Sky-Watcher Esprit 100EDX Field Flattener

Product Overview

The Esprit 100EDX Flattener is Sky-Watcher's dedicated field corrector for the Esprit 100EDX super apochromatic refractor. A fast triplet objective focuses onto a gently curved surface; a camera sensor is flat. The flattener resolves that mismatch, so stars hold their shape out to the frame edges instead of drawing short radial streaks in the corners while the centre of the frame stays sharp.

Sky-Watcher publishes a 44 mm corrected image circle for this corrector. That encloses the 43.3 mm diagonal of a full-frame sensor and covers APS-C with margin to spare, so framing choices are driven by focal length rather than by how much of the sensor the corrector reaches. On the Esprit 100EDX — published by Sky-Watcher as 100 mm aperture, 550 mm focal length, f/5.5, FPL-53 triplet — that is a wide, fast field with corners that hold up.

One name to keep straight: this part is published for the Esprit 100EDX, the super-apochromatic triplet. The Evostar 100EDX doublet is served by a different corrector (S20216, which also covers the Esprit 70EDX).

Who It's For

This is the corrector Sky-Watcher pairs with the Esprit 100EDX, and it is the right part if you own that telescope and want flat, round stars across a large sensor at the scope's native focal length and ratio.

It suits deep-sky imagers working at the 100EDX's native 550 mm — large nebulae, open clusters, mid-size galaxy fields and mosaic panels — where the goal is a clean frame edge to edge rather than a shorter, faster field from a reducing corrector.

Key Features & Design

• Matched to the Esprit 100EDX: Sky-Watcher publishes compatibility with the Esprit 100EDX specifically. The correction is computed for that objective rather than sold as a universal flattener.
• 44 mm corrected image circle: the published figure covers a full-frame sensor's 43.3 mm diagonal and leaves APS-C well inside the corrected zone.
• Flattening only: Sky-Watcher publishes no reduction factor for this corrector, so the telescope's native 550 mm and f/5.5 carry through it.
• Single-piece corrector: one optic in the imaging train between focuser and camera, with no separate elements to align.

Optical / Mechanical Design

What it corrects: field curvature across the imaging field of the Esprit 100EDX. The visible symptom it removes is corner stars that elongate radially, pointing away from the frame centre, while the middle of the frame focuses cleanly.

Corrected image circle: 44 mm, as published by Sky-Watcher. Sensor coverage follows directly from that number — a full-frame sensor measures 43.3 mm corner to corner and an APS-C sensor roughly 28 mm, so both sit inside the corrected field.

Resulting focal length and focal ratio: Sky-Watcher publishes no reduction factor and no altered figures, consistent with a flattener that works at native focal length. The Esprit 100EDX's published 550 mm and f/5.5 are what you image at with this part in the train. At 550 mm across a 44 mm circle, a full-frame sensor takes in roughly three and a half degrees on the long axis, which is why this pairing suits large targets and mosaic work.

Required backfocus and thread interface: Sky-Watcher does not publish a backfocus distance in millimetres or a thread size for this corrector on its product page, and no manual or datasheet is linked there. Those two numbers determine how the corrector spaces to your sensor, and corner star shape is what depends on getting the spacing right. Send us your camera, filter drawer or filter wheel and any adapters and we will lay the train out with you before you order.

Recommended Uses

• Deep-sky astrophotography at native focal length: large emission nebulae, open clusters, mid-size galaxy fields and star fields at 550 mm.
• Large-sensor imaging: the 44 mm circle supports full-frame and APS-C cameras, including cooled astronomical CMOS bodies in those formats.
• Mosaic panels: flat corners let adjacent panels blend without a ring of distorted stars at every seam.
• Narrowband and broadband alike: field curvature is a geometric error, so the correction applies regardless of the filter in front of the sensor.

Compatibility and Accessory Notes

Sky-Watcher lists one compatible telescope: the Esprit 100EDX. Their EDX correctors are published tube by tube — S20216 covers the Esprit 70EDX and the Evostar 100EDX, S20218 the Esprit 120EDX, S20219 the Esprit 150EDX — and refractors from other makers pair with their own. Tell us the exact model printed on your tube and we will confirm which corrector belongs on it.

The vendor does not publish the thread interface or the spacing distance to the sensor, so the adapters between this corrector and your camera cannot be specified from published data. Send us your camera model, filter wheel or drawer, and any off-axis guider in the train, and we will map the imaging train and tell you exactly what sits between the two.

Nothing about this part changes the mount side of the setup: it adds a short optic and roughly a pound behind the focuser rather than changing how the telescope mounts.

Good to Know Before You Order

• Backfocus is not published. Sky-Watcher gives no spacing figure in millimetres for this corrector. Corner star shape depends on hitting the correct sensor distance, and we can work that out with you from your specific camera and adapters.
• Thread interface is not published. No thread size appears on the vendor page, so the camera-side connection is confirmed case by case rather than assumed.
• Esprit 100EDX, not Evostar 100EDX. The two share a 100 mm aperture and a similar name but take different correctors; the Evostar doublet is covered by S20216.
• It is a flattener, not a reducer. No reduction factor is published, so focal length and focal ratio stay at the telescope's native values.
• Box contents are not published. The vendor states no spacer, adapter or case as supplied with the corrector.
• Weight figure is the vendor's shipping weight. Sky-Watcher lists 1.0 lb for this item and publishes no separate optic-only mass in a specification table.
• Vendor image note. Sky-Watcher states "Images subject to change" on this listing, which is normal for a recently released part.

Frequently Asked Questions

Is it difficult to set up?

Physically it is one optic that goes into the imaging train between the focuser and the camera, and it stays there. The part that takes attention is the spacing to the sensor, which Sky-Watcher does not publish for this corrector. Send us the camera and adapters you plan to use and we will lay out the train for you.

Does it change the focal length or focal ratio?

Sky-Watcher publishes no reduction factor, resulting focal length, or resulting focal ratio for this corrector. It flattens the field at the telescope's native 550 mm and f/5.5.

Will it cover my full-frame camera?

The published corrected image circle is 44 mm and a full-frame sensor measures 43.3 mm across the diagonal, so the sensor sits inside the corrected field. APS-C sits well inside it.

I have an Evostar 100EDX — is this the one?

No. The Evostar 100EDX doublet is published against S20216, the corrector shared with the Esprit 70EDX. This part is published for the Esprit 100EDX triplet. Send us the model printed on your tube and we will match it.

Does it fit the earlier Esprit 100ED?

Sky-Watcher publishes compatibility only with the Esprit 100EDX for this part. Earlier Esprit models have their own correctors in the range. Send us the model on your tube and we will confirm which one applies.

What weight does it add to the imaging train?

Sky-Watcher lists 1.0 lb for this item. It is a small addition behind the focuser next to the camera, filters and any guider.

Bottom Line

A dedicated flattener for the Esprit 100EDX with a published 44 mm corrected circle — enough to keep stars round across a full-frame sensor at the telescope's native 550 mm and f/5.5. Sky-Watcher leaves the backfocus and thread figures unpublished, so we confirm the spacing with you against your own camera and adapters and send it out ready to drop into the train.