BACK BAY AMATEUR ASTRONOMERS

Introduction

Here is some advice for those new to astronomy as a hobby, and for parents that are interested in steering their children toward a wholesome and educational pursuit. It is divided into several sections that discuss how to choose a first telescope, what you might try before you get a telescope and how to find your way around the sky as a novice.

I would caution parents not to make the common mistake of trying to push their children into the hobby before they are ready and not to succumb to the temptation of buying that inexpensive “department store telescope” in hopes of sparking an interest. Most kids go through a phase of real excitement about space and astronomy sooner or later. Be patient. And, be warned that buying the wrong telescope could easily discourage that natural interest. If you’ve already made that mistake, drop down to the section on “Fixing the Department Store Telescope” for some ways to salvage some use out of that purchase.

Before You Buy a Telescope

The night sky, of course, is something that can be enjoyed with the naked eye alone. Some objects can’t be appreciated any other way, short of photography. Our own Milky Way, for instance, is best enjoyed with the unaided eye. The Northern Lights, eclipses, artificial satellites and even the moon and planets can be enjoyed without optical aids. Indeed nearly all of our sky-lore, star names and constellations predate the invention of the telescope. So don’t put off the enjoyment of the night sky waiting for that first telescope. In fact we highly recommend that when you are ready to graduate to optical aid, you buy a decent pair of binoculars.

By all means seek out others with an interest in astronomy. Joining, or at least, meeting with your local astronomy club is the best way to get started.

Buying Binoculars

Binoculars are like two parallel telescopes. They each have an objective to form an image and a magnifier (occular) to view it. Binoculars use prisms to turn the image right side up. The two main designs are porro prisms and roof prisms. Roof prisms line up directly behind the objective allowing for a compact and usually lightweight design. They are typically more expensive than porro prisms. Porro prism binoculars use two right angle prisms offset from each other. This places the objective lens further apart. They are bulkier than roof prisms, but generally perform better optically, and are less expensive.

Most optical prisms are made from BK-7 (borosilicate) glass or BAK-4 (barium crown) glass. Bak-4, while more expensive is the preferred glass, yielding brighter images.

The ratio of the focal length of the objective to the focal length of the eyepiece gives the magnification power. With binoculars, we specify the objective size and the power. 7x35 binoculars have a 35mm objective, and are 7 power.

If you divide the objective number (35) by the power (7) you find the exit pupil. The exit pupil is the little cone of light you see when you hold the binocular away from your eye. For 7x35 binoculars, the exit pupil is 5mm. In order for a binocular view to appear as bright as a normal view, the exit pupil has to be at least as large as your eye’s pupil, if the exit pupil is smaller than your eye’s pupil, the image will be dimmer than normal. The human eye pupil is about 2 to 3mm in daylight and up to 7mm when dark-adapted at night.

Another important consideration in the choice of binoculars is the field of view. This is generally expressed in a field size in feet at 1000 yards distance. But for astronomical use, we want to know the angular size (how much of the sky) we see. If the specs give a value of say, 390 feet at 1000 yards you must convert that to an angle; near seven and a half degrees in our example. (Do you remember your trigonometry? Dividing 390ft by 3000ft gives you the Tangent of the angle.) That’s the true field. Multiply by the magnification to get the apparent field ... how big the view appears. Different eyepiece designs offer different apparent fields. Wider fields are nice, but beware that the image quality at the edge of the field will go down as field size goes up (as a rule).

If you are going to hold the binocular by hand, weight and magnification is critical. At higher magnifications, any unsteadiness will be greatly magnified. Probably 10x is the maximum magnification for a hand held binocular, 7x is better. You, of course, want a large objective to admit maximum light, but don’t lose sight of the exit pupil size and how it affects image brightness.

If you wear eyeglasses, your binoculars need at least 17mm of eye relief if you are to view without removing your glasses. Eye relief is the distance between your eye and the eyepiece lens when the whole field of view is visible. It varies with the eyepiece design.

Recommendation? For casual binocular astronomy, consider a 7x50 porro prism binocular. Insist on Bak-4 glass, with anti reflection coatings on all air to glass surfaces and about a 50-degree apparent field of view. If you will put the binocular on a mount or tripod, go with the largest objective that you can afford, but keep the factors we noted above in mind.

Choosing a Telescope

The most important thing you can learn about telescopes is that it is not power (magnification) that is important, but aperture. Aperture, the diameter of the objective lens or primary mirror of a telescope, determines what you can see. The larger the aperture, the more light the telescope can gather. And that’s what it is all about, collecting light from rather faint celestial objects and delivering it to the eye. Power is an adjustable quantity, changed by changing the eyepiece, and is limited only by, you guessed it, aperture. Any telescope can achieve nearly any magnification, but the laws of physics limit the “useable” magnification. A good rule of thumb is that the maximum magnification of a telescope is about 50 times the aperture in inches, and the most useable magnification will be in the range of 10-20 times the aperture in inches.

Perhaps you’ve seen the ads for 60mm Refractors that promise 750x? Well, now you know that the maximum effective magnification for that 2.4-inch scope is really about 118x and the most used range will be about 35x. They are not being dishonest exactly, because they are providing an eyepiece and barlow (magnifier) combination that yields a magnification of 750x. But that image will be just a blurry blob of light that speeds through the eyepiece so fast that you can’t be sure you saw anything at all. As a rule, avoid any telescope seller that advertises a telescope by its “power”. They are capitalizing on a common misconception to exaggerate the usefulness of their product.

You’ll want to consider several factors in choosing a telescope such as cost, portability, and optical design. As a general rule, it’s usually a good idea to get the largest aperture scope you can both afford and handle. You should also consider how you want to use the scope and what you are interested in observing, this will have a large impact on the choice of a telescope. You will be faced with choosing from among different types of telescopes and different strategies for pointing the scope. If you intend to view objects other than the moon and planets, you will have to get away from the city lights, consider how you will transport your scope.

Telescopes use either a lens to refract (bend) light to a common focus, a curved mirror to collect and focus light or a combination of both mirrors and lenses. Telescope mounts are divided into those that are referenced to the horizon or those that are aligned to the movement of the sky. There is no perfect or even “best” telescope or telescope design. Each type has strengths and weaknesses. The best telescope for you, is the one you will USE!

Types of Telescopes

A Refractor telescope uses a lens, a curved piece of glass, to bend light to a focus. Light from a distant star arrives as parallel light rays. The shape of a telescope lens is such that it bends the parallel light at differing angles so that all the light arrives at a single point where it can be magnified and sent to the eye. But light is made up of different wavelengths that we see as different colors. Each wavelength, or color, bends at a slightly different angle when passing through a lens consisting of a single type of glass. This tends to spread the light of the focused image into a rainbow of colors. This is called chromatic aberration, and it is the principal defect to be dealt with in a refracting telescope. But a lens made of two different types of glass, with different optical qualities and opposing shapes, can offset most of each other’s chromatic aberration. A third component can increase the quality of the image even more. But each additional piece of glass reduces the amount of light that gets through. To keep the image bright and prevent scattering, the purity of the glass must be kept very high, and of course, the surfaces must mesh together as perfectly as possible, and must be held together in a way that does not induce light loss or scattering. Optical coatings can improve the performance and offset the disadvantages of multiple lens systems somewhat but as the design becomes more complex, it quickly becomes very expensive to produce a quality refractor.

Once assembled, a refractor is a rugged design that requires no maintenance or optical alignment. Since there is no central obstruction in the light path, a necessary part of most other designs, quality refractors are noted for superb contrast and sharp images. They are often the telescope of choice for those interested in observing planets or studying close double stars. Commercial refractors range in price from under a hundred dollars to tens of thousands of dollars. Most are less than five inches in aperture, with 60mm and 90mm being very common sizes. (1 inch = 25.4mm). The focal length of a refractor is the distance from the lens to the formed image (a point inside the eyepiece). The focal ratio is the focal length of the scope divided by the aperture of the scope in similar units. A typical refractor has a focal ratio of around 10 to 15 expressed as “f/10” or “f/15”.

A Reflector telescope uses a curved mirror to collect light. Astronomical mirrors normally consist of a very thin (just a few molecules thick) coating of a reflective metal such as aluminum deposited on a curved piece of glass. The reflective surface is on the “first surface” of the mirror, i.e. the light being reflected does not have to travel through the glass like it would on an ordinary vanity mirror. The curve ground into the glass is shaped to focus the light to a common point and can be spherical or parabolic. The distance from the mirror’s surface to the focused image is called the focal length of the mirror. The focal length is determined by the depth of the curve in the glass, the deeper the curve, the shorter the focal length. The image of the reflected light is formed along the central axis of the mirror, so to avoid having the observer placed in the light path, the cone of reflected light is intercepted by a second mirror and directed sideways. This secondary mirror, also known as the diagonal mirror because of its flat forty-five degree angle, is itself in the light path. But the secondary mirror presents less of an obstruction than the observer’s head and torso would. This optical design is called the “Newtonian” reflector in honor of Sir Isaac Newton who is credited with its invention. Other types of reflecting telescopes direct the light back through a hole in the primary mirror to a focal point behind the mirror by the use of a curved secondary. There are other designs as well, but they are much rarer.

Large mirrors are easier and cheaper to make and support than large lenses. Therefore, reflectors can be made much larger than refractors. Typical reflecting telescopes range from 4.5 inches to 12.5 inches in aperture, but scopes as large as 30 inches are not all that uncommon. Since long focal length reflectors of the Newtonian design would place the observer very high off the ground, they tend to have focal ratios of f/7 or less with the most popular ratios being f/5 and f/4.5. But as the focal length gets shorter, that is, as the curve ground into a parabolic mirror gets deeper, the task of producing a uniform and ultra-smooth surface becomes more difficult. To maintain quality at reasonable prices, f/4 is about a practical limit, but f/5 offers a significantly better chance of avoiding imperfections in the figure of the mirror. Parabolic mirrors suffer from “coma”. A point source of light hitting a parabolic mirror near the center of the optical axis will be reflected accurately as a point. But the further the light strikes from the center of the mirror the more flared the reflected image appears. This is coma. The sharper the curve of the mirror, i.e. the shorter the focal length, the more pronounced this effect becomes. In a quality mirror of f/5, the effect is small enough not to be noticeable, below that however, it is a factor to consider. Newtonians, especially certain types that are disassembled for transport require frequent collimation (alignment of the optical elements.)

Catadioptic telescopes, sometimes called compound telescopes, use some combination of reflective and refractive elements in their design. Two popular designs you will see in today’s market are the Schmidt Cassegrain (SCT) and the Maksutov Cassegrain sometimes just called a Maksutov, Mac for short. They both use a spherical primary and a type of lens that corrects the “spherical aberration” of the primary (the tendency of a spherical surface to reflect light hitting it at different distances from center to a different focal point). The SCT uses a complex aspheric corrector plate and an adjustable secondary, the Mak uses a simple curved lens called a meniscus and a fixed secondary that is sometimes just a reflective spot on the meniscus. In both, the light is directed back through a hole in the primary mirror to a point behind the scope. They are both typically longer focal length instruments; Maks, perhaps f/17, SCTs are usually f/10. These scopes are compact, sturdy and versatile. They are superbly suited to photography and have a cacophony of accessories available. But they are more expensive than Newtonian reflectors of similar aperture.

Telescope Mounts

There are two main types of telescope mounts. The Altitude-Azimuth or “altaz” moves in two directions, up and down, left and right. The Equatorial mounting is tilted to align with the rotational axis of the earth. It must be aligned, set parallel to the earth’s axis of rotation, but once set, can easily track a celestial object through the night. The equatorial mount is essential for astrophotography, but is not suited for terrestrial viewing.

Two popular types of equatorial mounts are the fork mount used by most SCT and Mak designs and the German equatorial mount (GEM). In both, the base of the mount is set at an angle equal to the observer’s latitude, and the “polar” axis is pointed north. Each axis is lockable and capable of being gear driven by motors. Some modern telescopes with a fork mount can be configured in either altaz or equatorial mode.

Altaz mounts are usually tripod mounted, with locks in both axis. Some of the better mounts have slow-motion hand controls. A simple type of altaz mounting was developed by an amateur astronomer named John Dobson, and the mounting he developed takes his name. A “Dobsonian” telescope is a tube with Newtonian optics that sits atop a simple box called the rocker box. The telescope tube has a bearing attached that sits on a bearing surface on the rocker box. The rocker box itself sits on a ground board and pivots around a central bolt that connects the two. On these bearings, usually made of counter-top material bearing against Teflon, the scope can move in its two axis. A variant on this mount is the “Truss-Tube Dob”, where the solid telescope tube is replaced by removable poles or trusses connecting a mirror box containing the primary mirror to an upper tube assembly containing the secondary mirror and focuser.

Go-To or Not Go-To

Some telescopes are equipped with computer controlled drives, that can draw from an onboard database of celestial objects and drive the telescope to any object in its repertoire on command. Some of these drives require a little preliminary “training” of the drive, and most require a set-up procedure where the telescope is aligned to certain guide stars. The latest computer assisted scopes use GPS technology to make the alignment unnecessary and just about idiot proofs the process of finding celestial objects. There are two schools of thought about “Go-to” technology in the amateur community. One school holds that the thrill is in the chase and that finding celestial objects by intimately learning the sky leads to a real sense of accomplishment and fulfillment. The other school holds that enjoying the view of celestial splendors is what’s important; they would rather spend their limited time studying an object rather than searching for it.

Shopping for the Telescope

Your first stop should undoubtedly be “Skywatch”, the free public observing program of the Back Bay Amateur Astronomers! That’s where you can “try before you buy”. If you are not in the Tidewater Virginia area, contact your local astronomy club. You can find club listings on the Internet.

Step two: stop by a book store and pick up a copy of Sky & Telescope Magazine or Astronomy Magazine, in them you will find the latest offerings of the major telescope manufacturers. It is well worth a phone call to get a copy of the free catalog offered by Orion Telescope and Binoculars (1-800-676-1343). Not only are they a reputable source of entry-level telescopes, their catalog is an excellent source of information, as is the learning center on their Website: www.telescope.com.

Outlets like Wal-Mart, K-Mart, and Target etc. usually sell low end “beginner” scopes; small refractors and reflectors that experienced amateurs group into the category: Department Store Telescopes. That’s the polite term. These scopes are kept inexpensive by cutting corners; they typically have less stable mounts, too-small finders and very poor quality eyepieces. For a beginner, especially a youngster, these compromises very often lead to frustration and loss of interest.

The next step up is the science stores at the mall. They offer both the department store telescope and a variety of offerings leading up to serious amateur instruments. Keep in mind that selling telescopes is a sideline for them, the sales staff is unlikely to be knowledgeable about telescopes and these stores are typically in a high overhead environment. You can do the math. One should definitely do their homework before shelling out mall prices. That’s not to say that you shouldn’t buy a scope from them, if you know what you want and they have it at a good price, by all means go for it.

Some of the major brands ...

Meade Instruments Corporation offers a full range of telescopes from the smallest inexpensive refractor to professional grade state of the art techo-marvels. They have earned a superb reputation for optical quality, and offer the flashiest electronic gizmos that come on telescopes, all in sleek attractive packages. Their ETX line revolutionized the small telescope market. The LX200GPS series of Schmidt Cassegrain Telescopes are the ultimate in “state of the art”. Meade's new "AutoAlign" feature of their standard "Autostar" computer control practically aligns the telescope for you. Meade has a wide variety of products for every budget and every use.

Celestron International is Meade’s main competition in most market areas, and mirrors (no pun intended) much of Meade’s product line. Also known for superb optics, and innovation, Celestron offers quality refractors, and a complete line of Schmidt Cassegrain telescopes. Their latest technology is packed into their "CPC" line (Celestron Professional Computerized Telescope). With their new "SkyAlign" system the user need not even know the names of the alignment stars!

Orion Telescope is perhaps the premier mail order source for telescopes and binoculars. Noted for their flashy catalog, excellent customer service and entry level product line, Orion is a good first stop for the novice buyer. They started as a mail order warehouse selling other manufacture’s products and have now evolved their own product line. Orion is not a manufacturer, but rather an importer. Their “SkyQuest” Dobs and the “StarMax” Maks are some of the hottest products around. The "Inteliscope" version of their dob offers "push-to" technology and large aperture at very affordable prices. The Orion "Star Blast" (4.5 inch EQ Reflector) and the XT4.5 SkyQuest (4.5 inch Dobsonnian) are two examples of excellent scopes under $200!

TeleVue and Takahashi are the mass production kings of quality refractors. Their products are expensive, but well built. Both firms enjoy superb reputations.

Of course there are many other manufacturers, most build high end and custom telescopes. The majority of beginners will probably find what they want from the firms mentioned above.

Each of the companies listed above have Websites where you can peruse their product lines and find authorized dealers. Here in Tidewater, you might want to check out MRO Computers and Astronomy in Chesapeake, VA or Eagleton’s in Norfolk. Orion is the principal distributor of their own product, but the others can be ordered from any of several dealers. Astronomics, Lumicon, Scopetronix, and Adorama are the names of a few.

Eyepieces

When choosing eyepieces the factors to consider include the focal length (which determines magnification), the apparent field of view, eye relief (distance from the surface of the eyepiece to your eye when the image is in focus), and the optical correction of the eyepiece. Eyepieces come in three barrel sizes, .965, 1.25 and 2.00 inch. They can range in price from about $30 to over $500.

You need at least two or three eyepieces in your collection but it’s not unusual for an amateur to have a dozen or more. Faster telescopes are more demanding requiring more complex eyepiece designs to provide sharp, well-corrected images. Longer focal lengths can get away with less corrected designs. Start your eyepiece collection in the middle of the “most used” range of magnification values (10-20 times the aperture in inches), then fill in the low power end. You will use high power the least, so that should be the lowest priority when choosing eyepieces to buy.

There is no perfect eyepiece or eyepiece design. Make sure your telescope can accept the barrel diameter. 1.25 inch eyepieces are the most common.

Eyepieces determine your telescope’s magnification. Magnification = the telescope focal length / eyepiece focal length.

Eyepieces also determine the true field you will see in the sky. Manufacturers will indicate the apparent field of view offered by their eyepiece design (this is the angular diameter of the eyepiece’s “field stop” which is a metal ring inside the eyepiece barrel that limits the field size). You can estimate the approximate true field by dividing the apparent field of an eyepiece by its magnification in your scope.

Here’s an example; let’s say we have an 8-inch f/10 telescope like the popular SCT. The focal length in millimeters is 2032mm (“f/10” means the focal length is 10 times the aperture or 80 inches. That equals 2032mm since each inch equals 25.4mm). If we use a 26mm fl eyepiece, it will give a magnification of 78x (2032/26). Let’s say our eyepiece has an apparent field of 52 degrees. The true field will be 40 minutes of arc. (52/78=.66 degrees. Multiply by 60 to get the number of minutes.)

If you wear glasses, consider buying eyepieces that offer 15-20mm of eye relief, any less and you will not be able to get close enough to see the entire field without removing your glasses. Telescopes can be focused to compensate for near-sighted or far-sighted viewers, but not for those with astigmatism.

The differences in eyepiece designs are in the number of elements, the quality of the glass and the quality and number of anti-reflective coatings. The lens shape and barrel designs can be manipulated to produce greater eye-relief or wider fields. Many factors can contribute to the correction of field curvature, chromatic aberrations, internal reflections, brightness, sharpness and contrast. The best advice is to try before you buy.

Learning the Sky

Even with a “go-to” scope, there is no substitute for familiarity with the night sky to enhance your enjoyment of astronomy. At first it may seem like a daunting task, but you don’t need to know every detail to be an informed sky watcher. In fact once you learn to identify about a dozen constellations, and can name and point to ten bright stars, you’ll feel like you are pretty comfortable navigating the heavens. Some people manage that in one or two nights.

You’ll want a good star map and a planesphere, they are essential. I would suggest Deep Map 600 from Orion Telescope and David Levy’s Guide to the Stars Planesphere available at Barnes and Noble.

There are many wonderful books that can help you learn the sky: Turn Left at Orion by Guy Consolmagno and Dan Davis, Touring the Universe Through Binoculars by Phil Harrington, Nightwatch by Terry Dickinson or Skywatching by David Levy are just a few.

Visit the planetariums. The Chesapeake Planetarium is located at 300 Cedar Road, in the city Municipal Center. They have free public shows each Thursday evening at 8pm. Call 547-0153 for reservations. The Virginia Beach Planetarium is located inside Plaza Middle School at 3080 S. Lynnhaven Road. They have free public shows each Tuesday evening at 7pm. Call 431-4067 for reservations.

Join a local astronomy club.

BBAA - Back Bay Amateur Astronomers
VPAS - Virginia Peninsula Astronomy Stargazers
NAS - Norfolk Astronomical Society

Fixing the Department Store Telescope

That inexpensive 60mm refractor that looked like such a good purchase and promised such wonderful things languishes in the closet because it proved too difficult to see anything with. What to do?

You’ll have to spend a little bit of money, but you can upgrade your telescope to a useable instrument relatively easily. First, think about adding a BB gun sight to replace that tiny finder that you probably can’t see much with anyway. These reflex sights project a red dot into the sky, and once aligned to the telescope will enable you to easily point the scope. The “astronomical” versions sell for about $40 but a BB gun sight will work as well and should cost about half that.

Upgrading your eyepiece set to 1.25 inch eyepieces will make a world of difference. If your scope did not come with a “hybrid diagonal” that accepts the larger barrel eyepieces, you can buy a quality one for around $25. You can also purchase reasonably good eyepieces for about $30 each. First buy an eyepiece that will give you a magnification equal to about 10-15 times the aperture in inches. That will be your most used range. For a 60mm f/15 scope (900mm focal length) look for an eyepiece that yields about 35x. Divide the scope’s focal length by the eyepiece focal length to get magnification. So for a 900mm focal length, a 25mm eyepiece yields 36x. Perfect! Then if you buy more eyepieces, consider a 40mm for low power and something in the 17 to 15mm range for high power.

Dealing with the stability of the scope may just require some tightening of clamps, or perhaps you can use the tripod in its shortest configuration to increase its stability. And try rubber pads under the legs to reduce vibration.

Perhaps the most important enhancement will be the adjustment of your expectations! You will not see “Hubble Quality” views through even the largest amateur telescopes. The small refractor is quite enough, however, to see detailed views of the moon and decent views of Jupiter and Saturn. Double stars, and bright globulars are other possible targets. Once you calibrate your expectations and understand your scope’s limitations it can provide years of enjoyment. It might help to know, that most of the serious and dedicated amateurs of today started their love affair with the night sky with a small refractor of dubious quality.

Never use an eyepiece filter for solar observing. The cheap thread-in filter that may be labeled as a solar filter could shatter from the heat of magnified sunlight. If your telescope came with one, please destroy it. The safest way to view the sun is by projecting its image onto a screen. 

Never leave a telescope unattended in the sunlight where youngsters might point it at the sun and always supervise children using a telescope.