The AT115EDT has become one of the most popular imaging refractors we sell — 115mm of FK-61 triplet glass at 805mm f/7, enough aperture and focal length to frame mid-size deep-sky targets with real detail. But like any refractor at f/7, the field curves. Stars in the center are sharp. Stars at the edges stretch. That's not a flaw in the scope — it's what a curved focal plane does to a flat sensor. And it's exactly what a dedicated field flattener is designed to fix.
The AT115EDTFF is a 1× flattener built specifically for the AT115EDT. It doesn't reduce the focal length or change the focal ratio — you keep the full 805mm at f/7. What it does is flatten the field so stars are round and sharp from center to edge across your entire sensor, including full-frame chips. The AT115EDT delivers the light. The flattener makes sure that light lands correctly on every part of your sensor.
Why 1× Instead of a Reducer?
The AT115EDT has a strong following among imagers who value its 805mm focal length for the image scale it provides. At 805mm, mid-size targets — the Rosette Nebula, the Crab Nebula, galaxy pairs, the Veil complex — have enough scale to show structural detail without being too large to frame. A 0.8× reducer would drop that to 644mm and widen the field, which is great for larger targets but changes the image scale you bought the scope for. The 1× flattener preserves everything about the AT115EDT's native optical performance and just fixes the field curvature.
Connection and Installation
The scope side of the AT115EDTFF has an M63 threaded connection that threads directly onto the AT115EDT focuser drawtube — or onto the Camera Angle Adjuster if you're using the rotation feature for framing. No barrel insert. No adapter between the flattener and the focuser. Just threads — secure, repeatable, and square to the optical axis every session.
The camera side terminates in 48mm T-threads. DSLR and mirrorless bodies connect via a T-ring for your specific camera mount (sold separately). Dedicated CMOS and CCD astronomy cameras typically have 48mm T-threads built in and connect directly without a T-ring.
Back focus is 55mm from the flattener's shoulder to the sensor. ZWO filter wheel + OAG + camera stacks are designed for 55mm and bolt together directly. A DSLR with a standard T-ring typically hits the spacing naturally. Dedicated cameras with different flange distances may need 48mm spacer rings to fine-tune.
Inline Filter Thread
The top section of the flattener body unscrews, exposing a 2" filter thread on the lower section. Thread in a light pollution, UHC, or narrowband filter without a separate filter drawer or filter wheel. For one-shot color camera setups with a single broadband filter, this keeps the imaging train short and the back focus math simple.
Features
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1× dedicated field flattener — no focal reduction. Maintains the AT115EDT's native 805mm f/7. Flattens the field for round stars from center to edge without changing image scale or field of view.
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Full-frame sensor coverage. Corrects field curvature across full-frame (36×24mm) sensors and smaller. Round stars in the corners, not just the center.
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Optically matched to the AT115EDT. Designed for the AT115EDT's specific focal length and field curvature. Delivers better edge correction than generic flatteners because the optical correction is tuned to this scope.
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M63 threaded scope-side connection. Threads directly to the AT115EDT focuser drawtube or Camera Angle Adjuster. No barrel insert, no adapter — secure, tilt-free positioning every session.
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48mm T-thread camera connection. DSLR and mirrorless bodies connect via a T-ring (sold separately); dedicated CMOS/CCD astronomy cameras attach directly — 48mm T-threads are standard on most.
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55mm back focus. From the flattener shoulder to the sensor. Standard DSLR T-ring spacing typically hits this naturally. ZWO filter wheel + OAG + camera stacks are designed for 55mm.
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2" inline filter thread. Unscrew the top section to expose a 2" filter thread for inline filter use without a filter drawer.
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Fully multi-coated optics. Maximizes light transmission and minimizes internal reflections.
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Lightweight at 9.5 oz. Minimal impact on balance or focuser load.
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Protective covers included. Thread on covers for both ends.
Observing Tip
The AT115EDT at 805mm f/7 with the 1× flattener is a versatile imaging platform — long enough for galaxy detail and planetary nebula structure, wide enough for mid-size emission nebulae. If you're running a one-shot color camera with a dual-narrowband filter, thread the filter into the flattener's 2" filter thread and skip the filter drawer entirely. Fewer connections mean less chance of tilt, less back focus to calculate, and a stiffer imaging train. Simpler is usually better.
Frequently Asked Questions
What's the difference between this and a 0.8× reducer/flattener?
The AT115EDTFF is a 1× flattener — it corrects field curvature without changing focal length. You keep 805mm at f/7. A 0.8× reducer/flattener would both flatten the field and reduce to ~644mm at ~f/5.6 — faster exposures, wider field, but different image scale. Choose the 1× for full-resolution work on smaller targets. Choose the 0.8× for wider fields and shorter exposures.
Will this work with other AT scopes?
It's designed for the AT115EDT. The optical correction is matched to the AT115EDT's specific focal length and field curvature. It may work with other refractors that use a 2" focuser, but performance with other scopes is not guaranteed.
Does this cover a full-frame sensor?
Yes. The corrected image circle covers full-frame 36×24mm sensors with flat, round stars from center to edge. Owners have confirmed flat fields on APS-C sensors with filter wheel and OAG stacks.
How does the M63 thread connection work?
The AT115EDT focuser drawtube has M63 threads on its back end. Remove the 2" visual adapter and the flattener threads directly onto those exposed threads. If you're using the Camera Angle Adjuster, the flattener threads onto the adjuster instead. Either way, it's a direct threaded connection — no barrel, no adapter, no chance of tilt.
Can I use a filter wheel with this?
Yes. A ZWO filter wheel + OAG + camera stack is designed to hit 55mm back focus. Thread the flattener onto the scope, attach the filter wheel to the 48mm T-threads, and the system is at the correct spacing.
How critical is the 55mm back focus?
Very. If the spacing from the flattener's shoulder to your sensor is off by more than a couple of millimeters, you'll see elongated stars toward the edges. M48 spacer rings in 1mm, 2mm, and 5mm increments let you dial it in precisely.
Final Thoughts
The AT115EDT is already one of the best values in triplet imaging refractors. The AT115EDTFF turns it into a flat-field imaging system at the full 805mm f/7 — round stars from center to corners, full-frame coverage, and no compromise on focal length or image scale. If you're imaging with the AT115EDT and want flat fields without reducing, this is the flattener that was matched to your scope.
