll try as best as I can to cover as much as I can, as clearly as I can.
Telescope Types
Many different optical telescopes have been created. There are three main types, refractors,
those using lenses, reflectors, those using mirros, and catadioptrics, using both mirrors and
lenses. Each type is broken up into different designs, each of which is used to solve inherent
problems. I'd like to go through them all, however, that's complicated and boring to many. Let's
begin.
Refractors
Refracting telescopes, what most people picture when they think of a telescope, were first
invented in the early 17th century by the Dutch, and first pointed towards the heavens by
Galileo soon after. The refractor design basically uses a set of convex lenses to bend light
to a point and magnify it's brightness and scale. Producing good-quality lenses is extremely
difficult. Two surfaces must be ground and polished to an exact shape using glass with
absolutely no internal flaws. A lens can only be supported on it's edges so large, heavy
ones are easily distorted by their own weight, ruining images. Another flaw of lenses has
to due with the wave nature of light. Shorter wavelength light (like blue light) is refracted
more than longer wavelength (redder) light. This is called color dispersion, it's what makes
a prizm break up light into a rainbow. Unless corrected, images will be discolored and
distorted beyond recognition, especially on shorter focal length scopes. To help solve this
problem, called chromatic aberration, a concave (negative) lens using glass of different optical characteristics is
placed directly behind the first lens. This is called an achromatic design. This second lens
brings the blue and red wavelengths of light closer together at the focal point, but only
closer. The apochromatic refractor was created to better solve this problem. One or more
additional lenses and/or lenses of low dispersion glasses are used to bring colors to a
closer focus. Most do an excellent job of this, others not so good. This color usually only
shows up on objects like brighter stars, the planets, and the moon and typically at higher power.
A well made refractor is a big investment though some good ones are available at lower prices.
Their lack of a central obstruction (secondary mirror) allows them to produce the sharpest,
most contrasty images you can find. Planetary views are usually unmatchable but if it's very
faint deep-sky objects you want, a refractor is not ideal due to cost. This high cost per
inch of aperture pushes them out of reach for most people.
Reflectors
The reflecting telescope was invented by Isaac Newton. The Newtonian reflector uses a concave
mirror at the bottom of a tube to collect light and focus it to a point. A second mirror placed
near the top of the tube at a 45-degree angle reflects it out the side. This design makes
people unfamiliar to it scratch their heads, not knowing where to look. The main advantage
of a reflector is the relative ease in their manufacture. The parabolic surface is much easier
to make precisly, red light reflects at the exact same amount as blue light, a mirror can be
supported on the back as well as the sides, and light does not have to travel through glass
eliminating the need for perfectly clear homogeneous glass. The newest and largest telescope
mirrors aren't even made of glass but opaque ceramics.
A Newtonian Reflector telescope is the least expensive design available. You can find a good
quality scope which gathers ten times the light of a 4" Takahashi refractor for a good deal
less money. There is a compromise optically though. The secondary mirror, being in the light
path, blocks some of the light going to the primary mirror. If made too small in an attempt to
minimize light loss, you may not reflect light from the outer edges of the primary out the side
creating light loss at the field edges, vignetting. Light loss means less contrast and less
sharpness. Every scope is a compromise.
Catadioptrics
Catadioptrics, the most popular being the Schmidt-Cassegrain, use a combination of lenses and
mirrors to bring light to a focus. In a Schmidt-Cassegrain, a fast (~f/2) concave primary
mirror on the bottom of the tube reflects up to a convex secondary placed at the top of the
tube. The secondary reflects the light back down to the eyepiece through a hold in the primary.
The lens used is a slightly curved piece of glass called a corrector plate over the top opening
to eliminate optical defects in the primary. This glass must be figured precisely to form a
specific curve making the design more difficult and more expensive to produce. This design is
extremely compact for its aperture. The secondary mirror is actually a hyperbolic magnifying
mirror used to increase the effective focal length of the primary usually to around f/10 making
them extremely compact. An SCT has a much narrower field of view and a larger loss of contrast
due to toe larger secondary mirror.
Another popular type of catadioptric is the Maksutov-Cassegrain using a heavy miniscus-shaped
corrector plate with an aluminum-coated portion inside as the secondary. The Mak-Cass is somewhat
heavier than the SCT and usually has a longer focal ratio, around f/13. The Mak-Cass costs a bit
more than the Schmidt-Cass but the quality is quite good. As with an SCT, they have a narrow field
and due to their large central obstruction, may not be the best for picking out planetary detail.
There is also a Maksutov-Newtonian using the same type of corrector plate as the Mak-Cass, but
with the optical setup of a Newtonian. A well-made Mak-Newt can have near refractor-like sharpness
and contrast and produce no diffraction spikes on brighter objects like on classical Newtonians.
Mak-Newts are around $1000 for a 6-inch, almost 3-times that of a regular Newtonian of the same
aperture.
The Magnification Myth
First and foremost before you go shopping, any telescope with packaging that mentions magnifying
power (something like "650X" or "650 power") isn't worth its weight in raw sewage. Most objects
in the sky need relatively little magnification to see clearly. Raising the magnification makes
images dimmer and fuzzier.
I commonly get questions from visitors at star parties like, "How far can you see?" A scope is
not rated on how far it can see, but how MUCH it can see. The maximum magnification for any
telescope is at best around 60X per inch of aperture. Atmospheric conditions can blur images
even more, limiting the magnification to an even lower value. Increasing magnification just
magnifies the blur. Final thought, any 60mm scope advertized as having a magnification of 650x
is garbage no matter what magnification you use it at.
Choosing
No optical design is better than another in the long run. Some perfom better on certain objects
in certain situations, some are simpler to use, and some are easier to transport than others.
Again, think about what you really need to do, what you really want to do, and where you intend
to do it. Refractors and SCT's are typically more portable, but may be more complex and more
expensive. Newtonians are the least expensive design but may be too large for your car, or
even your spouse. More aperture is better, but not always, depending on your location. As
far as skill level, each optical design is basically as easy to use as the next one.
Catadioptrics may be a complicated design, but the manufacturer takes care of the difficult
parts for you. A Newtonian requires optical alignment, or collimation, typically before each
use to maximize performance (all scopes require collimation, Newtonians tend to lose alignment
easiest depending on design). This is usually quite easy as long as the mirror cell is made for
easy adjustment. Most of them are, some, surprisingly, are not. What is it you'll be concentrating
your observing time on? If it's high-power planetary views, refractors are ideal. If you plan to
view faint deep-sky objects from a dark-sky site, a medium to larger size Newtonian is basically
the only choice.
Mounts
A scope must be mounted on something ridged. You can't hold a telescope and expect to be able to
hold it steady. The mount used for a telescope must be a ridged support which allows smooth
movement and all axes. There are two types of mounts, alt-azimuth and equatorial.
Alt-Azimuth
Alt-Az mount, short for altitude-azimuth, is the simplest of all the mounts. The azimuth axis
moves side-to-side, while the altitude axis moves up and down. The most famous of these mounts
is the Dobsonian mount. They're extremely simple in design, and are very, very rigid. Because
of their simplicity, it's somewhat difficult to make a Dobsonian which is truely bad. The most
difficult part in their construction is smooth movement. A Dobsonian mount is stabilized by
gravity. The point of rotation in altitude must be precisely at its center of gravity. Teflon
bearings are used to provide just enough fristion to prevent unwanted movement, but not so much
as to prevent slight adjustments. Alt-azimuth mounts are less than ideal for high power views
as their axes are not lined up with Earth's rotational axis making movement by hand or the
attachment of a motor drive more complicated. Their cost and simplicity in basic use and setup
is ideal for beginners or those who like to setup quickly. They are also ideal for deep-sky
viewing allowing more money to be spent on bigger, better optics.
Equatorial
An equatorial mount can simply be thought of as an alt-az mount with its azimuth axis pointed
directly at one of the celestial poles. These axes are right-ascension, right to left, and
declination, up and down. This allows you to easily follow an object's movement by adjust
only the right-ascension axis. Pointing them at objects can be confusing for beginners.
Unless you're at the North or South Poles, either axis can be up and down or right to left
depending on where in the sky you're pointing it. Most EQ mounts have setting circles allowing
objects to be located by their celestial coordinates with ease once aligned.
The two types of EQ mounts most amateurs will encounter are the German and the fork mount.
A german mount is the one which make newcomers scratch their heads due to its awkward setup.
Most EQ mounts must also be balanced which reduces stress on motors ar gears, but most come
with counterweights making this much easier than balancing a Dobsonian mount. Equatorial mounts
are the most expensive of the two types. Precisely made mounts that have smooth movement and
rock-solid rigidity push $1500 all the way up to around $8000. Cheap ones are small and not very
good for high-power use, jiggling at even the slightest touch. Equatorial mounts are also more
bulky and must be assembled on site and manually aligned to the pole. Typical visual use only
requires that it be pointed toward the pole while photography requires it to be dead on to prevent
field rotation.
Choosing
When deciding what mount you would like to get, think about some of the same things you would when
deciding on optical design. Dobsonians are great for quick setup and straightfoward use, but can
be frustrating at high power. Equatorials must be transported in pieces and assembled on site.
Alt-Az mounts are relatively inexpensive while EQ mounts can be costly. If you want the ease
of a Dobsonian mount and the tracking capabilities of an equatorial mount some Dobsonian scopes,
those made with Sonotube, can either be adapted for us on larger EQ mounts, or computerized drive
systems offered by some smaller companies can be attatched. These drive systems are quite
expensive and may still use the stock mount, making them less than ideal for photography.
Remember what you intend to do later on.
Some mounts made today come with automatic "go-to" motor drives and software. This system
is built into the mount, but it typically comes together with a scope as a unit. Making it
extremely easy to find nearly any, a go-to mount can be an invaluable tool for the beginner.
The only thing you need to find on your own is two or three bright stars for alignment, but
finding bright stars is easy, finding faint objects manually requires skill. Not everybody
is so patient in the field. If you are one of these people try not to abuse the computerization
early on. If you let the scope do everything, you may not learn many of the key elements of
astronomy. You may also lose valuable observing time if something breaks or your battereis go
dead. I usually take pride in being able to find objects faster than someone with many years
more experience than me. If you have one, don't abuse it early on.
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