A few basics about telescope types for beginners
Posted 15 January 2007 - 09:57 PM
Some things to know about SCTs and MAKs.
Though they have short tubes, these actually have long focal lengths (fl) and high f/ratios (the typical Meade or Celestron SCT, for example, is an f/10, while MAKS are around f/14). I will not attempt to explain what this means technically, but only the performance outcome. Long focal length, high f/ratio (also called “slow”) scopes, offer a relatively narrow field of view (FOV) to the viewer. This makes MAKs, especially, very good for looking at planets and the moon, but if you want to look at an object like the North America Nebula, you will only see a small part of it, and thus have no sense of what you are seeing. At the same time, you will not be able to view things in their larger “context,” or place in the sky, due to the relatively small FOV. MAKs and SCTs, because of their long focal lengths, also produce a higher magnification per millimeter of eyepiece length, which means that the standard supplied 26mm eyepiece in a Meade SCT produces about 77X due to the fact that magnification is calculated by dividing the focal length of the telescope by the length of the eyepiece, which is given in millimeters (e.g., 2000mm/26mm=76.9X). In a short fl scope with a low f/ratio (less than f/6, as in many short tube refractors and Dobsonian reflectors these days), on the other hand, the same eyepiece will produce a proportionately lower magnification. One advantage to MAKs and SCTs is that, due to optical principles I will not attempt to explain having to do with the narrow FOV and long focal lengths, relatively inexpensive eyepieces perform adequately in them, as they will not in short fl, low f/ratio scopes. Final point: MAKs are competitively priced up to about 6 inches of aperture; after that their cost rises steeply, unlike SCTs which are relatively affordable up to 12 inches.
Some things to know about refractors.
In the old days (ie., when I was young), refractors were very long, as in the classic Unitrons. That long length, which meant long focal lengths and also high (or "slow") f/ratios, reduced the chromatic aberration (CA) that refracting instruments are prone to. CA can get in the way of your perception of fine detail on bright objects: for example, when looking at the moon, you will see a purple haze around CA affected craters and whatnot, and CA will blur your view of detail on bright planets like Jupiter while "smearing" out bright stars. The old refractors also made use of double lens systems to help reduce CA, along with their long fl and high f/ratios, and are called achromats. But because of their long fl and high f/ratio, these achromats offer a narrow, “portal” like view, which, like MAKs, makes them good for planetary and lunar viewing, but not so good for broadly extended objects like nebulae. Another problem with the old, long focus achromats is that they can be difficult to view from as you get nearer to zenith: that is, as the long tube points higher, the eyepiece heads lower: you end up on your knees. The long tube achromats are also unwieldly and thus tricky to transport.
In recent years, short focal length, low f/ratio (or “fast”) refractors have become very popular as viewers and astrophotographers (“imagers”) alike have come to desire highly portable refractors that can be easily transported to dark sites (the suburban skies are more light polluted now than in the old days), and which also offer wide field (“rich field”) views of extended objects like the North America Nebula, while offering nice crisp planetary and lunar images, due to the lack of the central obstruction that SCTs, in particular, are affected by (the central obstruction is the apparatus housing the secondary mirror that reflects light back through the primary mirror to the eyepiece: since the obstruction adds extra diffraction to the system, visual contrast is reduced relative to an obstructed telescope). The problem here is that once the focal length and the f/ratio are reduced to produce a short tube, wide field instrument, all that CA that the long tube achromat helped suppress comes flooding back in. So special optical designs and special CA reducing glass are required to produce what is called an “apochromatic” refractor (APO). Problem solved, and apochromats are wonderful scopes. But there’s another problem: those special designs, and much more significantly, that special glass (generally known as ED glass--there are many varieties) are very, very expensive, which means that an apochromat costs hundreds of dollars per inch of aperture (if you get a bargain model) and a thousand or more per aperture inch for a premium scope (that is, up to about 6 inches: after six inches the sky seems to be the limit because over 6 inch apochromats are rare, very expensive to build, and very heavy). What this all boils down to is that your fine apochromat is not going to have much aperture (rarely does anyone have one larger than 6-7 inches) and while it is going to give you magnificent, crisp wide field views and magnificent, crisp planetary and lunar views, it will not do as well on deep sky objects (DSOs), like galaxies, that are so faint that they need aperture for greater light gathering ability. Another thing to consider is that short fl, fast f/ratio scopes require expensive, specially designed eyepieces to avoid all sorts of optical problems that would otherwise distort what you see. The famous 31mm Tele Vue “Nagler” (known reverently as “the holy hand grenade”), for example, offers undistorted, extremely wide field viewing for fast scopes, but it costs over $600.
Note: There are many inexpensive short tube, fast achromats available (scopes without special ED glass), but they suffer from CA. Many viewers aren't really bothered by CA, however, and others have been satisfied with violet fringe reducing filters with their short tube achromats, however, so this offers an inexpensive alternative to a fast, short fl apochromat. It should be added that beyond 5 inches, any refractor, apochromat or achromat, is a large and heavy instrument.
Some things to know about reflectors.
If you want an affordable and transportable large aperture telescope (generally 10 inches or beyond) that doesn't have the relatively narrow FOV of an SCT, you need to go to a reflector. (A ten inch apochromatic refractor, for instance, may cost $100,000 or more; a ten inch Dob--even a truss design Dob thanks to Meade’s Lightbridge series--may cost less than $1000). Many many people today have Dobsonian mounted Newtonian reflectors for this reason. Today’s “Dobsonian” differs from my youth’s reflectors in having a much simpler, lighter, cheaper and more transportable mounting system than the German Equatorial Mounted (GEM) reflectors of my youth (and that Parks still specializes in manufacturing today). They also differ in often (but by no means always) having a short focal length and fast f/ratio in order to offer nice wide field viewing. Thus, a good modern Dob can both “go deep” for DSOs (if it has sufficient aperture) and offer wide field viewing. Due to the advent of “truss Dob” construction, a very large scope of this kind can be disassembled for transport and be easily reassembled. The Dob’s major drawback is that it does not ordinarily come equipped with a tracking system and it takes some practice to push and pull a Dob as you view to keep an object in the eyepiece at higher magnification. It also is not optimized for astrophotography. But various tracking devices, generally costly but not in comparison with an APO, are now available. The new fast Dobs do require expensive eyepieces, though, because, as above, their fast f/ratios and short focal lengths require specially designed eyepieces to avoid optical distortions.
Ultimately, you alone can decide what your budget is, and, just as importantly, what you want to view and how important ease of transport is to you. Finally, I have made no effort to distinguish between relative brand qualities. This can be a subjective and controversial matter. As a very general rule of thumb, you do get what you pay for in this hobby, but there is now an increasingly competitive market that is offering high quality at lower prices than ever before (especially in regard to recently introduced apochromats).
I hope these bits of information help.
Posted 15 January 2007 - 10:01 PM
Posted 15 January 2007 - 10:23 PM
This, basically, refers to the alignment of your optical system. You want a nice straight line in your light path, and it doesn't matter which kind of scope you have: it must be collimated. Many people like refractors because, most of the time, they do not require collimation after they have been collimated at the factory. Reflectors, SCTs, and MAKs, do require collimation by the user, especially truss Dobsonian reflectors which have to be collimated every time they are reassembled. An uncollimated scope will not perform to its best ability, and a really uncollimated scope will show you only mush. I will not attempt to explain how to collimate a scope here because there are many threads on Cloudy Nights on how to do this. You also should read your instruction manual that comes with your scope. I'll only say this: most of us are very intimidated by our first collimation, and most of us make a hash of it the first time around. But fear not, it is an easily learned skill and even I am no longer afraid to collimate my SCT.
Mirror cleaning. Ouch. A Newtonian reflector's primary mirror, though it lies at the bottom of the telescope tube, is inevitably going to collect dust, and possibly gunk. Many of us are very obsessive about seeing anything on our optics and want to wipe that junk off immediately. Do not do so. Mirrors, especially, are very delicate, and should only be cleaned when the viewing has been affected (dust does not affect viewing). There are many threads on CN explaining how to clean a mirror. Always be careful.
SCTs and MAKs are sealed, so their mirrors stay pretty clean, but they have glass plates at the front (which are really lenses called "corrector plates") that collect dust and dirt and whatnot. They are not as delicate as mirrors, but you can wreck their coatings by not following proper advice on cleaning. (I'll add that they must be pretty tough because I cleaned my corrector plate before I read my manual and before I looked into the matter on Cloudy Nights and managed not to wreck it.) The famous Dr. Clay has a web page that explains how to clean your corrector plate. He advises avoiding it until you have to. I haven't taken that advice.
Refractors are much like SCTs in this regard. If you must clean them, follow instructions and be careful.
Posted 15 January 2007 - 10:24 PM
Thanks for this great post.
Posted 15 January 2007 - 11:29 PM
Posted 15 January 2007 - 11:46 PM
Posted 15 January 2007 - 11:52 PM
You covered the "long and short" of it quiet well. I was wondering if you had any comments for those few "mid range scopes" (f6-f8) that are out there. Are they a good compromise between the two and how do they preform?
Posted 19 January 2007 - 11:50 AM
Thank you for a great effort , which I hope MANY people find very useful .
There's a lot of information there , which for ME , would have probably seemed easier to digest had there been a few more " breathing spaces " in the text .
Perhaps binoculars COULD have earned a mention in some capacity , too ! :-)
Good stuff though , and very well done !
Regards , Kenny
Posted 24 January 2007 - 01:04 AM
Posted 06 March 2007 - 04:18 PM
Nope! I can see I have lots to learn...
Posted 07 March 2007 - 10:40 PM
Posted 13 March 2007 - 09:58 AM
Posted 15 March 2007 - 09:46 AM
I am a new enthusiast in the field of startgazing and all. I am planning to buy my first Telescope. I have seen several ones on the net. And also got few ideas. First let me say my expectations from the scope:
1.) I want that scope through which, I can see the starts, galaxies, Nubela, Planests, Sky Deep objects etc very clearly and with all colors as we see some pics on the inernet.
2.) I want it should be computerised.
3.) Not so bulky and not so heavy.
Now, I would like to with what kind of telescopes I can get this kind of colorful views and to see deep objects?? I have seen Celestron 130 SLT on the net. It has 5.21 " aperture. With this telescope can I see deep objects with all colors?? Or I should go for higher one?? Or for general telescopes this kind of view is simply not possible??
I am eagerly waiting for your answer.
Posted 15 March 2007 - 11:07 AM
I want that scope through which, I can see the starts, galaxies, Nubela, Planests, Sky Deep objects etc very clearly and with all colors as we see some pics on the inernet.
Not going to happen. No such creature exists.