You've got 115mm of ED triplet glass pointed at the sky. The AT115EDT's optics deliver clean color correction — three FK-61 elements working together the way they should. But point a camera at the focal plane and the triplet shows you what it doesn't correct natively: field curvature. Center stars are tight and round. Corner stars are not. They stretch, they soften, they remind you that a refractor designed for visual use doesn't automatically produce flat-field images. The ATREDT15V2 fixes that. It flattens the field and reduces the focal length at the same time — converting 805mm f/7 into 644mm f/5.6 with sharp, corrected stars across your sensor.
That focal reduction matters more than the number suggests. At f/5.6, your exposures run approximately 36% shorter than at f/7 for the same signal. On a narrowband target at 5-minute subs, that's real time saved — or, looked at another way, significantly more signal per frame. The wider field of view means objects that were tight fits at 805mm now have room to breathe. The Rosette Nebula, the Orion Nebula, the Veil Nebula — they all frame more comfortably at 644mm than they do at native focal length.
The ATREDT15V2 threads into the focuser drawtube or camera angle adjuster of the AT115EDT via M63×1 threads. This is a secure, rigid connection — no barrel slip, no adapter stack flexing under your camera's weight. The camera side uses 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 reducer shoulder to the image plane, which is the standard spacing for most DSLR T-ring combinations and dedicated camera setups.
Fully multi-coated with broadband anti-reflection coatings on all optical surfaces. Slip-on rubber dust covers are included for both ends — scope side and camera side — to protect the coated glass between imaging sessions.
Important: Which Reducer for Which Focuser
The AT115EDT has been produced with different focuser versions over its production run. This reducer — the ATREDT15V2 — is designed for the 2.5-inch focuser with M63×1 drawtube threads. If your AT115EDT has the newer 3.2-inch focuser with M92 drawtube threads, you would want to consider the AT115EDTRFv3 instead. However, this model will work the the 3.2" focuser, it just won't produce as large an image circle.
What's Included
- Astro-Tech ATREDT15V2 0.8× reducer/field flattener
- Slip-on rubber dust cover (scope side)
- Slip-on rubber dust cover (camera side)
Features
-
0.8× focal reduction — Converts the AT115EDT from 805mm f/7 to 644mm f/5.6. Approximately 36% shorter exposures for the same signal, and a meaningfully wider field of view for framing large deep-sky objects.
-
Field flattener integrated — Corrects the native field curvature of the AT115EDT triplet. Stars stay sharp and round from center to corner across your imaging sensor. No separate flattener needed.
-
M63×1 scope-side connection — Threads directly into the AT115EDT's 2.5-inch focuser drawtube or camera angle adjuster. Rigid, secure, no barrel slip or flexure under camera weight.
-
48mm T-threads on camera side — DSLR and mirrorless bodies connect via a T-ring for your specific mount (sold separately). Dedicated CMOS and CCD astronomy cameras attach directly — 48mm T-threads are standard on most.
-
55mm back focus — Standard spacing from reducer shoulder to image plane. Most DSLR T-ring combinations and dedicated camera setups reach this distance naturally. If your setup differs, shim with T-thread spacers to hit 55mm precisely.
-
Fully multi-coated optics — Broadband anti-reflection coatings on all surfaces. Contrast is preserved and internal reflections are controlled — important for narrowband work where stray light shows up fast.
-
Dust covers included — Slip-on rubber covers for both ends. Protect the coated surfaces when the reducer isn't on the scope.
-
Compact, lightweight — Adds minimal weight and length to your imaging train. The AT115EDT's balance point barely shifts.
Observing Tip
Getting the back focus right is the single most important thing you can do with this reducer. At 55mm, the field correction is optimized — sharp stars to the corners. Too short or too long, and you'll see elongated stars at the edges that no amount of post-processing will fix. Measure from the reducer's shoulder (the flat face where it meets the T-ring or camera adapter) to your sensor. A set of T-thread spacers (AT4220 and AT4230) lets you dial in the exact distance. It's worth spending ten minutes with a ruler before your first imaging session — the results on screen will tell you immediately whether you got it right.
FAQ
Do I need a T-ring to connect my camera?
It depends on your camera. Dedicated CMOS and CCD astronomy cameras (ZWO, QHY, Player One, and similar) typically have 48mm T-threads built in and connect directly to the ATREDT15V2 — no T-ring needed. DSLR and mirrorless cameras (Canon, Nikon, Sony, and others) connect via a T-ring for your specific lens mount. The T-ring is sold separately.
Can I use filters with this reducer?
The ATREDT15V2 does have an internal filter threads like the V3 version. Just unthread the knurled ring on the top of the reducer to expose the fitler threads on the underside of it.
What happens if my back focus is off?
You'll see elongated or distorted stars at the edges of your images — the field correction is optimized for 55mm. Too short and stars stretch one way; too long and they stretch the other. Use T-thread spacers to fine-tune the distance. Measure from the reducer shoulder to your sensor before your first session.
Can I use this with other telescopes?
It may work with other f/7 refractors that have a 2.5-inch focuser with M63×1 threads, but we can only confirm performance with the AT115EDT. The optical design is matched to the AT115EDT's focal length and field curvature. Using it with other scopes may produce usable results, but we can't guarantee corner correction.
How does this compare to the 1× field flattener?
The 1× flattener (AT115EDTFF) corrects field curvature without changing the focal length — you stay at 805mm f/7. The 0.8× reducer (this product) both flattens the field and shortens the focal length to 644mm f/5.6. Use the 1× flattener when you want maximum image scale and don't need faster exposures. Use the 0.8× reducer when you want a wider field and shorter exposures. Most AT115EDT imagers end up using the reducer more often.
Final Thoughts
If you're imaging with an AT115EDT that has the 2.5-inch focuser, the ATREDT15V2 is the reducer that was designed for it. The optical correction is matched to the AT115EDT's triplet, the M63 threading gives you a rigid connection without adapter stacks, and 644mm at f/5.6 opens up the wide-field deep-sky targets that are cramped at native focal length. It does two things — flattens the field and speeds up the system — and it does both well. Get the back focus right at 55mm, and the corners of your frames will tell you the rest.
