Rich's Telescope Primer Part 2
Sep 21 '00 (Updated Mar 26 '07)
The Bottom Line This is the second part of an equipment overview. The intent is to have a summary of what is in the market today.
This is the second part to the Telescope Primer Article I have written to tell what different types of basic telescope equipment are and what they do. These are intended as a supplement to my article on Picking a Telescope.
PART 2
Telescope Types, Continued.
- Maksutov Newtonian Reflectors (MAK-NEWT)
- Class Performance
- Variations
- Unobstructed Reflectors
- Schiefspieglers
- Unonbstructed Newtonians
- Other variants
Mounts:
* German Equatorial Mounts
- Traditional Versions
- Computer Controlled
* Fork Mounts
- Traditional Versions
- Computer Controlled (GOTO)
* Dobsonian Mounts
- Traditional Versions
- Computer Controlled (PUSH-TO)
- Alt-Azimuth Mounts
2. D. Maksutov Newtonian Reflectors (MAK-NEWT)
The Maksutov-Newtoninan is a concept similar to the Schmidt Newtonian. In this case, it uses the dished-in shaped Maksutov Corrector and a spherical primary mirror. However, the Newtonian architecture, with the secondary mirror mounted much closer to the eyepiece allows these telescopes to use incredibly small secondary mirrors, even at fairly low focal ratios. As a result, 6" diameter MAK-NEWT telescopes such as the Intest MN 61 (which used to be offered by Orion) have gained a reputation for performance comparable to APO telescopes of equal aperture. At f/6, these telescopes appear to be a reasonable and far more affordable alternative to APO telescopes for larger sizes. It should be pointed out, however, that the price for this is increased weight- they are fairly large telescopes and are quite ungainly compared to MAK or SCT equivalents.
2. Di Class Performance
These telescopes are relatively rare and are only offered under the Intes and Intes Micro brand names. Orion offered these a few years ago but has since discontinued these models, but Intes scopes are still available from ITE (www.iteastronomy.com) in the US or APM (Markus Ludes, in Germany) internationally. I have had a chance to examine a earlier Intes telescope first hand and was able to see the way it was constructed and how it performed. The optical tube was a light neutral gray color and had the specked paint finish the orange-tube Celestrons had. If the color had been changed and the front corrector didn't show the obvious indigo sheen of modern coatings, I would have guessed the telescope must have been a prototype from Celestron circa 1982. This isn't a bad thing- it felt very solid and when I was finally able to focus it on something, the view was magnificent. However, this is where the significance of this being an early model comes in- although the 2" diameter focuser moved beautifully and smoothly, it stopped about a quarter inch short for focusing any eyepiece I could lay my hands on. So, the trick was to lock the eyepiece in half-way out of the focuser. From what I understand, this was fixed on the Orion version and the currently available Intes telescopes. In any case, it looks like this design is has cleaner images than the Schmidt Newtonians have had in the past, even though they can't get to the extremely low focal ratios the Schmidt Newtonians were built for.
2. Dii Variations
The scarcity of these scopes makes them a curiousity at the moment. Since Orion has picked up a full line of Maksutov Cassegrains from Taiwan, it is possible they may consider adding one of these at reasonable cost.
2. E. Unobstructed Reflectors
All reflectors with a secondary mirror in the light path pay a penalty in performance due to light diffraction around this mirror and any support legs. Many types of telescopes have been designed with very exotic optics to try to reduce the size of this secondary mirror. For example, Ritchey-Chretien telescopes use hyperbolic primary and secondary mirrors to reduce the size of the secondary mirror and achieve a flat photographic plane. These are so difficult to manufacture that this design is almost unknown outside observatory telescopes.
Theoretically, the best performance possible for a telescope would be a single reflecting element which would focus incoming light beams directly at an eyepiece with nothing interrupting the light path anywhere between the incoming light and the eyepiece. As a point of interest, this telescope was actually built long ago. The design works by holding the eyepiece at the front of the telescope off to the side. If the observer is using their right eye, the telescope will be setting looking up just to their right and the view they are looking at will be over their right shoulder. For simple parabolic or spherical mirrors with very long focal lengths, this works surprisingly well considering the light path isn't coming to a true focus. However, the view isn't really that sharp since the light can never be brought to a true focus. In fact, what this telescope really requires is a very special mirror- one which has its center of curvature outside the piece of glass.
All unobstructed reflectors need to take the light path outside of the incoming light path so that nothing is in the way, so the primary mirror can't be a straightforward axisymmetric or "Bowl" shape. Instead, it needs to be shaped more like a curled leaf. This single factor is what stands between an entire class of telescopes and practical realization. The problem is the grinding processes needed to make these mirrors are very difficult. So difficult, in fact that it is easier to grind one huge mirror and cut the sections out with the right curvature than it is to machine them individually. However, the price of a mirror goes up exponentially as its diameter increases. Therefore, these telescopes are very expensive. Except for ITE (www.iteastronomy.com/) and an experimental run by Orion a couple of years ago, about the only people making this type of mirror are amateur telescope makers.
The alternative solution to this problem is to try to use matched groups of other mirrors and bounce light off of them multiple times to make simpler symmetrical mirrors to get the light to come to a focus. Many different types have been attempted. Some telescopes with as many as four mirrors in the primary optics have been attempted to solve this problem. Generally, most of these telescoped end up with very long focal lengths. Although some are very good for planetary viewing, most have very narrow fields of view.
2. Ei Schiefspieglers (SSP)
These telescopes are from the German term for "Tilting mirror." This type of telescope is virtually unknown in the US, although there are several German and Swiss brands sold in Europe. There are several different variations on these telescopes, but essentially what they do is use two symmetrical mirrors to simulate the focusing effect of a single off-axis mirror. These telescopes operate at long focal ratios of f/15 and sometimes longer to minimize distortion in the image. From what I hear, they do a respectable job on planetary images, but have no realistic hope of capturing large dim deep sky objects. Some amateur versions reflect light three and even four times between the mirrors to try to focus it better. As a result, many of these have f ratios over f/20.
I can only guess at why these telescopes have enjoyed success in Europe and not the United States. If I were to make hypothesis, it would be the differences in population density between the US and Europe combined with light pollution. High magnification is a strong defense against light pollution, and large diameter telescopes perform poorly in areas brightly lit by fuel stations and the like. However, either small diameter or long focal length will dim images so that light pollution doesn't wash contrast out so strongly. The only objects easily in a light polluted areas are bright planets, stars, and the moon. The SSP performs well at this and has gained a following. In the US, where getting out to dark countryside skies is a real option, there is much stronger demand for wide angle telescopes capable of illuminating dim objects such as nebulas, galaxies, and comets. Of course, the amount of aperture you could get for what the SSP would cost would easily put y0u into higher resolution, and if you are really bothered by light pollution, there are Light Pollution Reduction Filters.
2. Eii Unonbstructed Newtonians
These are an ideal solution to the unobstructed telescope. However, at this time the main mirror has to be cut from a larger mirror. In essence, a single 34" mirror could realistically produce six 8" off-axis telescope mirrors. However, a good quality mirror that large is worth far more than the six conventional 8" telescope mirrors. To make matters worse, breaking up the original mirror will cause the daughter mirrors to flex slightly, so they will be of poorer quality than the original shape. As a result, this approach is rare and expensive. There are a few individuals who have slowly ground this mirror shape on their own. As mentioned previously, ITE and Orion have offered these.
2. Eiii Other variants
Other approaches to unobstructed telescopes abound. However, until someone can mass-produce a true off-axis mirror economically, off axis reflecting telescopes will remain a unique curiousity.
3. Mounts
The telescope mount is a huge part of what makes the telescope itself useful. Because of Earth's rotation and the apparent motion of everything in the sky it causes, a telescope needs to be able to be moved in fine increments to track objects. Several different types of mounts have been used for different purposes. The mount is usually a much larger piece of equipment than the telescope itself, unlike in the case for photographic equipment. The reason for this is the mount also needs to hold the telescope steady. As a result, mounts and tripods can be expected to weight three to five times as much as the telescope itself, and sometimes more.
Several different flavors of mounts have been explored at different times, but all mounts ultimately fall under two basic types: Equatorial and Altitude-Azimuth (alt-az) mounts. Sometimes the hardware for each type may be the same and the difference is in how they track an object across the sky. An Equatorial mount is the simplest to operate, and the most accurate, but is more difficult to set up than an alt-az. An equatorial mount has one axis aligned to be parallel with the earth's axis. This allows the mount to track objects as the cross the sky by only moving the one axis. The step of aligning the axis may be slow, but this one has the simplest job to automatically track objects in the sky with a motor.
In contrast, an alt-azimuth mount is mounted with one axis pointed at the ground and one pointed across to the horizon. Turning either axis alone will not track an object in the sky. The altitude of an object and its heading change constantly as it crosses the sky, and change at different rates compared to each other depending on where the object is at the moment. As a result, both axes will need to be adjusted continously to track an object across the sky. Additionally, this also comes with another error- as seen from a fixed point of earth this way, an object will appear to rotate slightly as it crosses the sky. An alt-azimuth mount will not correct for this the way an equatorial mount does automatically. Until the last 15 years, the tracking solutions for making an Alt-azimuth mount automatically track objects in the sky were so complex they could not be performed in real-time. Modern computers have now made this possible along with the ability to track to any object.
3. A German Equatorial Mounts
The German Equatorial is a mainstay of telescope mounts. These reliable pieces of equipment are unmatched in tracking performance and flexibility. However, they do have some drawbacks. Mainly, the mount has a central axis which is aligned with the north or south pole (If you are in the northern or souther hemisphere, repesctively). This axis is usually driven by a worm gear and is the one an electric motor turns to automatically compensate for earth's movement and keep objects steady in the telescope's eyepiece.
The second axis is mounted on top with a drive gear or a spring drive on one side. Theoretically, this axis should only be used when the telescope is first aligned with an object. In practice, small adjustments are often needed to correct error from time to time, unless you do a very careful polar alignment. The telescope will go on one side while a counterweight is mounted on the other. It is the adjustable counterweight design which makes these mounts versatile since different telescopes can be put on one side and all that is needed is an appropriately placed counterweight to balance the equatorial axis. However, this adds weight and can cause problems if the counterweight bounces on its arm and cause the telescope to only slowly stop shaking once it has been jarred.
3. Ai Traditional Versions
The German Equatorial has been a favorite standby for many years. The most common basic mounts have simple twist-knobs on each axis for driving the gears by hand to track objects. Each axis has a release lever so the gears can be disengaged from the axis and the telescope can quickly move to a new object.
These mount come in different sizes depending on the needed capacity for the telescope (i.e. the heavier the telescope, the bigger the mount). The basic mounts can either be attached to tripods for easy movement (the most common version) or they can be mounted to tall pier poles. The piers are usually intended for more permanent installations. Some very robust versions of this mount include the Celestron CG-4 and CG-5, the Vixen Great Polaris and Super Polaris (also sold by Orion), the Orion SkyView, and others. High end versions of this mount include The Losmandy GM-8 and G-11 mounts (these are see-to-believe pieces of equipment). Astro-Physics also has high-end German Equatorial mounts (which are the bulk of their product line these days).
Some equatorial mounts such as the Celestron CG-5 have their counterweight arms linked to the telecope so that both turn at the same time. This feature makes it possible to mount a second smaller telescope or camera in the place of a counterweight and thus have all of the instruments track perfectly together without adding more weight.
Another feature available on larger equatorial mounts are polar finder scopes. These are small telescopes which mount in a hole through the polar axis. They make it possible to quickly get an accurate polar alignment. This is expecially easy in versions such as Orion's, which actually has a small inlaid star map in the finder scope to overlay on the visible stars to get the alignment right.
Some care needs to be taken if you are buying an equatorial mount for general use. For example, Meade's LXD 500 series mounts have aluminum worm gears and will quickly wear. At the moment, I am wondering what is going on at Meade because of these remarkable systems they put out with basic flaws in critical components, but remarkable attention to detail elsewhere.
3. Aii Computer Controlled
One of the obvious upgrades to a telescope system beyond motorizing it so it can be moved by remote control is to give it the ability to know where it is pointed. This is accomplished through computer control and optical encoders. Optical encoders are devices similar in concept to the ones that tell your computer how fast and how far the mouse is rolling when you move it with your hand. In the case of a telecope mount, the encoders are designed to give a precise report on where the telescope is pointed. This information is used to automatically move the mount to new objects. Computers can take the form of small hand-held units, or using your own laptop and commercial astronomical software to guide the telescope. This can get the telescope onto an object when nothing else will, but you can expect to spend lots of time tinkering, also.
3. B. Fork Mounts
Fork mounts are the lightest and most weight efficient telescope mounts because they do not require large counterweights to the telescope. However, in some configurations even these mounts need to have counterweights placed on the telescope, but this is an equipment-specific requirement I'm not going to get into.
These mounts are some of the most basic possible and the easiest to use. Most recently, these mounts have been the basis for most robotic computer driven mounts. One design note is fork mounts may have one arm or two. Mounts with only one arm and the telescope mounted on the side are still referred to as fork mounts, even though they technically are no longer a fork since they operate the same way.
3. Bi Traditional Versions
In its basic configuration, a fork mount is simply a rotating base with two arms sticking up from it. The telescope mounts between the arms on a pair of bearings. These can be found with simple worm gears or even jack screws for changing the telescope's altitude in the sky. As a result, this type of mount will be found on the lowest end of all telescopes (interestingly enough, it is also the preferred configuration for the highest end of all- observatories).
The Fork configuration keeps the telescope much closer to remaining in one location than a German Equatorial, and so this mount is far more convenient for observing from a fixed location. The Celestron and Meade SCT telescopes have all been sold with fork mounts and this is the most commpn place they are seen on high quality telescopes. In more recent years, they have been offering these scopes on German Equatorials, though, since some extremely good quality mounts of that type are now available.
Fork mounts are typically available only for a specific telescope and are included with it. The only significant exception to this is the Apogee MPFM (multi-purpose fork mount) which is designed to accept almost any small telescope. Because of this, fork mounts should be regarded more as an option when buying a telescope as a system rather than an option for re-mounting a telescope. Dual arm forks are designed to accept one type of telescope geometry and special work is required to install other telescopes. In comparison, standard mounting hardware for just about any telescope is readily available for all of the major makes of German Equatorial mounts. Because of this, people with multiple telescopes who want to just have one high quality mount for all of them do not use fork mounts.
The equatorial mounts found on SCT telescopes are all made with a right ascencion axis which is geared to counteract earth's movement as the main large drive in the base of the mount. The secondary or "Declination" axis is only used to move the telescope betweein objects. This way, the telescope moves in the same path as a German Equatorial, but you don't have the unusual movement of the telescope about the hub depending on which direction it is pointed. In later years, the drive electronics for these telescopes have improved and many users have electric motors on both axes and digital encoders added so they can find objects more easily.
3. Bii Computer Controlled (GOTO)
In the last couple of years, there has been a huge change in this area, and I have changed my mind about GOTO control. Previoulsy I had thought having things given to an observer wasn't a good thing, but finally my wife pointed out that even when you know where it is, you can spend a few minutes getting a scope pointed. One of the main reasons I became interested in the sky was to see things for myself and find them for myself. For example, when I found the double star cluster near Perseus named "Double Cluster" (for obvious reasons) I found it because I was out with a friend observing one night and I noticed a bright something along the band of the Milky Way in the country sky. I put the lowest power eyepiece I had into the small C90 I had taken with me out into the countryside for maximum illumination. I lined the telescope up by eye and looked. The eyepiece was filled with an extraordinary site- hundreds of stars in two apparently adjacent star clusters filled the view. I went to my copy of the Cambride Star Atlas and discovered it was Double Cluster. No automated machine can match the feeling of finding something for yourself. In a way, it makes it yours in a way the other experience doesn't. And, the only way you can qualify for the object finding certificates such as the Messier certificate offered by the Astronomical League is to use manual equipment. The computer driven mounts are remarkable devices, I will attempt to be fair to them here.
The fork mount has been used for some very significant developments in recent years to produce what is generally called a "GOTO" telescope. The term comes from the way these mounts work- essentially, the user need only follow some simple instructions such as lining up a telescope on two obvious stars after pointing it north. You select what you want to look at on the computer with the telescope and it will automatically point to the object and track it.
The first of these on the market was the Celestron Ultima 2000 with a twin arm supported 8" SCT. Since then Celestron has introduced the single arm NexStar 5i and 8i 5" and 8" telescopes as well as the Nextar 8, 9.25 and 11 GPS scopes with GPS location ability. Off of forks, they now have an entire series of GOTO German equatorials. Unlike the Ultima 2000, the NexStar telescopes have no "manual" mode. The telescopes are permanently locked in place on the mounts and nothing will not track without electrical power. However, these mounts are all very sturdily designed and do not rock and the NexStars even have a little space to hang the handset in. Little details like simple and sturdy mounts to the tripod have been thought through and all of the battery compartments are on the top side of the mount so they are easy to get to (you will be going through batteries). The mounts are quiet, relatively easy to use, and look like they are built to last.
Meade has several different computer driven fork mounts. The largest mounts are the LX200 series mounts which are roughly the same capability of Celestron's Ultima 2000 except these are available for 7" MAK and 8", 10", 12" and 16" SCT telescopes. Like the NexStars, the LX200s, can be moved by hand for transport, but will lose their orientation if you do this while observing, so they must be moved using the motors. I have found these mounts are quite loud. You may think I am exaggerating, but if you hear one, see if you don't agree they sounds like RC cars from Radio Shack. Other than these nits, they appear to perform fairly well. The only note I'd make is the people I've heard using them for photography appeared to be doing very frequent corrections to the drive.
Meade has also been selling small Maksutovs and refractors with a miniature fork mount called the ETX series. The telescope sizes are 60 and 70mm for the refractors and 90mm, 102mm, and 127mm for the Maksutovs (the 127mm is labled a 125 for marketing reasons- the telescope is 5" diameter and hence 127mm in real life). These are very pretty telescopes, but after looking at them, they have quite a few serious flaws. I would not recommend any of them with their mounts. The Maksutovs are OK telescopes, but all three of the people I know who have bought one had to return it due to defects get a replacement. I have taken one apart and the bizarre thing I found was all of the highly loaded parts, like the main mirror cell, are plastic, and lightly loaded parts like the secondary mirror's light baffle are heavy weight metal. All I can say is it is absolutely strange, and the large injection molded cells must have been very expensive to produce compared to having them made from die-cast aluminum. The entire barrel, forward cell, and lens cap all thread together and are threaded in the same direction, so hold on to the forward cell when unscrewing the lens cap if it is on tight or you might find you are taking the telescope apart by accident. Given the miniscule size of the refractors, I don't know what Meade is trying to do other than perhaps alienate people from astronomy who might otherwise find a lifelong love.
As for the ETX series fork mounts, they have many serious flaws. First, they have their tripod mount points in the center of a plastic plate in the bottom. This makes the telescope mount so springy that it wil bob and bounce indefnitely once touched. Second, the mount holes are in the center and use small screws. It takes some care to get this right in the home, much less at twilight at an observig site. Third, the gearing is all plastic- these machines are not designed for long life and if you don't release the clutches fully before moving it (and there is no indicator to tell if you have), you can easily strip the gears. Forth, the little machines are just as loud as the LX200. Fifth, the drive motor will, in some orientations, cause the telescope to vibrate and thus blur the image. Sixth, the telescopes need to be removed from the tripod to change batteries. Seventh, the telescope can't point down more than 45 degrees. This may not sound like a serious problem if it is operating in alt-azimuth mode, however, if the telescope is being used equatorially for photography (as the dual mount back end of the telescope is designed to), then it can't see anything near the southern horizon at lattitudes lower than 45 degrees (or in otherwords, even Toronto, at 43 degrees north latitude, is too far south for an equatorially tracking ETX to see the southern horizon). This takes me back to what I was asking earlier: What is going on at Meade? They come up with neat ideas and build beautiful looking hardware, but they just don't seem to care about getting it right.
3. C Dobsonian Mounts
Technically, the Dobsonian mount used with Dobson-inspired Newtonian Reflectors is just a gearless and driveless version of an alt-azimuth fork mount. The main idea behind this mount is basic simplicity.
3. Ci Traditional Versions
These mounts are usually made of plywood and consist of a base similar to a lazy susan with a bolt holding on the top fork part. The two fork arms are really just a cradle into which the telescope will set on two large cyclindrical arms like an old-time cannon on its carriage. The entire idea behind this mount is to make it possible to push the telescope to point at something and stay pointed at it. The telescopes therefore will need to be roughly balanced about their own pivot so they won't nose over or pop-up to stare at the zenith. The right amount of friction will be just enough so the telescope doesn't move accidently, but not so much that it takes so much force to start it moving that you always overshoot. Do do this, the bearings will be lined with materials to give them this magic amount of friction. Different people have used formica, teflon, felt, rubber, steel, wax, and any number of other combinations to adjust the feel. The main platform rests on pads which are also optimized for the right amount of friction.
3. Cii Computer Controlled (PUSH-TO)
In a way, adding a computer to a Dobsonian is sort of like taking a television along on a Safari since these are opposing concepts. However, I have seen it done. Most recently, Orion (www.telescope.com) has made their main line of Dobsonians compatible with a $149 controller with built-in encoders and are calling it the "Intelliscope." I have to admit my curiousity is killing me. The telescope is modified with an optical encoder for each axis and attached to a small computer. The computer tells the user if they are pushing the telescope in the right direction and when it should be on-target. Before you go out and do this, be aware that the money it takes to have done it would have bought a larger telescope, but at the rate prices are going, this isn't a much bigger scope these days.
Alt-Azimuth Mounts
Technically these aren't really Dobsonian mounts, but are rather a new development where other kinds of telescopes are given a mount with the same sort of push-to simplicity of a Dobsonian. The best ones I have run across come from Universal Astronomics, http://www.universalastronomics.com/ and I have mounted the little C90 on one for my wife. The mount has teflon bearings and a handle. To point at something, take the handle and push the telescope there. The C5 I put together for my father is on one of these, also. The mounts are extremely light, so he doesn't have to move a heavy object, since he was a paratrooper in the army for years and a 40 lb. telescope to move isn't a help.
All sorts of telescopes and binoculars can be mounted on these and similar mounts, so the far-edge of the GOTO spectrum is alive and well. My wife really enjoys going out and looking around for objects and finding them. Any schmutz in the sky is fair game.
Summary:
This is all I have for now. I'll get back to this review from time to time to update it. Let me know what you think about this review- it has taken some time to put together and I appreciate the encouragement and support the memebers of epinions have given me on this effort.
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Member: Rich W.
Location: Tucson, AZ
Reviews written: 137
Trusted by: 40 members
About Me:
Dad, Engineer, Scientist, Astronomer, Traveler; order may vary.
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