AT90EDT 90mm f/6.7 FPL-53 ED triplet apochromatic refractor

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This Astro-Tech AT90EDT refractor has:

• 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).
Highest Useful Magnification:
This is the highest visual power a telescope can achieve before the image becomes too dim for useful observing (generally at about 50x to 60x per inch of telescope aperture). However, this power is very often unreachable due to turbulence in our atmosphere that makes the image too blurry and unstable to see any detail.

On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture or less) is usually more enjoyable than fruitlessly attempting to push a telescope's magnification to its theoretical limits. Very high powers are generally best reserved for planetary observations and binary star splitting.

Small aperture telescopes can usually use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.
Visual Limiting Magnitude:
This is the magnitude (or brightness) of the faintest star that can be seen with a telescope. The larger the number, the fainter the star that can be seen. An approximate formula for determining the visual limiting magnitude of a telescope is 7.5 + 5 log aperture (in cm).

This 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 magnitudes).

Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

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.

Focal Ratio:
This is the ‘speed’ of a telescope’s optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera.

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.

An f/5 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.

This is the ability of a telescope to separate closely-spaced binary stars into two distinct objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25-cent coin at a distance of three miles! In essence, resolution is a measure of how much detail a telescope can reveal. The resolution values on our website are derived using the Dawes’ limit formula.

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 seconds!

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.

1.29 arc seconds
This is the diameter of the light-gathering main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see.
The weight of this product.
12.7 lbs.
Telescope Type:
The optical design of a telescope.  Telescope type is classified by three primary optical designs (refractor, reflector, or catadioptric), by sub-designs of these types, or by the task they perform.
Based on Astronomy magazine’s telescope "report cards", scopes of this size and type generally perform as follows . . .
Terrestrial Observation:
Observing terrestrial objects (nature studies, birding, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial observing. Scopes with apertures under 5" to 6" are generally most useful for terrestrial observing due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes. 
Lunar Observation:
Visual observation of the Moon is possible with any telescope. Larger aperture scopes will provide more detail than smaller scopes, thereby getting a higher score in this category, but may require an eyepiece filter to cut down the greater glare from the Moon's sunlit surface so small details can be seen more easily. Lunar observing is more rewarding when the Moon is waxing or waning as the changing sun angle casts constantly varying shadows to reveal craters and surface features by the hundreds.  
Planetary Observation:
Very Good
Binary and Star Cluster Observation:
Very Good
Galaxy and Nebula Observation:
Terrestrial Photography:
Photographing terrestrial objects (wildlife, scenery, etc.) is usually possible only with refractor and catadioptric telescopes, and convenient only when the scope is on an altazimuth mount or photo tripod. Most reflectors cannot be used for terrestrial photography. Scopes with focal ratios of f/10 and faster and apertures under 5" to 6" are generally the most useful for terrestrial photography due to atmospheric conditions (heat waves and mirage, dust, haze, etc.) that degrade the image quality in larger scopes.
Lunar Photography:
Photography of the Moon is possible with virtually any telescope, using a 35mm camera, DSLR, or CCD-based webcam (planetary imager). While an equatorial mount with a motor drive is not strictly essential, as the exposure times will be very short, such a mount would be helpful to improve image sharpness, particularly with webcam-type cameras that take a series of exposures over time and stack them together. Reflectors may require a Barlow lens to let the camera reach focus. 
Planetary Photography:
Star Cluster / Nebula / Galaxy Photography:
1 year
Reviews from Cloudy Nights (
These reviews have been written by astronomers just like you and posted on the Cloudy Nights astronomy forums . . .
The Remarkable AT90EDT

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1. James on 3/11/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
This ruggedly built refractor is an excellent bargain for the price. Optical performance is impeccable, construction top-notch. If is overbuilt. Used as an imager it must be paired either w/ the AT Field Flattener or I have also found the Takahashi .8x reducer/flattener to be a good match. For visual use it provides stunningly crisp stars. A delight to use backed by great customer service.
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2. John on 10/2/2012, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
I've become a fan of long-focus refractors, but even at that, I've found myself returning to this chubby little shorter focal length refractor on many a night. I've found the views in it remarkably crisp and sharp. It's done an excellent job for me on double stars, and I've found it performs surprisingly well on deep sky objects. Naturally M31 or M33 are easily within its grasp, but it does rather well on the Leo Triplet, including pulling the elusively dim and thin NGC 3628 out of a dark sky. The focuser is more than adequate for my visual use, and can be adjusted so that it operates very smoothly.

Overall, it's a steal at its current price -- it competes very well with the more well known brand names in the 85 to 90mm range. Drop a 10mm to 20mm eyepiece in the focuser and just sit back and admire the crisp heavens. Highly recommended -- and I should know -- I've owned three of them. And the third one ain't goin' nowhere this time!

John Nanson
Manzanita, Oregon
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General Accessories
Altazimuth Mounts (1)
Voyager Altazimuth mount
by Astro-Tech
Tube Rings (1)
114mm i.d. Astro-Tech tube rings, pair
by Astro-Tech
Visual Accessories
Star Diagonals (1)
1.25" 99% Reflectivity dielectric mirror diagonal
by Astro-Tech
Photographic Accessories
Tele-Compressors (Photo/Visual) (1)
2" Field flattener for imaging with Astro-Tech and TMB refractors
by Astro-Tech
  • Collimatable lens cell
  • Retractable lens shade
  • Dual-speed 2.7" rack and pinion focuser with 2” and 1.25” brass compression ring eyepiece/accessory adapters
  • Quick release foot for finderscope
  • Dust covers
  • Hard case.
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Astro-Tech - AT90EDT 90mm f/6.7 FPL-53 ED triplet apochromatic refractor

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Astro-Tech - AT90EDT 90mm f/6.7 FPL-53 ED triplet apochromatic refractorClose-up of the focuser drawtube, showing the focusing scale; the dual-speed focusing knobs; the 2" and 1.25" compression ring eyepiece holders; and the camera angle adjuster and drawtube lock knobs.
Our Product #: AT90EDT
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The Astro-Tech AT90EDT is an exclusive new 90mm apochromatic triplet refractor with FPL-53 glass at a price that might surprise you . . .

. . . our 34th year