A corrector plate and two mirrors fold the light three times
The primary mirror is spherical, which is cheap to make but blurs the image. Bernhard Schmidt's thin corrector plate, a lens with a subtle curve, cancels that error at the front of the tube. A convex secondary mirror on the back of the corrector multiplies the focal length about five times and sends the light back through a hole in the primary. Focusing moves the primary mirror itself, which lets the focus range accommodate cameras and diagonals but can shift the image slightly.
Celestron's EdgeHD adds lenses in the baffle tube to flatten the field and remove coma for imaging. Meade, the other long-time maker, closed in 2024.
Long focal length and a large obstruction: high power, narrow fields
An f/10 SCT gives high magnification with ordinary eyepieces, ideal for planets, small galaxies and planetary nebulae. The trade-off is field: even with a 2-inch diagonal an 8-inch shows only about 1.3°, too small for the Pleiades or the whole Andromeda Galaxy. The large secondary lowers contrast slightly compared with an unobstructed telescope of the same aperture.
| Telescope | Aperture | Focal length | Ratio | Obstruction | Resolves | Widest field | Max power |
|---|---|---|---|---|---|---|---|
| StarSense Explorer DX 5-inch | 127 mm | 1,250 mm | f/9.8 | 37% | 0.9" | 1.2° | 254x |
| C6 / NexStar 6SE | 150 mm | 1,500 mm | f/10.0 | 36% | 0.8" | 1.0° | 300x |
| C8 / NexStar 8SE | 203 mm | 2,032 mm | f/10.0 | 34% | 0.6" | 1.3° | 350x |
| C9.25 | 235 mm | 2,350 mm | f/10.0 | 36% | 0.5" | 1.1° | 350x |
| C11 | 279 mm | 2,800 mm | f/10.0 | 34% | 0.4" | 0.9° | 350x |
Obstruction figures are approximate, as a share of the aperture's diameter. Widest true field uses the largest eyepiece the focuser takes: a 46 mm field stop for 2-inch focusers, 27 mm for 1.25-inch. Highest useful power is 2x per millimeter of aperture, capped at 350x by the air.
Current Schmidt-Cassegrains from Celestron
Celestron, owned since 2005 by Taiwan's Synta group, makes almost every SCT now sold.
| Model | Maker | Aperture | Ratio | US price | Notes |
|---|---|---|---|---|---|
| StarSense Explorer DX 5-inch | Celestron | 127 mm | f/10 | $650 | Phone-guided; manual |
| NexStar 5SE | Celestron | 127 mm | f/10 | $899 to $1,029 | Go-to |
| NexStar 6SE | Celestron | 150 mm | f/10 | $1,039 to $1,199 | Go-to |
| C8 OTA | Celestron | 203 mm | f/10 | $1,400 | Tube only |
| NexStar 8SE | Celestron | 203 mm | f/10 | $1,499 to $1,699 | Go-to; the best-known 8-inch |
| NexStar Evolution 8 | Celestron | 203 mm | f/10 | $1,999 to $2,399 | Built-in battery and Wi-Fi |
| C9.25 OTA | Celestron | 235 mm | f/10 | $2,000 | Tube only; prized for planets |
| EdgeHD 925 OTA | Celestron | 235 mm | f/10 | $3,099 | Flat field for imaging |
| C11 OTA | Celestron | 279 mm | f/10 | $3,300 | Tube only |
| EdgeHD 1100 OTA | Celestron | 279 mm | f/10 | $4,499 | Flat field for imaging |
Prices are from US dealers in November 2026; tariffs on Chinese-made optics raised most prices in 2025.
Best for planets, small targets and observers short on space
An SCT is the most aperture you can carry in one trip and point with go-to: a NexStar 8SE reaches all 110 Messier objects from a dark site and 95 from the suburbs, the same as an 8-inch Dobsonian, from a package that fits a car trunk. It is a strong planetary telescope once cooled, which takes 45 to 60 minutes for an 8-inch. Limits: narrow fields, dew on the corrector (use a dew shield), and an f/10 focal ratio that makes deep-sky imaging slow without a 0.63x reducer or HyperStar lens.