The AT90EDX and AT90CFT are triplet APO refractors with essentially perfect on-axis optics — guaranteed .95 Strehl or higher. But point a camera at them without correction and the field curves. Stars in the center are sharp. Stars at the edges are not. The scope didn't do anything wrong — a flat sensor and a curved focal plane just don't agree. That's where a field flattener earns its place in the imaging train.
The AT90FF is a dedicated 1× flattener for the AT90 triplet series. It doesn't reduce the focal length or change the focal ratio — you keep the full 540mm at f/6. What it does is flatten the field across the sensor so stars are round and sharp from center to edge, all the way to the corners of a full-frame chip. The AT90EDX delivers clean, color-free optics. The AT90FF delivers that quality across your entire sensor.
Dedicated — Not Generic
This flattener is optically matched to the AT90EDX and AT90CFT's specific focal length and field curvature. A generic 1× flattener designed for f/5–f/7 refractors in general will sort-of work, but it won't deliver the same edge-to-edge correction as a flattener that knows the exact optical system it's sitting behind. One reviewer put it simply: the images with the dedicated flattener were "superb across the entire field," compared to "okay" with a generic unit. The difference is the matching.
Connection and Installation
The scope side of the AT90FF has an M63 threaded connection that threads directly onto the AT90EDX or AT90CFT focuser drawtube — or onto the Camera Angle Adjuster if you're using the rotation feature for framing. No adapters between the flattener and the focuser. No barrel insert that can introduce tilt. Just threads — secure, repeatable, and square to the optical axis every time.
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. If you're using a ZWO camera with a ZWO filter wheel and off-axis guider, the ZWO system is designed so the combined stack hits 55mm automatically — just bolt the pieces together. A DSLR with a standard T-ring typically hits the spacing naturally. If you need to fine-tune, 48mm spacer rings in 1mm, 2mm, and 5mm increments let you dial it in precisely.
Inline Filter Thread
The top section of the flattener body unscrews from the main housing, exposing a 2" filter thread on the lower section. This lets you thread a 2" light pollution, UHC, or narrowband filter directly into the imaging train — inline, without a separate filter drawer or filter wheel adding back focus and potential tilt. If you're shooting from a light-polluted site with a one-shot color camera and a single broadband filter, this is a clean, simple way to get the filter into the path.
Features
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1× dedicated field flattener — no focal reduction. Maintains the AT90's native 540mm focal length and f/6 focal ratio. Flattens the field for round stars from center to edge without changing your 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. No vignetting on APS-C or full-frame chips. Stars are round in the corners, not just the center.
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Optically matched to the AT90EDX and AT90CFT. Designed for the specific focal length and field curvature of the 90mm f/6 triplet. Delivers better edge correction than generic flatteners because it's tuned to this optical system.
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M63 threaded scope-side connection. Threads directly to the AT90 focuser drawtube or Camera Angle Adjuster. No barrel insert, no adapter — repeatable, 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 and bolt together directly.
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2" inline filter thread. Unscrew the top section to expose a 2" filter thread. Thread in a light pollution or narrowband filter without adding a filter drawer to your imaging train.
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Fully multi-coated optics. Maximizes light transmission and minimizes internal reflections for cleaner images.
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Lightweight at 9.5 oz. Minimal impact on balance. Won't overload the focuser or shift your imaging train's center of gravity.
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Protective covers included. Slip-on rubber covers for both the 48mm T-thread end and the M63 scope-side threads.
Observing Tip
If you're switching from a DSLR to a dedicated astronomy camera, the back focus math changes even though the target is the same 55mm. A DSLR T-ring has a built-in flange distance that gets you close to 55mm with just the ring. A ZWO or QHY camera body is much shorter — which is why they're designed to stack with a filter wheel, off-axis guider, or spacer rings to make up the distance. Measure the sensor-to-flange distance of your camera (it's in the specs), subtract from 55mm, and fill the gap with spacers or accessories. Get this right once and mark your configuration — it stays right forever.
Frequently Asked Questions
What's the difference between this and the 0.8× reducer/flattener?
The AT90FF is a 1× flattener — it corrects field curvature without changing the focal length. You keep the full 540mm at f/6. The 0.8× reducer/flattener both flattens the field and reduces the focal length to 432mm at f/4.8 — faster exposures, wider field, but different image scale. If you want to preserve the native focal length and image scale of the AT90, use the 1× flattener. If you want faster, wider imaging, use the 0.8× reducer.
Does this cover a full-frame sensor?
Yes. The corrected image circle covers full-frame 36×24mm sensors. Owners have confirmed flat fields with no vignetting on the ASI2600MC Pro (APS-C sized sensor) and similar cameras, and the product description specifies full-frame coverage.
How does the M63 thread connection work?
The AT90EDX and AT90CFT focuser drawtube has M63 threads on its back end. You 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 from a loose fit.
Can I use a filter wheel with this?
Yes. A ZWO filter wheel plus off-axis guider plus camera stack is designed to hit 55mm back focus automatically. Thread the flattener onto the scope, attach the filter wheel to the 48mm T-threads, and the system is at the correct spacing. One CN owner runs an ASI294 + ZWO filter wheel + ZWO OAG on the same flattener without issues.
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 of the frame. M48 spacer rings in 1mm, 2mm, and 5mm increments let you dial it in. Measure once, mark your configuration, and it stays consistent.
Final Thoughts
You bought the AT90EDX or AT90CFT because the optics are exceptional. The AT90FF makes sure those optics translate to your sensor — all of your sensor, not just the center. It threads on, it stays square, it flattens the field from center to corners on a full-frame chip, and it doesn't change your focal length or focal ratio. If you're imaging with an AT90 and you want flat fields at the native 540mm f/6, this is the flattener that was designed for the job.
