What good is a telescope without a sturdy mounting? Absolutely no good at all. Without an adequate mount, a telescope becomes a source of frustration rather than a pleasure. This is especially true in the case of the Schmidt Cassegrain since its relatively long focal length produces a fairly high magnification with any eyepiece. It is at higher magnifications that "the shakes" become most obvious and serious. In addition to supporting the scope, an SCT's mount contains the motors and electronics that allow it to point at objects and, once pointed, track these objects as they move across the sky due to Earth's rotation.
Meade's and Celestron's SCTs are currently available in two flavors of mount, fork and GEM. The GEM, the German equatorial mount, must be properly aligned on the North Celestial Pole (or just Polaris) before it can track the stars. The fork, on the other hand, can be used in either of two modes. In equatorial mode, it is aligned on the Celestial Pole, just like a GEM, and also like a GEM, tracks objects by countering Earth's rotation. Today's go-to-equipped fork mounts can also be set up in alt-azimuth mode. This does not require any kind of polar alignment for the scope to track. In "alt-az," a fork needs the services of two motors to counteract the Earth's rotation, one for altitude (up/down) and one for azimuth (right/left). Alt-az tracking is a complicated business only made possible by the computers contained in go-to mounts, which can accurately "stair-step" a telescope across the sky.
Most of the SCTs Meade and Celestron sell are equipped with fork mounts. The reason is that the fork is easy to produce, is easy to use, and provides a reasonably stable mounting for a short-tubed Schmidt Cassegrain. The fork, which has not changed much since the days of the Orange Tube C8 seen in Plate 4, is just that, a large metal fork attached to the SCT's OTA on either side by means of declination (altitude) bearings. Today's forks contain some electronics and a motor in one of the fork arms that drive the telescope in declination/altitude during go-tos. In the past, almost all SCTs also featured mechanical slow-motion declination controls. These were knobs located on a fork arm and were designed to allow a user to manually move the telescope slowly and precisely in declination (north-south)/altitude. Some Meade telescopes still feature declination "slo-mo." Most Celestrons do not. With the move to motorized go-to forks, there is less need for mechanical slow motion. Another common feature before go-to was a graduated dial, an analog declination "setting circle," to assist in finding objects. The tube is held stationary in declination with a lock lever or knob.
The fork sits on the telescope's drive base, which contains most of the electronics and controls needed to run the telescope. The fork can swivel 360° in azimuth (or right ascension, RA [east-west], in equatorial mode). Before go-to, most SCTs had manual RA slow-motion controls as well as declination slow-motion controls. As with declination slow-mo, this feature is disappearing. Having an RA slow motion is still handy, though, since it can be used to track objects across the sky when power is unavailable. Even if the scope does not have a right ascension slow motion control, it will have a right ascension lock to secure the tube during go-to and tracking. Like declination setting circles, RA setting circles have disappeared from many telescopes with the advent of go-to.
Look at all those switches, lights, and connectors! The fork-mount Schmidt Casseg-rain's control panel (Plate 8) is usually located either on the top or the side of the drive base, but some models, including Celestron's NexStars and CPCs, place some connectors on the top of the drive base and switches and power indicators on the side.
Wherever they are, these complex-looking panels are intimidating for a beginner. They become less scary when they are boiled down to a few important indicators, sockets, and switches. First, there will be a receptacle for a power cable. The type of current required is usually 12 volts direct current (DC) and may be supplied either
Plate 8. (RCX Control Panel) The drive control panel of a modern SCT, the Meade LX400-ACF. Credit: Image courtesy of Meade Instruments Corporation.
by a DC source like a battery or by some kind of alternating current (AC)-powered DC supply. Somewhere in the area of the power connector, there will be an on-off switch—usually one that is way too small for convenient use by gloved hands in the wintertime. There may also be a little red (to preserve night vision) power-on indicator, usually an LED.
Next up is an RJ ("telephone-style") connector for the hand control (HC). There are several styles/sizes of RJ connector—RJ-11, RJ-12, RJ-45, and more—which is good, since it allows telescope makers to use different sizes of connectors for different purposes, ensuring SCT users do not plug the telescope's HC into the wrong socket, which could be disastrous. The usual plug style used for an HC is RJ-12, which can handle as many as eight wires, enough for all HC signals and power.
On many scopes, there will be yet another RJ jack labeled "autoguide" (or similar). This allows a telescope's aim to be automatically fine-tuned, "guided," during picture taking using Santa Barbara Instrument Group's relay-switch-closure autogu-ider format, which has become a standard in the telescope industry. Plug a cable from an autoguiding-capable camera into this RJ-12 port, and the camera will detect and correct any small drive gear errors that would otherwise spoil long-exposure images.
On some fork-mount go-to scopes, there will also be an RS-232 (serial) jack on the control panel. If present, this will usually be an RJ-11-style connector. Recently, however, Meade and Celestron have migrated this RS-232 port to the hand control for most models. RS-232 serial data are used for a variety of functions on a fork-mount SCT, including sending the telescope on go-tos using an astronomy program running on a laptop personal computer (PC), autoguiding the telescope serially if a dedicated autoguide port is not available, and updating telescope firmware.
If the CAT in question is a Celestron, there will be another RJ port on the drive base, one labeled "PC." You'd think that would be the place to connect a laptop computer to control the scope via "planetarium" software. Makes sense, right? But, the Celestron engineers had other ideas apparently. This port, which uses an RJ-45 connector, is instead used for two very different purposes. One use is for upgrading the telescope's motor control firmware. Celestron go-to scopes use two separate computers, one in the HC and one in the mount (the motor control board). The PC port is also used when operating the telescope via the NexRemote software program rather than a hand control (see Chapter 10).
Finally, both Meade and Celestron fork-mount telescopes feature "auxiliary" ports. These are used for a variety of functions, most often for operating accessories such as motorized focusers. Beware of plugging anything into these ports that should not go there since these receptacles are "hot." They supply power to devices that need it and can damage anything that does not.
That is the control panel. But, what is inside the drive base? We do not recommend opening the base of a modern telescope. It is generally a maze of easily pinched and disconnected wires. It is also full of circuit boards that can be damaged by static electricity. Also in there, however, are the same things that have been in there since the days of the Orange Tube C8: a drive motor and drive gears (Plate 9). The motors on today's telescopes are of two types: steppers or servos. These motors differ in one respect; steppers, as their name implies, move in distinct steps, while servos move continuously. Stepper motors were originally developed for use in computer print-
ers and are therefore easy to control with computers. Servo motors, on the other hand, feature smoother operation since they do not "step along." In practice, both types work well for fork-mounted CATs.
These motors are coupled to the telescope's main drive gears, either directly or through a gear train. However they are linked to it, their purpose is to drive a smaller gear that turns a large gear that is attached to the fork. In older fork-mount SCTs and some current less-expensive telescopes, the gears used for the telescope drive were both spur gears. In spur gear systems, a small gear with straight teeth is attached to the motor and drives a larger gear of the same type coupled to the fork. Spur gear systems work fairly well, are inexpensive, and can be highly accurate. Their drawback is that the gears' teeth cause tiny random variations in the telescope's tracking speed. This is not a problem for the visual observer, but it means imagers guiding manually must monitor the scope's aim very carefully and be ready to push a HC direction button when these random variations show up. An autoguider may have trouble with the sudden, random errors introduced by spur gears.
In all but today's least expensive Meade and Celestron telescopes, the smaller gear in the drive system is now a worm-type gear. A worm gear is a cylindrical, helically cut (slanted) gear renowned for smooth precision. The helical nature of the worm ensures good contact with the gear it is meshed to and delivers an accurate drive rate. More important, much of the spur gear system's randomness is eliminated. Like any mechanical gear system, worm gear drives still show some error, but this is usually periodic error, a slow and regular variation that is easy for an astrophotographer to "guide out" using the scope's HC or an autoguider. One thing to remember when talking about Meade and Celestron SCTs' worm gear drives is they are only really half a worm system. In these scopes, the larger gear the worm drives is actually a big spur gear. In a true worm system, this larger gear would be the "wheel" and would have helically cut teeth, like the smaller gear.
Although the half-worm system works decently, the very nature of Meade and Celestron forks tends to limit their drive accuracy. They have to be designed both to track at an accurate "sidereal" rate and, when called on, zip the scope across the sky for go-tos at speeds as high as 8° per second—much faster than the tracking rate of 15° per hour. Unfortunately, they must do that without relying on high-cost gears, motors, and couplings. For that reason, it is common to find some looseness and backlash in the companies' fork drives that limit them during demanding applications like long-exposure imaging.
If a fork-mount telescope is to be used in equatorial mode—which is a requirement for serious imaging—it requires one additional component, a "wedge." The wedge, shown in Plate 5, is a simple wedge-shaped metal affair that has a single purpose: It tips the telescope over so it can be aligned on the North Celestial Pole. When tipped in this fashion, the up-and-down movement of the tube in the fork tines becomes movement in declination (north/south). When swiveling on the drive base, the scope moves in RA (east/west) rather than left/right.
What makes a fork mount a good choice for an amateur astronomer? Comfort. Forks are incredibly comfortable to use for visual observing when set up in alt-az mode. Nothing is more pleasant for just looking than an alt-azimuth SCT. When mounted on a wedge for equatorial use, a Schmidt Cassegrain's eyepiece can be placed in some odd positions. When pointing to far northern targets (or far southern ones south of the equator), for example, the eyepiece will be nestled up against the fork arms and difficult to get at. In alt-az mode, the worst it gets is when the telescope is pointed at the zenith. In that position, the ocular will be up against the drive base, but still not as difficult to access as the eyepiece on a north-pointing wedge-mounted scope.
Should your Schmidt Cassegrain be a fork-mounted Schmidt Cassegrain? If you want ease of use and want to keep the price down without sacrificing strength or features, the answer is "yes." There are some reasons to consider an alternative to a fork, however. In addition to gearing/drive deficiencies, forks are not as stable as other mounts. One look tells the story: The thing is a big metal fork, like a big tuning fork. The fork mount is naturally prone to vibration. Need to set a fork mount scope up in equatorial mode? That makes the vibration problem even worse. Tipped over on its base, the fork is off balance, and the whole shebang, and not just the fork, is prone to a right good case of the shakes. Finally, have you seen large-aperture fork-mount telescopes like a 12 or 14-inch? In person? To support large OTAs, these forks must be huge—and heavy. Modern go-to SCT OTAs cannot be easily separated from their forks, either, so the whole, heavy thing has to be lifted onto a tripod in one big piece.
So, maybe fork-mount Meade or Celestron SCTs, as pretty and appealing as they appear in the ads, are not quite the thing for all amateur astronomers. What then? Do not worry. There is an alternative: the GEM seen in Plate 10 . The GEM has a lot of fans, especially among photographers and other serious amateurs. Although it has been overshadowed by the fork mount for Schmidt Cassegrains, the German mount has been offered as an option since shortly after the first Orange Tubes began rolling out of California. Although it has not and probably will not displace the fork
Plate 10. (German mount) A C8 CAT mounted on Orion's Atlas EQG German equatorial mount. Credit: Author.
mount as the most popular support for the SCT, it has some advantages that make it worthy of consideration.
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