Sky-Watcher Esprit ED and EDX triplets: flatteners, 0.77x reducers and two 70EDX figures

Flattener in the box, or not

Both generations share four optical formats: 80 mm at 400 mm (f/5), 100 mm at 550 mm (f/5.5), 120 mm at 840 mm (f/7) and 150 mm at 1050 mm (f/7). The EDX range adds a 70 mm tube.

The difference that matters at checkout is the flattener. Sky-Watcher USA’s Esprit ED pages, now marked discontinued, list a field flattener among the included accessories and say the two-element corrector “guarantees a flat field across the entire imaging plane”. The global 120EDX page and its siblings describe the matched flattener as optional, and Sky-Watcher USA sells one per tube, such as the 120EDX flattener. If you are pricing an EDX for imaging, add it.

Sky-Watcher gives no reduction factor for these flatteners, so this site keeps the native focal length with them. Their rated circle is 44 mm on the global pages and accessory pages; the spec tables on the US tube pages, for example the 120EDX, list 43 mm. The Esprit 80EDX flattener is rated for a 32 mm image height, enough for an APS-C diagonal of 28.3 mm but not a 43.3 mm full-frame one.

The 0.77x reducers on the 120 and 150

Two reducers turn the long Esprits into mid-length astrographs, and Sky-Watcher publishes the result for both. The Esprit 120 unit takes 840 mm f/7 to 647 mm f/5.4; the Esprit 150 unit takes 1050 mm to 810 mm, also f/5.4. The global 120EDX and 150EDX pages list the same 0.77x option. With an ASI2600MC Pro, the 120’s field grows from 1.60° × 1.07° to 2.08° × 1.39°, and the 150’s from 1.28° × 0.86° to 1.66° × 1.11°.

The rated fully illuminated fields are 34 mm (120) and 36 mm (150). An APS-C sensor fits inside both. A full-frame sensor reaches past them; on the 120 page Sky-Watcher says the reducer “can handle full frame as well”, which is a weaker promise than full illumination.

The reducers specify 95 mm (120) and 90 mm (150) of back focus, or 55 mm once the supplied spacer and M48 ring are fitted.

What f/5.4 buys on the same aperture

The reducer pages advertise a 68% cut in imaging time. The arithmetic behind it is (7 ÷ 5.4)² = 1.68, the factor by which signal per pixel per second rises on an extended object; the matching exposure is about 0.60 of the f/7 time.

That gain comes from image scale, not from extra light. The 150 mm objective collects the same photons from a galaxy at f/7 or f/5.4. The reducer packs them into pixels that each cover 0.96″ instead of 0.74″, so each pixel fills faster and fine detail is sampled more coarsely. Whether 0.74″/px is too fine for your sky is a sampling question.

Esprit 70EDX: 490 or 455 mm

skywatcher.com and the spec table on the US 70EDX page both give 490 mm at f/7. The bullet list on that same US page says 455 mm at f/6.5. This site uses 490 mm.

The gap matters for framing: with an ASI2600MC Pro, 490 mm gives 2.75° × 1.84°, while 455 mm would give 2.96° × 1.98°. One possible reading is that 455 mm describes the tube with its reducer: 490 × 0.92 = 451 mm. Sky-Watcher does not say so, and its own description of that accessory is split, 0.92x on the global page versus “1.0X Flattener” on the US accessory page. The FOV table below shows a 451 mm row computed from the 0.92x factor; treat it as provisional until a plate solve confirms your focal length.

The ED 150 has a smaller split of the same kind: its US bullet says 1050 mm and its spec table 1040 mm. The reducer page’s “native f/7 (1050mm)” supports the first.