Celestron CG-4 German Equatorial Mount,
By Chuck Hawks
The present day Celestron CG-4 evolved from Celestron's Vixen-supplied Super Polaris German Equatorial mount and (although now manufactured by Celestron's corporate partner Synta), it is essentially equivalent to the Vixen GP2 mount. CG-4 mounts are supplied with Celestron's Onmi XLT series telescopes. These include 102mm, 120mm and 150mm achromatic refractors; 102mm ED refractor, 127mm Schmidt-Cassegrain and 150mm Newtonian telescopes. The heaviest of these optical tubes (the 150R, a 6" refractor) weighs 16 pounds.
That should be a clue as to the size of telescopes for which the CG-4 mounting system was designed. It is what I would call a medium size German equatorial mount with a maximum load rating of around 20 pounds. It is supplied with two counterweights of four and seven pounds, for a total of 11 pounds. Used with telescopes weighing 10-12 pounds, such as a typical 4" refractor, it is a very solid mount.
CG-4 with Omni XLT 102 scope. Photo courtesy of Celestron.
Here are the features and specifications of the Celestron CG-4:
The CG-4 mount head is designed to accept Vixen quick-release, dovetail mount rails. The Vixen rail is secured in place by a bolt with a large finger knob and there is a smaller, secondary bolt to prevent the scope from sliding out of the mount should the main bolt loosen. It is a good system, better than found on some more expensive mounts.
The mount head is secured to its tripod by a large under bolt that is tightened by a captive finger knob. This center bolt runs through the top of the tripod and into the bottom of the CG-4 mount base. It is very long and serves a dual purpose. The lower end is threaded so that the leg spreader, which also serves as an ocular tray, can be slid onto the lower portion of this shaft, where it is held in place by another large finger knob. This knob is not captive, so you must keep track of it and its washer.
It is easy to set up a CG-4 or break it down for travel. To take the mount apart, first remove the telescope from the mount using the Vixen quick release system. Remove the counterweight shaft stop screw and slide the counter weight off the shaft. (Replace the stop screw so you do not lose it.) The 20mm diameter weight shaft simply unscrews from the mount head, where it is secured by a large hand nut. Back off the azimuth adjusting finger screws at the front of the mount a turn or two and then loosen the big captive finger knob on the mounting bolt that secures the mount head to its tripod until the head is free and lift it off the tripod. (Do not remove the bolt from the tripod.) Unscrew the lower finger knob that secures the spreader tray in place and remove the tray, afterward reattaching the knob to prevent loss. Fold the tripod legs and you're finished. The main pieces are the tripod legs, tray, mount head, counterweight shaft and counterweight, none of which are particularly heavy. The stainless steel tripod legs are the heaviest part and they weigh about 12 pounds. Reassemble in reverse order.
I acquired my CG-4 with an Omni XLT 102ED (4") refractor. It is a solid mount that quickly dampens minor vibrations. You can lightly touch the mount or focus the telescope without the image dissolving into jiggle blur, something that cannot be said for lightweight mounts.
Setting the angle of the mount to the viewing latitude is done by "push/pull" hand screws at the base of the mount head, a method common to most German equatorial mounts. An accurate and easy to read scale assists in this. Do not over-tighten the adjustment screws; it is possible (although unlikely) to crush the part they push against if you really reef on them.
Slewing a CG-4 manually is a very smooth operation. The mount head glides on ball bearing races and it is easy to point a properly balanced telescope at any part of the sky. Set the RA and Dec paddle locks and fine aiming adjustments are made using the manual slow-motion controls. These feature large plastic knobs on flexible metal stalks for convenient operation. The slow motion controls work efficiently with little take-up and allow an object to be precisely centered in even the highest magnification eyepiece.
I used the CG-4's predecessor, the legendary Celestron/Vixen Super Polaris mount, years ago under C6 and C8 telescopes. The Super Polaris was a good mount for its time, but the CG-4 is better in every way. The heavy-duty steel tripod, for one thing, is a big improvement over the wooden surveyor's type tripod that came with the Super Polaris mount.
Motor Drive Set DA
For visual observing, a motorized EQ mount is not necessary. It is easy to keep objects in the field of view using just the RA slow motion control. If you do your tracking manually, you are not battery dependent. However, if more than one person is to use the telescope, the Celestron Motor Drive Set DA (2011 MSRP $119) is a convenience. It is supplied with RA and Dec motors, a battery pack that holds eight "D" cell alkaline batteries, a Dec clutch so that you can revert to the manual Dec slow motion control if desired, all connecting cords and a four-way, four speed hand controller. The operating speeds are sidereal, 2x, 4x and 8x.
The Motors are bolted to the mount using pre-drilled holes and connected to the slow motion shafts by setscrews. Installation is so easy that even I could do it by simply following the supplied instructions, one step at a time.
I prefer to adjust declination manually, so normally I only plug-in the RA motor. This eliminates one of the two motor connecting cords. I keep the Dec motor cord neatly bundled and zip-tied, simplifying hook-up and leaving one less wire to accidentally snag in the dark. The Motor Drive Set DA works well and battery life is good; something like 20 hours on a set of alkaline batteries. It is great for keeping the subject in the field of view at star parties, when several people are sharing each other's telescopes.
Polar Finder Scope
The optional Polar Finder CG-4 #94223 (2011 MSPR $49.95) is a small finder scope with an alignment reticle for both the Northern and Southern hemisphere. It mounts in and aims through the center of the CG-4 mount head and simplifies accurate polar alignment, a boon for astro-photographers. Its eyepiece can be focused to your eye.
I have owned and used the Polar Finder, but for visual use in the Northern hemisphere it is unnecessary if you have correctly set the mount for the latitude of your observing site, leveled the tripod and pointed the north leg of the tripod at the North Star. That simple procedure will align the mount close enough for visual observation and star hopping. However, on the occasions that you need a more accurate polar alignment, for example if you intend to use the CG-4's setting circles, the Polar Finder is the way to go.
The CG-4 is an excellent medium size German equatorial mount. It is the only Celestron equatorial mount that is not computer controlled, so it is particularly attractive to those who prefer to "do it themselves."
The CG-4 is an outstanding value. Consider that the equivalent Vixen GP2 mount with an aluminum tripod costs $700 at Optics Planet and an Orion SkyView Pro mount with tripod costs $380 (with shipping extra) from Orion. I have not used the latter mount, but it is supposedly very similar to the CG-4. Pretty hard to go wrong with a Celestron CG-4 for only $270!
Copyright 2009, 2011 by Chuck Hawks. All rights reserved.