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Telescopes: Frequently Asked Questions

Here are brief answers to many questions asked by telescope shoppers. Some of the questions require complete chapters in many of the excellent books about telescopes. One of the best is the second edition of "Star Ware" by Phillip S. Harrington, John Wiley & Sons, Inc., ©1998.

What is the best telescope to buy?
The "best" telescope is the one you will use the most! Ask yourself, "What do I want to see -- earth objects, space objects, or both?" The answer should reveal whether you need a "spotting scope" or an astronomical telescope - or one that will handle both terrestrial and space objects!

What are my choices in telescope type?
All telescopes collect light and focus the magnified image on a single point. We classify telescopes by their intended application and by their optical design. The two main applications are terrestrial (spotting) and astronomical. The two principle optical designs are reflecting and refracting. Reflecting telescopes use curved, front-surface mirrors to collect light rays and focus them on a single point. Refracting telescopes use curved lenses to collect light and magnify the image by bending the rays as they pass through glass of different composition and thickness.

What do we mean by "terrestrial" and "astronomical?"
We call objects associated with the planet Earth "terrestrial." This includes birds, whales, people, ships, landscapes and butterflies. We regard space objects beyond Earth's atmosphere as "astronomical." Examples include the Moon, planets, comets, stars, galaxies, nebulae, and the Sun.

What are the differences between terrestrial spotting scopes and astronomical telescopes?
Terrestrial telescopes should be compact, portable and deliver a "correct image" - one that is right-side up and oriented from left-to-right as we see objects with the unaided eye. Astronomical telescopes must be longer to accommodate great magnifications and have the large apertures to gather as much light as possible from objects at enormous distances and often with low light levels. Astronomers do not require "correct images," so special image-erecting optics are not as important for celestial observation.

Which telescopes serve for astronomical and terrestrial viewing?
The "double-folded optical path" designs of Maksutov-Cassegrain and Maksutov-Gregorian reflecting telescopes provide striking versatility and compactness for portability. These reflecting telescopes are about one-third the length of similar-aperture and focal-length refracting telescopes and yield comparable images to the best-corrected refractors. They eliminate color aberration associated with refracting telescopes and render very sharp, high contrast images prized by astronomers. Maksutov-Gregorians require no additional optical attachments to provide a correct image. Maksutov-Cassegrains deliver correct images with the addition of an erecting prism attachment. When used with a "star-diagonal" right-angle finder, they yield a right-side-up picture which is reversed from left-to-right.

Is there such a thing as a "wrong" telescope?
Yes! A junk cheap telescope is wrong. Even a technically excellent optical instrument can be the wrong telescope for you. It may be incorrect for your use, too awkward to carry, too big to store in your residence, too expensive for your wallet or too complicated for you to operate. You will also find that a very basic instrument will frustrate you by its limitations, especially if you possess a good mechanical aptitude coupled with a burning desire to pursue serious amateur observational astronomy.

How can I avoid "junk" telescopes?
Always avoid telescopes sold with outlandish claims of magnification! These telescopes are "breaking laws" - the laws of optical physics. Every ethical telescope manufacturer will warn you away from 60mm "department store telescopes" advertised as capable of , for example, "450x magnification." Such claims are nonsense for a refracting telescope with a 60-90mm aperture, when useful magnification, even in excellent telescopes, cannot exceed about 2x per millimeter. Figure it out -- the manufacturer that makes ridiculous claims about the capabilities of its products is more serious about snagging your money than telling the truth! What does that indicate about the quality of the instrument?

What are other possible mis-matches in telescopes I should avoid?
l) Because they are designed to point at the heavens, Newtonian and Dobsonian designs are astronomical telescopes unsuited for continual terrestrial use. 2) Small spotting scopes do not have adequate aperture for deep-space observation. 3) Large-aperture refractors are long, heavy, expensive and difficult to transport. Any telescope designed for a particular application occasionally may be put to use for other purposes with varying degrees of inconvenience.

Is magnification a telescope's most important feature?
Emphatically NO! Although telescopes are optical tools engineered to magnify distant objects, aperture is the engine that drives useful magnification. Aperture is the diameter of the primary mirror on reflectors, or front objective lens in refractors. This diameter is the real source of an astronomical telescope's "horsepower," and along with focal length, is used to calculate focal ratio.

What is focal length?
Focal length is the distance, stated in inches or millimeters, from the main lens or primary mirror to the focal point - where the light rays converge to form a sharp image. Longer focal lengths give more magnification.

What is focal ratio?
Focal ratio is the focal length divided by the aperture. In photography, "focal ratio", or "f-stop", indicates the ability of the lens to gather light for exposure. In telescopes, it is the combination of aperture and magnification that determines how bright an image appears. A fast lens with a small aperture cannot gather sufficient light from a faint, distant nebula, but a large enough aperture lens of a high focal ratio may provide excellent viewing of dim, distant objects..

How do you calculate magnification?
Divide the focal length of the telescope optical system by the focal length of the eyepiece. The result is magnification (x). With a refracting telescope, the objective lens focal length alone may be used for this calculation. In some reflecting telescope designs, the net focal length may be greater than that of the primary mirror alone, as the secondary mirror may add to the system focal length. This combined number, the "system focal length", usually appears on the ring locking the front lens in place at the front of the optical tube.

Can you have too much magnification?
Yes, magnification can be a handicap, particularly if the telescope aperture is not large enough to gather sufficient light to support the magnification. With a given aperture, brightness decreases as magnification increases. Magnification also decreases field of view, emphasizes movement and vibration, reduces depth of field and exaggerates optical aberrations. Finally, if the magnification increases beyond the telescope's resolving power, the result is a blurry, useless image. We call this "empty" or "useless" magnification.

What is resolving power?
Resolving power, measured in arc-seconds, is the ability to discriminate between two close points, such as double stars, or to reveal detail. The celestial sphere is divided into 360º, each degree is divided into 60 "arc-minutes." These arc-minutes are further divided into 60 parts, each called an "arc-second." From directly overhead (0º) to the unobstructed, or "true" horizon (if on a plain) is 90º, or one-quarter of the celestial sphere. The following is an example of how the theoretical limit of resolution can be applied.
If the telescope can resolve 1.0 arc-seconds, objects 1.0 arc-second apart can be recognized as two entities. The Moon measures about 0.5 degree, or 1,800 arc seconds. If enough giant beads were lined up at the visible side of the full Moon's equator, each just 1.0 second-of-arc from the next, they would be distinguishable as circular bulges resembling a string of beads belting the equator. But, if the number of these giant beads were increased so they were separated by only 0.8 second-of-arc, the "string of beads" would change to a band with fuzzy edges. Telescope manufacturers always list the theoretical capability of resolution. Some meet or exceed that design limit, some do not.

Can you change magnification of a telescope, and if so, how?
Yes. Two easy ways to change magnification: 1) Change to a different focal length eyepiece. Shorter focal lengths increase magnification, longer focal lengths decrease magnification. 2) Add a "Barlow" lens between the telescope and the eyepiece. A Barlow lens system can double or triple magnification by increasing the focal length of the instrument. Excellent quality, multi-coated Barlow lenses are a good way to double your stable of eyepieces at modest cost and with minimal sacrifice of image quality.

What Do Eyepieces Do?
Besides their affect on magnification, eyepieces impact optical quality. They can increase or decrease field of view, depth of focus and brightness. Cheap eyepieces can diminish the performance of an otherwise superb telescope. Very expensive eyepieces can help a good optical system perform at, or even above the design limit, but they cannot rescue a bad optical system. Most of our telescopes from 60mm to 203mm all employ interchangeable 1¼" outside diameter Plossl eyepieces of tested, dependable quality. With few exceptions, cheap telescopes with exaggerated claims of magnification use .965" eyepieces of inferior quality. Some super-quality apochromatic refractors employ special .965" eyepieces of superb quality, but they are a rarity.

Can the aperture be too large?
Yes - and no. Too bright an image destroys detail. Large aperture also means heavier, longer, more awkward instruments, especially with refractors. Diminished contrast and resolution may result from apertures that are too large for the telescope focal length. Dark backgrounds associated with smaller apertures improve planetary views. But properly selected filters placed in the eyepiece also can improve detail, and there are simple ways to reduce aperture by restricting part of the opening through which light enters. Once you purchase a telescope, there is no easy way to increase aperture, so be smart, buy the largest aperture telescope you think you may need, but first, make sure you can afford it, lift it and store it!

What are the best combinations of magnifying power and aperture?
Astronomers agree that useful magnification in perfect "seeing" conditions with excellent optics can never exceed 50x-60x per inch of aperture. Perfect seeing conditions are so rare that most of us will never be lucky enough to get more than 40x per inch, even with a superb instrument. Another reliable measure of the "Maximum Limiting Visual Power," is to double the aperture in millimeters. Therefore, a top quality 60mm telescope, in "perfect seeing conditions," may achieve 120x - not the ridiculous 250x, 375x or 450x advertised for "junk" telescopes.
How much magnification do I need?
For terrestrial observation, 10x-60x will cover 99% of needs. The rule - use the least magnification required to see the object you wish to observe. To clearly view Saturn's rings and Jupiter's Galilean moons, 40x is a good magnification with which to start. Less than 200x will suffice for most astronomical observation. Experienced observational astronomers agree that atmospheric air currents limit useful magnification to 300x even for the largest amateur instrument. This means that a 10" - 15" scope will deliver the maximum useful magnification. The greater light gathering ability of even larger "light bucket" telescopes used at no greater than 300x magnification may be helpful to capture extremely distant, very dim objects and to obtain larger apparent fields of view. These very large telescopes are expensive and heavy, but they always gather a crowd at star parties.

What are the advantages and disadvantages of reflectors vs. refractors?
Per dollar spent, reflecting telescopes have three big advantages over refracting telescopes -- 1) Better color rendition, 2) Larger apertures and 3) Shorter tube lengths. At a huge increase in cost, refractors can match the reflecting telescope's good color rendition -- they can even have large apertures - but they still are burdened with long, heavy optical tubes. Compact reflector telescope easily challenge the best reflectors at planetary observation, and for much lower cost per inch of aperture. Only the most highly corrected apochromatic refractors can produce images that compare in color accuracy, sharpness and contrast to the best reflectors we have in stock. These fine refractors produce stunning images with superb contrast, definition and background separation - the same qualities of excellent brand name telescopes we carry in our online store!

What types of reflectors are available?
Other common reflecting designs are the Newtonian, Dobsonian and Maksutov-Newtonian. Newtonian and Dobsonian reflectors have long optical tubes open at one end, a parabolic (preferred) or spherical primary mirror, and a flat (plane) secondary mirror. Newtonians usually fit on tripods or piers with altazimuth or German equatorial mounts. The open end exposes the primary mirror to dirt, moisture, and air currents. In the hunt for large aperture, short-focus tubes at the lowest cost per inch, Dobsonians are the choice of many astronomers. Dobsonians are Newtonian reflectors with close-to-the-ground, altazimuth or fork mounts. Maksutov-Newtonian rich-field astronomical telescope combines the folded optical path with a short focal length, large aperture design. Its unique viewfinder rotates between the observation setting and the "finding" position to utilize the eyepiece as both a finder/guide and observing ocular.

How important is the "mount?"
Very! When you magnify an image, you also magnify movement. The mount must easily support the instrument, reduce vibration and provide a smooth, secure means of aiming at the object you wish to observe. For astronomical use, the mount should incorporate an alignment system to match the orientation of the axis of its movement to the celestial axis. (This is easier than it sounds). Astronomical telescopes may also include one or two "clock drives" to match the movement of the telescope to the Earth's rotation. This keeps the telescope fixed on celestial objects and reduces or eliminates their apparent drift from view.

What are the different kinds of mounts?
For small spotting scopes, simple camera-tripod mounts attached with a ¼"-20 screw may be adequate. The choices are wider for astronomical telescopes: Altazimuth mounts allow the telescope to rotate side-to-side or to pivot up-and-down. German equatorial mounts with counterweights provide qualities of stability and a proven orientation system that greatly enhances ease of astronomical viewing. Some German equatorial mounts have an alignment telescope mounted inside the polar axis to simplify alignment with the celestial axis. Fork mounts feature pivots on either side of the optical tube and usually rest on "wedges" to align the telescope with the celestial axis. One major manufacturer has introduced a fork mount with a support only on one side! Conveniences added to astronomical mounts may include multi-speed clock drives on one or two axes, slow-motion hand controls, and computer-interfaced guiding systems which slue the scope to a specific astronomical object.

I like astronomy, do I need a clock drive or computer guidance system?
The choice is yours. Inexpensive clock drives attach easily to most German equatorial mounts, helping to keep celestial objects in view as they move in synchronization with Earth's rotation. Computer guidance systems are a significant portion of a telescope's purchase price, yet contribute nothing to the telescope's all-important aperture and optical precision. In fact, if part of the cost of a telescope is the computer system, consider the possibility of spending that money on larger aperture or better optical quality to see deeper into space. Computer guiders with "go-to" capability are useful to amateurs who do not "know the sky" and do not have time to learn. Once you "know the sky" the "computer system inside your head" will become very reliable.

Can I attach my 35mm SLR, a video camera or digital camera to my telescope?
If your camera takes interchangeable lenses, there is probably a T-mount adapter that will permit attachment to any telescope. Some smaller telescopes require a device which fits over the eyepiece.

How Do I Know Which Manufacturers To Trust?
Our products, our claims, our specifications and our traditions speak for themselves. We offer the finest telescope brands Celestron, Meade and other top brand telescopes!

Astronomical Telescope -- Search the Heavens
You want to view the moons and color bands of Jupiter, the rings and moons of Saturn, the mountains, valleys and rills of Luna - Earth's moon, and visually savor the glorious beauty of the Orion Nebula. You plan to hunt for nebulae and comets, and see every one of the 110 objects listed in the fabled catalog of Charles Messier, the 18th century French astronomer and comet hunter.

You can scan the heavens with our astronomical telescopes that are up to 10 times as powerful and superior in every way to the best instrument Galileo ever used! He achieved his discoveries never seeing the heavens with more than 30 power. You can have up to 300x true, usable, magnification to use when and where you wish! Galileo's "cannon" was more than seven-feet long.


Terrestrial Viewing - "Spotting Scopes"

Modern terrestrial spotting telescopes should:
· Give a "Correct" Image
· Be Compact and Portable
· Be "Rough and Ready"
· Offer "diffraction-limited"optics

1. Erect image -- When the magnified image appears just as you see it with the unaided eye, we say, "the image is correct." Most telescope images are upside down and reversed. An "erect" image is right-side up, but is flipped, left-to-right, like a mirror-image.

For astronomical use it does not matter which way a space object is oriented, but terrestrial subjects such as upside-down or reversed birds, buildings, ships, people and animals, are very disconcerting. Most telescopes require extra optical components to"flip" the images. These "added accessories" cost extra, add weight, reduce light transmission - and they can accidentally fall off the instrument!

2. Compact and portable.

A field instrument that is too heavy to carry comfortably defeats the whole concept of portability. All of our small telescopes and most spotting scopes are lightweight, very sturdy and perfectly suited for outdoor use by hikers, hunters, birders and naturalists.

Larger telescopes require larger stands for rewarding viewing. Keep in mind, as you magnify the image, you also magnify distracting vibration. Budget your purchase to include a sturdy tripod suited for the telescope to minimize vibrations. (See "Accessories").

3. Be "Rough and Ready".
Get a strong one - if you pay more, you get better construction of the scope.

4. Offer "Diffraction-limited Optics."
Many of our high end astronomical and spotting scopes meet industry tests for the coveted premium rating "diffraction-limited optics" This standard assures clean, sharp images with minimum flare - the kind of images delivered only by finest optics.

Astronomical Viewing

Astronomical Telescopes should:
· Be Bright
· Be Sharp (High Resolution)
· Offer Sufficient Magnification
· Give Good Color
· Meet Your Capabilities in Size, Weight and Expense
· Be Mounted on a Stable, Easily-aimed Support
· Offer "Diffraction-Limited Optics"

Reflector or Refractor?
· Color rendition is therefore superior in reflectors.
· For a given power of magnification, reflecting telescopes are more compact than refractors
· Reflecting telescopes "give more bang for the buck."

"Light Grasp"
Knowledgeable astronomers value a telescope's "light grasp," or ability to collect light. The telescope "aperture" -- diameter of the "objective"-- or main lens, determines "light grasp." A 60mm (2-½") telescope has about 80x the light grasp of a normal human eye, which typically has an aperture of only 6-7 millimeters. A 150mm telescope gathers nearly 500x the light of a human eye. It is the light grasp of a telescope that makes dim objects visible to the eye. The dimmer the object you want to see, the more light grasp you require.

Because light loss increases greatly with distance, astronomical telescopes require larger apertures. Individual stars are so far away that even the most powerful amateur instruments will show them only as brilliant points of light. Jupiter's four largest moons and Saturn's rings appear crisp and beautiful at 40x magnification. Our own moon will reveal considerable detail at 30x magnification.

"Limiting Magnitude"
The higher the magnitude, the dimmer the star. "Limiting magnitude"describes the faintest star that can be seen with a given telescope under ideal conditions. The unaided human eye can see nothing fainter than the sixth magnitude. A 60mm aperture can detect stars of just below 11th magnitude and the 150mm aperture is capable of capturing 12.9 magnitude stars.

Reflector Design Advantage
With a large primary reflecting mirror (for excellent light grasp) and a much smaller secondary mirror, Maksutov telescopes "double-fold" the light path to fit into a compact tube. Powerful, precise, brilliantly color-corrected telescopes can be constructed which are about one-fourth to one-third as long as older refracting and reflecting telescope designs with comparable apertures. Some telescopes like LOMO ASTELE™ Maksutov telescopes optimize the classic designs, and despite their moderate price, are among the very best in the world. Of course, there are other types of useful telescopes, but Maksutov designs have won the confidence of the amateur astronomy community for their traits of power, compactness and image quality -- and affordable prices.

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