CHOOSING OPTICS FOR SKYWATCHING
There is a mind-boggling variety of binoculars and telescopes on the market for skywatchers. To help sort them out, I like to divide the instruments first into four size classes: 1) handheld wide-angle binoculars, 2) higher-powered "tripod binoculars," 3) "portable telescopes," and 4) larger "transportable telescopes." Each class of instruments and each type has strengths and weaknesses for various purposes. The best class and type for a person depends on the answers to these questions:
Don't buy any instrument until you have done your homework by reading and by going to star parties held by amateur astronomy clubs. To find your local club go to Clubs and Organizations. One word of caution about star parties: Don't contract aperture fever or technitis. There is an element of competition among amateur astronomers regarding aperture size and bells and whistles.
For a good commercial site on skywatching equipment go to Orion Learning Center.
Here are the top ten reasons that beginners should start with handheld wide-angle binoculars: 1) They have wide fields of view to make it easier for you to find your way around the sky and to recognize star patterns. 2) They are small, lightweight instruments. 3) Compared to telescopes they enable you to look straight toward the object and to view with both eyes, which is more comfortable. 4) They give you a correct image rather than a mirror or upside down image given by telescopes. 5) Because of their size a number of popular objects in the night sky are best viewed with binoculars. 6) Since they don't require a tripod, they are highly portable, and they require no setup time. 7) They are simple to use, and you don't have to learn technical stuff as you do with telescopes. 8) You don't need much observing skill or knowledge of the sky. 9) They are general-purpose instruments. You can use them for birding in the daytime and skywatching at night. 10) You may already have a pair of binoculars on the closet shelf, and if not, you can buy a new pair for a modest amount and there are no accessories that you need to buy.
| Binoculars are real astronomic instruments and far more useful than those cheap little department store telescopes with their shaky tripods and absurd magnification claims. |
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Handheld binoculars, as I define them for astronomy use, are typically 7 or 8-power. The most often recommended handheld binoculars are 7X50s. If you are young enough to fully utilize the 7 mm exit pupil, the 7X50s might suit you. If you are middle-aged (the pupils shrink with age), 8X40-42s with a 5 mm exit pupil may be a better choice, and they are lighter and lower priced. They provide a good balance between FOV and power for observing star patterns. I use Orion Scenix ($89) 8X40s that have a good 9-degree angle of view. For a lower price one might try the Orion Outsider ($64.95) with a 9.4 degree FOV and 10.5 mm eye relief or the Kronos EWA 8X40s which I.T.E. sells online for $60 plus $15 S&H. They have a 9.5 FOV and 17 mm eye relief making them suitable for those wanting to use their eyeglasses. (Kronos EWA)
When you want to view objects (as opposed to star patterns) such as the Moon, planets, nebulas, and star clusters, you need more power without losing too much FOV. What I call the "tripod class" of binoculars are those that need to be put on a tripod for concentrated viewing. They start with 10X50s, which are the second most often recommended binoculars for skywatching.
The binoculars I most often use for a quick look at objects are my 12X50 relatively lightweight Minolta Activa binoculars with 5.5-degree AOV and a price of about $135 online. I handhold them to scan the sky, and put them on a tripod or brace them against a wall for concentrated viewing. If a beginner lives under moderately dark skies and can see the major constellations with the naked eye, these might be a good choice to begin with. I also like them for terrestrial viewing.
I have 15X70s, 20X60s, and 20X80s (least favorite because of size, weight, price, and undistinguished optics) all of which give nice binocular views of the Moon and open star clusters; however, I prefer a rich (wide) field telescope such as an 80mm refractor with a focal length of 400 to 600mm (see below). You need about 40X to get a good look at Saturn's rings, the Trapezium, many binary stars, globular clusters, etc.
Zoom binoculars sound like a good idea, but they are not generally well regarded for skywatching because of the small FOV, less durability, and somewhat compromised optics. However, some people like some of them. I have the Bausch & Lomb Legacy 8-24X50 Zoom ($125 online) that are a great value. They have a 4.7 degree FOV (only half that of my 8X40 Scenix) at 8X and 3.1 at 24X (better than most zooms). The way zooms work is that the magnification is increased is by decreasing the exit pupil with a little lever connected to irises on each eyepiece. Recall that the exit pupil (the size of the beam of light coming out of the eyepiece) equals the size of aperture divided by the power. In this case the aperture is 50mm, and when the power is 8X, the exit pupil is 6.2mm. To get 20X, we close the iris down to a 2.5mm exit pupil.
Choose a sturdy tripod with a panhead with a quick-release for the dovetail shoe. To enable you to look up near the zenith with binoculars, the maximum height at the top of the tripod head should be about 2 inches above your height. The Orion Paragon HD-F2, which comes with a fluid-type pan head and extends up to 69-1/2" ($90). You will also need a binocular adapter or bracket. To make it easier to find things in your binoculars, you might try a "Z" bino bracket (instead of a conventional adapter) including a red dot finder from Burgess Optical online ($25 plus $10 S&H).
I buy binoculars on the Web, but one must look out for inaccurate or misleading descriptions. If you are considering an off-brand, look for an explicit money-back guarantee. For binocular reviews by owners go to Binoculars.
The number of different types, sizes, and grades of telescopes will make your head spin. For an introduction to telescopes I refer you to these sites: Choosing a Telescope, Choosing Your First Telescope, and Choosing a Telescope for Astronomy.
There are four important optical specs to consider when choosing a telescope. The first is aperture or diameter of the main lens (in refractors) or primary mirror (in reflectors); the more aperture the more light-gathering power the scope has and the more usable magnification can be attained, which is about 40X or more per inch of aperture depending upon the quality of scope. I might add that I seldom go over 100X with my small scopes because higher power means fuzzier, shakier images and a smaller field of view and because of poor sky conditions. If you see a 2 or 3-inch department store scope that claims to go up to 500X or more, have a good sardonic laugh.
The second spec is the focal length (FL), which is the distance from the main lens or mirror to the focal point or the point where the light rays converge. The third spec is the focal ratio, which is the FL divided by the aperture. A telescope with a low focal ratio such as f/5 has a relatively wide field of view making it easier to find things, and such a "fast" lens means less exposure time if you do photography. A scope with a long FL such as f/12 has a smaller field of view but is better for high magnification.
The fourth spec and one not often stated is resolving power, which is the ability to show fine details and depends on the previous specs plus optical quality. (One arcane measure of optical quality is the wave error or irregularities of the surface of the lens or the prime optic. For example, a 1/8-wave prime optic means that the height or depth of the surface irregularities do not exceed 1/8 of the wavelength of yellow light.) Good optical quality also means freedom from several types of aberrations such as chromatic aberration, which is common in lower-priced refractors. You can determine the probable optical quality of a particular make and model by talking to experienced stargazers, reading reviews and owners' reports, and looking through one at star parties.
To get an excellent explanation of how the optical system works in a basic telescope go to Basic Telescope Optics.
In addition to the optical specs one needs to consider the dimensions and weight of a scope. For example, I have a bad back, and my weight limit is 25 lbs. per piece of equipment. I must haul my scope in a sedan, so I am limited as to the tube length. I also live in a condo without much storage space, so my 6-inch scope on a Dobsonian mount is stored in the master bedroom, which some spouses wouldn't tolerate.
All telescopes need to be properly collimated, which means that the optical elements must be properly aligned. To learn all about aligning a Newtonian telescope (which is the type that most often needs to be collimated), go to Collimation.
You may see an ad for a refractor with a FL of 600mm and an aperture of 100mm. What is the focal ratio? It is an f/6 (600mm / 100mm) scope, which tells you that it is a short tube or rich-field (wide field of view) scope. It comes with a 20mm eyepiece, which will give a magnification of 30X (600mm / 20mm), and a 10mm EP that will raise the magnification to 60X. Adding an accessory called a 2x Barlow lens will double the magnification, so with the 10mm EP you can kick it up to 120X. Since the aperture is about 4 inches, the highest usable magnification will be about 160X (4 x 40).
Refractors come in many different designs with different levels of optical performance. The most common and inexpensive refractor is the achromat, which means that the lens is a doublet consisting of a crown glass element paired with a flint glass element. You can expect some color aberration in the form of violet fringes around bright objects like the Moon. This is particularly true with short tube refractors. However, the resolution from a well-made achromat may be very good. The false color won't bother you as much after you find out how much you pay for an apochromat (APO), which literally means "color-free." "Apochromat" is a standard for correcting chromatic and spherical aberration and coma rather than a specific optical design. APOs may have from two to four lens elements and include a fluorite element or an ED (extra-low dispersion) glass element. A less expensive intermediate correction is often called a semi-apochromat, which is a semi-meaningful term.
Reflectors also come in many different designs. The simple Newtonian reflector usually has a parabolic mirror meaning that the shape of the concave mirror is a parabola, which means that all of the light rays are brought to focus at the focal point. The Dobsonian design is a simple open tube Newtonian OTA (optical tube assembly) on a simple box-like mount on a "lazy Susan." The Cassegrain scopes usually have very short OTAs with long focal lengths. They really do "do it with mirrors," by having a secondary mirror that folds the light back through the center of the primary mirror. The popular Maksutov and Schmidt-Cassegrain designs have spherical mirrors and corrector plates that correct for spherical aberration. One advantage of reflectors is that they are not subject to chromatic aberration; therefore, the optics can be a lot simpler and less expensive especially for large apertures.
A telescope has two main parts. The objective or light gathering part consisting of a lens system or a mirror that forms an image. The second part is the eyepiece which magnifies the image. Here are the numbers we look at in choosing an eyepiece: 1) The diameter of the barrel. Most amateur scopes have 1.25-inch eyepieces, but some can accommodate both 2-inch and 1.25-inch eyepieces with an adapter. Ignore the cheap scopes having 0.965-inch eyepieces. 2) The focal length. This determines the magnification for a particular scope. 3) The apparent field, which may be around 40 or 50 degrees for low-priced EPs. To find the true field of view in a particular scope, we divide the apparent field by the power (the telescope's FL divided by the EP's FL). An example: EP's apparent field is 50 degrees, and its FL is 10mm. In a rich-field telescope having a FL of 500mm the true field of view will be 50/(500/10) or 1 degree. In a compound scope with a FL of 1500mm the true field of view will be 50/(1500/10) or 1/3 degree. This shows why it is easier to find things in a rich-field telescope, but the power will be lower. 4) The eye relief, which should be 15mm or better for eyeglass wearers. There are several eyepiece designs, but the 4-element Plossl is the most popular. The ultra wide designs having apparent fields of view up to 90 degrees are the bulkiest, heaviest, and most expensive.
For more information about telescope designs go to Rich's Telescope Primer.
My idea of a portable scope (also known as a "grab 'n go" scope) is that it can be set up quickly, and if you want to move it, you can easily pick up the scope and mount (and tripod) and move it if a tree or something is in your way. It is also easy to transport and store. Small refractors don't require much cool down time, so they are also "quick look" scopes. Decent quality portable scopes are a good choice for a first scope because you will always need one even if you buy a larger "transportable" scope later.
The portability of a telescope depends upon the weight and bulk of the mount and tripod as well as the tube. Alt-az mounts are lighter and simpler than equatorial mounts. However, there are few good choices available, and they are expensive. I have considered making a Dobsonian type of mount and a tripod, see Alt-Az Mount, for my portable scopes.) Photo tripods with panheads are simple alt-az mounts, but they are often too light for scopes weighing over about 5 lbs. or having small fields of view. When the scope is pointed upward, it becomes unbalanced and wants to rotate back making it hard to hold with the friction control. On some heads you can attach a coiled spring to counteract the rotation.
German equatorial mounts (GEM) are heavier and more complicated to set up and use, but they are useful for finding and for tracking an object, which is needed for astrophotography. These mounts have a declination axis and a polar axis, which by a process of polar alignment must be pointed toward the North Celestial Pole (NCP) or at least toward Polaris (about 3/4 degree from the NCP). Go to Polar Alignment. To learn how to use the setting circles to find objects go to Using Setting circles. When you want to sweep from a guide star to a target, you need the coordinates of some bright stars to use as guide stars. Go to Bright Stars Suitable for Setting Circles. GEM setting circles are crude, but they are sufficiently accurate if you make short sweeps from guide stars to targets. Edmund Mag 5 Star Atlas ($6.95) published by Edmund Scientific is a good little atlas for beginners, and it gives good instruction for using an EQ mount with its setting circles to find things. It also gives the coordinates of bright stars to use as guide stars. To order, go to Edmund Scientific's Books and Atlases. Although my EQ mount and tripod weighs 16 lbs. and my AZ-3 alt-az mount and tripod weighs only 10 lbs., the AZ-3 should have been designed with a counterweight, which would equalize the weights of the two mounts. (Instead of a counterweight, I use a coiled spring to keep my AZ-3 from torquing back.)
Most EQ mounts can be used like an Alt-Az mount if you don't want to bother with doing a polar alignment. Just point the polar axis straight up and turn the mount for the azimuth adjustment. Raise and lower the scope on the declination axis for altitude adjustment. The advantage of this over a regular Alt-Az mount is that you can make fine adjustments and lock your position.
Dobsonian mounts are a popular type of alt-az mount for Newtonian scopes. They sit on the ground and eliminate the need and cost of a tripod. Since Newtonian scopes have the focuser at the front end of the scope, the eyepiece is at a convenient height on medium length Newtonian tubes.
"Go to" computerized mounts are becoming more and more popular. Once they are properly aligned one can select objects, and the scope will slew to them automatically with a little bit of luck.
The "short tube" or rich-field refractors are popular "quick look" scopes, especially a group of light f/5 80mm achromat clones such as the Orion ShortTube 80 selling for $199, which has often been on sale for $159. It comes with a 45-degree prism diagonal, which is needed for terrestrial viewing, but a 90-degree mirror diagonal is needed to avoid wrenching your neck when looking up high in the sky and to avoid raising the scope above your head. It has a correct-image 6X26 finder scope. (Most straight-through finder scopes give an upside down image, which is confusing for a beginner.) A "1X" or reflex finder such as the excellent Rigel QuikFinder or the Stellarvue red dot finder is a big help to home in on objects you can see with the naked eye. Although a sturdy photo tripod with a good panhead can be used for these scopes, I would prefer the light (12 lb.) equatorial mount or GEM ($79.95) that can be purchased with this scope. The problem with the photo tripod is that the scope is unbalanced and tends to flop back when the scope is pointing high. With my tripod I have attached a coiled spring (7/16" diameter and 2-1/2" long) with coathanger wire to the lower part of the head and the upper part so that it comes under tension when the scope is raised and prevents it from torquing back. The spring goes on to hooks in the wire so that I can remove it. Like any short tube achromat you can expect flaring false color around bright objects. These scopes also make good fast telephoto lenses.
Orion offers an apochromatic SkyView Pro 80mm ED refractor, which has had great reviews. It sells for $819 and comes with a free single-axis drive. The price may cause a beginner some "sticker shock," but when you look at the price of other "apo" scopes, this is a bargain. Unfortunately, the sturdy EQ mount weighs about 35 lbs., which is too much for me, but you do need a solid mount to take advantage fully of the scope's capability. They offer the tube-only option for $499.
The second type of portable scope is the small Maksutov-Cassegrain type of compound scope. They are very compact, have long focal lengths with focal ratios of around f/12 to 15, and have no color aberration. Consequently, they are better for higher magnifications, but the down side is that they have smaller fields of view. The problem is reduced by having a good finderscope and a "1X" finder. The more power you use the shakier the image gets, so you need a mount with smooth movements and a solid tripod. With these scopes you may need up to an hour (depending upon the temperature differential) of cool-down time before use. Some of these scopes such as the Meade ETX-90EC are "go to" or computer controlled and automated scopes.
The reviews and owners' reports on Orion's small Maks (the StarMax on equatorial mounts and the Apex with no mount) are from good to glowing, and the prices are unbeatable. (I have the Apex 127, which I like after changing the finder and diagonal.) The StarMax 102mm with the EQ-2 mount and tripod sells for $389 and has a total weight of 22 lb. making it quite affordable and portable. I would replace the finder with the Orion 9x50 right-angle, correct-image finder to make starhopping easier, and I would add a Rigel QuickFinder. The StarMax 127mm with the EQ mount and tripod sells for $569 and has a total weight of 36.1, which is a little too heavy for my back.
The third type of portable scope is the small (4.5-inch and 6-inch) Newtonian reflector on a Dobsonian, equatorial, or computerized mount. The 4.5-inch f/4 Orion StarBlast on a simple Dobsonian mount ($169) weighing in at only 13 lbs and having a wide field of view is a good scope for kids. Adults need to set it on a three-legged stool or table for comfortable viewing. Celestron sells a comparable Newtonian, the Firstscope 114 EQ Short on a minimal equatorial mount. It has a built-in Barlow lens, which in effect doubles the FL doubling the power of any eyepiece. Adorama sells the scope and mount for only $139.95. I have the "go to" version, NexStar 114GT, which has a computerized mount. Adorama sells the NexStar 130GT for only $359.95. I would prefer it since it doesn't have a built-in Barlow lens as in the 114GT and has a larger mirror. The weight is a reasonable 18 pounds. I am a little leery of these inexpensive "go to" scopes because I wonder about the life expectancy of the electro-mechanical parts. Also, the AA batteries are good for about one long viewing session, so one needs a portable lead-acid battery unit.
The f/8 Orion SkyQuest XT4.5 Dobsonian ($199) weighs 17.6 lbs. The next step up is the SkyQuest XT6 weighing in at 34.4 lbs. and selling for $359. For an additional $149 you can have a computerized object locator (digital setting circle). I would add a right-angle correct image finder and a Rigel QuickFinder. They do offer the most "bang for the buck."
A couple of disadvantages of the Newtonian scopes is that they may require frequent collimation (alignment of the mirrors), especially the shorter ones, and they have open tubes letting dust get on the mirror.
What I call "transportable" scopes covers a wide range of types and sizes. The smallest including the mount may weigh about 40 to 50 lbs. and fit in your car trunk, but you may need a van to haul the larger ones. They include 6-inch refractors, the popular 8-inch and larger Schimdt-Cassegrains (compact tubes but with heavy, bulky mounts and tripods), tubed Dobsonians, and larger trussed Dobsonians. If you get to the point of buying one of these scopes, make sure you can comfortably lug the weight and can accommodate the size in your vehicle and storage space.
The larger the scope the less often you will use it. I have a 6-inch Maksutov-Newtonian scope on a Dobsonian mount, which doesn't take long to set up, but I only use it when I go to star parties because it takes time to cool down and is somewhat heavy and bulky. I use my portable scopes around home.
If you are thinking about making your own Dobsonian scope or if you interested in John Dobson's interesting bio, check out Building Your Own Telescope, which assumes you will buy the mirror ready to install. If you are really gung-ho and would like to make your own mirror or just see what is involved, go to Amateur Telescope Making. Mirror making needs to be done in a class where you can give each other the courage to go on and the hope that you will have something better than an expensive glass ashtray when you finally finish.
I hope that I have made it abundantly clear that buying a telescope involves consideration of many factors and that one should not rush the decision before having the necessary information. It is something like choosing a mate.