Celestron NexStar 102SLT and 102GT: Computerised 4-Inch Refractors Explained

This site used to have two posts about Celestron’s computerised 4-inch refractors: a product description of the NexStar 102SLT and a used-sale listing for an older NexStar 102GT. They are now combined. The specifications below come from Celestron’s current NexStar 102SLT product page.

Verified specifications (NexStar 102SLT)

• Optical design: refractor
• Aperture: 102 mm (4.02 inches)
• Focal length: 660 mm; focal ratio f/6.47
• Eyepieces in the spec table: 25 mm (26x) and 9 mm (73x)
• Finder: StarPointer red dot finderscope
• Diagonal: 1.25-inch star diagonal
• Highest useful magnification: 241x
• Lowest useful magnification: 15x
• Limiting stellar magnitude: 12.5
• Resolution: 1.37 arc seconds (Rayleigh), 1.14 arc seconds (Dawes)
• Light-gathering power: 212 times the human eye
• Mount: computerised altitude-azimuth single fork arm
• Instrument load capacity: 8 lb (3.6 kg)
• Power: 12 V DC from eight AA batteries (not included)
• Database: over 40,000 objects, including 220 of the best deep-sky and solar system objects
• Alignment: SkyAlign, Auto 2-Star, 1-Star, 2-Star and Solar System Align
• Software: Starry Night Special Edition and the SkyPortal app
• Total kit weight: 15.5 lb (7.03 kg)

A note on the eyepieces

Celestron’s product page is inconsistent. Its specification table lists 25 mm and 9 mm eyepieces (26x and 73x), while its “included items” list says 20 mm and 9 mm. Check the box contents when you buy. With the 660 mm focal length, a 20 mm eyepiece would give 33x.

What “computerised” means

The NexStar+ hand control drives the motors to point at objects from its database and then tracks them as the Earth turns. You first align the telescope on the sky. SkyAlign lets you centre any three bright objects, so you do not need to know star names to get started. Solar System Align uses a single bright object such as the Moon or a planet.

Magnification and its limits

Magnification equals the telescope’s focal length divided by the eyepiece’s focal length. Orion’s AstroView manual gives a general rule of about 2x per millimetre of aperture as the useful limit, which for 102 mm is about 204x. Celestron quotes 241x as the highest useful magnification. Either way, it is only reachable on very steady nights.

What the numbers mean

• Light-gathering power, 212x: Celestron says the 102 mm lens collects 212 times as much light as the human eye.
• Limiting magnitude, 12.5: the faintest stars visible through the telescope under good skies.
• Resolution, 1.14 arc seconds (Dawes): how close two stars can be and still appear separate.
• Focal ratio, f/6.47: a fairly short refractor, giving a wider field of view than long-focus models, which helps when scanning star fields.

Setting up for your first night

Orion’s AstroView refractor manual offers practical advice that applies to any small refractor:

• Practise focusing in daylight on a distant object before your first night out.
• Allow at least half an hour for the telescope to cool to the outdoor temperature.
• Give your eyes about 30 minutes to dark-adapt, and use a red light.
• Set up on grass rather than asphalt, and avoid viewing over rooftops.

The older NexStar 102GT

The second post was a used listing for a NexStar 102GT, a computerised 102 mm refractor from Celestron’s earlier GT range. The seller described it as including a NexStar+ hand controller, a red dot finder, a 25 mm eyepiece, the optical tube with a dovetail mounting and a steel tripod. We could not find a current Celestron page with verified 102GT specifications, so treat any figures from old listings with caution.

Checking a used GoTo refractor

• Power up the mount: both axes should move smoothly in every direction from the hand control.
• Try an alignment: a successful SkyAlign or Solar System Align shows the electronics and motors work.
• Inspect the lens: look for scratches, haze or fungus.
• Check the load: the 102SLT mount is rated for 8 lb; do not add heavy accessories.
• Batteries: motorised mounts use power quickly, so carry spares.

What you can see

With 102 mm of aperture and a limiting magnitude of 12.5, expect:

• The Moon: craters and mountains in sharp detail
• Planets: Jupiter’s bands and moons, Saturn’s rings and Venus’s phases, as Orion’s manual describes for a similar refractor
• Deep-sky objects: bright clusters, nebulae and galaxies, which appear as grey glows

Safety

Never look at the Sun through the telescope or its finder without a professionally made solar filter covering the front. Young children should use it with adult supervision.

Quick summary

• 102 mm, 660 mm, f/6.47 refractor on a computerised single-arm mount.
• Database of over 40,000 objects; SkyAlign makes setup easy.
• Highest useful magnification 241x; load capacity 8 lb.
• Celestron lists both 25 mm and 20 mm eyepieces in different places, so check what is included.

Jargon buster

• GoTo: a mount that points to objects automatically.
• SkyAlign: Celestron’s alignment using any three bright objects.
• Fork arm mount: a mount that holds the tube from one side.
• Dawes limit: a measure of how close two stars can be resolved.