As mentioned in Chapter 1, a new user's first impression of a Schmidt Cassegrain tube is that it is short and fat, like a little beer keg. The SCT is very compact due to its "folded" optical system. Seen in Figure 1, light enters the corrector end of the tube, passes through the all-important corrector lens, strikes the primary mirror, and is reflected back up the tube to the secondary mirror. The secondary sends this light down the tube again and out through a 1.5-inch diameter hole for viewing. The SCT's convex, magnifying secondary mirror allows the Schmidt Cassegrain to pack a lot of focal length into its short tube, which is only about 17-inches long. Standard Meade and Celestron Schmidt Cassegrains have focal lengths of approximately 2,000 mm, and if not for these SCT tricks—the folded optics and the magnifying secondary—the tube would need to be approximately that long, about 80-inches, or over 6.5 feet.
Almost all the SCTs Meade and Celestron produce today have focal ratios of f/10 (for an 8-inch SCT, 2,000/200 mm [focal length/mirror diameter] = 10 or f/10). Meade sold an f/6.3 focal ratio version for a while, but it was never very popular with consumers despite the wider fields of view its shorter focal length yielded, and it was phased out a few years ago. Recently, however, Meade has begun to offer an alternative to f/10 again. Its top-of-the-line LX400ACFs feature the slightly faster focal ratio of f/8.
Physically, most of Meade's and Celestron's SCT tubes are made of thin-walled aluminum. Both companies have also used carbon fiber for some of their top-of-the-line telescopes at times. The tubes' interiors, whether made of carbon fiber or aluminum, are painted a flat black to reduce unwanted reflections. At one end of the tube is the "corrector assembly," and at the other end is a "rear cell." The corrector assembly is designed to, naturally, hold the corrector and secondary mirror securely and maintain proper alignment. The rear cell contains the primary mirror, focusing mechanism, and the "rear port," a hole onto which eyepieces, star diagonals, cameras, and other accessories can be mounted.
Probably the most striking part of the SCT OTA is its big glass corrector lens (Plate 6). This lens does not look much like a lens, and some beginners mistakenly assume it is just a flat piece of glass designed to hold the secondary mirror in place and keep dust out of the tube. The corrector is thin (about 5 mm thick) and very gently curved, but it is a lens, all right. At least one person we know of broke a corrector and replaced it with a piece of nice, flat window glass. This person was mightily flummoxed when he looked through his "repaired" scope and found its images were a blurry mess. As we know from our discussion of the Schmidt camera, the corrector plate has the important job of removing image-destroying spherical aberration. In the center of the corrector and extending through it is the mounting for the secondary mirror.
This secondary holder both supports the CAT's small convex mirror and provides a means for adjusting its tilt so that the telescope's optics can be aligned, or collimated. Look closely at the secondary holder, and three screw heads equally spaced around its circumference in a triangular pattern will usually be obvious (Plate 6). These screws, Allen screws on Meade scopes and Phillips screws on Celestrons, are used to adjust the telescope's collimation.
Moving around to the CAT's back end, take a look at the rear (or "mirror") cell. In addition to the central hole, the rear port, there is usually at least one knob, the focus control, that moves the telescope's primary mirror forward and back in the tube to focus the telescope. If the CAT in question is one of Meade's LX200ACF's, there will also be a second knob, which is used to lock the primary mirror in place during picture taking. If the scope is an LX400 ACF, neither knob will be present. The 400's primary mirror is permanently fixed, and the scope is focused by moving a (motorized) secondary mount.
The rear port is surrounded by a raised and threaded metal lip. The size and threading on 8-inch rear ports is the same on all modern Meades and Celestrons (2-inches, 24 threads per-inch). Some equipment, such as camera adapters, threads directly onto the port. Other items, mainly diagonals and eyepieces, require the use of a "visual back" (Plate 7). This is an adapter that is composed of a threaded ring and a barrel. The ring screws onto the rear port and snugs the barrel up against the rear cell. The visual back's barrel has an inside diameter of 1.25-inches, allowing "American standard" eyepieces, star diagonals, and other small accessories to be inserted into it and secured with a set screw.
Look through the rear port and into the OTA interior, and the first thing that you will notice is an eye staring back, reflected by the secondary mirror at the corrector end of the OTA. The other thing is that the rear port does not look out onto the surface of the primary mirror; instead, it opens into a metal tube that is the same diameter as the rear port and extends about halfway into the telescope. This is the SCT's "baffle tube," which serves two purposes. First, it prevents the contrast destroying "sky flood" that would happen if light passing through the corrector at oblique angles were allowed to bypass the telescope's mirror system and "flood" the
Plate 7. (Rear Cell) SCT rear cell assembly showing rear port, visual back, and focus knob." Credit: Author.
eyepiece with unwanted light. The baffle tube blocks the eyepiece from these contrast-spoiling light rays. The other important job the baffle tube does is to provide something for the primary mirror to slide up and down on during focusing.
Other telescope designs focus by moving the eyepiece in and out, just like Galileo's little refractor did—but not Schmidt Cassegrains. Meade's and Celestron's standard SCTs focus by moving the primary mirror back and forth. The focus knob on an SCT's rear cell is attached to a threaded rod that screws into the scope's primary mirror holder. This simple system allows the mirror to be moved up and down on the baffle tube in small, fine increments. Turning the control clockwise moves the mirror down the tube. Counterclockwise moves it up the tube. Note that in most SCTs the mirror is not riding directly on the baffle tube. Instead, a sleeve/hub is inserted into the primary's central hole, and that is what actually moves up and down on the baffle. Also, note that turning the focus control to both ends of its range does not move the mirror very far. It does not have to move very far; a small amount of movement has a large effect on focus due to the magnifying effect of the secondary.
Why don't Schmidt Cassegrains focus by moving the eyepiece, like other telescopes? Moving mirror focusing has some advantages. Not having to move the rear port and visual back in and out to focus makes for a more stable mounting for heavy items such as cameras. The moving mirror system also gives the SCT a large focus range. Almost any eyepiece, even insanely long focal length or homemade oculars, will come to focus in a Schmidt Cassegrain. Remember how we said that a Newtonian will sometimes need to be modified by moving its mirror up the tube before a camera will focus? The reason that is not necessary with SCTs is because of the moving mirror-focusing system and its huge amount of focus travel, or "back focus."
Alas, the moving mirror focus system is not all to the good. Meade and Celestron SCTs are pretty well put together, but they are not Swiss watches. There is generally a small amount of space between mirror and baffle, and the mirror rides just a little loosely on the tube. That causes the primary to tilt slightly when the focus control is turned since the threaded rod is on one side of the mirror and is pushing up or pulling down. When that happens, images move slightly in the field, which is annoying but not debilitating. Most new SCTs display a "focus shift" of only about 45 arc seconds, about the diameter of the planet Jupiter as seen from Earth.
A more serious problem with moving mirror focusing (for astrophotographers) is mirror flop. Unfortunately, a CAT's primary mirror may move slightly even when it is not being focused. When the scope's attitude changes significantly, when it tracks across the local meridian (the imaginary line that divides the sky in half from north to south), for example, the primary may shift a little bit. If an image is being exposed when the mirror flops in this fashion, the picture may be ruined; stars in the frame will come out as little lines rather than dots. Fortunately, there are several simple means of eliminating or at least reducing flop and shift, which are covered later.
SCTs larger than 10-inches are very similar to the 8-inchers, but one way in which they differ is the size of their rear ports and baffle tubes, which are larger in diameter, almost 3-inches rather than 1.5-inches. That allows big CATs to use long focal length wide-field eyepieces without suffering the "vignetting" that cuts off the edge of the field of view in some long focal length eyepieces when they are used on 8-inch scopes. Unfortunately, not many accessories can be used with the larger rear port. One reason is that, unlike the 8-inch scope ports, Meade and Celestron use different-size big backs (3.25-inch 16 tpi [threads per inch] for Meade, 3.3-inch 16 tpi for Celestron), do not ask why. Luckily for big CAT owners, standard SCT accessories of all types can be used on the larger scopes with the aid of a "rear port reducer." This item is supplied as standard equipment with all Schmidt Cassegrains bigger than 10-inches.
What else does the rear cell do? It provides a place to mount a finder. Even if the SCT has go-to, as most do these days, a finder scope will be needed to help locate two or three go-to alignment stars. An f/10 2000-mm focal length 8-inch SCT has a narrow field of view, even when long focal length (low-magnification) eyepieces are used. This field of view is so narrow that it is surprisingly difficult to get even the Moon centered in an eyepiece without a finder. Finders are of two basic types. One is a small, low-power telescope with a magnification of about 6x to 12x. Some recent CATs use nonmagnifying zero-power ("unit power") finders instead. These employ a red light-emitting diode (LED) and an optical window to "project" a dot or bulls-eye reticle on the night sky for aiming.
Back at the front of the CAT, take another peep down the tube, this time focusing on the primary mirror. Looks pretty, doesn't it? All bright and shiny? Due to the semi-sealed nature of the SCT's tube (it is not exactly airtight), dust and dirt on the mirror are not usually a problem. Theoretically, an aluminum-coated first-surface mirror will need recoating every 10 to 15 years. Maybe that is the case with Newtonians, but luckily it is not the case with SCTs. I have a 1973 Orange Tube C8 at the university where I teach astronomy labs, and its mirror at least looks as shiny as it did the day it rolled off the line at Celestron's Torrance, California, plant. That is good because getting SCT mirrors recoated normally requires the scope to travel back home to Celestron or Meade.
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