In 2014 BBAA club member Nick Anderson wrote this great guide on getting started in the amateur astronomy hobby. We have updated the formatting but left the original text intact as the advice was great then as it is now.
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.
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.
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.
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!
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.
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.
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.
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 ...
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.
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.
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.
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. One of the safest ways 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.
Now that you have acquired a telescope or binoculars you can use the tips below to help you observe the night sky.
There are many methods to find celestial objects, here are a couple of hints on how to SEE them once you locate them.
The prevailing wisdom in the amateur community is that cleaning of your scope’s primary mirror should be done only rarely. An accumulation of dust will make very little difference in the performance of your scope and every time you handle your primary optics you risk disaster.
Granting the above statement as true doesn’t mean that you should be afraid to clean your mirror when the need arises. Here’s some advice that might help.
Use only pure distilled water, and lots of it. You’ll need a clean spray bottle, keep one just for this purpose. And you’ll need to mix a very diluted soap solution. I can’t emphasize enough how little soap you should use. One very tiny drop of concentrated dish washing liquid to a couple of quarts of distilled water is all you need. Have 100% cotton pads standing by, it’s best to buy the sterile cotton that comes in a roll and cut it into pads.
With most scopes you will have to remove the mirror cell from the scope. You should mark the cell and the tube to assist re-assembly in the same orientation later. It is a good idea to wear latex gloves in case you can’t keep your fingers from contacting the mirror. Scopes vary, but the mirror cell is usually secured in the tube with three or four screws that enter radially from the circumference of the tube. Remove the fasteners and slide the mirror cell (mirror and supporting back) from the tube.
Support the mirror nearly vertical. Use the clean spray bottle filled with pure distilled water to spray the mirror, wetting it thoroughly. Get any visible solid matter off the mirror this way. Lay the mirror flat and pour the soap mixture over the mirror. Let it pool up in the center of the mirror and use the cotton to gently swirl it around. Then rinse with copious amounts of pure distilled water. Don’t skimp on the rinse water, more is better. Stand it back up and dry the mirror with the cotton. Be sure to change the cotton frequently and use gentle, short strokes. If you notice colored streaks, you’ve left soap film on the mirror; rinse it again.
Now would be a great time to “spot” your mirror to facilitate collimation.
After your mirror is dry, reinstall it in your tube. You should then collimate your optics, which is another subject for another time. Keep your tube covered when stored to reduce the dust accumulation (but don’t cover a dew soaked mirror until it dries).
Placing a spot in the center of your primary mirror is the best way to facilitate collimation of your Newtonian or Dob. Don’t worry about degrading the image, the center of the mirror is in the shadow of the secondary and doesn’t contribute to the image.
You will have to remove the mirror cell from the scope. You should mark the cell and the tube to assist re-assembly in the same orientation later. It is a good idea to wear latex gloves in case you can’t keep your fingers from contacting the mirror. Scopes vary, but the mirror cell is usually secured in the tube with three or four screws that enter radially from the circumference of the tube. Remove the fasteners and slide the mirror cell (mirror and supporting back) from the tube.
Using a compass, draw a circle on newspaper the same diameter as your mirror and cut it out. Use a pencil to slightly enlarge the pinpoint that the compass left in the center of your circle. Gently lay the newspaper disk on the mirror and align the edges. Use a magic marker to mark the center of the mirror through the hole in the paper, then remove the paper. Place a loose leaf reinforcement ring on the mirror centered on your mark. You can either blacken the inner hole of the reinforcement ring and then remove it, or blacken the ring and leave it on. Both methods work equally well. Use an indelible marker.
Replace the mirror in its original orientation. You are now ready to collimate.
The popular SCT telescope has a closed optical tube assembly that helps to keep the primary mirror dust free longer. The “corrector plate” or front lens of the scope, however, is a “dust magnet.” Still, dust will effect your scope’s performance very little and you shouldn’t be cleaning your corrector too often. When you do clean it, some simple precautions will help you to avoid disaster.
The best way to keep your optics clean is to keep them covered when not in use, but NEVER cover a dew soaked corrector plate until it dries. When returning the scope from outdoors, check to insure its dry and cover it outside before bringing it into the warm house or car (where condensation can quickly form). Keep your rear cell sealed as well to prevent the introduction of dust borne air into the optical tube assembly (OTA).
Forced air is the best way to remove dust from your corrector plate. But, be wary of the canned compressed air, as it can sometimes propel liquid or particles at your scope. If you use these, don’t shake the can, hold it upright, test it on your hand first and don’t allow the spray to continue too long. I use a simple squeeze bulb to force air onto my corrector plate. Point the scope down and force air toward the corrector at a sharp angle, be careful not to actually contact the glass with the bulb.
If you need a liquid to remove stubborn material try using pure distilled water. Dampen a clean cotton ball with the water and use another to dry it. There is disagreement among sources as to the proper motion to use when cleaning a corrector plate so as to not damage the delicate coatings. I use the Celestron recommended method of swiping radially outward from the center obstruction toward the edge of the plate. Be careful not to let liquid reach the edge of the plate where it might enter the tube. Use gentle, uniform strokes. Others recommend a circular swirling motion. Check your manufacturer’s recommendation. If all else fails, you can use a very diluted soap solution using a tiny drop of dish washing liquid in lots of distilled water or a commercially prepared lens cleaning solution.
Cleaning the primary of an SCT is not for the faint of heart. You will have to remove your corrector plate. The corrector is secured to the tube with a retainer ring that is fastened with screws. There are usually shims under the retainer, or under the corrector itself. It is important to mark these shims and the corrector plate. The corrector plate is designed to offset inherent aberrations in the spherical mirror and the orientation is factory set for optimum correction. Great care should be taken to insure that the corrector is reinstalled properly. Note also that the edges of the glass are thin in a Schmidt corrector and can easily be crushed or cracked by too much (or uneven) pressure from the securing screws. Once access is gained to the primary it can be cleaned with the same methods used for the corrector outlined above. Dust can be removed from the optical tube with a small vacuum cleaner. Removing the primary from the OTA is not recommended.
Collimation of an SCT is accomplished by adjusting the secondary mirror (three screws on the front of the obstruction in the middle of the corrector). But that is another subject.
The purpose of collimation is to align all of the optical elements of your telescope so that the optical axes line up. The collimation requirements of different scopes vary; a truss tube Dob must be collimated before every use, a refractor might never need realignment.
Most reflectors are of the Newtonian design where a parabolic primary is combined with a flat diagonal to collect light and direct it sideways to a focuser where an eyepiece then magnifies the image. Collimation aligns the primary, diagonal and focuser.
The procedure below uses a set of collimation tools that are available from several sources. The Tectron tools I use can be purchased for about $100. The set contains a “Sight tube” with cross hairs, a “peep sight” eyepiece called a “chesire” and an autocollimator.
The first step, and one that should only have to be accomplished once, is to “square the focuser” to the optical tube. The centerline of the focuser must be perpendicular to the centerline of the optical axis of the primary, so it is assumed that the focuser must be perpendicular to the center of the tube. One way to do this is to remove the secondary mirror from the spider assembly and replace it with threaded rod that will extend down into the axis of the tube. Adjust the spider so that this rod is centered between each spoke. Then sighting through the focuser, use the rod to mark a vertical line on the tube opposite the focuser. Insert a chesire (peep sight) eyepiece into the focuser with the pinhole inside and flush with the inner diameter of the tube. Measure the distance from the top of the tube to the peepsight. Then mark the same dimension on the vertical line opposite the focuser and highlight the intersecting point. Remove the threaded rod and the chesire and install a cross hair sight tube in the focuser. Shim or adjust the focuser to align the crosshairs on the mark you made across the tube.
Next reinstall the diagonal (secondary) mirror. (We assume that no offset will be imposed on the secondary mirror). Slide the sight tube in or out until it is just larger than the apparent diameter of the secondary. Rotate the secondary until it appears round when viewed through the sight tube. Move the secondary toward or away from the primary (adjust up or down), keeping its image round, until it is centered in the sight tube. (Ignore all of the reflected images) adjust the diagonal until its round shape is centered in an annular ring (the wall of the sight tube). Now look at the reflected image of the primary in the secondary. Tilt the secondary until the image of the primary is centered. Most secondary holders have three screws to adjust the tilt of the diagonal mirror. Be careful, they often also secure the holder to the spider and if loosened together, will allow the mirror to fall.
The last step is to adjust the primary. Put the chesire eyepiece into the focuser and find the center spot or ring that you placed on your mirror previously. Your focuser should be at mid travel for this adjustment. Use the collimation screws to tilt your primary to center the pinhole over your mirror’s center dot.
Collimation can be checked by using the “star test.” Well-collimated optics will focus stars to pinpoints (if “seeing” allows). Allow time for your mirror to cool to ambient temperature and select a moderately bright star (about third magnitude) that is high off the horizon. Keep the image in the center of the field. The star should “snap” into sharp focus from both sides of “out of focus.” The shadow of your secondary should be centered in the ‘out of focus’ star. Use a moderately high power eyepiece (one that yields a magnification equal to 15 to 20 times your scope’s aperture in inches) and be sure the image is in the center of the field. “Touch up” your collimation to achieve the best possible star image. At High power a star should focus to a sharp point with faint concentric rings around it, the rings should appear round.
Collimating an SCT is very easy in principal and quite difficult in practice. Fortunately, these scopes hold collimation quite well and if treated gently may go years without requiring adjustment. But they are very sensitive to small misalignments and benefit greatly from precise optical collimation.
The SCT is collimated by adjusting the secondary mirror which is mounted in the middle of the corrector plate and constitutes the secondary obstruction. There are three adjusting screws located 120 degrees apart on the front of the secondary obstruction. Often, there is a plastic cap that must be pried off to expose the screws.
Take care in adjusting these screws. To move the mirror requires loosening one side while tightening the other. Be careful not to loosen all three at once as the mirror can come loose and fall into the tube with catastrophic results.
SCT’s are collimated using a star image, normally a fairly bright star half way to the zenith is a good candidate. Position yourself where you can turn the screws while observing the image. Unfortunately, this procedure is best accomplished without a diagonal making it quite difficult to do alone. But it is better to accomplish it with a diagonal than not at all. Do course adjustments with the star defocused, adjusting to center the shadow of the secondary in the image and make the surrounding rings concentric and circular. Check the in-focus star image for a sharp pinpoint when you think you’ve achieved collimation. The star should “snap” into focus from both sides (inside and outside) of focus.
Some have reported success with an “artificial star” for collimating during the day. You might try a sparkling object like a Christmas tree ornament if you have a sufficiently large test area. Hang the ornament as far away as possible and use the reflection as the point source of light.
Dew is formed on optics when the temperature of the glass falls below the “dew point” of the air around it. So fighting dew is a matter of keeping your optics warmer than the surrounding air. The two methods used to do this are to 1. Limit the exposure of the optics to the atmosphere and 2. Apply heat to the glass.
There are many sources of information about how to fight dew and many clever devices for warming or shielding optics have been developed. Our own Kent Blackwell is the author of an S&T article that describes one way to build an anti-dew heating system. There are commercial products and home made systems aplenty. This article is to make you aware of the fact that, if you are going to observe here in Tidewater, you must deal with dew or limit your observing to the time it takes dew to form and send you packing.
There are methods to remove dew once it has been formed, but most will agree that dew prevention is better than dew removal. The first line of defense against dew is the dew shield. By extending the telescope tube beyond the objective of a refractor or corrector plate of a compound telescope we delay the formation of dew by slowing the cooling of the glass. This method can also be applied to the Telrad or finder scope. The dew shade should be kept flat black on the inside to reduce reflections, benefits from lining with a moisture absorbent material such as felt and should not be so long as to cause vignetting of the optical path. Dew shields are a MUST for SCTs, their corrector plates are very exposed and radiate their heat rapidly!
Applying heat to optics is a controversial subject. Optical performance is very dependent on temperature equilibrium. Even slight temperature variations can deform the shape of a mirror or lens and induce aberrations that degrade the image. But once dew forms, observation ends, so pick your poison.
Every glass surface is subject to dewing. Keeping eyepieces covered, or in your pockets will prevent them from forming dew. If they fog from your breath or proximity to your face just fan them to facilitate evaporation (in this case, your body has warmed the air near the surface of the glass so rather than warming the glass you need to cool the air). Once in the focuser, keep dew from forming on eyepieces by applying heat, or keeping a cover on them when you’re away from the scope.
An anti-dew heating system can be purchased (Kendrick Astro Instruments is a popular source) or fabricated. They run on battery power, and should provide just enough gentle heat to keep exposed optics above ambient temperature. I use a 12 volt battery to power heat ropes or resistors on my secondary mirror, telrad, finder objective, finder eyepiece and the eyepiece in my focuser. My SCT has a heated dew shield. (from Roger W. Tuthill Inc.)
Once dew forms, it can be removed with a hair dryer. There are battery powered devices that serve this purpose well. Another trick is to turn your optics to the ground, they will clear eventually. Both of these responses offer only temporary relief, however, unless they are warmed your optics will dew up again.
A telescope that is on an equatorial mount must be placed parallel to the Earth’s axis of rotation to realize its full potential, a process that is called “polar alignment”. Once aligned to the pole, this type of telescope mount can track a star’s diurnal motion with the movement of just one axis.
There are two common designs for equatorial mounts; the German Equatorial Mount and the Fork Mount. They both operate from the same principle. The “polar axis” is tilted by an angle equal to the observer’s latitude and then pointed at the celestial pole. We are fortunate in the Northern Hemisphere to have a “pole star”, Polaris, that is very close to the north celestial pole. It is so close in fact, that for casual observing, just aiming the polar axis at Polaris will suffice. But if you intend to use your mechanical setting circles to find objects or if you will be attempting to do guided photography, you need a more accurate alignment.
Some commercial telescopes come equipped with devices meant to facilitate polar alignment. It might be a sight that is offset from the polar axis of the scope by the same amount that Polaris differs from the true pole. The sight is “set” for the date and time so that when it is centered on Polaris, the scope it is attached to will be aimed precisely at the pole. Another method is an etched reticle that can be used in a finder scope and that has a pattern that when aligned with Polaris (or a circumpolar asterism) places the scope’s polar axis parallel to the pole. In any case, truly accurate polar alignment will require refinement by a more exact process such as “declination drift”.
Declination (or star) drift is a tedious process that only need be accomplished if accurate alignment is a must. With your right ascension drive running, center a star on the celestial equator and on the meridian (as near as possible to due south and zero degrees declination) and watch for drift. If the star drifts south, the polar axis is too far east. If the star drifts north, the polar axis is too far west (in the eyepiece, with the drive off, drift is from east to west, north is the direction 90 degrees clockwise from drift). Adjust as necessary until there is no drift north or south. Repeat the procedure with a star 20 degrees above the eastern horizon. If the star drifts south the polar axis is too low. if the star drifts north, the polar axis is too high. Adjust the position of the polar axis until there is no drift on either star. If your mount is “square” and the ground firm, this method can provide near perfect alignment.