Celestron CGEM II 925 SCT

9.25" Schmidt-Cassegrain Optics: 2350mm at f/10

The telescope is especially at home as a solid platform for long exposure photography with optional cameras. The C9.25 is the Goldilocks of optical tubes, not too big and not too small. The aluminum tube is 559mm (22") long and 271.8mm (10.7") in diameter, weighs 20 lbs (9.07 kg) and mounts with its CGE dovetail bar.

The 234.95mm aperture and 2350mm focal length give 1127x the light gathering power of the eye, a limiting stellar magnitude of 14.4 and a resolution of 0.59 arc seconds (Rayleigh) or 0.49 arc seconds (Dawes). The secondary obstruction is 36% by diameter and 13% by area.

The included 25mm eyepiece gives 94x; the highest useful magnification is 555x and the lowest is 34x. A 6x30 finderscope and a 1.25" star diagonal are included.

StarBright XLT Coatings, Focusing and Collimation

StarBright XLT is Celestron's coating system, built from three parts: enhanced multi-layer mirror coatings of aluminum, SiO2 and TiO2, with reflectivity fairly flat across the spectrum for both imaging and visual use; multi-layer anti-reflective coatings of magnesium fluoride and hafnium dioxide; and water white glass instead of soda lime glass for the corrector. Water white glass transmits about 90.5% of light uncoated, 3.5% more than soda lime glass, and reaches an average transmission of 97.4% with the StarBright XLT anti-reflective coatings. These coatings let you discern subtle details while viewing the Moon and planets as well as faint galaxies and nebulae.

Focusing moves the primary mirror, which slides on the primary baffle tube; the focus knob sits on the rear cell just below the star diagonal, and it takes about 30 turns to go from close focus to infinity. Make the final focus turn counterclockwise so the mirror moves against gravity, which minimizes any mirror shift, and start on a bright object such as the Moon or a planet, since out-of-focus stars are very diffuse.

Celestron collimates every telescope at the factory and checks it before shipping. If it is jarred in transport, you can adjust the tilt of the secondary mirror with the three collimation screws on the secondary housing, the only adjustment available to the user. To check, defocus a star at least 20 degrees above the horizon with a high-power eyepiece: the dark shadow of the secondary should sit centered in the bright rings on both sides of focus. If you are unsure whether it needs adjustment, it is likely fine and best left alone. Before checking collimation, let the telescope reach thermal equilibrium with its surroundings, allowing 45 minutes if you move it between large temperature extremes, such as from a heated house into the cold night air.

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 11 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 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 9.25" tube is Fastar/HyperStar compatible: the secondary mirror can be removed and replaced with a lens assembly, sold separately and not included, so that a camera works at the f/2 focus of the primary mirror at the front of the telescope. Celestron's SCT manual says f/2 needs exposures about 25 times shorter than f/10, with a field of view 5 times larger and the object 1/5 the size. The secondary mirror should never be removed except to install the Fastar lens assembly.

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.