There's a specific kind of frustration that comes from imaging with a refractor and reviewing your frames the next morning. The core of M33 is exactly right — color balanced, detailed, pinpoint sharp in the center. But the stars at the edges are soft, stretched, pulling away from round. Your scope didn't fail you. Your focal plane did.
The AT115EDT is a triplet apo, and like all triplet refractors, it produces a curved focal plane. The better your seeing and the more you push your sensor to the edges, the more you'll notice it. The AT115EDTFFv3 corrects it — one piece of equipment, threaded directly into the M92 drawtube of your 3.2" focuser — and your 805mm f/7 system flattens out. Stars stay round from center to corner. Focal length stays exactly where you set it.
That last part matters. The 0.8× reducer/flattener for the same scope corrects to 43.8mm of image circle and brings your system to 644mm f/5.6. If you want more speed and a wider field, that's the right tool. But if 805mm f/7 is where you want to be — if that's the plate scale you planned for, the focal length you chose for your targets — the 1× flattener is the piece that lets you stay there. As the manufacturer specifies, the corrected image circle reaches 60mm. A full-frame sensor diagonal is 43.3mm. Your corners are covered, and then some.
Features
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1× magnification — 805mm f/7 preserved. The flattener corrects field curvature without altering focal length or f/ratio. The system you planned is the system you image with.
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M92 male telescope connection. Threads directly into the AT115EDT's 3.2" focuser drawtube — no adapters, no extension tubes, no guesswork.
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60mm corrected image circle (manufacturer-specified). Full-frame sensors fit well within the corrected zone. Coverage extends to medium-format for larger imaging systems using the M78 connection.
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Three camera-side thread options. M78×1 female, M54×0.75 male, and M48×0.75 male. T-ring users connect at M48. Dedicated astronomy camera users and larger-format setups step up from there.
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Internal 2" filter thread. Unscrew the upper cell to reveal a standard 2" filter thread. Keeps your imaging train short, eliminates a filter drawer, and puts a 2" narrowband filter exactly where it belongs — permanently installed and out of the way.
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55mm back focus. Industry-standard spacing from flattener shoulder to sensor. Compatible with standard T-rings, spacer sets, and most astronomy camera adapters without modification.
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Fully multi-coated optics. Anti-reflection coatings on all air-to-glass surfaces. High contrast, minimal ghosting, no light scatter from uncritical surfaces.
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Thread-on end caps included. Both scope-side and camera-side caps protect your optics in storage and transport.
Under the Night Sky
At 805mm focal length, the AT115EDT lives in a productive zone — long enough to show genuine galaxy structure, wide enough to take in large emission nebulae whole. With a flat field, you start to see what this scope was built to do.
Point it at the Orion Nebula (M42). At 805mm on a full-frame sensor, you get the full core of the Orion Molecular Cloud — the bright inner nebula, the fainter outer skirt, the Running Man (NGC 1977) tucked into the upper corner of the frame. Without a flattener, the stars at those corners drag and soften. With the AT115EDTFFv3 in the train, they stay tight. The Trapezium at center is pinpoint; the field stars at the edges are pinpoint too.
For galaxy season, the Bode/Cigar pair — M81 and M82 — fit in a single full-frame field at this focal length, comfortably framed with room for the surrounding star field. M82's irregular dust structure and M81's spiral arms are both in play at 805mm. The flattener means you lose nothing to corner softness on a target this important.
M33 (Triangulum Galaxy) is a classic test of a flat field: it spans nearly the full short axis of a full-frame sensor at 805mm, and its spiral arms reach out toward the corners of the frame. Any field curvature shows up as a soft outer ring. With the AT115EDTFFv3, that ring disappears. The outer HII regions in the spiral arms — NGC 604 among them — resolve cleanly across the full field.
Observing Tip
The internal 2" filter thread is more practical than it might look in the specs. Thread a 2" dual narrowband filter into the upper cell and leave it there. The filter is protected, your imaging train stays as short as it was, and you have one fewer connection to re-seat when you're imaging after midnight and temperature has dropped. For Ha/OIII work on emission nebulae — the Rosette, the Lagoon, the Veil — the internal filter position keeps flexure out of the equation.
Frequently Asked Questions
Why choose the 1× flattener instead of the 0.8× reducer/flattener?
If you want to preserve your native focal length and f/ratio, the 1× flattener is the right tool. It corrects field curvature without compressing the image. The 0.8× reducer/flattener shortens your focal length from 805mm to 644mm and speeds up your system to f/5.6 — useful if you want a wider field or faster exposures. The choice comes down to which focal length serves your targets. If 805mm is where you want to be, don't give it up.
Does this work with the AT115EDT's 3.2" focuser?
Yes — the AT115EDTFFv3 is designed specifically for the AT115EDT's 3.2" focuser. The M92 male connection threads directly into the drawtube. No adapter rings required.
Does the 1× flattener cover full-frame sensors?
Yes. The corrected image circle is 60mm as the manufacturer specifies — a full-frame sensor diagonal is approximately 43.3mm. Your sensor falls well within the corrected zone, with margin beyond full-frame for future-proofing.
Can I use this with a DSLR or mirrorless camera?
Yes. Connect your DSLR or mirrorless camera using a standard 48mm T-ring (M48×0.75 thread, sold separately) at the camera-side connection. Dedicated astronomy CMOS cameras with 48mm T-thread connect directly — no additional T-ring needed. Cameras with 2" nose pieces can also work with the appropriate adapter.
What is the required back focus distance?
55mm from the shoulder of the flattener to your imaging sensor. This is the industry-standard spacing and is compatible with most T-rings, standard spacer sets, and astronomy camera adapters without modification. If you're using a dedicated camera, verify your adapter stack achieves this distance.
Can I use 2" filters with this flattener?
Yes — the upper cell of the AT115EDTFFv3 unscrews to reveal an internal 2" filter thread. Any standard 2" filter threads in directly. This eliminates the need for an external filter drawer and keeps your imaging train compact.
Final Thoughts
A lot of astrophotographers choose the AT115EDT because it's the scope that doesn't require compromises — enough aperture to image seriously, enough portability to set up regularly, enough focal length to frame galaxies with real detail. The AT115EDTFFv3 is the piece that removes the one constraint the physics of a triplet imposes. Field curvature isn't a defect in the AT115EDT — it's inherent to the design of every triplet refractor. The flattener corrects it cleanly, and your 805mm f/7 system delivers what you expected when you bought it: flat, sharp stars corner to corner, on every target you choose.
