Celestron CGEM II 700 Maksutov-Cassegrain Telescope

7" Maksutov-Cassegrain Optics: 2700mm at f/15

The Mak is a catadioptric telescope: light passes through the corrector lens to the primary mirror and back to the secondary before reaching the eyepiece, and images are bright, sharp and virtually free of chromatic aberration. Instead of a separate secondary mirror, the Mak has a secondary spot of reflective coating applied to the inside of the corrector plate, so it does not need the occasional collimation a Schmidt-Cassegrain does; the spot is also significantly smaller than a Schmidt-Cassegrain secondary, which gives the Mak its signature high-contrast views.

The 180mm aperture and 2700mm focal length give 661x the light gathering power of the eye, a limiting stellar magnitude of 13.9 and a resolution of 0.64 arc seconds (Rayleigh) or 0.52 arc seconds (Dawes). The secondary obstruction is 22% by diameter and 5% by area. The tube is 571.5mm (22.5") long and 222.25mm (8.75") in diameter, weighs 19 lbs (8.6 kg), has StarBright XLT coatings and connects directly to the CGEM II with its CGE dovetail bar.

Astronomers in urban areas will appreciate the Mak's ability to bring out more contrast in celestial objects, and transporting the whole system to a dark site is not a problem either.

2" Accessories, Finder and Focusing

The kit comes with a 2" visual back, a 2" star diagonal with 1.25" adapter, a 28mm wide-field 2" eyepiece that gives 96x, and a large 8x50 straight-through finderscope. The tube accommodates larger 2" accessories, such as 2" Barlow lenses, Celestron's 2" eyepiece and filter kit, and larger wide-field eyepieces. The highest useful magnification is 420x and the lowest is 67x.

The finderscope slides into the dovetail mounting bracket on the top left of the rear cell. Align it during the day on a distant object at least a quarter mile away, using the two adjustment knobs on its bracket.

The focus knob sits just to the right of the visual back. The mechanism is very fine and can take quite a few turns to go through the entire range of focus, so the first time, turn it slowly and work from one stop to the other. The larger, heavier Maksutov corrector plate takes longer to cool down and adjust to ambient air temperature than a Schmidt-Cassegrain corrector of the same aperture.

The CGEM II Mount: 40 lb Payload and Permanent PEC

The CGEM II is a fresh update to the venerable CGEM mount, capable of carrying Celestron's higher-end SCT optical tubes, up to 11", securely and vibration free, for both imaging and visual observing. The CGEM II carries 40 lbs. Its features include high-torque motors to smoothly drive heavier loads, low-cog DC servo motors, permanently programmable periodic error correction (PPEC), a dedicated autoguider port, large, easy-to-access polar alignment knobs and an ergonomic design that disassembles into compact, portable pieces. The latitude range is 15° to 70°, and the mount slews at nine speeds, tracking at sidereal, solar and lunar rates.

Celestron designed the CGEM II to be ergonomically friendly, with large altitude and azimuth adjustment knobs for quick and easy polar alignment, and internal R.A. and DEC motor wiring for a clean look and an easy, trouble-free setup. The saddle connects directly to both CG-5 and CGE dovetail bars.

To balance in R.A., loosen the R.A. lock lever with the counterweight bar horizontal and move the counterweight until the telescope stays put; in Dec, loosen the DEC clutch with the tube level and slide the tube in the saddle. Celestron's tip for imaging is a very slight imbalance, toward the counterweight side when the tube is west of the mount and toward the tube when it is east, so the worm gear always pushes against a slight load.

Tripod, Polar Alignment and All-Star

The steel tripod has 2" (50.8mm) legs with graduated markings on the lower section and weighs 20 lbs (9 kg); with the mount, the height range is 1346mm to 1651mm (53" to 65"). The equatorial head weighs 40 lbs (18.14 kg). This kit includes one 17 lb and one 12 lb counterweight.

Polar alignment is done not by moving the telescope in R.A. or Dec but with the mount's altitude and azimuth adjustments; both azimuth knobs push against the alignment peg on the tripod head, so you may have to loosen one while tightening the other. A bubble level built into the mount helps you level the tripod.

All-Star Polar Alignment lets you polar align accurately using any bright star, even when the celestial pole cannot be seen. After a two-star alignment with at least one calibration star, you slew to a bright star from the Named Star list, ideally high in the sky and near the meridian, and choose Polar Align, then Align Mount. The mount syncs on the star and slews to where it should be if accurately polar aligned, and you center it with the mount's latitude and azimuth adjustments, not the hand control's direction buttons.

NexStar+ Hand Control and Software

The NexStar+ hand control has a 2 line, 18 character backlit LCD and 19 LED backlit buttons, and its database holds over 40,000 objects, 100 user defined programmable objects and enhanced information on over 200 objects. New observers can start with the built-in Sky Tour, which finds the most interesting objects in the sky and slews to each one.

The mount slews at 9 speeds up to 4.5° per second and tracks at sidereal, solar and lunar rates in EQ North and EQ South modes. Alignment procedures are 2-Star, 1-Star, Solar System, Last Alignment and Quick Align; after a two-star alignment you can add calibration stars to improve pointing accuracy.

The hand control's USB port connects to a computer for point-and-click slewing from software and for firmware updates; updating the hand control with Celestron Firmware Manager may resolve some problems. The mount's software is Starry Night Special Edition and the SkyPortal app.

Imaging: Permanent PEC, Autoguiding and Tracking Past the Meridian

Periodic Error Correction improves tracking by reducing the amplitude of the worm errors. It works in three steps: the mount indexes its worm gear, you guide with an autoguider for at least 10 minutes (one full worm revolution) while it records your corrections, and then you play them back. The mount retains the recording, and PEC is for advanced astrophotography and still needs careful guiding.

The CGEM II tracks well past the meridian for uninterrupted imaging through the best part of the sky. The R.A. slew limits can be set anywhere from 40° above level to 20° below, so the mount can keep tracking past the meridian without flipping, or stop before an imaging cable runs out of length, and the Meridian setting (Favor Current, Favor West, Favor East or Disable) chooses which side of the mount the tube stays on. The CGEM II images through the meridian without needing a meridian flip until the object is a full 20° past it.

The mount has a hand control port, one AUX port for optional accessories and a dedicated autoguider port on its electronics panel. Built-in calibration routines compensate for the mechanical variation found in every German equatorial mount; they are worth running, and most need doing only once.

The drive has low cog DC servo motors with integrated optical encoders, the low cog design for smooth running and the encoders for accuracy, turning steel worm gears supported by precision bearings against brass worm wheels. All cabling is housed internally, which also removes the need to connect or disconnect an external Dec motor cable at setup.

Other features are Northern or Southern hemisphere compatibility, adjustable autoguiding rates, a hibernate mode that powers the mount down without losing alignment, and continual firmware development. All-Star polar alignment needs no extra equipment such as a polar finder scope (an optional one is available) and can usually be completed in about 15 minutes, even without a clear view of Polaris.

Power

Power is 12V DC at 3.2 amps. The supplied car battery adapter plugs into a car's cigarette lighter outlet or a portable power supply, or you can use one of Celestron's optional AC to DC adapters; use only adapters Celestron supplies, as others may damage the electronics and void the warranty.