• 90mm f/6.67 fully multicoated triplet apochromatic optics using an FPL-53 ED element and all Ohara glass• collimatable lens cell• dual-speed 2.7” rack-and-pinion focuser with 11:1 ratio fine focusing • both 2” and 1.25” brass compression ring eyepiece holders • retractable lens shade/dew shield • long-lasting white paint finish with anodized grey trim • aluminum-frame foam-fitted hard case • two-year warranty
That the performance of this Astro-Tech triplet apochromatic refractor is superb shouldn’t surprise you. Astro-Tech has quickly developed a well-deserved reputation for producing excellent small optics. However, what might surprise you is the more-than-reasonable price of this exceptional 90mm mid-sized refractor. Knowledgeable astronomers have “known” for years that you couldn’t get a true FPL-53 triplet apochromatic optical system with a collimatable lens cell in this size/focal ratio range for anywhere near this price. With this new AT90EDT, Astro-Tech is happy to prove those doubters wrong.
Other manufacturers often use FPL-51 ED glass or its equivalent in their more-expensive triplet designs to keep their manufacturing costs down. Some competitive systems in the AT90EDT’s price range are even only simple doublets. In comparison to this approach, Astro-Tech uses more-expensive true FPL-53 ED (Extra-low Dispersion glass) element in the triplet optics of the AT90EDT. The result is the finest possible control of spurious color, for images that are free of all vestiges of the annoying faint color halos around bright objects visible in lesser scopes – despite the Astro-Tech’s fast focal ratio, and even at very high magnifications.
Those exceptional optics are even more impressive when you consider the package they come in. The finely-machined scope has a dual speed 2.7” rack-and-pinion focuser with a microfine 11:1 fine-focusing ratio. The focuser can rotate a full 360° to put your camera or eyepiece in the most comfortable observing position. The supplied 2” and 1.25” eyepiece holders use non-marring brass compression rings that won’t scratch your eyepiece barrels. The Astro-Tech AT90EDT has a retractable lens shade and comes in a locking aluminum-frame carrying case.
If this Astro-Tech AT90EDT was simply a 90mm ED doublet system, it would still be fairly priced. But – a 90mm fast focal ratio triplet using true FPL-53 glass and a dual-speed 2.7” rack and pinion focuser at this price . . . is a steal!
Features of this Telescope . . .
Apochromatic triplet refractor optics: 90mm (3.54”) aperture, 600mm focal length, f/6.67 focal ratio three-lens optical system using the finest quality FPL-53 ED (Extra-low Dispersion glass) element to reduce spurious color halos and fringing to vanishingly low levels. All three lens elements are made from the finest quality Ohara glass. Ohara is well-known as Japan’s premier manufacturer of specialized optical glass. The lens cell is fully collimatable for peak performance. Fully multicoated optics: The triplet objective lens has the latest state-of-the-art antireflection multicoatings on all air-to-glass surfaces for high light transmission and excellent contrast. This can easily be seen by looking into the objective lens of the scope. Virtually no reflection of your face will be seen. It’s a sure sign that the high transmission coatings are doing their job, by letting virtually all the light enter the scope, rather than reflecting some light back to your eye. Internal light baffles: Contrast-enhancing knife-edge light baffles in the lens shade and the optical tube provide truly dark sky backgrounds and high terrestrial contrast. The edges of the lenses are blackened to reduce internal reflections and further improve image contrast. Dew shield: A self-storing retractable dew shield slows the formation of dew on the lens in cold weather to extend your undisturbed observing time. It also improves the contrast, similar to the effect of the lens shade on a camera lens, when observing during the day or when there is excessive ambient light at night, such as a neighbor’s backyard security light. The tube measures a very compact 21” long with the dew shield retracted, and only 24” long with it extended. Dual speed microfine 2.7” rack-and-pinion focuser with 2” eyepiece holder and 1.25” eyepiece adapter, both with compression rings: The precision-made focuser has dual-speed focusing. There are two coarse focusing knobs. The right knob also has a smaller concentric knob with 11:1 reduction gear microfine focusing ratio. This provides exceptionally precise image control during high magnification visual observing and critical 35mm or CCD imaging. The focus knobs have ribbed gripping surfaces so they are easy to operate, even while wearing gloves or mittens in cold weather. The 2.7” focuser drawtube terminates in a built-in camera angle adjuster as standard equipment. It is not an optional extra-cost accessory, as it is with some other scopes. The camera angle adjuster lets you rotate the focuser to line up a camera in either a landscape or portrait orientation (or any orientation in between), as well as put a star diagonal and eyepiece into the most comfortable observing position. A knob on the camera angle adjuster lets you lock the focuser at whatever angle is most convenient for you. The extra-long 120mm (4.7”) travel 2.7” diameter focuser drawtube has a scale marked in 1mm increments so you can note individual focuser positions for easy return to the correct focus when switching between visual use and photography. A lock knob on top of the focuser lets you lock in your photographic focus. The 2.7” drawtube has a 2” eyepiece holder that uses a non-marring soft brass compression ring to hold a 2” star diagonal and accessories in place. The compression ring won’t scratch the barrel of your star diagonal and accessories as an ordinary thumbscrew can. A standard equipment 1.25” accessory adapter slips into the 2” eyepiece holder to let you use a 1.25” star diagonal or image erecting diagonal. Like the 2” eyepiece holder, the 1.25” adapter also uses a non-marring soft brass compression ring to hold 1.25” star diagonals and accessories in place. If you are partial to 1.25” eyepieces and need an outstanding diagonal for this scope, consider the Astro-Tech 1.25” dielectric diagonal (#AT1D 1.25" 99% Reflectivity dielectric mirror diagonal). It's a Sky & Telescope Hot Product for 2007. An Astro-Tech guaranteed 1/10th wave peak-to-valley accuracy 2” quartz star diagonal (#AT2DQ Astro-Tech One-piece machined 2" diagonal with 1/10th wave quartz mirror) is recommended to deliver all of the scope’s exceptional optical performance to your eye. Tube finish: The optical tube and lens shade are finished in a durable high gloss automotive-style white paint, the same as that used on scopes three times the price of the AT90EDT. The focuser and trim are anodized gun metal grey. There is an anodized dark green identifying band around the camera angle adjuster portion of the focuser body. Unlike other manufacturers who skimp on production costs by putting a 90mm lens in an 80mm tube, Astro-Tech has designed and built a tube specifically for these exceptional 90mm optics. While this may cause a slight weight penalty (the scope weighs 11.5 lbs.), the solid feel and precision build quality of the AT90EDT tube are well worth the slight extra weight. Tube rings: No tube rings are supplied for the 114mm o. d. optical tube, as some observers may already have rings for this popular tube size. If rings are needed, however, optional 114mm Astro-Tech #AT114R hinged tube rings are readily available. Other supplied accessories: A slip-on metal dust cap is standard. There is a dovetail shoe on the upper left side of the optical tube for installing an optional multiple reticle red dot finder (#ATF). Shipping/storage case: The scope comes in a foam-fitted aluminum-frame locking hard case with carry handle. Astro-Tech is one of the very few manufacturers to provide a case at no charge for protection during shipping and as a storage convenience when the scope is not in use. Unfortunately, FedEx, UPS, and the Postal Service are very good at treating packages roughly. Occasionally, your scope can arrive in perfect condition, but with the walls of the shipping case dented in transit from rough handling, or the aluminum frame sprung, rendering the appearance of the case less than pristine. Damage to the shipping case in such instances is not covered by warranty. Two year warranty: As an expression of the confidence Astronomy Technologies has in the quality of their products, the Astro-Tech AT90EDT triplet is protected by a two-year limited warranty against flaws in materials and workmanship (shipping case excluded).
is the formula that we use with all of the telescopes we carry, so that
our published specs will be consistent from aperture to aperture, from
manufacturer to manufacturer. Some telescope makers may use other
unspecified methods to determine the limiting magnitude, so their
published figures may differ from ours.
Keep in mind that this
formula does not take into account light loss within the scope, seeing
conditions, the observer’s age (visual performance decreases as we get
older), the telescope’s age (the reflectivity of telescope mirrors
decreases as they get older), etc. The limiting magnitudes specified by
manufacturers for their telescopes assume very dark skies, trained
observers, and excellent atmospheric transparency – and are therefore
rarely obtainable under average observing conditions. The photographic
limiting magnitude is always greater than the visual (typically by two
The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.
Fast f/4 to f/5 focal ratios are generally
best for lower power wide field observing and deep space photography.
Slow f/11 to f/15 focal ratios are usually better suited to higher power
lunar, planetary, and binary star observing and high power photography.
Medium f/6 to f/10 focal ratios work well with either.
system can photograph a nebula or other faint extended deep space object
in one-fourth the time of an f/10 system, but the image will be only
one-half as large. Point sources, such as stars, are recorded based on
the aperture, however, rather than the focal ratio – so that the larger
the aperture, the fainter the star you can see or photograph, no matter
what the focal ratio.
Dawes’ limit only
applies to point sources of light (stars). Smaller separations can be
resolved in extended objects, such as the planets. For example,
Cassini’s Division in the rings of Saturn (0.5 arc seconds across), was
discovered using a 2.5” telescope – which has a Dawes’ limit of 1.8 arc
The ability of a telescope to resolve to Dawes’ limit is
usually much more affected by seeing conditions, by the difference in
brightness between the binary star components, and by the observer’s
visual acuity, than it is by the optical quality of the telescope.