Celestron C9.25 XLT Reviewed: Highly Recommended!

Optics

The C9.25 uses a 9.25” (235mm) f/2.5 primary mirror and a secondary mirror that amplifies the focal ratio by about 4x as opposed to the normal f/2 + 5x combination in most SCTs, for a total system focal length of 2350mm and an f/10 focal ratio. This results in the tube being proportionally a little longer than other SCT optical tubes. However, the main difference is that the primary mirror is slightly easier to make to high tolerances, and collimation is made easier by the longer focal ratio primary, resulting in better images on average with C9.25s compared to other SCTs of similar apertures. The slight change to the optical design was done partly to better illuminate full-frame cameras and reduce vignetting problems seen in the C8, which cannot quite fully illuminate a 2” field without some vignetting, an issue the C9.25 does not suffer from.

The C9.25 is also just the right size to make it practical for planetary use. Your average air cell is about 10 inches wide, so a scope at or above 10” of aperture is looking through more than one column of turbulent air and is thus more affected by atmospheric seeing – another reason, I suspect, why many people rate the planetary views with the C9.25 as equal or better than larger, equally good quality SCTs.

Unfortunately, increasing the length of the tube makes the C9.25 a heavy and rather unwieldy scope compared to a C8—20 pounds for the C9.25 versus 12.5 pounds for the C8. You’re nearing the weight of Celestron’s C11, which, as specified by the manufacturer, clocks in at 27.5 pounds. In practice, all three scopes are a couple of pounds heavier than advertised—and that’s before you even add accessories. The tube length is only 2 inches shorter than a C11, despite being a full five inches longer than the C8. So if you want better portability for your given aperture, you’re better off going with a C8 or C11.

The main advantage of the C9.25 over the C8 or C11 is that it grants you about 33% more light-gathering ability over the C8, but the 2350mm focal length is not going to box you in when viewing deep-sky objects and won’t require a monster mount like the C11 does. The C9.25 is the largest SCT we would really recommend as an only telescope; anything bigger is going to have a focal length of at least 2500mm, which is really too claustrophobic for deep-sky use, even when coupled with an f/6.3 reducer, so you’ll definitely want a fast Newtonian, a refractor, or even a smaller SCT for low-power wide-field views—you may even want this with a C9.25.

There is an EdgeHD version of the C9.25 that has improved cooling, better edge-of-field correction for astrophotography, and mirror support knobs to prevent image shift and mirror flop. However, until recently, there was no 0.7x reducer available for the Edge 9.25 due to the complications of designing it combined with low demand, so you may be hard-pressed to find any actual users of these scopes since most astrophotographers use reducers with their SCTs.

Mechanics

As with most Schmidt-Cassegrain telescopes, you focus the C9.25 XLT by turning a knob at the back of the scope, which slides the primary mirror along a threaded rod. This system keeps the telescope compact and any accessories attached to the threaded back fixed in place. However, it can result in some “image shift” as the mirror rocks back and forth when focusing, as well as “mirror flop,” where the mirror sags and ruins long exposures. The latter is a major reason why you should get the EdgeHD 9.25 instead of the standard XLT for deep-sky astrophotography, as the EdgeHD has locks to keep the mirror in place once you have reached focus.  Like all industry-standard Schmidt-Cassegrain telescopes, the C9.25 will accept a reducer-corrector, T-adapter, and other Celestron or third-party supplied accessories attached to the back.

Collimating the C9.25 XLT is a task that doesn’t need to be done often but requires pointing the scope at a bright star to do; our collimation guide explains in more detail how to go about this process. The stock Philips head screws at the front of the telescope do require a screwdriver to adjust collimation, but stay pretty secure when tightened. The same cannot be said of the all-too-tempting thumb screws that some users buy, ensuring that they constantly have to fiddle with collimation even just throughout an observing session, as these thumb screws cannot be tightened enough to keep the secondary mirror fixed in place.

The C9.25 XLT is offered with either a Vixen or CGE-style dovetail rail bolted to the bottom. The CGE dovetail will fit any mount saddle that accepts CGE or Losmandy plates, and almost any mount capable of holding the C9.25 XLT will have such a saddle, which will be more rigid than a narrower Vixen dovetail bar. Get the CGE dovetail.

Accessories

The C9.25 XLT optical tube comes with a 6×30 finderscope, 1.25” visual back, 1.25” prism star diagonal, and 25mm (94x) E-Lux Plossl eyepiece. If you buy the Advanced VX, CGEM, CGX, CGX-L, or CGE Pro kits where the C9.25 is bundled with one of these mounts, you will get the same accessories. The 6×30 is fine for aligning a GoTo mount, but if you plan on using the scope manually, something like a 9×50 or Telrad is preferable; additionally, the 6×30 can be uncomfortable to use even for aligning the mount. The 1.25” visual back, prism diagonal, and 25mm Plossl are all fine, but the field of view is rather narrow with 1.25” eyepieces, and you will really want a 2” diagonal and low-power eyepieces for most use.

Mount Recommendations

The C9.25 is spec’d at about 20 pounds. In actuality, it weighs a few more pounds than that, and once you add heavy eyepieces, a finderscope, and a dew shield, you are probably going to end up with a scope nearing thirty pounds.

If you’re looking for a lightweight setup, Celestron’s Advanced VX (or an older, used CG5) will work just fine for visual use and planetary/lunar astrophotography. Beware that the Orion Sirius/Skywatcher HEQ5 has 1.75” tripod legs versus the VX’s 2” legs and really can’t support the weight of the C9.25 adequately.

For deep-sky astrophotography, we have seen people get away with EQ6-class mounts like the Sky-Watcher EQ6R, but even with good autoguiding, you’ll have to discard some frames. You need a mount with at LEAST the capacity of 50 pounds, preferably more if you can get it. 

You could put the C9.25 on one of the various manual alt-azimuth mounts like the Explore Scientific Twilight II, but we find that tracking with a focal length above 2000mm by hand is rather tedious and annoying, even at low magnification.

Should I buy a Used Celestron C9.25 XLT?

A used C9.25 XLT can be a great scope—if you can get your hands on one! These telescopes are often hard to pry away from their owners. Older C9.25s differ little from new units apart from a lack of HyperStar compatibility. When buying a used C9.25 XLT optical tube, be sure to check that the mirrors are free of corrosion and the front corrector plate is free of fungus (which can cause permanent damage). Additionally, avoid Schmidt-Cassegrain telescopes with broken front corrector plates, as these cannot be repaired or replaced without replacing all the optics, which will likely cost more than what the telescope is worth. Missing accessories are not a major concern since they are typically not too expensive to replace.

Aftermarket Accessory Recommendations

To get the most out of your C9.25 XLT, you’ll need a 2” mirror star diagonal, like Apertura’s dielectric model, which screws directly onto the back of your telescope. While the stock 25mm E-Lux Plossl works fine, we’d recommend supplementing it with a low-power wide-angle eyepiece such as Apertura’s 38mm SWA (62x), as well as a mid-range eyepiece between 12-18mm focal length like Explore Scientific’s 14mm 82-degree (168x) and a 10mm or shorter focal length high-power eyepiece like Explore Scientific’s 8.5mm 82-degree (276x). But keep in mind that these are just recommendations; cheaper Plossl and wide-angle designs such as SuperView, SWA, and goldline/redline oculars will work great at f/10 too. A Barlow lens may also be a good idea in lieu of a high-power eyepiece, especially if you want to use the C9.25 XLT for planetary imaging, for which a Barlow is a must.

We also recommend investing in an Orion UltraBlock UHC nebula filter (or another good quality UHC) for any telescope — in the C9.25 XLT’s case, preferably in a 2” size format that will screw onto a 1.25” adapter for use with either size eyepiece — to increase contrast when viewing nebulae no matter what kind of sky conditions you have. Additionally, you’ll want to invest in a dew shield for your C9.25 XLT; its thin Schmidt corrector plate cools down quickly and attracts dew easily after some time set up under damp conditions, and the dew shield has the additional benefit of protecting the corrector from bugs, fingerprints, and the worst sources of glare such as moonlight or nearby streetlights to increase contrast at the eyepiece.

Astrophotography Capabilities

The C9.25 XLT’s focal length needs to be reduced to f/6.3 and 1480mm for deep-sky imaging and will need very accurate guiding and tracking. Its large size and long focal length necessitate a big mount, which makes it a challenging choice as an astrophotography rig. The EdgeHD version would be a better choice due to its flatter field and mirror locks, which the regular C9.25 XLT does not possess. The C9.25 XLT can also accept Starizona’s HyperStar f/2 conversion kit to reduce its focal length to 470mm. 

Planetary astrophotography is a task for which the C9.25 XLT is ideally suited; using a 2-3x Barlow lens and high-speed planetary video camera such as the ZWO ASI224MC with laptop capture software will enable you to take stunning pictures of the Moon and planets such as Mars, Jupiter and Saturn.

What can you see?

The Celestron C9.25 XLT optical tube has enough light-gathering power to make deep-sky objects appear especially vibrant. Its field of view is wide enough for smaller open star clusters like M35, M11, M46, and M38 to fit in the eyepiece at low power; however, larger ones such as the Double Cluster, Pleiades (M45), or the Beehive (M44) are a little too big and too close for comfort. You can also observe globular clusters like M3 and M22 all the way down to their centers with high magnification, revealing unique features like dust lanes on M13 or the out-of-round shape of M92.

Planetary nebulae such as the Cat’s Eye Nebula will show greenish and blueish colors with the C9.25 XLT, while larger planetary and emission nebulae such as the Dumbbell (M27) or the huge Orion Nebula (M42) look great even from suburban environments, though the view is enhanced by dark skies and/or a UHC nebula filter. Under dark skies with little light pollution, the C9.25 does a decent job on galaxies, with the Virgo Cluster displaying hundreds of galaxies and the telescope revealing details including dust lanes in many galaxies like M65 and hints of spiral arms in some galaxies such as M51 if you take your time observing them.

The C9.25 XLT is also, of course, a powerhouse for planetary viewing. With it, you can easily see the phases of Venus and Mercury, as well as the thousands of mountains, craters, and ridges on the Moon. Mars shows its polar ice cap and dark markings on its surface when it is close to Earth. Jupiter’s Great Red Spot, colorful cloud bands, and moons are visible when they transit in front of the planet, with their shadows following close behind. Saturn’s rings and Cassini Division can be seen on a steady night, along with even the Encke gap with some scrutiny and a few cloud bands on the ringed planet alongside a half dozen or so of its small moons. Additionally, Uranus’ teal disk can be spotted, along with perhaps one or two of its moons, while Neptune may only appear as a star-like point, but its moon Triton is more easily distinguished than those of Uranus. Pluto is just barely within reach of the C9.25’s light-gathering grasp due to its limited aperture, but it can still be seen if you know where to look under dark skies.