Nexstar 127SLT Optical Tube Overview
The Celestron NexStar 127SLT computerized telescope is a 127mm f/12 Maksutov with a focal length of around 1500mm. At least that’s what Celestron says.
In practice, the primary mirror of the Celestron NexStar 127SLT Mak is slightly undersized, stopping it at about 120 mm in aperture. This is a minimal reduction, but it does make the scope a half f-stop slower – f/12.5.
Focusing the NexStar 127SLT is accomplished in the same manner as pretty much all Maksutov- and Schmidt-Cassegrains: by turning a knob that slides the primary mirror on a threaded rod inside the tube. This system works, but it can cause “image shift” due to the mirror slightly wobbling on the rod. Luckily, the 127SLT has a relatively low amount of image shift, so it shouldn’t be a problem for visual use or astrophotography.
The Celestron NexStar 127SLT has a 1.25” visual back with a different thread system than Schmidt-Cassegrain accessories use. You can buy an aftermarket adapter that allows the use of Schmidt-Cassegrain visual backs, adapters, or a 2” diagonal. A 2” diagonal will vignette slightly with the 127SLT and long focal length eyepieces, but will allow you to achieve a wider true field of view with a 2” wide-angle eyepiece (anything with a field stop of over 35mm or so will vignette, so keep this in mind when shopping). An f/6.3 reducer meant for Schmidt-Cassegrains will vignette severely and thus should not be used with the 127SLT.
The NexStar 127SLT has a Vixen dovetail plate mounted on the right side of the optical tube, and it’s sufficiently long that you can slide the scope on its mount to balance the scope if it is slightly front-heavy or back-heavy. You can also put the scope on a manual alt-az mount or an equatorial mount if desired.
A Fairly Good Set of Accessories
The Celestron NexStar 127SLT Mak comes with two eyepieces: a 25mm Kellner for low power (approximately 60x) and a 10mm Kellner for medium-high power (approximately 150x). These eyepieces work pretty well, but Plossls would’ve been a little better and are usually what to expect at the 127SLT’s price point.
The 127SLT’s diagonal is a prism, and a nice one at that—unlike the cheap diagonals supplied with many entry-level scopes, which are cheap mirrors that aren’t very flat and tend to offer dim and fuzzier images.
The NexStar 127SLT’s finderscope is the same generic “StarPointer” red dot LED device that comes on almost every beginner scope nowadays. It is well-made and sufficient for aligning the 127SLT’s GoTo system. You probably won’t use it again during your observing session once alignment is complete.
NexStar SLT Mount Capabilities
The NexStar SLT mount is an evolution of the NexStar GT mount, which has been around since the early 2000s. The 127mm Maksutov optical tube is about at the limit of what the SLT mount can handle – the 102mm f/6.5 refractor and 130mm f/5 Newtonian sold with the SLT mount are really pushing its capabilities and can be wobbly as a result. The 127mm Maksutov optical tube is lighter and shorter than both of those, thus putting less strain on the mount. The SLT mount is a GoTo; you line it up with a couple stars when setting up the telescope, and then it loudly and automatically swings over to and tracks the target of your choice.
The NexStar SLT’s hand controller offers a database of about 4,000 objects. You will probably never observe all 4,000 of said objects because a good chunk of them are invisible or incredibly boring with a 120-mm aperture.
The SLT mount needs 8 AA batteries, which it will probably use up by the end of your second observation session or so. Obviously, that’s inconvenient and can be expensive. So I highly recommend biting the bullet and buying a dedicated DC power supply. But don’t forego the batteries—in the event your power supply disconnects for even an instant and the battery compartment is empty, the scope will shut down, and you’ll have to realign it before continuing on your way. The scope also lacks an internal clock, so you have to set the time and date again every time you use it.
Should I buy a Used Celestron NexStar 127SLT?
The Celestron NexStar 127SLT Mak computerized telescope is a great deal if you can get it used. Make sure to check that the electronics still work and that the mount moves smoothly. And if it’s an older unit with a non-USB hand controller, be prepared to pay a bit less.
The Celestron NexStar 127SLT has great optics, but it’s hobbled by its stopped-down aperture, less-than-adequate mount, and high price tag for what you get. There are certainly worse choices, but we have a lot of alternatives you might want to consider first even if you think the 127SLT is right for you.
- The Apertura AD8/Zhumell Z8/Orion SkyLine 8 offers nearly triple the light gathering ability and about double the resolving power of the NexStar 127SLT, with a shorter focal length and 2” dual-speed Crayford focuser permitting a much larger possible true field of view – perfect for viewing deep-sky objects. The included accessory bundle is of excellent quality, and the full-sized Dobsonian mount stands on its own and is rock-steady, making finding and tracking objects by hand a breeze.
- The Explore Scientific 10” Hybrid Dobsonian offers 4.5 times as much surface area – and thus light collecting power – as the 127SLT’s 120mm primary mirror, and more than double the resolution, blowing it away when it comes to views of all types of targets. For those looking for a portable telescope, the 10” Hybrid Dobsonian is just the ticket, as it collapses into a small cube in just seconds. The buttery-smooth Dobsonian mounting is rock-solid, will maintain balance with almost any accessories thanks to the huge side-mounted altitude bearings, and is easy to aim around the sky and nudge to track targets at high power.
- The Orion SkyQuest XT8 features just under triple the light-collecting surface area of the NexStar 127SLT, and almost double the resolving power thanks to its large 8” primary mirror. The XT8’s included accessories are pretty sparse – just a single eyepiece and red dot finder – and the focuser is a single-speed 2” Crayford, but it’s a great telescope that’s fairly affordable and easy to use – though you get a bit less for your money compared to offerings from other brands.
- The Sky-Watcher Virtuoso GTi 150P offers a significant (64)% boost in light gathering ability compared to the NexStar 127SLT, and like the 127SLT, it features fully motorized tracking and GoTo – but controlled with your smartphone or tablet instead of a hand controller, and with the ability to push the telescope around the sky manually. The collapsible tube of the Virtuoso GTi 150P makes it just as compact as the NexStar 127SLT, and the tabletop mount is quick to set up and use. The 150P’s short focal length also allows it to attain a much wider possible true field of view, perfect for viewing deep-sky objects. The cheaper Heritage 150P is identical to the Virtuoso GTi 150P apart from the lack of electronics, should your budget not permit the Virtuoso GTi or the GoTo not interest you.
- The Sky-Watcher Virtuoso GTi 130P has slightly more light gathering ability than the NexStar 127SLT, in addition to a shorter focal length yielding a much wider achievable field of view. The GTi 130P can be aimed manually even with its electronics powered on, and its fully motorized tracking and GoTo capabilities are controlled entirely with your smartphone or tablet. The collapsible tube makes it super-compact too. The all-manual Heritage 130P is identical to the GTi 130P apart from lacking GoTo and makes for a superb budget alternative.
- The Celestron Astro-Fi 130 rides atop an updated version of the SLT mount supplied with the NexStar 127SLT, with beefed-up tripod legs for extra stability and ditching the hand controller for the SkySafari or Celestron SkyPortal app on your phone or tablet. The 130mm f/5 optics – the same as the Heritage/Virtuoso 130mm reflector and 130SLT – provide a wider field of view and slightly brighter images than the 127SLT, too. You can’t aim this scope manually, however, unlike the Virtuoso GTi telescopes, and the price is a bit high.
- The Celestron Astro-Fi 102’s aperture is a little smaller than that of the NexStar 127SLT, but between the steadier tripod and the ease of use thanks to the Astro-Fi mount and its smartphone/tablet control, you probably won’t be missing the 127SLT’s extra aperture much. The Astro-Fi 102 is perfect if you must have a Maksutov-Cassegrain telescope but don’t like some of the compromises that the 127SLT makes.
- The Apertura AD10/Zhumell Z10/Orion SkyLine 10 has more than double the resolving power and quadruple the light gathering ability of the NexStar 127SLT, with an easy-to-use and sturdy Dobsonian mount, a wide assortment of high-quality accessories, and the same great features of the AD8/Z8, but with a negligibly heavier or wider tube and base. We highly recommend either this scope or the smaller 8” version for the performance and value for the money.
- The Celestron StarSense Explorer 8” Dobsonian offers some computerized capabilities, in the form of the StarSense Explorer technology that uses your smartphone to help with locating targets in the night sky, albeit without any motor drives or automatic slewing. The StarSense Explorer 8” doesn’t come with a lot else in the way of accessories or features – the handles on the tube and cutouts in the base are nice, but the focuser is a single-speed and only one 1.25” Plossl eyepiece is included. However, it’s still an excellent telescope especially if you must have some sort of computerized functionality, and it easily bests the NexStar 127SLT in convenience, stability, and performance.
- The Sky-Watcher 8” FlexTube Dobsonian is a little more compact than a regular 8” or 10” solid-tubed Dobsonian thanks to its innovative collapsible strut tube, which is handy if you store the scope in a small space or transport it in a smaller vehicle. The included accessories are fairly decent, though the focuser is only a single-speed 2” Crayford unit. However, the 8” FlexTube is quite heavy and the collapsible tube needs a shroud to keep stray light and moisture away from the optics.
Aftermarket Accessory Recommendations
The included eyepieces with the 127SLT are adequate, but you might want something with slightly higher magnification in the 6mm range (250x), such as a 6mm goldline. A dew shield and some sort of rechargeable power supply are also good ideas and will keep you out observing longer.
What can you see with Celestron NexStar 127SLT Mak?
The NexStar 127SLT’s f/12.5 focal ratio makes it a little more suitable for deep-sky than some of the competing GoTo Maksutovs, but it really still limits what you can see. The best deep-sky objects to observe with small scopes are large nebulae, which will be so spread out that they’re simply invisible with the 127SLT, and open clusters, some of which won’t fit in the field of view even at low power. The scope’s meager 120mm aperture means you can’t resolve many globular clusters, and planetary nebulae big and bright enough for a 120mm scope are few and far between.
Really, the 127SLT is a lunar and planetary scope, and for that, you don’t need the GoTo at all. Mercury and Venus’ phases can be seen; the 120mm aperture is enough to resolve a few dark patches and the ice cap on Mars; and Jupiter and Saturn display a wide array of features and moons. Uranus and Neptune, invisible to the naked eye, are uninteresting with the 127SLT and appear as little more than bluish dots.
The 127SLT’s only significant advantage over a Dob is its portability; it’s very light at only 18 lbs, which means you can pick it up and move it fully assembled, in some cases with just one hand. However, the GoTo and the complete lack of manual controls mean you aren’t going to be able to dodge trees with the scope unless you re-align every time you move to a new spot. You might as well just lug a heavier scope.
127SLT and Astrophotography
The lightweight, alt-azimuth SLT mount combined with the 127SLT’s f/12.5 focal ratio means it is utterly useless for deep-sky imaging. However, you can do a decent job imaging the Moon and planets with an inexpensive CMOS camera and a 2x Barlow lens; the only other prerequisite is a laptop with a decent amount of RAM and lots of hard drive space.
If you have a DSLR, you can also use it with a 2x or, preferably, 3x Barlow lens with or without a laptop (a laptop allows for more control over the camera) and get equally good results, but the DSLR will strain the mount more, so if you don’t already have a camera, I’d choose a CMOS instead.