Sky-Watcher 0.77x Reducer/Flattener for Esprit 150ED

Product Overview

The Esprit 150ED Reducer/Flattener is Sky-Watcher's 0.77x corrector for the Esprit 150ED. It flattens the curved focal surface the triplet objective produces, so stars stay round out to the corners, and shortens the system at the same time. Sky-Watcher publishes the change as a drop from native f/7 (1050mm) to f/5.4 (810mm).

On the largest tube in the Esprit line, that shift matters most in time. Sky-Watcher publishes a 68% reduction in exposure time compared with native f/7 — a 1050 mm f/7 system is a demanding place to collect photons, and f/5.4 turns the same target into a materially shorter project.

The frame widens with the reduction. Sky-Watcher publishes a 2.54 x 1.7-degree field of view on a full-frame camera and 1.66 x 1.11 degrees on APS-C, backed by a 36 mm fully illuminated field the vendor states is compatible with both APS-C and full frame sensors.

This is the reducing corrector for the Esprit 150ED. The Esprit 150EDX Flattener (S20219) is a separate part that flattens at native focal length and publishes no reduction.

Who It's For

This is for Esprit 150ED owners who want the tube's aperture without its native 1050 mm framing — large emission nebulae, extended cluster fields, bright galaxy groups, anything that crowds the edges at f/7. At 810 mm and f/5.4 the same telescope covers 2.54 x 1.7 degrees on a full-frame sensor.

It also fits anyone whose clear nights are the scarce resource. The published 68% exposure reduction against native f/7 is the difference between finishing a narrowband target over a season and finishing it over a few weeks.

If maximum image scale on small targets is the aim instead, a flattener working at the native 1050 mm is the part for that.

Key Features & Design

• 0.77x reduction, published end to end: Sky-Watcher states the result as f/7 (1050mm) dropping to f/5.4 (810mm) on the Esprit 150ED, so both working figures come from the vendor.
• Reducer and flattener in one optic: curvature correction and focal reduction in a single unit, adding one element to the imaging train.
• 36 mm fully illuminated field: Sky-Watcher's published figure, stated as compatible with both APS-C and full frame sensors.
• Two backfocus configurations: 90 mm back-focus as supplied, or 55 mm with the back focus spacer and M48 ring installed — 55 mm being the spacing most cooled astro cameras are built around.
• Built-in 2" filter cell: a 2-inch filter mounts into the light path inside the corrector rather than in a separate drawer.
• Complete in the box: Sky-Watcher lists the reducer, metal thread-on caps, back focus spacer and M48 thread adapter as included.

Optical / Mechanical Design

What it corrects: field curvature across the Esprit 150ED imaging field, combined with a 0.77x focal reduction. Uncorrected, the corners of a large sensor show stars stretched radially while the frame centre is sharp; this optic flattens that surface onto the sensor plane and shortens the system in the same pass.

Resulting focal length and focal ratio: Sky-Watcher publishes the change as native f/7 (1050mm) to f/5.4 (810mm), stated for the Esprit 150ED specifically.

Corrected image circle: 36 mm fully illuminated, which the vendor states is compatible with both APS-C and full frame sensors. An APS-C sensor's roughly 28 mm diagonal sits inside the fully illuminated circle; a full-frame sensor spans 43.3 mm, so its extreme corners fall outside the 36 mm figure in the region Sky-Watcher covers with the full-frame compatibility statement. The published fields follow — 2.54 x 1.7 degrees full frame, 1.66 x 1.11 degrees APS-C.

Required backfocus: 90 mm as supplied, or 55 mm with the back focus spacer and M48 ring installed. 55 mm is the distance most cooled astronomical cameras and filter wheels are dimensioned to, so that is where most imaging trains settle.

Thread interface: M48, via the included M48 thread adapter that pairs with the 55 mm back focus configuration.

Filter mounting: the built-in 2" filter cell holds a 2-inch filter in the light path without adding a drawer or extra spacers to the train.

Recommended Uses

• Wide-field deep-sky astrophotography: large emission and reflection nebulae, extended cluster fields and galaxy groups that run past the frame at 1050 mm.
• Narrowband imaging: the faster f/5.4 ratio offsets the transmission cost of narrow filters, and the built-in 2" cell carries the filter in the light path.
• Limited clear-sky sites: the published 68% exposure reduction against native f/7 puts more finished signal into each usable night.
• Full-frame and APS-C imaging: the 36 mm fully illuminated field is published as compatible with both, at 2.54 x 1.7 and 1.66 x 1.11 degrees respectively.

Compatibility and Accessory Notes

Sky-Watcher publishes this corrector for the Esprit 150ED, and the f/7 (1050mm) to f/5.4 (810mm) figures are stated for that telescope. Other Esprit models pair with their own correctors — including the separate Esprit 150EDX Flattener — and refractors from other makers have theirs. Send us the exact model printed on your tube and we will confirm which corrector belongs on it.

The box covers most of the spacing work: reducer, metal thread-on caps, back focus spacer and M48 thread adapter are all included. With the spacer and M48 adapter fitted, the corrector presents 55 mm of back focus, which is what a typical cooled camera plus filter wheel is built to consume. As supplied without them, it presents 90 mm, and that distance is filled by whatever sits between corrector and sensor.

Send us your camera, filter wheel or drawer, off-axis guider and adapters and we will add the imaging train up with you and confirm which of the two configurations your setup wants. Filters thread into the built-in 2" cell; cameras with an internal filter wheel simply leave that cell empty.

Good to Know Before You Order

• The published figures are for the Esprit 150ED. 1050mm to 810mm and f/7 to f/5.4 are stated for that telescope. Other refractors reduce by their own numbers with their own correctors.
• Backfocus sets corner star shape. The corrector presents 90 mm as supplied and 55 mm with the back focus spacer and M48 ring installed; the rest of the train is built to whichever figure you use.
• 36 mm is the fully illuminated field. Sky-Watcher states compatibility with both APS-C and full frame sensors, with a full-frame sensor's outer corners lying beyond the 36 mm fully illuminated circle.
• Filter cell takes 2-inch filters. The built-in cell is sized for 2" filters; other filter sizes go elsewhere in the train.
• This is the reducing corrector, not the flattener. The Esprit 150EDX Flattener is a separate part working at native focal length.
• Vendor publishes no barcode for this SKU. The UPC we carry is 050234202152.
• Vendor lists 2.0 lb for this item. That is the published weight; no separate optic-only mass appears in a specification table.

Frequently Asked Questions

Is it difficult to set up?

The reducer threads into the imaging train once and stays there, and the parts that set the spacing — the back focus spacer and the M48 thread adapter — come in the box. The one decision is which backfocus you build to, 90 mm or 55 mm, and then adding camera, filter wheel and adapters up to that figure. Send us your components and we will do that arithmetic with you.

What focal length and focal ratio do I end up with?

Sky-Watcher publishes the Esprit 150ED as dropping from native f/7 (1050mm) to f/5.4 (810mm) with this reducer/flattener fitted.

How much time does it save?

Sky-Watcher publishes a 68% reduction in exposure time compared with native f/7.

Will it cover a full-frame sensor?

Sky-Watcher publishes a 36 mm fully illuminated field described as compatible with both APS-C and full frame sensors, quoting 2.54 x 1.7 degrees on a full-frame camera and 1.66 x 1.11 degrees on APS-C.

What comes in the box?

Sky-Watcher lists the reducer, metal thread-on caps, back focus spacer and M48 thread adapter.

Can I use filters with it?

Yes — a built-in 2" filter cell mounts filters into the light path, so a 2-inch filter needs no separate drawer.

Bottom Line

Sky-Watcher's own 0.77x corrector for the Esprit 150ED: f/7 (1050mm) becomes f/5.4 (810mm), exposure time drops by a published 68%, the frame opens to 2.54 x 1.7 degrees on full frame, and the back focus spacer, M48 adapter and thread-on caps are in the box. Tell us what camera and filters you run and we will confirm the 55 mm or 90 mm spacing with you before it ships.