Celestron NexStar 127 SLT Review
- •Sharp Optics
- •Some Astrophotography Capabilities
- •Fully Computerized
- •Lightweight & Portable
- •Not A Lot Of Aperture For The Money
- •Long Focal Length Limits Target Options
- •Mount Is Not The Sturdiest Nor Most Up-To-Date
The Celestron NexStar 127SLT isn’t a terrible scope, but is limited for deep-sky viewing and fairly expensive.
Celestron’s NexStar 127SLT (Star Locating Telescope) is one of the larger scopes in the SLT lineup, and consists of a nominally 5” Maksutov-Cassegrain paired with a GoTo mount. It’s a lot cheaper than the 5” SCT models Celestron offers or the computerized Maksutov-Cassegrains of equivalent aperture from other companies.
The Optical Tube
1500mm Focal Length
The NexStar 127SLT is nominally a 127mm Maksutov-Cassegrain with a 1500mm focal length and focal ratio of f/12. However, due to the design of the internals and an undersized primary mirror, the 127SLT is actually stopped down to 120mm in aperture, which also slows the scope to f/12.5. All of the “127mm” Maksutov-Cassegrains made by Synta (offered by Celestron, Sky-Watcher, Orion, and a few other companies) suffer from this flaw. The loss of 7mm of aperture is hardly noticeable but is still annoying. However, the optics in the NexStar 127SLT are tack sharp as with most Maksutov-Cassegrains and the telescope does not require collimation. A Maksutov-Cassegrain uses a meniscus corrector lens, spherical primary mirror, and an aspheric secondary mirror (actually just an aluminized portion of the corrector) to achieve sharp images, as well as a long focal length with a very physically short tube.
As with most catadioptric telescopes, you adjust the focus by turning a knob at the back of the 127SLT to move the primary mirror back and forth inside the tube. Your eyepieces or camera attach to the back. The 127SLT has a proprietary 1.25” visual back which is also threaded for T threads to attach a DSLR or mirrorless camera. The tube attaches to its mount with a Vixen-style dovetail bar, which means you could use the 127SLT on another mount if you wish.
The NexStar SLT Mount & Tripod
The NexStar SLT mount is an upgraded version of the old NexStar GT mount which is pushing 20 years old by now (the SLT has been around for about 15 years) as of 2022/3. The SLT mount is an alt-azimuth GoTo mount that moves up-down and left-to-right with internal gearing and electronics which can automatically find, move to, and track any object in its 40,000 object database, or more if you plug it into a computer. The newer Astro-Fi mount is similar to the SLT but ditches the hand controller for control via an app on your smartphone/tablet and has a sturdier tripod, something to consider if you’re in the market for a computerized telescope.
Setup of the SLT mount requires entering the date, time, and aligning the telescope/mount on 2 or 3 bright stars. The SLT mount’s alt-azimuth design and cheap gearing are nowhere near accurate enough for long-exposure astrophotography, not that a Maksutov-Cassegrain is good for that task anyways. The SLT mount runs on AA batteries by default but will burn through them fast – a rechargeable power supply is a must.
The NexStar 127SLT comes with a 1.25” prism star diagonal and two 1.25” Kellner eyepieces: a 25mm for 60x and a 10mm for 150x magnification. These are arguably all you need; the 127SLT can only handle up to around 200x magnification anyway and a lower-power eyepiece will achieve only a marginally wider field of view than the stock 25mm. The provided eyepieces are fairly good, though you do have to get your eye pretty close to the 10mm to take in the whole field of view.
Like most beginner and computerized scopes, the 127SLT has a red dot finder provided, which you only really will need to align the telescope on the sky during setup.
What can you see?
The NexStar 127SLT’s larger 120mm aperture makes it capable of resolving more on the planets than smaller Maks. However, it is noticeably inferior to a 6-8” or larger Dobsonian.
Mercury and Venus will only show their phases with any amateur telescope.
The Moon looks fantastic with the 127SLT, showing details mere miles across.
Mars will reveal its polar ice caps and any dust storms, and when it’s close to Earth the 127SLT resolves some dark markings on its surface.
Jupiter’s moons show up at low magnification along with its cloud belts. High magnification with the 127SLT on a clear and steady night will reveal the disks of Jupiter’s 4 large moons when they transit, along with their shadows, and the Great Red Spot on Jupiter itself.
Saturn’s rings show the Cassini Division within during steady seeing, along with cloud belts, as seen through the 127SLT. A few moons can also be seen.
Uranus can be resolved with high magnification as a fuzzy turquoise disk, but the 127SLT reveals little else and its moons are too faint to see.
Neptune is hard to distinguish from a star, but the 127SLT can just barely reveal its dim moon Triton under dark and steady skies.
Pluto is simply out of reach with telescopes smaller than 8-10” of aperture.
Light pollution affects the visibility of deep-sky objects beyond the Solar System with any telescope, and the NexStar 127SLT is no exception. The brightest globular star clusters like M13 can be resolved at high magnification, while open star clusters, although dazzling, can struggle to fit into the telescope’s limited field of view. A few galaxies show dust lanes and galaxy groups or clusters can be seen in large numbers under dark skies with the 127SLT. The Orion Nebula dazzles under almost any conditions, and there are plenty of double stars you can try splitting at high power.
The NexStar 127SLT’s long focal length and inability to accept 2” eyepieces and accessories limit its field of view quite a bit when viewing deep-sky objects. The maximum field of view with a 1.25” eyepiece at a 1500mm focal length is only about 1 degree across or twice the width of the full Moon in the sky.
The SLT mount is also not the steadiest with a wider and larger optical tube like the 120mm Maksutov-Cassegrain supplied, especially if you extend the tripod legs all the way, as the tripod is made of thin-walled and narrow steel tubing. Filling the legs with spray foam or sand helps to dampen vibrations significantly.
Lastly, for the price of the NexStar 127SLT you could get a larger 6”, 8”, or even 10” Dobsonian with a wider field of view, steadier mount, and much more light-gathering and resolving power than the 127SLT.
The Celestron NexStar 127SLT isn’t the best deal for the price, though it’s quite portable and delivers decent views if you don’t mind the compromises of small aperture.