Schmidt-CassegrainOptical tube onlyCelestron
A 279 mm f/10 aplanatic Schmidt-Cassegrain with a flat, corrected field — 2 800 mm of focal length aimed squarely at small galaxies, planetary nebulae and high-resolution planetary work.
A superb long-focal-length instrument that will expose every weakness in your mount, your seeing and your guiding before it shows you what it can do.
Sold as an optical tube only. No mount, tripod or head is included. For imaging in particular, the mount is usually the larger part of the total cost.
Where to buy
Links open at the retailer in a new tab. We do not publish prices, because a price we recorded last month is worse than no price at all.
The same four purposes are scored for every telescope on this site, using the same weights. A telescope that is excellent for one is often poor for another — that is the point.
Visual observing
6.4 / 10
Confidence: medium
Outstanding aperture and image quality, but the weight, cost and separate mount requirement make it a demanding visual choice.
| Factor | Score | Weight |
|---|---|---|
| Aperture | 7.7 | 30% |
| Optical qualityeditorial | 8.5 | 20% |
| Ergonomicseditorial | 5.5 | 15% |
| Portability | 5.1 | 15% |
| Mount usabilitySold without a mount — the mount is a separate purchase and a separate decision. | 2.0 | 10% |
| Valueeditorial | 6.0 | 10% |
Weighted total: 6.4. Read the full methodology.
Nebula imaging
4.1 / 10
Confidence: high
Even with the 0.7× reducer the system is slow and narrow-field; large nebulae simply do not fit.
| Factor | Score | Weight |
|---|---|---|
| Focal ratio | 1.0 | 25% |
| Corrected field | 7.5 | 20% |
| Image circle | 9.8 | 15% |
| Focal length fit | 0.0 | 15% |
| Mount demand | 0.0 | 10% |
| Imaging workfloweditorial | 6.0 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 4.1. Read the full methodology.
Galaxy imaging
6.4 / 10
Confidence: high
Aperture, focal length and a genuinely flat field are all in the right place; the demands it places on the mount are the only thing holding it back.
| Factor | Score | Weight |
|---|---|---|
| Focal length fit | 7.6 | 25% |
| Resolving power | 5.8 | 20% |
| Corrected field | 7.5 | 15% |
| Optical qualityeditorial | 8.5 | 15% |
| Mount demand | 0.0 | 10% |
| Imaging workfloweditorial | 6.0 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 6.4. Read the full methodology.
Planetary imaging
7.7 / 10
Confidence: high
Large aperture and 2 800 mm native focal length make this close to an ideal planetary instrument on nights that cooperate.
| Factor | Score | Weight |
|---|---|---|
| Aperture | 7.7 | 30% |
| Focal length fit | 10.0 | 25% |
| Optical qualityeditorial | 8.5 | 20% |
| Collimation & thermals | 4.0 | 10% |
| TrackingNo mount supplied — tracking quality depends entirely on the mount you pair it with. | 5.0 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 7.7. Read the full methodology.
Scores are relative to every telescope on this site, not absolute quality marks. A 6.5 for galaxy imaging and a 6.5 for visual observing do not mean the same thing. How scoring works.
The EdgeHD adds a field-flattening lens group inside the baffle tube of an otherwise conventional Schmidt-Cassegrain. The result is an aplanatic system: coma-free and, crucially for imaging, flat across a 42 mm image circle. A standard SCT of the same aperture produces visibly elongated stars in the corners of an APS-C frame; this does not.
Celestron also added mirror locks and tube vents. The locks stop the primary shifting when you change orientation — the single most irritating property of a moving-mirror focus system for imagers — and the vents shorten what would otherwise be a very long cooldown for a 279 mm mirror.
Very good, and rarely the reason anyone buys one. 279 mm shows globular clusters resolved across the core, structure in bright galaxies, and planetary detail limited by the atmosphere rather than the optics. The field is narrow, as it must be at 2 800 mm, and the practical obstacle is that you need a substantial mount before you can look through it at all.
Not the tool. At 2 800 mm, or 1 960 mm with the reducer, only the smallest planetary nebulae fit the frame, and f/10 or f/7 is slow for extended nebulosity. A small refractor will produce a better nebula image in a tenth of the time.
This is the EdgeHD’s purpose. 2 800 mm gives roughly 0.28 arcseconds per pixel with a 3.76 µm sensor — over-sampled for typical seeing, well matched on the best nights, and the flat field means the whole frame is usable rather than just the centre.
Be honest about the demands. Guiding must hold well under an arcsecond RMS, the mount must be rated far above the tube’s 12.7 kg, and flexure anywhere in the train shows immediately. Most owners run the 0.7× reducer at 1 960 mm and f/7, which is more forgiving and still delivers real image scale.
Excellent. 279 mm resolves to about 0.42 arcseconds, and the native 2 800 mm needs only modest amplification for a well-sampled planetary capture. On nights when seeing allows the full aperture to be used, this is among the best planetary instruments available in a transportable form.
12.7 kg of tube, 610 mm long and 310 mm across, on a mount that will itself weigh 20 to 30 kg. This is a set-up-and-leave instrument for most owners. Collimation is a secondary-screw adjustment done occasionally. Even with the vents, allow a good hour of cooling before high-resolution work, and fit a dew shield — a 279 mm corrector plate is a large surface to lose to dew.
| Aperture | 279.4 mm (11") |
|---|---|
| Focal length | 2800 mm (110.2") |
| Focal ratio | f/10 |
| Optical design | Aplanatic Schmidt-Cassegrain with integral field flattener |
| Type | Schmidt-Cassegrain |
| Glass / mirror | Schmidt corrector with StarBright XLT coatings; field-flattening lens group in the baffle tube |
| Central obstruction | 32% |
| Resolving power | 0.42"Calculated from aperture (Dawes limit), not a manufacturer figure. |
| Limiting magnitude | ≈ 14.2Estimated from aperture under dark skies. Real skies are usually worse. |
| Corrected image circle | 42 mm |
|---|---|
| Backfocus | 146 mm |
| Field correction built in | Yes |
| Usable visually | Yes |
| APS-C sensors | Supported |
| Full-frame sensors | Not fully covered |
| Reducer | Available (×0.7) |
| Corrector required | No |
| Autoguiding | Recommended |
| Mount included | No — optical tube only |
|---|---|
| Tracking | Depends on your mount |
| GoTo | Depends on your mount |
| Mounting interface | Losmandy-style dovetail |
| OTA weight | 12.7 kg (28 lb) |
|---|---|
| Tube length | 610 mm (24") |
| Tube diameter | 310 mm |
| Longest transport dimension | 700 mm (27.6") |
| Brand | Celestron |
|---|---|
| Model | EdgeHD 11 Schmidt-Cassegrain |
| Status | Current |
| Price band | FlagshipWe publish bands, not prices. Check the retailer for the current price. |
| Skill level | advanced |
Editorial notes
We list an accessory as included only when the product record says so explicitly. If a mount, eyepiece, camera, flattener, reducer, corrector, guider or adapter is not listed above, assume it is not in the box.
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