Celestron NexStar 90SLT Computerized Telescope

90mm Maksutov-Cassegrain Optics

The Maksutov-Cassegrain is a catadioptric design with both a lens and mirrors: light passes through the corrector lens to the primary mirror and back to a secondary before reaching the eyepiece, and the images are bright, sharp and virtually free of chromatic aberration. Instead of a separate secondary mirror, the secondary is a spot of reflective coating applied to the inside of the corrector, so the telescope does not need the occasional collimation of a Schmidt-Cassegrain, and the small spot gives the Mak its high-contrast views.

The 90mm aperture and 1250mm focal length give 165x the light gathering power of the eye, a limiting stellar magnitude of 12.3 and a resolution of 1.55 arc seconds (Rayleigh) or 1.29 arc seconds (Dawes), with fully coated optics. With the 25mm eyepiece the true field of view is 1 degree, or 53 feet (16 m) at 1000 yards. The larger, heavier Maksutov corrector takes longer to cool to the outside air temperature than a Schmidt-Cassegrain of the same aperture, so give it time outside before expecting the sharpest views.

Focusing and Collimation

Focusing moves the primary mirror, which slides on the primary baffle tube; the knob is on the rear cell just below the star diagonal. It takes many turns, about 25, to go from close focus (approximately 20 feet) to infinity. Start on a bright object such as the Moon or a planet, since out-of-focus stars are very diffuse, and make the final turn counterclockwise so the mirror moves against gravity, which minimizes mirror shift.

The Maksutov is collimated at the factory, and it does not need the occasional collimation of a Schmidt-Cassegrain. If the telescope is dropped or jarred severely, check it this way: with the telescope in thermal equilibrium (allow up to 45 minutes between large temperature extremes), center a star near the zenith in the 9mm eyepiece and cross slowly in and out of focus; a star pattern skewed to one side means it needs collimation.

The Computerized Fork Arm Mount

The single fork arm altazimuth mount tracks objects as they move across the sky, with DC servo motors and nine slew speeds up to 3 degrees per second. It tracks at sidereal, solar and lunar rates, in Alt-Az, EQ North and EQ South modes. The tube attaches by its dovetail bar to the clamp on the inside of the fork arm, held by a hand knob; make sure the bottom of the dovetail sits flat against the holder before tightening.

If the power fails, you can move the tube by hand, but when the telescope is powered on, always move it with the hand control: the manual warns that it loses its star alignment if moved by hand while powered.

The NexStar+ Hand Control

The NexStar+ hand control slews the telescope automatically to any of the objects in its 40,000 object database, which includes more than 600 galaxies, 300 clusters and dozens of binary stars, with catalog keys for Solar System, Stars and Deep Sky objects (the complete Messier catalog and select NGC objects). Sky Tour builds a list of the best objects currently visible for your time and location and slews to each one; OBJECT INFO shows coordinates and information about the selected object. The NexStar+ LCD has red backlighting and is less sensitive to low temperatures than the older hand control, and the direction buttons have raised edges you can find without looking.

The MOTOR SPEED button sets nine rates, from 9, the fastest, for slewing between objects and finding alignment stars, down to 1, half sidereal, for accurate centering in the eyepiece. Pressing the opposite direction button while holding a direction button jumps to the maximum rate. Never slew the telescope while someone is looking into the eyepiece.

SkyAlign and the Other Alignments

SkyAlign is Celestron's quickest alignment: with the time, date and your location entered (pick the nearest city from the City Database), you point the telescope at any three bright objects, center each in the finderscope and press ENTER, then center it in the eyepiece and press ALIGN. You don't need to know their names, and the Moon and the brightest planets work too. When the display reads Match Confirmed, the telescope is ready to find objects.

The manual's tips: level the tripod first, choose alignment objects as far apart as possible, and make sure the third does not lie in a line between the first two. Auto Two-Star Align asks you to center one known star and then picks and slews to a second; Two-Star Align uses two stars you choose; One-Star Align is the quickest way to find and track bright planets but is not meant for small or faint objects; Solar System Align uses the Moon or planets, which makes it handy in the daytime. Re-alignment lets you replace an alignment star later, for example after a few hours as the sky turns.

Power, Tripod and Accessories

The telescope runs on 8 AA batteries (not included) in the internal compartment at the base of the fork arm: squeeze the tabs on both sides of the cover while lifting, insert the batteries matching the polarity marks, and snap the cover back. The internal compartment powers the servo motors while avoiding the cord wrap of external battery packs. You can also use a PowerTank portable battery or a 12V AC adapter, both sold separately; the power requirement is 12 VDC, 750 mA, tip positive. The power switch is on the side of the fork arm.

The pre-assembled, adjustable steel tripod has a center leg brace and a quick-release accessory tray that snaps onto the brace without tools, and a bubble level on the tripod head to help you level it. A snap-on holster holds the hand control on any tripod leg. The fork arm sits on the tripod head and locks with a large captive knob between the legs.

The SLT is ready to grow with you: it is compatible with StarSense AutoAlign, the SkyPortal WiFi module and the PowerTank Lithium, all sold separately. The NexStar+ hand control has a port for computer control and firmware updates. An optional SkySync GPS accessory (93969) is also available.

Eyepieces, Star Diagonal and the StarPointer

Two 1.25" eyepieces are included: the 25mm gives 50x and the 9mm 139x. The highest useful magnification is 213x and the lowest is 13x. The star diagonal slides into the visual back and turns the light at a right angle so you can observe in a more comfortable position; loosen its thumbscrew to rotate it. It gives a right-side-up view. With the mirror star diagonal in place the image is the right way up but reversed left to right, which is normal when using a star diagonal.

Align the StarPointer red dot finderscope in daylight on an easily recognizable object at least a quarter mile away, such as a streetlight, a car license plate or a sign. First remove the small plastic tab under its battery cover. Center the object in the 25mm eyepiece, turn the finder's power dial all the way up, and without moving the telescope use its two adjustment knobs to put the red dot on the object. It won't need realigning unless it is bumped, dropped or removed; turn the red dot off when you aren't using it to save the battery.

Magnification is the telescope's focal length divided by the eyepiece's. Celestron's Telescope Basics manual suggests finding objects with the longest focal length (lowest power) eyepiece, which gives the widest field, then switching to the higher power eyepiece for a closer view. If bright stars twinkle or change color, the air is not steady enough for high magnification, and a lower power will give a steadier view.

What You Can See, Your First Night and Care

The 90SLT reveals details of the lunar surface, Venus and its phases, the polar caps on Mars, Jupiter and its four moons, and Saturn with its rings plainly visible. For your first night, try the Moon: from two days after New Moon to a few days before Full Moon you see the most detail in the craters and mountain ranges. Start with the 25mm eyepiece, then switch to the 9mm for a much larger view; you may need to refocus when you change eyepieces. Because the Earth rotates, objects drift across the eyepiece, faster at higher magnification. Position the telescope just ahead of the target and let it drift through the field.

Celestron's Telescope Basics manual suggests giving the telescope at least 10 minutes to cool to the outside air temperature, longer if the difference is large, and allowing about 30 minutes for your eyes to fully dark-adapt, using a red LED flashlight at low brightness if you need light. If you wear glasses, Celestron's Telescope Basics manual suggests checking whether you see the same field with and without them; if not, observe without them and refocus. For additional eyepieces, it suggests designs with at least 18mm of eye relief if you want to keep your glasses on.

To clean the optics, remove dust with a camel hair brush or a few seconds of pressurized air sprayed at an angle, then use an optical cleaning solution on white tissue with low-pressure strokes from the center outward, never in circles. Replace the lens covers after each session to keep contaminants out. Celestron includes Starry Night Special Edition software for your computer, to learn the night sky and plan observing sessions, and lists the SkyPortal app; you don't need either to use the telescope.