Optics
The optical design of all of the Celestron EdgeHD telescopes is very similar, but just different enough between aperture sizes that the scopes each have their own dedicated reducers; the Edge 9.25’s took longer to design than the rest and thus hasn’t been available for as long.
The EdgeHD telescopes are “aplanatic” Schmidt-Cassegrains (SCTs for short) with a series of corrector lenses in the internal baffle tubes to correct for the field curvature and mild coma seen with Schmidt-Cassegrains used for astrophotography, especially with larger-format camera sensors. This design is similar to the Meade ACF system, but a little more well-designed and versatile, and able to utilize Starizona’s HyperStar system to convert the telescope to an f/2 Schmidt camera. For visual work, you’re unlikely to notice much of a benefit from the EdgeHD optics, if anything at all, compared to the regular C9.25 XLT optical tube. Most eyepieces have their own edge-of-field aberrations and field curvature, which will negate the advantages of the coma-free, flat-field design of the EdgeHD 9.25’s optics.
The C9.25 design uses a slightly slower primary mirror (f/2.3 instead of f/2) and a more weakly curved secondary mirror that causes the scope to be less sensitive to misalignment and easier to manufacture to high tolerance than Celestron’s other SCT designs, so the C9.25 XLT or EdgeHD both tend to be slightly better performers than a typical C8 or C11, for instance.
With a focal length of 2350mm at f/10, the use of a 0.7x reducer to bring the scope down to 1650mm focal length and f/7 is basically mandatory for astrophotography with the C9.25.
Mechanics
The Celestron EdgeHD 9.25 uses the same moving-mirror focusing system in all SCTs and most Maksutov-Cassegrains, where the primary mirror slides along a threaded rod inside the telescope. This can cause “image shift” and “mirror flop” during long exposure astrophotography, which can ruin photos, so the EdgeHD 9.25 has a pair of mirror locking knobs that you tighten once you’ve properly set focus for your camera and guiding setup. The back of the scope also features air vents with HEPA-grade filters to assist in faster cooldown and a built-in carrying handle.
Collimating the EdgeHD 9.25 is achieved by adjusting the three small Philips head screws on the secondary mirror, and is required rather infrequently; thumb screws make collimation more convenient, but also hold collimation poorly and thus require it more often (or worse, they can slip while you’re out imaging).
One of the improvements Celestron has added to the EdgeHD optical tubes is a set of rear vents to help accelerate cooldown time; warm air trapped inside the tube can hinder the scope’s ability to achieve sharp images. These vents also have HEPA-grade filters so that they don’t let in pollen, dust, and other debris, which could contaminate the optics of the telescope and are difficult to remove. Another improvement comes in the form of an attached carry handle on the back end of the telescope tube.
The EdgeHD 9.25 has a CGE-style dovetail (negligibly narrower than a Losmandy D-style plate and thus functionally identical) for attaching to an equatorial mount; the top of the tube has holes for a second plate to which you can attach counterweights or a guide scope.
Accessories
The Celestron EdgeHD 9.25 includes a 2” screw-on dielectric mirror star diagonal with a 1.25” adapter. This diagonal is high-quality and works great for visual use. The included eyepiece is a 23mm Luminos, which provides an 82-degree apparent field of view much like UWA, Nagler, and ES82 eyepieces, albeit with slightly lower performance in faster telescopes (of which the EdgeHD 9.25 isn’t one, so there’s no need to worry). With the EdgeHD 9.25, the 23mm Luminos provides 102x and a true field of 0.8 degrees, or just over one and a half times the width of the full Moon in the sky.
Like the other EdgeHD telescopes and most of Celestron’s XLT optical tubes, the EdgeHD 9.25 includes a 9×50 straight-through finderscope with an unusual bracket (the only other finder made for this bracket is an illuminated reticle 9×50 right-angle finder designed to replace the stock unit). The 9×50 finder has a field of view of about 5 degrees (10 full Moons) wide and can show stars down to around magnitude 11 or so, depending on your light pollution conditions; this is quite a bit fainter than what you can see with your naked eye, and the 9×50 may even pick up some of the brighter deep-sky objects. The field of view in the finder is flipped upside-down, however, which can be a bit disorienting, and all you likely need it for is initially aligning the telescope on a GoTo mount. If you don’t use Celestron’s All-Star Polar Align and plate solve for your rig, you don’t need the finder at all, and removing it will help reduce weight.
Mount Recommendations
The Celestron EdgeHD 9.25 is offered with a number of mount options: for visual use, there’s the alt-azimuth fork-mounted CPC package, and for astrophotography, the EdgeHD 9.25 works great on mounts like the Sky-Watcher EQ6Ri Pro, Celestron CGX, Celestron CGX-L, or a Sky-Watcher CQ350 Pro. With a long focal length of 1645mm, even with an f/7 reducer, you need a beefy mount with top-notch tracking and guiding accuracy to work well.
Should I buy a Used Celestron EdgeHD 9.25?
A used Celestron EdgeHD 9.25 is not something that you’ll see often, but there’s no reason not to get one if there are any cost savings over a new purchase. Make sure that the mirrors, lenses, and front corrector plate are in good condition with no coating damage and you’re good to go.
Aftermarket Accessory Recommendations
Almost anyone with an EdgeHD 9.25 should plan on purchasing some kind of dew shield. A dew shield prevents condensation from forming on the front corrector plate on damp nights, which fogs up the image and can damage the Starbright XLT multi-coatings over time. It also cuts down on stray light, slightly improving visual contrast in a similar manner to a lens hood or shade on a camera lens. This is helpful if you deal with a lot of local light sources, light pollution, or light from the Moon. A basic plastic dew shield or even a homemade unit may be all you need, but heated plastic and metal dew shields are also available if you frequently observe or photograph in a very humid place.
If you’re considering doing visual observation with the EdgeHD 9.25, there are many options for eyepieces and accessories. The provided 2” star diagonal opens up a lot of possibilities. For low power, the Apertura 38mm SuperView offers 62x and a true field of 1.1 degrees across with the EdgeHD 9.25, a bit wider than the field offered by the provided 23mm Luminos and with a larger exit pupil more appropriate for deep-sky observing.
For medium to high magnifications, wide-angle oculars like the Baader Hyperions or Explore Scientific 82-degree eyepieces line may be worth considering. It’s a good idea to have at least three or four high-quality eyepieces to fully utilize the capabilities of the EdgeHD 9.25. An Orion UltraBlock 2” UHC filter or similar high-quality UHC nebula filter can also be helpful for viewing nebulae, even in dark sky locations, as it enhances the contrast between the target and the background sky.
For planetary imaging with the EdgeHD 9.25, you’ll want a high-speed, high-resolution astronomical CMOS video camera, such as the ZWO ASI224MC. Most good planetary cameras also double as guide cameras for deep-sky work.
For best results when planetary imaging, you’ll likely need to use a Barlow lens or focal extender to bring the EdgeHD 9.25 to an optimal focal length and f/ratio between f/20 and f/25. Some good options include the Apertura 2.5x Barlow lens or Tele-Vue 2.5x Powermate, either of which bring the scope to f/25 and 5875mm focal length. A 3x Barlow is probably pushing beyond what typical seeing conditions will allow, and a 2” format Barlow is needlessly heavy and expensive regardless of whether you plan on using it in conjunction with eyepieces too.
Those planning on using the EdgeHD 9.25 for deep-sky astrophotography with a separate guide scope should certainly invest in an additional dovetail rail to attach to the top of the main optical tube. Depending on the bracket you are using for a guide scope, the dovetail rail can be either CGE-style or Vixen-style, and will allow large guide scope brackets to attach directly to the EdgeHD 9.25 optical tube, keeping the center of gravity of the guide scope aligned with the mount and reducing the load on gears for more accurate tracking.
Another crucial astrophotography accessory for the EdgeHD 9.25, or really any telescope, is a Bahtinov focusing mask. These masks can be purchased or made at home. If you want to focus with even greater precision, a motor focuser such as the ZWO EAF, Celestron, or Pegasus units may be worth considering. Aftermarket Crayford focusers that attach to the back of the telescope are also available but may not be compatible with your needed back focus, spacing, and balance requirements.
Lastly, the EdgeHD 9.25 is able to be reduced to a focal ratio of f/7 for deep-sky imaging with a dedicated reducer which is practically required for most setups. Some astrophotographers may also want to consider the Starizona HyperStar conversion, which turns the EdgeHD 9.25 into an f/2 astrograph, but the HyperStar has limitations with regards to what cameras and filters can be used, and if you’re all in on f/2, a dedicated 8” RASA may be a simpler as well as an arguably more economical choice.
Astrophotography Capabilities
When paired with a good mount such as the Celestron CGX, CGX-L, or Sky-Watcher CQ350, as well as good polar alignment and autoguiding, the EdgeHD 9.25 is great for imaging smaller deep-sky objects like galaxies or globular clusters, though you’ll almost certainly want to reduce it to f/7 regardless of your mounting or target choice. At f/2 with a Starizona HyperStar, the EdgeHD 9.25 provides a similar focal length (470mm) to many small refractors but will give you more data and a better signal in a fraction of the exposure time – ideal for large star clusters and nebulae.
In addition, under good seeing conditions with a Barlow lens and a good CMOS planetary camera, the EdgeHD 9.25 makes for a great planetary imaging telescope, as with most larger Schmidt-Cassegrains. Compared to a C8 XLT or EdgeHD 8, the EdgeHD 9.25 is a little easier to focus and collimate accurately, which gives it more of an advantage on planets than such a small step up in aperture would suggest. However, there is little real-world difference between the EdgeHD 9.25 and a regular C9.25 XLT for planetary work, and a C11 XLT will blow away either with similar mounting requirements to the EdgeHD 9.25.
What can you see?
Compared to a C9.25 XLT, the EdgeHD 9.25 is pretty much identical in viewing quality, with the only differences being the EdgeHD’s slightly better edge-of-field correction with cheap eyepieces and slightly faster cooldown time thanks to its built-in vents. If you are looking for a strictly visual telescope, we strongly recommend considering the regular C9.25 XLT, a 10” Dobsonian, or a C11 XLT instead. However, the EdgeHD 9.25 is great if you are interested in both imaging and visual work and have the mount and storage space to support such.
Deep-sky views with the EdgeHD 9.25 are excellent, as is to be expected with any quality telescope of this size. Open star clusters that fit within the scope’s limited 1.1-degree or so maximum field of view, such as M35 or M11, look fantastic. However, larger clusters like the Pleiades (M45) or Beehive (M45) are simply too wide to fit in the scope’s field of view and are rather disappointing as a result. Globular star clusters reveal individual stars with the EdgeHD 9.25 at high power, as well as differences in shape, brightness, and size – though some are still out of reach at this aperture.
Galaxies require dark skies to be seen well with any telescope and appear as washed-out smudges under city skies. However, under good conditions, the EdgeHD 9.25 can show dust lanes and even spiral arms in quite a few galaxies, along with groups like the Leo Triplet and clusters like the Virgo Cluster. Emission nebulae such as Orion (M42), the Swan (M17) and the Lagoon (M8) show up even in light-polluted locales but are outright jaw-dropping under darker skies and/or with a UHC nebula filter. And there are plenty of colorful planetary nebulae to be seen with the EdgeHD 9.25 as well, brought out easily with a UHC filter at low power and revealing small details within on a steady night at high magnifications.
The EdgeHD 9.25 is also well-suited for observing planets. The phases of Venus and Mercury are easily visible, and the Moon looks amazing with myriad detail in its craters, mountain ranges, ridges, and smooth basaltic maria. Mars’ polar ice cap and a few dark markings are visible during the few months around opposition when it is closest to Earth and at its biggest and brightest. You can also see vivid cloud belts, storms, and the Great Red Spot on Jupiter, as well as the disks of all four Galilean moons – along with their shadows – when they transit in front of Jupiter. The rings of Saturn are magnificent, as is the razor-thin Cassini Division within and a handful of moons scattered around the planet. Saturn’s beige cloud bands and grayish poles are clearly resolved, and on a very steady night, the EdgeHD 9.25 can even bring out the Encke gap in the rings too. Uranus’ turquoise disk and a couple of moons can be spotted, Neptune is just barely resolved from a star along with its accompanying moon Triton, and Pluto is just barely within reach of the EdgeHD 9.25’s light-collecting power under dark skies if you can manage to find it.
