DR. SMITH'S BACK YARD ASTRONOMY
TELESCOPES
Some people have asked for information concerning purchasing a telescope. On this page, I will try to give you enough information to make a wise choice albeit laced with some of my own biases.
One can waste a bunch of money on books that may or may not be useful. One single "all-in-one" book, I highly recommend is "The Backyard Astronomer's Guide" by Terence Dickinson and Alan Dyer; the current 3rd edition is 2008. Check it out at the library. You can usually find good prices at Amazon.com for NEW and USED books. The authors cover all the same issues I discuss here plus much more with lots of tips and practical advice. His star charts are very good and easy to use.
First, DO NOT purchase one of the inexpensive telescopes you see in discount stores such as Walmart, Target, Costco or Sam’s Club no matter the brand (e.g., Meade, etc.). These are poor quality, “cheap” lines with many plastic parts. Also DO NOT buy any of the cute "tabletop scopes"; they are much too small and unsteady. In addition, I DO NOT recommend any of the "Short tube" or "Grab and Go" scopes again because they are too small and their tripods are way too flimsy.
You should expect to pay from $500 and up for a good quality telescope. Keep in mind that a GOOD telescope, whatever its type, will repay you with a lifetime of enjoyment. It can't be used up or worn out; it can be dropped and damaged. A POOR telescope, on the other hand, will wind up unused in the back of a closet after only a few weeks of shaky and frustratingly inadequate views. Try to make your first purchase a quality telescope, even if you have to wait a while to buy it. You'll be happier with a well-made telescope – from your very first look and for many years to come.
Invariably, one of the first things people ask is “What’s the magnification of that telescope?” In reality, the “aperture” of a telescope (the size of the lens or mirror) is what counts and not magnification. The larger the aperture the more light it gathers with the caveat that the larger the aperture the larger, heavier and bulkier the scope becomes. Think of the telescope as a "light bucket"; the bigger the bucket, the more light you can catch. “Magnification” is secondary and is provided by the eye-piece used.
There are two basic telescope designs; refractors and reflectors. There are two general kinds of reflectors; Newtonian and Catadioptric (Schmidt-Cassegrain) and each of these has some modifications/improvements (e.g., Dobsonian, Schmidt-Newtonian, Maksutov-Cassegrain or Newtonian, etc.).
REFRACTOR TELESCOPES
Refractor telescopes are what most folks think of when you mention telescopes. The very first telescopes, such as Galileo’s, were refractors and have an “objective lens” at the front of the tube that bends (refracts) light and delivers it to the eye-piece. I am not a big fan of refractors because good ones are extremely expensive; small inexpensive refractors are guaranteed disappointment.
REFLECTOR TELESCOPES
In Newtonian reflectors, the secondary mirror is tilted 45 degrees sending the image to the eye-piece at the front of the telescope.
Reflector telescopes gather light on a “primary mirror” at the back of the tube and reflect it back to a “secondary mirror”; the image is taken from this second mirror. They are called “Newtonian” scopes because Sir Isaac Newton invented the design. Initially the mirrors, which may be concave or convex, were polished "speculum metal" (2/3 copper and 1/3 tin capable of high reflective polish) but now are made of glass polished smooth and coated with a thin coat of highly reflective metals. The design has been optically improved by placing a “corrector plate” at the entrance of the tube (e.g., Schmidt- and Maksutov- variations). The Schmidt-variations are generally less expensive and preferable; the Maksutov-versions are more expensive but better suited for “serious” astrophotography.
The Dobsonian design is just a Newtonian on a basal mount that moves in altitude and asimuth (Altaz). Dobs are very inexpensive and easy to transport and setup. I definitely recommend a metal tube over a cardboard or fiber tube. To use a Dob effectively, you must know the night sky or be willing to make a concerted effort to learn it. I am not a “Dob” fan primarily because they do not track and they do not have motor-driven, robotic “GoTo”. There are devices that can be attached to Dobs to assist in locating objects.
SCHMIDT-CASSEGRAIN TELESCOPES
Schmidt-Cassegrain reflectors are a modification of the Newtonian design and like them gather and reflect light off of primary and secondary mirrors. The secondary mirror of a Schmidt-Cassegrain sends the image back through a hole in the primary mirror to the eye-piece. I have owned or operated all kinds of scopes but the Schmidt-Cassegrain is by far my favorite. Nearly all are currently sold with built-in robotic “GoTo” function, a feature I believe is absolutely essential and I would not recommend buy any scope that does not have it. My reason for saying this is that to use a non-GOTO scope will require you to know the night sky or be committed to learning it. Many purists would dispute that but, for a beginning astronomer, a “GoTo” scope will teach you more about the objects in the evening sky and stimulate your interest more than “star-hopping” or “setting circles”.
The Schmidt-Cassegrain design allows large aperture in a shorter, more compact scope. An 8 inch scope in this design, with “GoTo” is just about optimal. It is big enough to see the Moon, most of the planets out to Saturn (Neptune and Uranus appear star-like), comets, asteroids and many deep space objects (star clusters, nebulae, galaxies, and planetary nebulae). At the same time this scope is small enough to easily fit in the back seat of most cars and can be lifted and setup by young adults (mid-teens). The Meade LX 90 is a good example of such a scope.
I would like to say a few more things about “GoTo” systems. Nearly all major manufactures (Meade, Celestron, Orion, and others) have telescope models with “GoTo” function. Meade’s “AutoStar” system has developed over the many years and is a very reliable system. It is also easily updated by downloadable links on the internet and their scopes also allow “computer-driven” planetarium software such as Software Bisque “Sky” to control the telescope. Celestron’s “NexStar” system is similar but I have found it somewhat less reliable. I have not had experience with latter versions of this system.
In order for the “GoTo” system to operate, the telescope must be properly aligned. Meade and others are presently offering telescopes that align themselves. While alignment might sound difficult it is a relatively quick and easy process and, in my estimation, far cheaper than the extra cost you will pay for the automation. Of course, you will have to learn to recognize some constellations and some of the bright guide stars, but that is all part of the FUN!
Meade has recently advertised their new “LightSwitch Technology” that allows you to simply set the telescope down and turn it on; it studies the sky and aligns itself; sounds good. These scopes have been around long enough for the bugs to be worked out.
Finally, I do not recommend buying a used telescope unless you know and trust the seller. While scopes do not wear out, they can be dropped, scratched, electronics burned out, etc. A used scope from a reputable vendor that provides a warrantee is another matter.
Here are some reputable vendors. Oceanside Photo & Telescope (OPT) is at the top of my list because they simply are the best for courtesy, helpful information, and service. At the Starizona website, check out their “Telescope Basics” and follow some of the links therein for more information on buying a telescope. I have not had firsthand experience with Starizona but some WVAC members have. The Astronomics site has an excellent “How to Pick a Telescope” link (see especially the “Juggling Ps” page).
EYE-PIECES
Nowadays, telescopes with otherwise good optics are usually provided with very cheap eyepieces of marginal quality. Another reason for sticking to the better quality telescopes is that many of the cheap ones come with non-standard-sized 0.965 inch eyepieces rather than the typical 1.25 inch eyepieces. Your telescope will perform best with good quality eyepieces. These can range from economical to very expensive. Eyepieces and other accessories is another place where you can waste a bunch of money if you aren’t careful and selective with your purchases. Again, the WVAC star parties are a place to have a look through a variety of eyepieces.
Since, I just introduced the sizes of eyepieces, this might be a good place to discuss these a bit further. The barrel of the “American standard” eyepiece is 1.25 inches in diameter. You can manage quite well with these eyepieces which are generally of good quality and also the most economical. Refractor telescopes work well with the 1.25 inch eyepiece format.
Some telescopes will also allow the use of eyepieces with 2 inch diameter barrels. These are quite a bit more (sometimes considerably more) expensive but have brighter and wider fields of view. While I use only the 2 inch format, I would not recommend that you start with these. However, I will say that 8 inch and up Schmidt-Cassegrain telescopes work very well with the large format eyepieces. So, you might start out with the 1.25 inch format with an eye to the future of graduating to the 2 inch format.
Another “size issue” has to do focal length of an eyepiece. While eyepiece barrel diameter is measured in inches, focal length is measured in millimeters (mm). Normally, you will hear most users identify an eyepiece by the magnification it produces. Magnification, in this sense, is equal to the focal length of the objective divided by the focal length of the eyepiece. For example, a 25 mm eyepiece in a telescope with a 1200 mm focal length would magnify objects 48 times. A 4 mm eyepiece in the same telescope would magnify 300 times. The point here is that the larger the number on the eyepiece the less magnification it has and the smaller the number the higher the magnification.
All of the major makers of telescopes also have a line of eyepieces of varying price and quality. And there are many other sources that sell lines of eyepieces such as Orion Telescope & Binocular and Tele Vue to name a few.
Like telescopes, eyepieces come in a variety of types depending on the lens design and quality of materials used. The simplest, cheapest and practically useless are the Huygenian and Ramsden varieties. These should have an R, H, or AR engraved on the outside of the barrel of the eyepiece; they are only worth a few dollars. There are zoom eyepieces and while the idea is interesting, these are generally expensive and really not good for astronomical use. Avoided them too!
There is a whole lexicon of jargon relating to types of eyepieces: Keller design (three lens elements), improved Keller or RKE design, Orthoscopic or “Orthos” design (four lens elements), Plӧssl design (four lens elements), and modern wide-field and ultra wide angle designs. Generally, the more lens elements in the eyepiece the more expensive it will be. You won’t see many of these eyepiece design names in most telescope accessory catalogues. I will come back to this below.
So, the big question is “How many eyepieces do I need?” I think there is a very simple answer to that question and it is THREE!
When all is said and done, the number of eyepieces you will actual use in an evening of viewing is THREE. Unless there is some special circumstance, I will often use just ONE, at most, TWO eyepiece(s) at a star party. Yes, I have a box full of eyepieces that have accumulated over the years, but only THREE travel in my star party case.
So what are my THREE eyepieces? First, recall from above that I use only the 2 inch format. I will come back to the range of the THREE eyepieces that I recommend. The first eyepiece I use every night I set up is my Orion 50mm Plӧssl. I bought this eyepiece about 15 years ago. It has been my alignment eyepiece ever since because it has a wide field of view (FOV) and is relatively “low power” allowing me to acquire and center my alignment star quickly. Once, I have my alignment that eyepiece goes back in the box for the rest of the evening. Orion no longer sells this very useful eyepiece but it can occasionally be found on the internet. However, remember I am talking about the THREE eyepieces I use.
The second eyepiece I use on a nightly basis and often the one I stay with for an entire evening of view is my Tele Vue 31mm Nagler Type 5. It has an 82 degree apparent FOV and you almost fall into this eyepiece. My third eyepiece, if I want a bit more magnification, is a Tele Vue 22mm Nagler Type 4. I use this a lot when looking at the rings of Saturn or the bands on Jupiter.
The range of eyepieces that I recommend is as follows:
1. An eyepiece in the 30mm to 40mm range which will serve in alignment and general observing. Orion’s 32mm or 40mm “Sirius” or "HighLight” Plӧssl would be good choices here. Orion’s 32mm or 38mm 2 inch “Super-Wide Angle (70o)” would be another good choice in the larger format.
2. An eyepiece in the 15mm to 25mm range which will serve in increasing the magnification for the Moon and planets. You might think of the ends of this range as your second (20mm to 25mm) and third (10mm to 15mm) eyepieces, skipping the lowest range. Orion’s 20mm or 25mm “Sirius” or "HighLight” Plӧssl would be good choices here; the 17mm would also work here too.
3. An eyepiece in the 8mm to 15mm range which will serve for greatest magnification. Some telescopes work well with eyepieces in this range but others do not. You may want to test an eyepiece in this range below committing to a purchase. Orion’s 7.5mm, 10mm or 12.5mm “Sirius” or "HighLight” Plӧssl would be good choices here. Personally, I wouldn’t go below 10mm.
Finally, I DO NOT recommend buying sets of eyepieces. My reasons are that you will spend more time switching between the various eyepieces than in actually observing objects. Ultimately, you will likely settle on two or three of them that you use most often and the others are just wasted money. I think it is best to focus one’s resources on two or three really good eyepieces than spreading it out over a set most of which you won’t use.
Barlow lens can be used to bump up the magnification of an eyepiece. Orion’s “2X Shorty” increases the magnification by two times; a 10mm becomes a 5mm eyepiece. They also make a 3X Barlow. I am not a big fan of barlows and I don’t advocate their use unless you know they will work well with your equipment. I have several but use them may be once or twice in two or three years. Atmospheric distortion and vibration render barlows ineffective.
I also am not a big fan of filters although a good polarizing filter sure makes looking at the Moon an eye pleasing experience. Meade sells an adjustable 1.25 inch “Variable Polarizing Filter” that is a great accessory. You put it on your telescope then slip whatever 1.25 inch eyepiece you want into it than adjust the polarization. This beats the filters that screw onto the barrel of your eyepiece. Orion makes a “Neutral Density Moon filter” but the cost is almost two-thirds the price of the Meade filter without the versatility.
At some time, you may want to observe the Sun. NOTE YOU CANNOT DO THIS WITHOUT A PROPER SOLAR FILTER! Depending on the aperture of your telescope, these can be expensive. The very best solar filters come from Thousand Oaks Optical. You don’t want to skimp on these because a bad or faulty solar filter with just pin prick holes can cause damage to the eye. No matter where you get the filter, ALWAYS CHECK IT WITH A LIGHT SOURCE TO MAKE SURE IT DOES NOT HAVE ANY HOLES!
If you telescope didn’t come with a diagonal, I recommend getting one as the make observation a lot more comfortable.
