The first thing you notice about the AT90EDX is how quiet the view is. No color fringing on bright limbs. No softening at the edge. Just the object — clean, still, and sharp in a way that makes you stop adjusting and start looking. There's a moment, the first time you point a scope like this at Saturn or a tight double star, where you forget you're looking through a 90mm refractor. You're just looking at the thing itself.
That's what happens when you put FCD-100 glass in a triplet design with a guaranteed .95 Strehl ratio or higher, wrap it in a tube that weighs 12 lbs with the rings on, and hand it to someone who's spent years wondering whether a smaller scope could actually deliver. The AT90EDX isn't trying to replace a 130mm APO. It's trying to be the scope you actually bring outside on a Tuesday night — the one that's set up and cooled down while you're still thinking about whether to haul the big rig out of the closet. And once it's out there, it shows you things that make you glad you didn't stay inside.
The Design
The AT90EDX is a continuation of a design that started with Thomas Back. Tom laid the foundation for this scope when he created the TMB-92 Signature Series — a 92mm f/5.5 triplet that became one of the most respected small APOs ever made. When we discontinued the AT92 (our version of Tom's design), the optical prescription sat in the vault for years. We sent it to Roger Ceragioli, who studied the design, updated it to take advantage of current glass and coatings, and made changes to extract everything the 90mm aperture could give.
Every AT90EDX is tested on an interferometer at the factory and guaranteed to meet or exceed a .95 Strehl ratio. That's not a marketing claim — it's a specification. The test reports go to us, and we keep them on file. The Strehl measurement is taken at 550nm (green), which is where the human eye is most sensitive and where optical quality matters most for visual observing.
The Focuser
The AT90EDX uses the same 3.2" dual-speed rotating rack and pinion focuser we put on the AT115EDT and AT130EDT — the larger version with the wider inner diameter. The 3.2" bore means the full light cone from a 90mm f/6 objective passes through without contacting the focuser walls, even with the drawtube fully racked in. No vignetting, no clipping, no compromise. The focuser has two coarse knobs and a concentric fine-focus knob with a 10:1 reduction ratio for precise image control at high magnification. All knobs are ribbed for use with gloves.
The focuser rotates — a built-in Camera Angle Adjuster lets you frame your target without loosening the camera or disturbing focus. For imagers, that's one less adapter in the chain and one less source of tilt. For visual observers, it means your diagonal and eyepiece stay where you want them as the scope tracks across the sky.
The drawtube ends in a 2" compression ring holder — non-marring twist lock, not a thumbscrew. A 1.25" adapter with its own compression ring holder slips into the 2" fitting. Both hold accessories firmly without scratching barrels.
The Tube
The OTA is aluminum with our grey anodize finish. At 17.5" long with the dew shield retracted — 20.2" extended — it's short enough to ride in a carry-on or tuck into a corner of the car. The retractable dew shield doubles as a lens shade, blocking stray light from streetlamps and neighbors' porch lights. Inside the tube and focuser, knife-edge baffles kill internal reflections. The edges of the objective lens are blackened. These are details that show up as contrast — the difference between a faint galaxy that's "maybe there" and one that's clearly there.
The tube rides in hinged CNC tube rings with a carry handle and a Losmandy-style dovetail plate. Two Vixen finder shoes are installed on the focuser body. A padded soft case with carry handle, shoulder strap, and five cutout compartments is included — room for the scope, a diagonal, and a reducer or flattener.
What's Included
- AT90EDX optical tube assembly
- 3.2" dual-speed rotating rack and pinion focuser with Camera Angle Adjuster
- 2" compression ring eyepiece holder
- 1.25" compression ring eyepiece holder adapter
- Hinged CNC tube rings with carry handle
- Losmandy-style dovetail plate
- Two Vixen-style finder shoes (installed on focuser body)
- Retractable dew shield
- Slip-on metal dust cap
- Padded soft case with carry handle and shoulder strap
Features
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90mm f/6 FCD-100 triplet apochromat. 540mm focal length. The FCD-100 center element eliminates chromatic aberration — no purple fringing on bright stars, no false color on planetary limbs. Views are clean and color-free in a way that ED doublets at this aperture cannot match.
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Guaranteed .95 Strehl ratio or higher. Every scope is interferometer-tested at the factory. The guarantee means you're getting optics that perform at or near the diffraction limit — not a hope, a specification.
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3.2" dual-speed rotating rack and pinion focuser. The wider bore eliminates vignetting across the full light cone. 10:1 fine focus for precise control at high magnification. Built-in Camera Angle Adjuster rotates your imaging train without disturbing focus. Ribbed knobs for glove use.
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17.5" retracted length. Dew shield slides back to make the scope short enough for airline carry-on or compact car storage. 20.2" with dew shield extended. This is a genuinely portable telescope that doesn't ask you to sacrifice optical quality for size.
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TMB heritage design. Thomas Back laid the foundation with the TMB-92 Signature Series. Roger Ceragioli refined the design for current glass and coatings. The result performs at the level of the original — and, by some accounts, better.
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12.2 lbs with rings and handle. Light enough for an alt-az mount, a photo tripod with a fluid head, or a small equatorial. Heavy enough that the machining and build quality are obvious the moment you pick it up.
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Knife-edge internal baffles. In the tube and focuser. Combined with blackened lens edges and fully multicoated optics, they deliver the contrast that makes faint objects visible and planetary detail sharp.
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Non-marring compression ring holders. 2" and 1.25" twist-lock compression rings hold diagonals, eyepieces, and cameras firmly without scratching barrels. No thumbscrews to leave marks.
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Dedicated imaging accessories available. A 1× field flattener and a 0.8× reducer/field flattener are both designed specifically for this scope. The reducer drops focal length to 432mm at f/4.8 for faster widefield imaging.
Under the Night Sky
Ninety millimeters isn't a lot of aperture. It's enough. And when the optics are this good, "enough" goes further than most people expect.
Moon: This is where the AT90EDX first earns your trust. At 135× (4mm eyepiece), the terminator is a landscape — not a feature list. Crater walls cast shadows that look three-dimensional. The Alpine Valley rille shows in steady air. Clavius reveals the chain of craterlets across its floor. The Straight Wall is a knife edge. Push to 180× or 216× on a good night and you'll see detail in the lunar highlands that you'd swear requires more aperture. It doesn't. It requires good optics, and these are good optics.
Planets: Saturn's Cassini Division is easy. You'll see two or three cloud bands on the globe and the shadow of the rings where they cross the disk. Jupiter shows multiple belts, the Great Red Spot when the geometry is right, and festoons between the belts in steady seeing. The Galilean moons resolve from points to tiny discs at 200×+. Mars at opposition shows the polar cap and dark surface markings — Syrtis Major, Acidalia Planitia — though a 90mm scope will always leave you wanting more Mars. That's true of any scope under about 8 inches.
Double stars: The Dawes limit for 90mm is about 1.3 arc seconds. Epsilon Lyrae — the Double Double — splits cleanly into four components. Albireo is a gold-and-blue showpiece. Zeta Herculis, a tight pair that challenges small scopes, has been split with this scope (and the position angle confirmed) by at least one CN observer. The color fidelity of the triplet design makes colored doubles — Albireo, Almach, Ras Algethi — look vivid in a way that scopes with residual chromatic aberration cannot match.
Deep sky: From a dark site, M13 begins to resolve into individual stars at 135× and above. M42 shows the Trapezium clearly split with surrounding nebulosity and structure in the bright inner regions. M57 is a visible ring, not just a smudge. The Double Cluster in Perseus fills a low-power field with hundreds of resolved stars. M31's core is bright with hints of the dust lanes from a dark location. Galaxy hunting is limited by aperture — you'll see the Messier galaxies and brighter NGCs, but the AT90EDX is not a faint-galaxy scope. What it shows, though, it shows cleanly. No scatter, no flare, no false color muddying the view.
Community Says
Cloudy Nights observers on the AT90EDX:
"I don't think the views I saw in that AT90EDX could have been improved had a copy of the Test Report had been included."
"I have to tell you, honestly, that this 3.2" focuser that Astro-Tech has provided on several of the current ED refractors, including the newest editions of the AT115EDT and AT130EDT and the EDQ models, is my all-time favorite."
"Having had the AT90CFT (same optics as the EDX version, lighter build) now for going on two years, I would heartily recommend the AT90EDX from Astronomics. They are truly wonderful scopes... Simply put, mine is the best optics I've ever had. Looking through it I just see perfection!"
"Oh and please try looking through it someday, they are amazing visual instruments."
"I had the newest generation Stowaway prior to this scope and optically this AT90CFT is every bit as good."
Observing Tip
A 90mm f/6 scope cools down fast — fifteen minutes in most conditions, maybe twenty in winter. But "fast" is not "instant," and the difference between thermal equilibrium and almost-there shows up as soft, restless star images that can fool you into thinking the seeing is bad or the optics aren't sharp. Give it the full cool-down before you judge anything. If you're star-testing, wait until the intrafocal and extrafocal diffraction patterns are symmetrical and steady. When they are, you'll know the scope is ready — and you'll know why people say this design traces back to Tom Back.
Frequently Asked Questions
Is FCD-100 the same as FPL-53?
Not exactly, but close. FCD-100 is Hoya's designation; FPL-53 is Ohara's. Both are high-dispersion ED glasses with very similar optical properties. In practice, the chromatic correction and Strehl performance from either glass, when properly figured, are indistinguishable. What matters is the design, the execution, and the testing — not the brand stamped on the glass blank.
Can I image with this scope?
Yes — and there are two dedicated accessories for it. The 1× field flattener corrects field curvature for flat stars across the sensor at the native 540mm f/6. The 0.8× reducer/field flattener drops the focal length to 432mm at f/4.8 for faster widefield imaging. Both are designed specifically for the AT90EDX/AT90CFT optical path. The 3.2" focuser and Camera Angle Adjuster are built with imaging in mind.
What mount does this need?
For visual, any sturdy alt-az mount rated for 15+ lbs works well. A good photo tripod with a fluid head (like the Manfrotto 546B, which at least one owner uses) is a viable option for grab-and-go. For imaging, a small equatorial mount — iOptron CEM26, ZWO AM3, Sky-Watcher HEQ5 class — handles the 12-lb payload easily. The Losmandy dovetail plate fits any Losmandy-style saddle.
How does the .95 Strehl guarantee work?
Every AT90EDX is tested on an interferometer at the factory before it ships to us. The test report stays on file at Astronomics. If the measured Strehl is below .95, the scope doesn't go out the door. We don't ship the test reports with the scopes — if you're curious about your specific scope's number, call us.
Accessories
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Astro-Tech 1× Field Flattener for AT90. Corrects field curvature at the native 540mm f/6 for flat stars across the sensor. Essential for imaging.
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Astro-Tech 0.8× Reducer/Field Flattener for AT90CFT/AT90EDX. Reduces to 432mm at f/4.8 — faster imaging, wider field of view, flat field correction.
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Astro-Tech 2" 99% Dielectric Mirror Diagonal. Pairs naturally with the 2" compression ring holder. Dielectric mirror coating delivers near-total reflectivity with zero chromatic shift — important in a scope this color-free.
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Astro-Tech 20mm 100° XWA 2" Eyepiece. 27× at f/6, 4.3° true field. A grab-and-go widefield eyepiece for sweeping the Milky Way.
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Astro-Tech 9mm 100° Waterproof 1.25"/2" Eyepiece. 60× — a versatile medium-power eyepiece for open clusters, bright nebulae, and galaxy cores.
Final Thoughts
The AT90EDX exists because Thomas Back designed a 92mm triplet that people still talk about twenty years later, and because Roger Ceragioli thought it could be made better with modern glass. It exists because a 90mm f/6 APO with a .95 Strehl guarantee, a 3.2" focuser, and a retractable dew shield that fits in a soft case is the scope a lot of experienced observers have been waiting for — the one that goes from closet to sky in five minutes and shows them what they expected to see only through their 130. It won't replace a bigger scope for deep-sky work. It will replace the excuses for not going outside. And at the eyepiece, on the nights when the seeing cooperates, it will show you views that make you forget the aperture.
