Sky-Watcher Evostar ED, EDX and Evolux doublets: reducers and working focal lengths

Discontinued ED, current EDX

Sky-Watcher USA now titles every Evostar ED tube page “(Discontinued)”, while the EDX models replacing them are listed on the global site and, in the US, sold as limited editions. Optically, most pairs carry over. The 80EDX keeps 600 mm at f/7.5, the 120EDX keeps 900 mm at f/7.5, and the 150EDX matches the old 150DX at 1200 mm and f/8.

The 100 mm model is the exception. Sky-Watcher’s 100EDX page calls it an optical redesign that took the focal ratio from f/9 to f/7, so focal length drops from 900 mm to 700 mm. With an APS-C sensor such as the ASI2600MC Pro (23.49 × 15.70 mm imaging area), the field grows from 1.50° × 1.00° to 1.92° × 1.29°, and image scale coarsens from 0.86″ to 1.11″ per pixel.

The Evolux 82ED (530 mm, f/6.45) survives only as a used-equipment listing on Sky-Watcher USA, so its status is unknown.

Reducer focal lengths: published versus computed

Every tube here has a matched reducer/flattener, but the factor alone does not always reproduce the accessory page.

Tube	Factor	Published with reducer	Native × factor
Evostar 72ED	0.85x	355 mm, f/4.93	357 mm, f/4.93
Evostar 100ED	0.85x	f/7.56, no focal length	765 mm, f/7.65
Evostar 150DX	0.77x	930 mm, f/6.2	924 mm, f/6.16
Evolux 82ED	0.9x	477 mm, f/5.8	477 mm, f/5.81
Evostar 100EDX	0.92x	644 mm, f/6.44	644 mm, f/6.44
Evostar 150EDX	0.77x	not published	924 mm, f/6.16

A 2–6 mm gap does not change framing. The 100ED line deserves a second look, though: 0.85 × f/9 is f/7.65, not the f/7.56 printed on the reducer page. The tables on this page use the published focal length where one exists and native × factor otherwise. The reducer calculator does the same arithmetic for other factors.

What the exposure percentages mean

Focal ratio is focal length divided by aperture. A reducer keeps the aperture and shortens the focal length, so light from an extended object such as a nebula lands on a smaller image. With the same camera, each pixel fills faster, in proportion to 1/N². The object sends no more total light than before; it is spread over fewer pixels at a coarser scale.

equivalent exposure ratio = (N₁ ÷ N₀)²

Sky-Watcher’s accessory pages state savings as “reduces exposure time by” a percentage, and the numbers match the faster per-pixel rate rather than a shorter time. The 72ED reducer claims 38%: (5.8 ÷ 4.93)² = 1.38, a 38% higher rate, which means exposures 72% as long. The 150DX reducer claims 66%: (8 ÷ 6.2)² = 1.66, or 60% of the native time. The Evolux 0.9x reducer claims 24%: (6.45 ÷ 5.8)² = 1.24, or 81% of the native time.

The 100EDX flattener: 0.92x or 1.0x

One accessory serves both the Evostar 100EDX and the Esprit 70EDX, and Sky-Watcher describes it two ways. The global 100EDX page calls it a 0.92x focal reducer/field flattener with a 40 mm image height. Sky-Watcher USA’s accessory page calls it a “1.0X Flattener” with a 44 mm image circle, while the spec table on the US 100EDX page lists 0.92x, 644 mm and f/6.44 with a 43 mm illuminated field.

This site uses 0.92x. If 1.0x were right, an APS-C frame would span 1.92° × 1.29° instead of 2.09° × 1.40°, about 9% less in each axis. A plate solve of your own image decides it: focal length = 206.265 × pixel size (µm) ÷ measured scale (″/px), the inverse of the image scale formula.

Coverage and spacing

Sky-Watcher quotes coverage only with a reducer fitted, and the two sites differ by a few millimeters: 30 mm (global) versus 43 mm (US) for the 80EDX, 40 versus 43 mm for the 100EDX, 44 versus 43 mm for the 120EDX. The 150DX reducer is rated for a 34 mm fully illuminated field and the Evolux reducer for 36 mm. All of them cover an APS-C diagonal of about 28.3 mm; only the 44 mm figures reach the 43.3 mm diagonal of a full-frame sensor.

The 0.85x and Evolux reducers want 55 mm of back focus. The 150DX unit has 89 mm, or 55 mm once its spacer and M48 ring are installed. Sky-Watcher’s three-element refractors are covered on the Esprit page.