Astro-Tech AT90EDX f/6 Triplet APO Refractor OTA

Description

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.

Accessories

• 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.
• 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.
• 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.
• 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.
• 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.

Tech Details: