Changing with Time.
More and more telescopes are designed with portability in mind. From small refractors to large, light-bucket reflectors and everything in between, there's a trend to make today's telescopes easy to transport and set up. This translates into more frequent use whether the instrument is in the hands of an advanced amateur or a beginner. Portable telescopes, especially those with compact optical designs that pack a moderate focal length into a small tube assembly, are also very popular with globe-trotting observers seeking dark skies and ephemeral events such as solar eclipses.
Thus, I'm sure many people have shown interest in two of the latest offerings from Celestron International. The G-3 and G-5 are reengineered versions of two telescopes that have stood the test of time. The G-3 is an astronomical resurrection of the company's 311/42- inch f/11 reflector with a Maksutov optical design, which, in recent years, was available only as a spotting scope. The G-5, on the other hand, uses the same German equatorial mount as the G-3 but couples it to the company's venerable 5-inch f/10 reflector with Schmidt- Cassegrain. Both compact tube assemblies have track records dating back more than 20 years. I've used one of the Maksutov telescopes since the early 1980s to guide astrophotos.
I tested the G-5 with the assumption that, if the mount could handle the larger and heavier Schmidt-Cassegrain, it would certainly work well with the smaller Maksutov instrument. Completely assembled, the G-5 weighs just under 23 pounds, while the G-3 tips the scale at just over 21 pounds. Both telescopes offer focal lengths that are short enough to provide wide-field views of deep-sky objects and also long enough for high-power study of the Moon and planets. Both are supplied with a 25- millimeter eyepiece, which yields 40 [yen] with the G-3 and 50 [yen] with the G-5.
The G-5 I tested was on loan from the company. Its optical quality was excellent, which is consistent with the telescope Sky & Telescope reviewed in the October 1993 issue, page 39. The coated optics produced crisp, detailed images of the planets and high-contrast deep-sky views. The focusing mechanism is very smooth and when moving back and forth through the focal point had the smallest lateral image shift of any Schmidt-Cassegrain telescope I have ever used. A 6 [yen] 30 finder is standard.
At first I thought the G-5's German equatorial mount was the same as those I had seen with cheap department-store telescopes. But close inspection revealed mechanical components machined to tight tolerances and nicely finished. Furthermore, field tests proved that the lightweight mount is fully capable of carrying the 5-inch tube assembly.
To aid with polar alignment, the mount can be rotated in azimuth, and there is a large, hand-operated screw for fine adjustment of the polar- axis elevation. The chrome-plated counterweight shaft is 911/42 inches long, 11/42 inch in diameter, and rotates when the telescope is moved in declination, which makes it ideal for carrying a camera for guided photography. Two counterweights come with the instrument. The larger one is sufficient for balancing the instrument for visual work. It was necessary to add the smaller counterweight only when a 35-mm camera was attached to the scope or used for piggyback photography.
Standard equipment includes an adjustable aluminum tripod than can raise the mount between 28 and 47 inches from the ground. A triangular accessory tray attaches to the tripod legs and adds considerable stiffness to the assembly. The tray is about 10 inches in diameter and has a 1-inch-high lip, which keeps eyepieces and other accessories from rolling off. I found the tripod to be one of the best combinations of rigidity and light weight of any I have used.
The mount has manual slow motions on both axes, but I imagine most people will purchase the optional motor for the worm-gear drive on the right-ascension axis (no motor is available for the declination axis). The drive is powered by four D batteries, and even the low-cost, nonalkaline ones that came with the drive provided four full nights of observing. A green light-emitting diode on the hand control blinks when battery power is low.
A spring and thumbscrew hold the motor against a 2-inch-diameter, fine- pitch spur gear on the end of the worm shaft. For manual operation the motor can be disengaged by a small cam lever on the side of the mount. Even with the drive engaged, the telescope can be moved in right ascension thanks to a slip clutch. The 3-inch-diameter worm gear is well protected by the setting circle and a plastic cowling.
With the mount assembled in the conventional manner, the position of the drive motor on the west side of the polar axis restricts sky coverage somewhat. This is because the declination slow-motion control knob can strike the motor when the telescope is pointed to high altitudes. It is possible, however, to turn the tube assembly on its mounting saddle so that the knob will not interfere with the motor.
The drive's hand control has two sets of push buttons that move the telescope east and west at either two or four times the sidereal rate. There is no adjustment for fine-tuning the drive's normal tracking rate, but my tests revealed the fixed speed to be very accurate as well as stable over a wide range of temperatures. The drive's push-button motions are positive and well suited to centering objects in an eyepiece and for guiding long-exposure photographs.
The drive has between 12 and 15 arcminutes of periodic error that accompany each 10-minute rotation of the worm. While this is considerable, it is smooth and free of abrupt jumps. Most of the error was in the east-west direction and thus easy to guide out with the 2 [yen] push buttons. The small amount of periodic displacement in the north- south direction was more difficult to correct because it required a very gentle turn of the declination axis's manual slow-motion control. As with most worm-driven equatorial mounts, the G-5 performed best with the polar axis set slightly out of balance and the drive pushing the load "uphill."
Astrophotography with the G-5
While the G-5 is not intended as a high-power astro-imaging system, it is remarkably versatile for beginning and intermediate-level amateurs. The telescope is a proven performer when coupled to a 35-mm camera. Its nominal focal length of 50 inches (1,250 mm) provides good image scale for shots of the Moon and (with proper filters) the Sun. The mount and drive also allow high-magnification snapshots of the Moon and planets with an optional eyepiece-projection system. One of the scope's strengths is the huge range of photographic accessories available from Celestron and other companies.
Deep-sky photography through the telescope is more challenging. The mount is not heavy enough to carry a separate guidescope, so the best way to make long exposures is with an optional off-axis guider such as the Celestron radial guider. Because these guiders use only a portion of the scope's aperture, they require rather bright guidestars, and these are often difficult to find. The need for a bright guidestar is made even worse with an optional f/6.3 focal reducer. Indeed, the one night I tried the telescope with a focal reducer, I was unable to find any suitable guidestars around the deep-sky targets I wanted to shoot.
The G-5 mount can be configured for other astrophotography applications. It connects to the tube assembly with a standard 11/44-20 screw like those on conventional photographic tripods. This makes it a snap to attach a 35-mm camera body or a small photographic ball head in place of the scope. The ball head helps when framing a camera's field parallel to the horizon.
I took numerous unguided 5-, 10-, and 15-minute exposures with only a camera on the mount. Those made with normal and wide-angle lenses usually showed excellent star images, and I consider the mount capable of this type of photography right out of the box. Unguided exposures made with a 180-mm lens were less successful, since only about one in five produced round stars.
Amateurs with a knack for tinkering could fit a camera to the counterweight shaft. It would allow guided exposures when the telescope is equipped with a reticle eyepiece. Such a system is limited only by the weight of equipment added to the mount.
The G-3 and G-5 scopes come with a nice astronomical software package. TheSky by Software Bisque is a comprehensive computer program for creating sky charts that is widely respected in the amateur community. While the Level I version accompanying the scopes lacks some of the features found with the more advanced versions currently available, it is very user-friendly for beginners. The software is great for locating planets, naked-eye stars, and prominent deep-sky objects, and it is a solid aid in planning observing sessions. Information on upgrading the program to versions showing more stars and deep-sky objects is included with the software.
Eclipse chasers will find the G-5 an ideal combination of aperture and focal length in a compact, portable system. Its components break down into small, manageable pieces that are easily packed for airline travel.
The G-5 is priced only a little more than the cost of the tube assembly alone. The mount is good for visual work. It is an extremely cost- effective way to venture into the world of astrophotography, through the telescope and with conventional camera lenses. The system is made even more versatile by the ease with which the telescope and mount can be used independently of one another. In addition to the mount's astrophotography applications, many amateurs will find the telescope an excellent auxiliary instrument or guidescope on a large astro-imaging system.
North Carolina native and experienced astrophotographer Johnny Horne is a newspaper photography editor. He frequently reviews equipment for Sky & Telescope.
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|Title Annotation:||Celestron International G-3 and G-5 telescopes|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 1999|
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