The AT125EDL is a serious visual scope at 975mm f/7.8, but when you turn it toward astrophotography, two things work against you: the curved field stretches stars into small comets near the corners of your sensor, and the slow focal ratio means longer exposures to capture the same depth of signal. The AT125EDLRF fixes both. It reduces the focal length to 780mm, speeding the system to f/6.2 — a 56% increase in photographic speed — and flattens the field so stars stay round and tight from center to edge.
Designed for One Scope
This reducer/flattener is not a universal accessory. It's computed and optimized for the field curvature profile of the Astro-Tech AT125EDL 125mm f/7.8 doublet APO refractor. A generic 0.8x reducer can shorten focal length, but it won't correct the specific curvature of your scope. This one does. It may thread into other f/7.8 refractors with a 2.5-inch focuser, but optical performance is only guaranteed with the AT125EDL.
How It Connects
The scope side threads into the focuser drawtube or the camera angle adjuster (CAA) using M63×1 metric threads — a secure, repeatable connection with no set screws and 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 — a T-ring and camera body will put you at the correct distance without spacers. For dedicated astronomy cameras, measure your camera's back focus and add spacers if needed to reach 55mm.
What's Included
- Astro-Tech AT125EDLRF 0.8x reducer/field flattener
- Slip-on rubber dust cover for 48mm T-threads (camera side)
- Slip-on dust cover for 2" scope-side barrel
Features
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0.8x focal reduction — Converts the AT125EDL from 975mm f/7.8 to 780mm f/6.2. That's a 56% increase in photographic speed — the same target reaches the same signal-to-noise ratio in roughly two-thirds the exposure time.
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Field flattening — Corrects the native field curvature of the AT125EDL so stars are round and sharp all the way to the corners of your imaging chip.
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Fully multi-coated optics — Minimizes reflections and maximizes light transmission through the reducer elements.
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M63×1 scope-side thread — Threads securely into the focuser drawtube or camera angle adjuster. No compression ring, no wobble, no shift 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. No spacers needed for most DSLR setups. Dedicated astronomy cameras may require spacers to reach the correct distance.
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Compact and lightweight — 8 ounces. Adds minimal weight to your imaging train.
Imaging Tip
When you first set up the AT125EDLRF, shoot a short test exposure of a star field and inspect the corners at 100% zoom. If stars are elongated radially (pointing away from center), you're too far from the reducer — remove a spacer or check your T-ring stack. If stars are elongated tangentially (perpendicular to the radial direction), you're too close. The 55mm specification is measured from the shoulder of the reducer, not the rear element — make sure you're measuring from the right point.
FAQ
Will this work with my AT102EDL or AT150EDL?
No. Each EDL scope has its own dedicated reducer matched to its specific field curvature. The AT102EDL uses the AT102EDLRF, and the AT150EDL uses the AT150EDLRF. They are not interchangeable.
Can I use this visually?
The reducer is designed for prime-focus astrophotography. It will not reach focus with an eyepiece and is not intended for visual use.
Do I need a T-ring?
DSLR and mirrorless cameras connect via a T-ring for your specific mount (sold separately). Dedicated CMOS and CCD astronomy cameras typically have 48mm T-threads built in and connect directly without a T-ring.
What if I need to use a filter?
You can place a 48mm (2") threaded filter between the reducer and the camera. Be aware that adding a filter changes your back focus distance — account for the filter thickness when calculating your total spacing.
Final Thoughts
If you're imaging through the AT125EDL, this is the reducer that was designed for it. It flattens the field, speeds the system to f/6.2, and threads in with a secure M63 connection that doesn't shift between sessions. At 8 ounces, it adds almost nothing to your imaging train. The result is sharper corners, shorter exposures, and a wider field — exactly what the AT125EDL needs to go from a visual scope that happens to image to a proper astrophotography platform.
