Sky-Watcher Esprit 70EDX / Evostar 100EDX Field Flattener

Product Overview

This is Sky-Watcher's dedicated field corrector for two telescopes: the Esprit 70EDX and the Evostar 100EDX. A refractor objective brings light to focus on 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 stays sharp.

Sky-Watcher publishes it as a 1.0X flattener with a 44 mm corrected image circle. The 1.0X figure means it corrects without reducing: focal length and focal ratio stay at the telescope's native values. The 44 mm circle encloses the 43.3 mm diagonal of a full-frame sensor and covers APS-C with margin to spare, so framing decisions come down to focal length rather than to how much of the sensor the corrector actually reaches.

The dual fitment is worth reading carefully, because Sky-Watcher's naming runs close together here. This part is published for the Evostar 100EDX — the ED doublet. The Esprit 100EDX, the super-apochromatic triplet, has its own corrector (S20217).

Who It's For

This is the corrector Sky-Watcher pairs with the Esprit 70EDX and the Evostar 100EDX, and it is the right part if you own either telescope and want flat, round stars across a large sensor at the scope's native focal length and ratio.

On the Esprit 70EDX — published by Sky-Watcher as 70 mm aperture, 455 mm focal length, f/6.5 — that means very wide fields with corners that hold up: large emission nebulae, star fields, comets, mosaics. On the Evostar 100EDX doublet it means a longer-focal-length field with the same corner correction across a full-frame sensor.

Key Features & Design

• Matched to two telescopes: Sky-Watcher publishes compatibility with the Esprit 70EDX and the Evostar 100EDX. The correction is computed for those objectives rather than sold as a universal flattener.
• 1.0X flattener: no reduction. The telescope's native focal length and focal ratio carry through the corrector unchanged.
• 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.
• Single-piece corrector: one optic in the imaging train between focuser and camera, with no separate elements to align.
• One part for a triplet and a doublet: Sky-Watcher publishes the same corrector against the 70 mm Esprit triplet and the 100 mm Evostar doublet, so a two-telescope setup built from those models needs one flattener rather than two.

Optical / Mechanical Design

What it corrects: field curvature across the imaging field of the Esprit 70EDX and the Evostar 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.

Magnification factor: 1.0X. Sky-Watcher publishes no reduction, so the Esprit 70EDX continues to image at its published 455 mm and f/6.5 with this part in the train, and the Evostar 100EDX at its own native figures.

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

• Wide-field deep-sky astrophotography: on the Esprit 70EDX at 455 mm, large emission nebulae, open clusters, comets and star fields fill the frame with corners that stay clean.
• Longer-focal-length imaging on the Evostar 100EDX: the same corner correction applied to the doublet's field.
• 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 two compatible telescopes for this corrector: the Esprit 70EDX and the Evostar 100EDX. The name similarity across their range is the thing to watch — the Esprit 100EDX triplet takes S20217, the Esprit 120EDX takes S20218 and the Esprit 150EDX takes S20219, each published against its own tube. Tell us the exact model printed on your telescope 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.
• Evostar 100EDX, not Esprit 100EDX. The 100 mm telescope named for this part is the Evostar doublet. The Esprit 100EDX triplet is served by S20217.
• The 44 mm figure is published for these two telescopes. Other refractors pair with their own correctors, and the figure is not carried across to them.
• It is a flattener, not a reducer. Sky-Watcher publishes 1.0X, so focal length and focal ratio stay at each 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.

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 fit both telescopes, or do I need two?

Sky-Watcher publishes this one corrector against both the Esprit 70EDX and the Evostar 100EDX. If you own both, this is the single part for the pair.

Does it change the focal length or focal ratio?

No. It is published as a 1.0X flattener, so the Esprit 70EDX images at its native 455 mm and f/6.5 and the Evostar 100EDX at its own native figures.

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 Esprit 100EDX — is this the one?

The 100 mm telescope Sky-Watcher names here is the Evostar 100EDX doublet. For the Esprit 100EDX triplet the published corrector is S20217. Send us the model printed on your tube and we will match it.

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 1.0X flattener published for both the Esprit 70EDX and the Evostar 100EDX, with a 44 mm corrected circle — enough to keep stars round across a full-frame sensor at each telescope's native focal length. 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.