Astro-Tech AT115EDTRFv3 0.8× Reducer/Field Flattener
The AT115EDT is a triplet apochromat — three air-spaced elements that bring color under tight control and deliver sharp, high-contrast images at f/7. What it doesn't do on its own is flatten the field for imaging. At 805mm, the field curvature is gentle enough to be invisible visually, but a camera sensor at the focal plane sees it in the corners: stars that are sharp in the center going slightly soft toward the edges. The AT115EDTRFv3 corrects that, and while it's at it, reduces the focal length to 644mm at f/5.6 — roughly 36% faster, with a wider field of view. One element, threaded in, and the scope changes character in the best possible way.
This is the V3 — the third generation of a reducer/flattener matched specifically to the optical geometry of the AT115EDT. It threads directly into the M92 drawtube of the 3.2" focuser with no intermediate adapters. Set your camera spacing to 55mm from the reducer shoulder to your sensor, and the triplet becomes a 644mm f/5.6 astrograph with a corrected 43.8mm image circle.
The camera side offers three direct threading options: M78×1 female, M54×0.75 male, and M48×0.75 male. Dedicated CMOS and CCD astronomy cameras connect directly via whichever thread matches their back plate — no T-ring needed. DSLR and mirrorless cameras connect via a T-ring for your specific mount, combined with a step adapter to M48. The three-thread design handles the majority of imaging setups without additional hardware.
Unscrew the top knurled ring and you'll find internal 2" filter threads. Narrowband, broadband, or light pollution rejection filters thread in directly — no filter drawer, no extra back focus to manage. The filter lives in the reducer, rotates with the camera, and keeps the imaging train as compact as possible. Fully multi-coated with broadband anti-reflection coatings on all surfaces. Thread-on dust caps for both ends are included.
Features
0.8× focal reduction. 805mm f/7 becomes 644mm f/5.6 — about 36% faster. That's the difference between a 10-minute sub and a 6.5-minute sub for equivalent signal. On a night with three good hours, those saved minutes add up to real data.
Field flattener integrated. The AT115EDT is a triplet with native field curvature. The V3 corrects that curvature across a full 43.8mm image circle — the same full-frame coverage as the AT130EDT version, sharp from center to corner.
M92 scope-side connection. Threads directly into the AT115EDT's 3.2" focuser drawtube. No adapters, no shimming, no backfocus arithmetic at the scope end.
Three camera-side thread options. M78×1 female, M54×0.75 male, and M48×0.75 male. Dedicated CMOS and CCD cameras connect directly. DSLR and mirrorless cameras connect via a T-ring plus step adapter to M48 (T-ring sold separately).
Internal 2" filter threads. Top knurled ring unscrews to accept standard 2" astronomy filters — narrowband (Hα, OIII, SII), broadband LRGB, and light pollution filters all fit. No external filter drawer required.
Fully multi-coated optics. Broadband anti-reflection coatings throughout. Maximum light throughput and controlled internal reflections — important when imaging faint emission nebulae with narrowband filters.
55mm back focus. Standard spacing from reducer shoulder to image plane. Works with most camera and adapter combinations without custom spacers.
Includes thread-on dust caps. Scope-side and camera-side covers protect coated surfaces when the reducer is off the telescope.
Under the Night Sky
At 644mm and f/5.6, the AT115EDT reaches targets it couldn't quite frame comfortably at native focal length. The Heart Nebula (IC 1805) — about 2.5° across — fits on an APS-C sensor with room for context. The Rosette Nebula (NGC 2237), just over 1.3°, sits squarely in the frame. The North America Nebula (NGC 7000) fills the sensor edge-to-edge with its characteristic landmass shape. M42 and the full Orion complex, including the faint outer nebulosity, land at an ideal scale for wide single-frame captures.
For galaxy imaging, 644mm is effective at resolving structure in nearby groups. M81 and M82 frame together on APS-C with both galaxies showing detail. The Virgo Cluster core — M84, M86, and the surrounding Markarian's Chain — fits in a single frame with enough angular separation to read each member individually. The 115mm aperture gathers enough light to show dust lane hints in M82 in a reasonable number of subs.
At f/5.6, narrowband imaging becomes genuinely efficient. An hour of Hα data on the Elephant Trunk Nebula (IC 1396) pulls up the pillar structure clearly. The Cygnus Wall section of NGC 7000 shows sharp emission edges in 5-minute subs. This is the kind of result the AT115EDT was capable of all along — the reducer/flattener just removes the friction.
Community Says
"I didn't know much about this new v3 except that it was supposed to be bigger. This thing is bigger and it's amazing. The image circle from this is bigger than anything I have. It covers my full frame dslr 100% and works great with my asi2600mc duo. I love that it has the 2 inch filter holder built in. Images are flat, the scope is faster (f/5.6 when using this reducer). It's a great product." — Clayton O., February 2025 ★★★★★
"Cloudy skies are keeping me from using it as much as I would like so far but it seems to work great. Thanks for all the help putting it together!" — Fred G., May 2025 ★★★★★
Observing Tip
For your first light with the AT115EDTRFv3, aim at a field with a mix of stellar magnitudes — the Double Cluster in Perseus works well. Check the corner stars at full resolution. If they're round and tight, your 55mm back focus spacing is dialed in and you're ready to go to work on faint targets. If you see elongated stars or coma patterns in the corners, adjust your spacing by 1–2mm at a time until the field flattens. Getting this right once means every session after that is plug-and-shoot.
Frequently Asked Questions
Does this work with the AT115EDT Version I (2.5" focuser)?
No. The AT115EDTRFv3 requires the M92 drawtube of the AT115EDT with the 3.2" focuser. It is not compatible with the earlier 2.5" focuser. If you're not sure which version you have, measure the focuser opening — Version II units have the wider 3.2" aperture and the M92 drawtube.
Does it cover full-frame sensors?
Yes. The corrected image circle is 43.8mm, which covers the full 35mm full-frame sensor format (diagonal ~43.3mm) edge to edge. Stars are sharp and flat across the entire frame.
Do I need a T-ring to connect my camera?
Dedicated CMOS and CCD astronomy cameras (ZWO, QHY, Player One, and similar) typically have M54 or M48 back plates and connect directly — no T-ring needed. DSLR and mirrorless cameras connect via a T-ring for your specific lens mount plus a step adapter to M48. T-rings are sold separately.
What filters fit in the internal threads?
Standard 2" astronomy filters with 48mm filter threads. This includes narrowband filters (Hα, OIII, SII), broadband LRGB sets, and light pollution rejection filters. If a filter fits a standard 2" filter wheel slot, it will thread into the AT115EDTRFv3.
Can I use this with the AT130EDT?
No — use the AT130EDTRFv3 for the AT130EDT and AT130EDX. Each reducer/flattener is matched to the specific focal length and field curvature of its scope. Using the wrong version will produce incorrect field correction.
Final Thoughts
If you own the AT115EDT and you're imaging, this is the most efficient upgrade available to you. No new scope, no new mount, no new camera — just one precision element threaded into the focuser drawtube, and your 805mm f/7 triplet becomes a 644mm f/5.6 astrograph with corrected field corners. The targets that were slightly too large to frame at native focal length — the Heart Nebula, the Rosette, the great Cygnus emission fields — move into range. The exposures run a third shorter. And the field flattener that the AT115EDT optical design never included, because triplets don't, gets quietly supplied. It's a logical completion of a scope that was already doing most things right.
