Celestron C5–C14 and EdgeHD: reducers, back focus and as-designed focal lengths

Two reducer systems that don’t cross over

The classic reducer/corrector (#94175) shortens focal length and focal ratio by 37%, taking f/10 to f/6.3 and f/11 to f/7. Its compatibility list names the C5 through C14 and adds “No EdgeHD tubes”. Celestron does not print the reduced focal lengths, so the tables here compute them: 1280 mm for the C8, 1764 mm for the C11 and 2463 mm for the C14 (0.63 × f/11 = f/6.93, which Celestron rounds to f/7).

Each EdgeHD has its own 0.7x lens. The EdgeHD 800 reducer is a four-element design optimized for APS-C sensors, with 105 mm of back focus. The EdgeHD 1100 and 1400 units have five elements and keep the native 146 mm, so the same T-adapter works with or without them. Computed working focal lengths are 1422 mm (EdgeHD 8), 1645 mm (9.25), 1960 mm (11) and 2737 mm (14).

One number doesn’t add up: Celestron calls the EdgeHD 14 f/7 with its reducer, but 0.7 × f/11 is f/7.7. The FOV table uses 2737 mm.

The EdgeHD manual says the 0.7x reducer makes the angular field 43% larger and halves exposure times. Both follow from the factor: 1 ÷ 0.7 = 1.43, and 0.7² = 0.49. The classic reducer’s 0.63 gives (0.63)² = 0.40, so an f/6.3 exposure needs about 40% of the f/10 time for the same per-pixel signal on a nebula, at a scale 1.59 times coarser.

Back focus is a spec on EdgeHD, a range on classic SCTs

The manual explains the difference. On classic SCTs the focus knob moves the primary mirror, and focusing at different distances behind the tube changes the effective focal length. EdgeHD optics are optimized for one distance: 133.35 mm (5.25 in) behind the baffle-tube lock nut on the EdgeHD 800, and 146.05 mm (5.75 in) on the 925, 1100 and 1400, with the sensor within ±0.5 mm. Celestron publishes no back focus or image circle for the classic tubes, and the spec table above shows that gap.

Holding a camera, filter wheel and off-axis guider to ±0.5 mm is a spacing exercise worth planning on paper; the back focus guide walks through the arithmetic.

As-designed focal lengths

The manual’s imaging table lists the focal length and ratio each EdgeHD reaches at its specified back focus, and none matches the product page exactly:

Tube	Product page	Manual
EdgeHD 8	2032 mm, f/10	2125 mm, f/10.456
EdgeHD 9.25	2350 mm, f/10	2321 mm, f/9.878
EdgeHD 11	2800 mm, f/10	2788 mm, f/9.978
EdgeHD 14	3910 mm, f/11	3857 mm, f/10.846

The EdgeHD 8 gap is the largest at 4.6%. With an ASI2600MC Pro, 2032 mm gives 39.7′ × 26.6′ at 0.38″ per pixel, while 2125 mm gives 38.0′ × 25.4′ at 0.36″. This site keeps the product-page values in its tables; a plate solve of your own frames shows which one your spacing produces.

The manual also says the EdgeHD 800 covers a 42 mm circle but is optimized for the central 28 mm, with relative illumination falling to 84% at the corners of an APS-C frame. The ASI2600MC Pro’s imaging area has a 28.26 mm diagonal, right at that edge.

The C5 numbers

Celestron sells no bare C5 tube, so the C5 row comes from the C5 spotting scope page: 127 mm, 1250 mm, f/10, and a 2.72 kg weight for the whole package. The reducer page lists the C5 as the optic inside the NexStar SE, and the NexStar 5SE page gives slightly different figures: a 125 mm aperture and a 51 mm secondary. This site keeps the C5-branded values.

At these focal lengths small sensors are the natural match. An ASI533MC Pro behind a reduced C8 covers 30.4′ square at 0.61″ per pixel, room for small galaxies such as the Whirlpool.