The AT90CFT and AT90EDX are excellent visual scopes at 540mm f/6, but if you're imaging through either one, you already know the two things an f/6 refractor does to your photos: it curves the field and it makes you wait. Stars that are pin-sharp in the center turn into little comets near the corners. And at f/6, you need longer exposures to pull faint nebulosity out of the background. The AT90CFTRF fixes both problems in one piece of glass. It reduces the focal length to 432mm, speeding the system to f/4.8 — a 56% increase in photographic speed — and it flattens the field so stars stay round and tight all the way to the corners of your imaging chip.
Designed for Two Scopes
This reducer/flattener is not a universal fit. It's designed and optimized specifically for the Astro-Tech AT90CFT (carbon fiber tube) and AT90EDX (aluminum tube) 90mm f/6 triplet APO refractors. The optical correction is matched to the curvature profile of those two scopes, which is why the stars come out round across the field instead of trailing into elongated shapes at the edges. A generic 0.8x reducer can reduce focal length, but it won't correct the specific field curvature of your scope. This one does.
How It Connects
The scope side threads into the focuser or camera angle adjuster using M63×1 metric threads — a secure, repeatable connection that doesn't shift between sessions. No set screws, no compression ring, no wobble. The camera side has standard 48mm T-threads, connecting to a T-ring for DSLR and mirrorless bodies, or directly to dedicated CMOS and CCD astronomy cameras, which come with 48mm T-threads built in.
Back focus is 55mm from the shoulder of the reducer to the image plane. That's the standard DSLR spacing — meaning a T-ring and camera body will put you at the correct distance without spacers or adjustments. For dedicated astronomy cameras, you may need a short spacer to reach the same 55mm. Check your camera's back focus specification and add spacers if needed.
Built-In Filter Holder
The top cap of the reducer unthreads from the body, exposing a 48mm filter holder on the underside. This lets you place a 2" threaded filter — a light pollution filter, narrowband filter, or UV/IR cut — directly into the optical path without adding a separate filter drawer to your imaging train. One less piece of hardware between the scope and the sensor, and one less source of spacing error or tilt.
What's Included
- Astro-Tech AT90CFTRF 0.8x reducer/field flattener
- Slip-on rubber dust cover for 48mm T-threads (camera side)
- Slip-on dust cover for scope-side barrel
Features
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0.8x focal reduction — 540mm f/6 becomes 432mm f/4.8 — A 56% increase in photographic speed (light per pixel). Shorter exposures to reach the same signal, or deeper images in the same exposure time. Either way, your total imaging time per target drops significantly.
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Field flattening corrects edge-of-field curvature — Refractors curve the focal plane. Without correction, stars in the outer third of the field elongate into streaks or comets. The flattener brings the entire field into focus on a flat imaging sensor, producing round stars corner to corner.
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Fully multi-coated optics — All air-to-glass surfaces are multi-coated to minimize reflections and maintain contrast. Important in a reducer that sits in the optical path of every photon reaching your sensor.
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M63×1 threaded scope connection — Threads securely into the AT90CFT/AT90EDX focuser or camera angle adjuster. No compression ring, no play — a solid, repeatable mechanical connection that eliminates tilt between sessions.
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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 (standard DSLR spacing) — A T-ring and DSLR body reach focus without spacers. Dedicated astronomy cameras may need a short spacer depending on their native back focus distance. The product manual includes spacing diagrams.
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Integrated 48mm filter holder — Unthread the top cap to access a built-in 2" filter holder. Accepts standard 48mm threaded filters — narrowband, UHC, UV/IR cut — without adding a separate filter drawer to the imaging train.
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Lightweight (9 oz) — Adds minimal weight to your imaging train. Won't stress the focuser or shift balance on a light mount.
Observing Tip
Get the 55mm back focus right before you shoot a single frame. Measure from the shoulder of the reducer — the flat face where it meets the focuser — to your camera sensor. For a DSLR with a T-ring, 55mm is usually automatic. For a dedicated astronomy camera, add up the T-ring or adapter thickness, any spacers, and the camera's flange-to-sensor distance. If the total is less than 55mm, add spacers. If it's more, you'll need a shorter adapter or fewer spacers. Getting this measurement right is the single biggest factor in whether your corner stars are round or elongated. The product manual has a spacing diagram — use it.
FAQ
Q: Do I need a T-ring?
If you're using a DSLR or mirrorless camera, yes — you need a T-ring for your specific camera mount (sold separately). The reducer's camera side has 48mm T-threads that connect to the T-ring. If you're using a dedicated CMOS or CCD astronomy camera, no T-ring is needed — those cameras typically have 48mm T-threads built in and connect directly to the reducer.
Q: Can I use filters with this reducer?
Yes. Unthread the top cap of the reducer to expose a built-in 48mm filter holder on the underside. Standard 2" threaded filters (48mm) thread right in — narrowband, light pollution, UV/IR cut. The filter sits inside the reducer body, keeping your imaging train compact.
Q: What's the correct back focus distance?
55mm from the shoulder of the reducer (where it meets the focuser) to the image plane (your camera sensor). For a DSLR with a standard T-ring, this spacing is typically automatic. For a dedicated astronomy camera, you may need to add spacers to reach 55mm. Incorrect spacing is the most common cause of elongated corner stars — it's worth measuring carefully. The included product manual has a spacing diagram for common camera configurations.
Q: How much faster is f/4.8 compared to f/6?
About 56% faster in terms of light gathered per pixel per unit time. In practical terms, if you were shooting 5-minute exposures at f/6 to reach a certain signal level, you'd reach the same level in roughly 3 minutes and 12 seconds at f/4.8. Over a multi-hour session, that adds up to significantly more data or significantly less time on each target.
Q: Will this work for visual observing?
Technically yes — it will reduce magnification and widen the field, and the flattening will improve edge sharpness in wide-field eyepieces. But it's designed and optimized for photographic use with a flat sensor, not for visual observing through an eyepiece. Most AT90CFT/AT90EDX owners use the reducer strictly for imaging and observe without it.
Optional Accessories
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48mm T-Ring (for your camera mount) — Required for DSLR and mirrorless cameras. Available for Canon EF, Nikon F, Sony E, and other mounts. The T-ring threads onto the reducer's 48mm camera-side connection and adapts it to your camera body. Not needed for dedicated CMOS/CCD astronomy cameras.
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48mm Threaded Filters — Light pollution, narrowband (H-alpha, O-III, S-II), or UV/IR cut filters thread directly into the reducer's built-in filter holder. No separate filter drawer required.
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Spacer Rings (if using a dedicated astronomy camera) — Needed to achieve the 55mm back focus distance with cameras that have a shorter native back focus than a DSLR. Available in various thicknesses (e.g., 11mm, 16.5mm, 21mm). Check your camera's specs and add spacers to make up the difference.
Final Thoughts
If you own an AT90CFT or AT90EDX and you're imaging with it, this is not an optional accessory — it's the other half of the imaging system. The scope gives you the light and the color correction. The reducer gives you the speed and the flat field. Stars round to the corners, exposures cut by more than a third, and the whole thing threads together without adapters or guesswork. Dedicated, matched, done.
