The AT150EDL gives you 1200mm at f/8, and for a lot of imagers that's exactly the focal length they want — long enough to pull in galaxies and planetary nebulae at a real scale, without crossing into the territory where guiding turns fussy. The one thing a doublet can't do on its own is hold the field perfectly flat to the edge of a camera sensor. The center of the image snaps into perfect focus, but the corners never quite get there at the same time. The AT150EDLFF fixes that and leaves everything else alone: the focal length and f/ratio don't change, so it stays a 1200mm f/8 scope. What changes is the field — round, sharp stars from the center out to the corners of a full-frame chip.
Built for the AT150EDL
This flattener is designed specifically for the AT150EDL — it isn't a universal accessory. A generic flattener can thread onto the back of almost anything, but it won't correct the field of your scope the way one built for it does. This one will. It may physically thread onto another refractor with an M92 focuser, but flat, corrected corners are only guaranteed on the AT150EDL.
How It Connects
The scope side threads into the M92 drawtube of the AT150EDL's 3.2" rack-and-pinion focuser — M92×1, a solid metal-to-metal connection with no compression ring to slip and nothing to shim. The camera side gives you two threads to choose from, M48×0.75 or M54×0.75, so you can match your gear without a stack of step rings. Dedicated CMOS and CCD astronomy cameras thread on directly through whichever matches their back plate. DSLR and mirrorless bodies connect through a T-ring for your specific mount (sold separately) on the M48 side.
Back focus is 55mm from the shoulder of the flattener to your sensor — standard spacing that most T-ring-and-camera and dedicated-camera setups reach without custom parts. Hold that 55mm. It's the distance that keeps the corners corrected, so confirm your camera's sensor-to-flange depth and add spacers if you need them to land on the number.
What's Included
- Astro-Tech AT150EDLFF 1× field flattener
- Thread-on dust cap, scope side
- Thread-on dust cap, camera side
Features
-
1× magnification — your focal length stays put. 1200mm f/8 stays 1200mm f/8. You picked that focal length for your targets; the flattener doesn't trade it away. It only makes the field flat.
-
Flat field across a full-frame sensor. Corrects the doublet's native field curvature so stars stay round and sharp into the corners of a full-frame chip — not just the center crop.
-
M92 scope-side connection. Threads straight into the 3.2" focuser drawtube. No adapters, no shimming, no shift between sessions.
-
Dual camera-side threads (M48 / M54). Two direct options cover most imaging trains without extra hardware. Dedicated cameras attach directly; DSLR and mirrorless bodies connect via a T-ring (sold separately) on the M48 side.
-
Fully multi-coated optics. Helps preserve contrast while reducing internal reflections and ghosting.
-
55mm back focus. Standard spacing from flattener shoulder to sensor, compatible with most camera-and-adapter combinations without custom spacers.
-
Includes thread-on dust caps. One for each end, to protect the coated surfaces between sessions.
What You'll Frame
At the full 1200mm and f/8, a full-frame sensor gives you roughly a 1.7° × 1.1° field — tight enough to lay a galaxy across the frame, wide enough that you're not framing blind. M81 and M82 sit together with the dust lanes in one and the starburst core in the other. The Leo Triplet frames as a group. M51 has room for its spiral arms and the bridge to its companion. On the nebula side, Planetary Nebulae M27 and M57 show real structure at this scale, and globulars like M13 and M5 resolve cleanly with a flat field behind them. This is a focal length for detail on a single object rather than wide swaths of sky — and the flattener is what makes the whole frame usable, not just the middle.
Imaging Tip
For first light, shoot a 30-second frame of a rich star field and look at the corners at 100%. Round, tight stars mean your 55mm is right. If the corner stars stretch radially — pointing away from center — your sensor is too far from the flattener; pull a spacer. If they stretch tangentially — smeared sideways — you're too close; add a thin one. The 55mm is measured from the shoulder of the flattener, not the rear element, so reference your stack from the right point before you start chasing it with shims.
Frequently Asked Questions
Should I get the 1× flattener or the 0.8× reducer?
It comes down to what you want from the scope. The 0.8× reducer (AT150EDLRF) shortens the focal length to 960mm and speeds the system to f/6.4 — a wider field and shorter exposures, good for larger targets. The 1× flattener keeps the full 1200mm f/8 and simply flattens the field. If you like your current focal length and want sharp corners, get the flattener. If you want a faster, wider system, get the reducer. Some imagers keep both and switch by target.
Does the flattener cover a full-frame sensor?
Yes. It corrects the field across a full-frame chip, so stars stay round and sharp into the corners — not just within an APS-C crop. APS-C and smaller sensors sit comfortably inside the corrected field.
Do I need a T-ring to connect my camera?
It depends on the camera. Dedicated CMOS and CCD astronomy cameras (ZWO, QHY, Player One, and similar) usually have an M54 or M48 back plate and thread on directly — no T-ring needed. DSLR and mirrorless cameras connect via a T-ring for your specific mount (sold separately) on the M48 side.
What is the correct back focus, and how do I get it right?
55mm from the shoulder of the flattener to your sensor. Most dedicated-camera back plates and T-ring combinations land close to this on their own. If you add a filter wheel or off-axis guider between the flattener and camera, count its depth so the total still comes to 55mm. A millimeter or two shows up at the corners, so it's worth measuring once and noting your spacer stack.
Can I use this flattener visually?
No. The AT150EDLFF is for prime-focus astrophotography. It won't reach focus with an eyepiece — for visual observing, use the AT150EDL as it comes.
Can I add a filter?
Yes — you can thread a 2" (48mm) filter into the camera-side train between the flattener and the sensor. A filter shifts the optical back focus by about a third of its glass thickness, so account for it when you set your spacing to 55mm.
Final Thoughts
If the focal length of the AT150EDL already matches the way you like to image, the AT150EDLFF is the natural companion. It preserves everything that makes the telescope enjoyable while correcting the one thing a flat camera sensor notices immediately. Thread it in, set the spacing once, and enjoy a sharp, flat field every time you image.
