Celestron C5 XLT Review

I really try to remain impartial when reviewing gear. But it’s no secret that until recently I was pretty down on SCTs. The orange-tube C8 I bought new in 1981 had never been much good, optically or mechanically. A friend’s recent Meade 8” had been similarly poor, as had the 12” Meade I used on a university course. To be honest I thought it was the basic design and concept – something to do with a biggish central obstruction and fast primary with moving-mirror focusing. I was wrong.

The problem with the SCTs I had tried, I realised, wasn’t the fundamental design, but that old quality thing again. The recent, Chinese-made C8 XLT I reviewed in late 2013 had the same large obstruction, fast primary and moving-mirror focus as those other SCTs; it was even very slightly out of collimation. But nonetheless it was an excellent telescope, one I would seriously consider owning myself: it gave fantastic lunar and planetary views for its size and cost.

Over the last few years I have been trying out as many portable lunar and planetary scopes as I can. Given my positive experience with a modern C8, I was keen to try the smallest SCT Celestron make, the C5. The C5 has been around since the late Seventies in various forms and on various fork and equatorial mounts, but here I’m testing the basic OTA with the latest XLT coatings.

At A Glance

Telescope	Celestron C5XLT OTA
Aperture	127mm
Focal Length	1250mm
Focal Ratio	F 9.8
Central Obstruction (incl. holder/baffle)	48mm (37.8%)
Length	280mm w/o visual back and obj. cap (330 with).
Weight	~2.7 Kg

What’s in the Box?

The C5 OTA was delivered packed in its (standard) carry case.

Design and Build

Optics

As I’m sure you know, the C5 is a Schmidt-Cassegrain Telescope (SCT), a type that belongs to a class of telescopes that uses both refractive and reflective elements in combination – a catadioptric. In this case, the main mirror focuses onto a secondary which in turn reflects the light back through a hole in the primary, so it is indeed a variant on the Cassegrain; a corrector plate at the front supports the secondary mirror and corrects aberrations in the primary optics.

The SCT design has advantages in terms of compactness and light weight. The corrector is thinner than that in a Maksutov, so the C5 is lighter and more back-heavy than the Skywatcher Skymax-127, for example. It is shorter too. However, that compactness requires steeply curved optical surfaces – the primary mirror is F2; compare that with the F3 primary on a Takahashi Mewlon Cassegrain. Such a fast mirror can be hard to make well and close tolerances are required if performance is to be good. That thin corrector plate is made of float glass too, same as your windows, which can have inhomogeneities.

Another disadvantage of the design is the large central obstruction. In this case the secondary mirror and housing are 48mm across, equating to almost 38% of the width. A 38% obstruction is at or beyond the very top end of what is acceptable for visual use and is the largest I’ve ever reviewed; 33% is generally accepted as the maximum for a high-performance system (excepting the large obstructions found in some astrographs, where it matters less than for visual use). A large obstruction does reduce contrast, but the main effect in my experience is to worsen performance in poor seeing.

Like most commercial SCTs, the C5 is an F10 design. That means it has a 1250mm focal length, which is relatively long and means the maximum field of view with a 1.25” eyepiece like a 32mm Plossl is 1.2°, but in fact the usable field, due to off-axis aberrations, will be more like 1.0°.

The baffle tube is just 38mm clear aperture, so fitting a 2” visual back wouldn’t allow maximum benefit from 2” eyepieces with the widest field stops. Theoretically you could get about 1.5°, but again off-axis aberrations will limit the usable field to perhaps 1.25°.

So the C5 is never going to give you much more than a 1.25° field of view and this is a disadvantage of this (or any other SCT) compared to a refractor.

One big advantage of a long focal length, though, is that you can get high powers without using complex, expensive eyepiece designs – Plossls and Orthoscopics are adequate.

SCT optical configuration, from celestron.com

C5 Primary and baffle tube seen through the almost-invisible XLT-coated corrector.

XLT coated corrector plate and 38% central obstruction.

StarBright XLT

The first (70s and early 80s) C5s had an un-coated corrector, but this one has proper multi-coatings (as you can see in the photos). Celestron call this premium coating technology ‘StarBright XLT’, as distinguished from the earlier ‘StarBright’ coatings. StarBright XLT employs multi-layer mirror coatings and multiple layers of magnesium and hafnium fluoride on the corrector plate. Celestron also use a better, more transmissive glass for the corrector than in previous models. Overall, Celestron claim 83.5% transmission for the system, up from 72% for the original StarBright coatings.

The C5 is very compact and light weight for a 5” telescope.

Tube

The tube and castings are completely different in detail from the original C5, but the overall design remains much the same with solid metal front and rear castings and an aluminium tube. Build quality looks more mass-produced than the original C5 and isn’t up to the standard of the C8: the OTA is fixed together by screws, some of which aren’t seated all that well. Overall external build-quality is adequate but no more.

A plastic dew-cap is provided, but unlike the C8 cap this one is a simple push-fit.

Focuser

C5 Focuser, standard 1.25” and visual back.

Like many Cassegrains, the C5 focuses internally by moving the main mirror. This has advantages over an external focuser – it keeps the tube sealed, the eyepiece in one place and allows lots of focus travel. In many cases, though, it also comes with issues – notably the tendency to cause the image to move when changing focus direction (‘image-shift’) and mirror-tilt that can move the sweet spot when you back out and degrades the image as well. As we will see, this C5 (unlike some earlier versions) suffers from neither of these issues: the focuser is smooth and precise.

Mounting

The C5 on test has a Vixen dovetail bar running the length of the OTA. Mounted on my Vixen GP it needed just the smallest counterweight to balance it. The dovetail would easily allow the C5 to mount up on any small equatorial or altaz mount accepting the Vixen dovetail (an EQ5, Porta etc).

The bottom of the bar has two ¼-20 threads that can be used to mount the C5 on a photo tripod (it would have to be a hefty one); I used them to fix the OTA to a small Losmandy clamp which then slotted onto the upside-down dovetail atop the rings of my permanently mounted large refractor.

C5 Vixen-compatible dovetail with handy ¼-20 threads.

The C5 piggy-backed atop my TMB175/AP1200.

Accessories

The C5 OTA comes with a permanently-attached vixen dovetail bar, 1.25” visual back, star diagonal, 6x30 finder and a 25mm eyepiece as standard.

The finder is baffled and multi-coated and has good eye relief and a bright field; it’s identical to other Synta 6x30 finders. The eyepiece is a nice multi-coated Celestron E-Lux Plossl and the mirror diagonal looks decent quality too.

The C5 comes with a very useful soft case that is very well padded with dense ethafoam to provide lots of bump-protection. The case is light and properly carry-on portable at 39x29x20 cm. In reality this soft case is going to be a more useful travel case than most hard cases and really adds to the C5s travel-scope credentials.

Celestron C5 carry case – ready to go with you.

In Use – Daytime

The C5 is sold as a spotter and it does give excellent daytime views up to 100x and beyond. There is no chromatic aberration worth noting and sharpness is very good centre field (though not at the edge). I was able to watch birds in the copse 200m away as if through binos in my own garden.

In daytime, best focus is an absolutely crisp point and I note that the focuser is very precise and has almost no image shift; what tiny amount there is remains well damped at all times. Best focus is also exactly the same focusing out and then back, suggesting mirror tilt is well controlled.

The C5 makes a great budget long-telephoto lens and I got some crisp photos of a roosting pigeon about 50m away; a paparazzo could doubtless make good use of the C5 for getting snaps of A-lister indiscretions.

All in all, the C5 makes a very fine high-power spotting scope during the daytime.

Snap of a roosting pigeon through the C5.

In Use – Astrophotography

For astrophotography, the C5 suffers from two problems:

1) Off axis aberrations: field curvature and coma

2) Arcing reflections from bright stars just out of view

It might do better with a flattener and some judicious flocking of the baffle or focuser tube.

Below are an unprocessed image of the Moon (the best I could manage from about sixty shots) and a misty single frame of M42 to show those arcs from bright stars outside the field and the comatic stars towards the field edge (even on an APS-C sensor).

Moon through C5 XLT – unprocessed image is a bit soft and worsens off-axis, but overall not bad.

M42 through C5 to show the reflection arcs caused by bright stars just out of view: 45s Fuji XM-1 APS-C at ISO 3200

In Use – The Night Sky

General Observing Notes

Cool-down is slower than a typical doublet refractor, but not as lengthy as a Maksutov which has a much thicker corrector plate. Still, slow cool-down limits the C5’s use as a quick-look telescope.

I noticed that the C5 view through a lowish power (32mm plossl) was unpleasant due to secondary mirror shadow, especially on the Moon.

Star Test

The star test was excellent: virtually identical either side of focus with good, even illumination.

The Moon

As with other designs that have a large central obstruction, the C5 suffers when seeing is poor. On a night with a lot of fine turbulence and the Moon just past 1^st quarter, I had the C5 set up next to Takahashi’s little FS-60Q 60mm apochromatic refractor, with a 13mm Ethos in the C5 and a 6mm Ethos in the FS-60Q giving roughly 100x magnification in both.

Not only did the Tak’ cool completely within 15 minutes from a warm room, it delivered a much steadier (though also much dimmer) view. I also noted that under those conditions the little refractor actually resolved more detail, with Rima Huygens and rima Ariadeus more obvious than through the C5. The FS-60Q also gave a better view of the whole Moon, due to a flatter field and less off-axis aberrations generally.

Poor seeing affected the C5 more than my 175mm refractor, even though sensitivity to seeing generally worsens with aperture. At 100x, the view through the big refractor shimmered, but the image remained intact and detailed. Meanwhile the C5 was mushy, the whole image blurred. Blame that big secondary mirror.

In the early morning, when seeing is usually better here, I finally got some really stable conditions. The C5 gave a good view at last at 114x with an 11mm Tele Vue Plossl. The Moon was just a few days past full and I enjoyed picking out craterlets in Mare Criseum and exploring the deeply-shadowed mountains and embayments around its rim and the drowned craters Lick and Yerkes. Even so, the view wasn’t as crisp and contrasty as in a small APO and more unfocussed light was washing out from the bright limb into what should have been black space.

Jupiter

Unfortunately, Jupiter was the only planet around during the period of my test.

At a magnification of 114x with an 11mm TV Plossl, Jupiter was just a mushy ball in mediocre seeing, but in quite good seeing all the main features were easily visible: the polar hoods with hints of fine banding, the NEB and SEB (obviously), with darkened and thickened areas and hints of other cloud belts too. I could just about pick out the GRS when it came into view.

Overall, Jupiter in good seeing looked similar to a good 60-70mm APO, though perhaps not quite as crisp and contrasty and again with more stray light around the planet’s limb.

Deep Sky

Perhaps surprisingly, visual deep sky is the C5’s forte. That theoretically narrow field is really plenty wide enough for most things and the off axis aberrations still leave a decently flat portion almost a degree in width. Then again, the XLT coatings and big (for a tiny scope) aperture help to deliver lots of bright stars and better picked out nebulosity than you get in a small refractor. The high optical quality also helps out here: stars are nicely pinpoint and brilliant.

Clusters

M35 looked good with pinpoint stars off-axis until about 80% field width. At low powers on deep sky you don’t get the unpleasant secondary-shadowing effect you do with the Moon and so 39x with a 32mm Plossl gives great views of the deep sky through the C5.

M36, M37 and M38 in Auriga

These clusters are favourites of mine. The C5 filled them with plenty of stars, and off axis aberrations were confined towards the field edge, so didn’t spoil them.

The Pleiades

Unfortunately, the C5 suffers from the same problem as the Sky-Watcher Sykmax-127 Maksutov I tested earlier this year: bright stars just outside the field of view form arcs of reflection that intrude a long way into the field, spoiling the view. You can clearly see this on the image of M42 below. Even so, The Pleiades looked good through the C5, with bright stars.

M31

The Andromeda galaxy is big, so shouldn’t work in something like the C5, but actually the whole central section was nicely bright and picked out from the star fields in our own galaxy and I could start to make out the big dark lane to one side.

Bode’s Nebula

The two contrasting galaxies of Bode’s again looked good through the C5. No longer quite the faint blobs they are in a small refractor, dark skies and averted vision gave hints of their true shape and structure.

Globular clusters

The C5 is good for globular clusters. They benefit from the extra light gathering power and don’t tax the narrowish field of view. M15 looked like a bright fuzzy star at 39x and started to reveal its components at 114x, even under a murky, pre-Moon-rise sky.

Doubles

Rigel was split, despite extended plumes of seeing thrown off from the diffraction rings, but I had trouble with Epsilon Lyrae: blame that sensitivity to seeing again.

Nebulae

M42 was surprisingly nice, with detail and even some colour, but those arcing reflections did spoil the view a bit. Even so, the XLT coatings and the extra aperture gave the C5 an advantage over a small refractor here.

The Ring Nebula again worked well through the C5 – floating in a nicely populous star field, its shape was really obvious at 39x.

Other brighter planetaries, like The Crab and the Dumbbell, also looked good through the C5, especially if you are used to the dim views through a small refractor.

Celestron C5 XLT vs Takahashi FS-60Q

In one way this isn’t fair: the Takahashi is much more expensive. But it’s an interesting comparison of two valid but different ways to create a super-compact travel scope with eclipse potential. A list of main comparison points follows:

· The C5 is actually much shorter and more compact, but a kilo heavier

· The FS-60Q will pack just as small because it takes apart

· The longer focal length of the C5 means greater native image scale, but narrower field

· The C5 gathers a lot more light so is (a lot) better for most deep sky objects

· The C5 should have much better resolution, but this doesn’t work out in practice

· The FS-60Q cools much more quickly

· The FS-60Q deals with poor seeing much better

· The FS-60Q is much sharper on axis

· The C5 suffers from off-axis aberrations, too much so for serious imaging; the FS-60Q has a very well corrected field across a full-frame sensor

Overall, the Takahashi is much the better scope unless visual deep sky is your thing.

Summary

Given my good experience with the C8 XLT a year ago, I really wanted to like the C5; and I fully expected to – it’s just a smaller C8, after all.

The C5 does have a lot of positives: it’s very small and light and has excellent optical quality and a very good focuser. It’s sold as a spotter and indeed gives very good views up to 100x in the daytime; it makes a great terrestrial telephoto lens too.

Unexpectedly, I didn’t really like the C5 as a Lunar/planetary scope. Its biggest problem is extreme sensitivity to seeing due to the 38% central obstruction. On none of the evenings I tested it could I get a really clean lunar or planetary image out of the C5, even though my refractors were still delivering sharp, detailed views alongside. This optics weren’t fundamentally faulty or substandard: the C5 did finally manage clean views in really stable pre-dawn seeing.

The C5 suffers the usual SCT drawbacks of off-axis coma and field curvature, so for astrophotography you would really need a flattener. Then there are those arcing reflections. But that said, the C5 does have a real astronomy niche – for visual deep sky. The combination of lots of aperture for its size (2.5x the light gathering of an 80mm for example), good coatings and enough well-corrected field of view for all but the most extended objects, makes for great views of clusters, planetary nebulae and smaller bright galaxies.

Meanwhile, if you do want an ultra-compact catadioptric for the Moon and planets, I’d go for a Sky-Watcher Skymax-127: it’s cheaper and gives much better views, though it is a bit bigger and heavier.

I highly recommend Celestron’s C5 XLT as a terrestrial spotter or telephoto lens, or for visual use as a highly-portable deep sky scope. However, it doesn’t deliver the sharpest planetary views and is sensitive to poor seeing. For the Moon and planets buy a small APO or Maksutov instead.

Updated by Roger Vine Jan 2018