Overview Of The Infinity 90 Optical Tube
The Infinity 90 is one of the few inexpensive scopes on the market these days that doesn’t seem to bear superficial resemblance or directly copy anything else. It’s a 90mm f/6.7 achromatic refractor, a rather uncommon configuration to be sold as a beginner instrument.
With a focal ratio of f/6.7, there is of course a significant amount of chromatic aberration in the Infinity 90’s images, but certainly less than that of the Infinity 102 and considerably less than the Infinity 80. A longer achromat or an apochromat will of course crush the Infinity 90 but for the price and considering its portability the scope produces really excellent images.
The dew shield at the front of the Infinity 90 is entirely plastic, and the inside is almost shiny. Scuffing the interior up with coarse sandpaper (#80 or #120) and spraying it flat black, or simply installing some flocking paper, will greatly improve image contrast – you’d really be surprised by the effect it has!
The scope’s focuser is largely plastic, but works pretty well and has no trouble holding heavy eyepieces.
As with the Infinity 80 and 102 the Infinity 90 has a Vixen dovetail plate for use on more expensive astronomical mounts, but it attaches to the Infinity AZ mount using a simple ¼ 20 threaded hole and screw/washer.
The Infinity AZ refractors come with three eyepieces: A 26mm MA (23x), a 9mm MA (67x) and a 6.3mm MA (95x). The MA design is a derivative of the Kellner eyepiece and while cheap, they work pretty well (though a good Plossl or wide-angle design will dispense with them easily).
The Infinity scopes also come with a 2x Barlow lens, an Amici prism star diagonal, and a red dot sight.
The MA eyepieces are plastic-bodied, but the eyepiece lenses themselves are glass. The 6.3mm MA is rather hard to look through due to it small lens and low eye relief, and the view is less-than-sharp – a 6mm “goldline” eyepiece would prove far superior.
The included 2x Barlow seems to be entirely plastic, including the optics. It works, but you really don’t need to use it in the first place and it is definitely not the sharpest.
The plastic Amici prism diagonal provides correct non-inverted left-right and up-down images, making it useful for terrestrial viewing. However, the net effect on astronomical use is a dimmer-than-normal image with slightly more chromatic aberration than with a regular diagonal, and the Amici prism design produces a bright, distracting diffraction spike on the planets and bright stars.
The included red dot sight works reasonably well, though at 600mm focal length a real finderscope or reflex sight would be preferable for locating deep-sky objects. The shoe for the finder is the industry standard size, so a good 6×30, 8×40, or 9×50 finderscope bracket (or a bracket for some reflex sights) should slide right in.
The Infinity AZ Mount & Tripod
With a design resembling more of a pan-head photo tripod than an astronomical mount, the Infinity AZ is a little confusing to get the hang of, but it works quite well. Balancing is achieved by sliding the small ¼ 20 captive knob much like a photo tripod head screw, and if the scope is still back-heavy (if, say, you use a large ultra wide angle eyepiece or 50mm finderscope) you can tighten the altitude axis a bit using a wrench. Azimuth tensioning is adjusted via a small knob, and there are slow motion knobs for both axes. Keep in mind that the slow motion knobs on the mount are tangent arms and thus will run out of travel if you keep turning them in one direction.
The tripod is steel, and while some plastic is used in the construction the thing overall feels pretty sturdy and solid, far better than the cheap EQ-1, EQ-2, and EQ-3 mounts supplied with many entry-level instruments both in its sturdiness and ease of use.
The only major downside of the whole mount is that you may have trouble pointing the scope directly overhead; it’ll either bump into the knobs or the tripod legs.
Overall I would definitely recommend the Infinity 90AZ. If your budget is a little bigger and you aren’t too worried about the chromatic aberration, I might steer you towards the 102AZ instead – it’s only a little more expensive and less portable, but has a 2” focuser and larger aperture. However, if you would prefer better lunar/planetary performance there’s absolutely nothing wrong with getting the 90, and all three of the larger Infinity series refractors are fine telescopes.