Nexstar 127SLT Optical Tube Overview
The NexStar 127SLT 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 NexStar 127SLT is slightly undersized, stopping it down to about 120mm in aperture. This is a minimal reduction, but it does make the scope a half f-stop slower – f/12.5.
Focusing the 127SLT is accomplished in the same manner as pretty much all Maksutov- and Schmidt-Cassegrains: by turning a knob which 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 that shouldn’t be a problem for visual use or astrophotography.
The NexStar 127SLT has a 1.25” visual back with a different thread system than Schmidt-Cassegrain accessories use. You can buy an aftermarket adapter which 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 (which would reduce the scope to f/7.875) will vignette severely and thus should not be used with the 127SLT.
The NexStar 127SLT has no dew shield or anything more than a slight protrusion in the front of the corrector lens. I’d recommend a flexible plastic dew shield both to protect the corrector lens from dewing up as well as shield the scope from stray light.
The 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.
How Good Are The Accessories?
The NexStar 127SLT 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 fine as starters but Plossls would’ve been far better and are usually what I expect at the 127SLT’s price point. If/when you do decide to upgrade, I’d recommend a 32mm Plossl for low power and 9mm and 6mm goldline eyepieces for high power, as well as maybe a 15mm wide-angle eyepiece in the middle and perhaps a Barlow lens.
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, which 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.
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 lightweight and shorter than both of those and thus puts less strain on the mount.
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 120mm of the aperture.
The SLT mount takes 8 AA batteries, which it will probably consume by the end of your second observing 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 (Celestron even sells astronomy-dedicated ones for the purpose, the PowerTank). 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 re-align it before continuing on your way – the scope also lacks an internal clock, so you have to entire the time and date again every time too.
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 you in 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 (Uranus and Neptune, invisible to the naked eye, are uninteresting with the 120mm aperture so it’s not like it really matters). So why are you paying for a 120mm scope when you could get an 8” Dobsonian at nearly twice the aperture for the same price if not slightly cheaper?
The 127SLT’s only big 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 means 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 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 that 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.
While I recommend the Celestron NexStar 127SLT for the budget-conscious observer who wants a serious telescope that’s portable and has GoTo, I continue to say this as-always: Small GoTos are a pointless conundrum because you do not really need GoTo to see the things that are visible with them, and you can afford a bigger manual scope for the same price.