Large camera sensors make field curvature easy to see. A conventional refractor can produce a sharply focused star in the center of the image while stars farther toward the edge fall slightly outside that same focal plane. The telescope is focusing onto a gently curved surface, while your camera sensor is flat.
The Sky-Watcher S20216 is the matched 1.0x field flattener for the Esprit 70EDX and Evostar 100EDX. Its job is straightforward: correct that field curvature across a 44mm image circle while preserving the telescope's native focal length and focal ratio.
That last part is important. This is a 1.0x flattener, not a focal reducer. On the Esprit 70EDX, the telescope remains 455mm at f/6.5. On the Evostar 100EDX, it remains 700mm at f/7. You get the framing and image scale you chose the telescope for, but with a field designed for large-sensor astrophotography.
A standard 36 × 24mm full-frame sensor has a diagonal of approximately 43.3mm, so Sky-Watcher's 44mm image-circle specification is intended to cover full-frame cameras such as those using the Sony IMX455. APS-C and smaller sensors fall comfortably inside that field.
Features
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1.0x field flattener. Corrects field curvature without reducing the telescope's native focal length or focal ratio.
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44mm image circle. Sized to cover the approximately 43.3mm diagonal of a full-frame 36 × 24mm sensor, while also working naturally with APS-C and smaller formats.
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Matched to the Esprit 70EDX. Sky-Watcher designed the S20216 for the 70mm FCD-100 Esprit EDX triplet.
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Also matched to the Evostar 100EDX. The same flattener is officially compatible with Sky-Watcher's 100mm FPL-53 Evostar EDX doublet.
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Preserves 455mm f/6.5 on the Esprit 70EDX. Because the flattener is 1.0x, focal length, focal ratio, image scale, and exposure speed remain essentially unchanged.
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Preserves 700mm f/7 on the Evostar 100EDX. The flattener corrects the field without turning the Evostar into a shorter or faster optical system.
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Designed for large-sensor astrophotography. The advantage becomes increasingly important as sensor size grows and the camera records farther from the center of the telescope's native focal surface.
A Flattener, Not a Reducer
Field flatteners and focal reducers are often combined into the same accessory, which makes it easy to assume that every flattener shortens the telescope's focal length.
This one does not.
Sky-Watcher currently specifies the Esprit 70EDX / Evostar 100EDX Flattener as a 1.0x optical corrector. Its purpose is to improve the shape and focus of stars toward the outside of the image while leaving the telescope's basic imaging geometry alone.
On the Esprit 70EDX, you stay at 455mm and f/6.5. On the Evostar 100EDX, you stay at 700mm and f/7.
There is no reduction in focal length, no wider field created by the flattener itself, and no change in exposure speed from a reduced focal ratio.
What changes is the usable field.
Why a Field Flattener Matters
A camera sensor cannot refocus different parts of the image independently. It is one flat surface sitting at one focus position.
If the telescope's natural focal surface is curved, you can focus the center perfectly while the outer field remains slightly out of focus. Refocus for the corners and the center moves away from best focus instead.
A matched field flattener changes that geometry so the telescope's focal surface better matches the flat sensor.
The larger the sensor, the farther its corners reach off axis and the more demanding the correction becomes. That is why a flattener may seem unnecessary with a small camera while becoming much more important when you move to APS-C or full frame.
Full-Frame Coverage
Sky-Watcher specifies a 44mm image circle for the S20216. A 36 × 24mm full-frame sensor has a diagonal of approximately 43.3mm, putting the corners of a full-frame chip within the stated field.
That makes the flattener particularly relevant to cameras based on sensors such as the Sony IMX455, where you are paying for a large field and naturally want to use the corners rather than crop them away.
APS-C cameras, including models based on the IMX571, occupy a considerably smaller portion of the corrected field and are an equally natural match.
What It Does on the Esprit 70EDX
The current U.S. Esprit 70EDX is a 70mm f/6.5 triplet with a 455mm focal length. Because this is a 1.0x flattener, those numbers do not change when it is installed.
With a full-frame camera, 455mm provides a field of approximately 4.5° × 3.0°. That is wide enough for many of the large subjects that make a small refractor so enjoyable for astrophotography: the Andromeda Galaxy and its companions, the North America and Pelican region, large sections of the Veil, and expansive emission-nebula fields along the Milky Way.
An APS-C camera tightens the field to roughly 3° × 2° while remaining wide enough for targets such as the Pleiades, Rosette Nebula, and many larger nebula complexes.
The flattener does not make those fields wider. It helps make more of the field you already have usable.
What It Does on the Evostar 100EDX
The Evostar 100EDX is a 100mm f/7 doublet with a 700mm focal length. With the S20216 installed, it remains 700mm at f/7.
That gives the Evostar a distinctly tighter field than the 70EDX while still taking advantage of the same 44mm image-circle specification.
On a full-frame sensor, the field is approximately 2.9° × 2.0°. That's a useful size for subjects such as the Pleiades, Rosette Nebula, M81 and M82 together, larger galaxy groups, and medium-size emission and reflection nebulae.
The same accessory therefore serves two rather different imaging systems: a compact 455mm wide-field triplet and a 700mm 100mm-aperture doublet.
Compatibility Matters
This is not being sold as a universal refractor flattener.
Sky-Watcher USA currently lists the S20216 for exactly two telescopes:
- Sky-Watcher Esprit 70EDX
- Sky-Watcher Evostar 100EDX
Do not confuse the Evostar 100EDX with the Esprit 100EDX. The Esprit 100EDX uses its own dedicated field flattener.
Likewise, we would not assume that a flattener intended for an older Esprit ED or another Sky-Watcher refractor is interchangeable simply because the aperture or thread appears similar. Optical matching matters just as much as mechanical fit.
Camera Spacing and Adapters
Correct sensor spacing remains important with a field flattener. The distance from the rear optical surface of the flattener to the camera sensor affects how well the correction holds toward the corners of a large sensor.
Sky-Watcher USA's current S20216 product listing does not publish a production back-focus dimension or the complete camera-side thread specification, so we are not going to guess at those numbers.
If you need help building the imaging train for your camera, filter wheel, filter drawer, or off-axis guider, contact us and we can confirm the current Sky-Watcher specification and work out the correct adapter stack before you order unnecessary spacers.
Setting Up the Imaging Train
Once you have the correct published sensor spacing, test the system on a dense star field with stars reaching all four corners of the sensor.
Focus carefully near the center, take a short exposure, and compare all four corners. If the residual corner pattern is reasonably symmetrical, sensor spacing is one of the things to investigate. If one side or one corner looks substantially different from the others, tilt or mechanical squareness is more likely to be involved.
Make spacing changes in small increments and refocus after each change. Don't diagnose the system from one corner alone, and don't assume a particular direction of star elongation always means the camera is too close or too far away; optical systems differ.
Frequently Asked Questions
Is the Esprit 70EDX / Evostar 100EDX flattener also a reducer?
No. Sky-Watcher USA currently specifies the S20216 as a 1.0x field flattener. It corrects field curvature without intentionally changing focal length or focal ratio.
What focal length does the Esprit 70EDX have with this flattener?
455mm. The current U.S. Esprit 70EDX is 455mm at f/6.5, and the 1.0x flattener preserves those specifications.
What focal length does the Evostar 100EDX have with it?
700mm. The Evostar 100EDX remains f/7 because this is a 1.0x flattener rather than a focal reducer.
Will it cover a full-frame camera?
Sky-Watcher specifies a 44mm image circle. A standard 36 × 24mm full-frame sensor has a diagonal of approximately 43.3mm, so full-frame format falls within the stated image circle.
Will it work with an IMX455 camera?
The IMX455 is a 36 × 24mm full-frame sensor with an approximately 43.3mm diagonal, so its format falls within Sky-Watcher's stated 44mm image circle. As with any large sensor, correct spacing and a mechanically square imaging train become increasingly important toward the corners.
Will it work with an IMX571 or other APS-C camera?
Yes. APS-C is considerably smaller than the stated 44mm image circle.
Does this flattener work with the Evostar 100EDX?
Yes. Sky-Watcher USA specifically lists the S20216 as compatible with both the Esprit 70EDX and Evostar 100EDX.
Does it fit the Esprit 100EDX?
No compatibility is listed for the Esprit 100EDX. Sky-Watcher sells a separate dedicated flattener for that telescope.
Can I use it on an older Esprit ED?
Sky-Watcher USA currently lists the S20216 only for the Esprit 70EDX and Evostar 100EDX. We would not assume compatibility with an older Esprit ED or another refractor unless Sky-Watcher specifically confirms it.
What back focus does it require?
Sky-Watcher USA's current S20216 listing does not publish that dimension. Rather than give you a number copied from a different regional or pre-production configuration, contact us and we will confirm the current production specification for your imaging train.
Do I need a field flattener for visual observing?
This accessory is designed primarily for astrophotography with flat camera sensors. Its main benefit is correcting the imaging field toward the edges and corners of larger sensors.
Final Thoughts
The S20216 has one job: let the Esprit 70EDX and Evostar 100EDX make better use of a large camera sensor without changing the telescope's native focal length.
On the Esprit 70EDX, you keep the 455mm f/6.5 wide-field system you bought. On the Evostar 100EDX, you keep the 700mm f/7 image scale of the larger doublet. In both cases, the flattener is designed around a 44mm image circle that reaches the corners of full-frame format.
It is not a focal reducer, it does not make either telescope faster, and it is not intended as a universal corrector for every refractor with a compatible-looking thread.
If you're imaging with APS-C or full frame and want to use the field your camera is capable of recording rather than giving up the corners to field curvature, this is the matched Sky-Watcher flattener for the Esprit 70EDX and Evostar 100EDX.
