AT8D 8" F/5.9 Dobsonian reflector

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This 8” Astro-Tech Dobsonian reflector has:

• Diffraction-limited parabolic Newtonian optics
• dual-speed 2” Crayford focuser with 1.25” adapter
• innovative ball-bearing altitude motion with clutch
• smooth roller-bearing azimuth motion
• tube balance system
• 8 x 50mm fully-multicoated achromatic finderscope
• 2” 30mm SuperView 68° field eyepiece and 1.25” 9mm Plössl eyepiece
• battery-operated primary mirror cooling fan
• eyepiece tray
• free shipping

    Like all Dobsonian reflectors, this 8” f/5.9 Astro-Tech AT8D Dob gives you more light-gathering for your dollar than any other telescope type. It shows you faint deep space objects that are simply invisible in smaller scopes. With a visual limiting magnitude of 14, this 8” Astro-Tech is ideal for observing faint nebulas, galaxies, and star clusters from a dark sky site – all the faint fuzzies outside the solar system that fascinate us.

    The 8” Astro-Tech Dobsonian is easy to get to your favorite observing site, too, whether that be your own back yard or a dark sky site miles out of town. The Astro-Tech AT8D breaks down into two components – a 45.25” long x 9.25” diameter rolled steel optical tube and a 19.5” diameter x 26.75” tall wooden rocker box altazimuth base. It weighs only 54 pounds fully assembled. Assembling the AT8D takes just a moment, with no tools needed. Simply lift the optical tube, rest its circular altitude bearings in the semi-circular cutouts in the sides of the rocker box, put in an eyepiece, and you’re ready to observe.

    The performance of the BK7-equivalent optical glass mirrors of the 8” Astro-Tech Dobsonian is guaranteed to be diffraction limited – for sharp high contrast images of nebulas, galaxies, and star clusters. Planetary images are also sharp and crisp, although a neutral density filter is often required to allow glare-free observing. The mirrors are overcoated with quartz for long life.

    The 8” Astro-Tech Dobsonian is designed for visual observing only – to show you as much of the night skies as possible at an affordable price. Photography is not possible with a Dob.

    Under dark skies, the Orion Nebula becomes a glowing complex of filaments, filling the wide 1.7° field of the supplied 2” eyepiece. Globular clusters are often resolved to the core. Messier, NGC, and IC objects show detail and structure never visible in the smaller telescopes that are more common in this price range. As with any serious telescope, the performance of the 8” Astro-Tech on faint objects will be improved by a dark sky observing site. Light-polluted city and suburban sites are not ideal for observing very faint objects with any 8” scope.

    While it is in deep space observing of galaxies and nebulas from a dark sky observing site that the 8” Astro-Tech Dobsonian excels, significant planetary and lunar observing is also well within its capability. Given suitably good seeing, Encke’s Division and other minor features in Saturn’s rings occasionally become visible in the 8” Astro-Tech, as does Saturn’s faint crêpe ring. Low contrast banding and details in Saturn’s atmosphere begin to make themselves apparent, as well. Lunar features less than one mile across become visible, while Jupiter’s four Galilean moons start to show as tiny discs. Small details in the atmospheres of Jupiter and Saturn and on the surface of Mars reveal themselves at high powers given suitably good seeing.

    Simply made, but with precision optics, this Astro-Tech 8” Dobsonian reflector will reward you with bright deep space and solar system views and years of trouble-free observing enjoyment.

This Telescope’s Optical System . . .

  • Type: Newtonian reflector with parabolic primary mirror.

  • Primary mirror: 8” (203mm) diameter, 1200mm focal length, f/5.9 focal ratio. The mirror is made of B270 “water white” optical crown glass that is free of internal stress and striae. B270 glass is equivalent to BK7 in performance and optical quality. The thermal stability of B270 glass is generally better than the soda lime float glass used for the mirrors of most reflectors in this reasonable price range. The mirror is ground and polished under computer control for guaranteed diffraction limited performance, coated with 91% reflectivity aluminum, and overcoated with a protective layer of silicon dioxide (quartz) for long life.

  • Primary mirror cell: Fully adjustable 3-point flotation system metal cell holds the primary mirror. The open frame of the cell allows the mirror to cool down to ambient temperatures quickly, so you can start observing sooner. Large hand-tighten color-coded push-pull collimation and collimation lock knobs make it easy to collimate the primary mirror with no tools required.

  • Primary mirror cooling fan: A lightweight 4.7” diameter low-vibration cooling fan is built into the mirror cell to assure faster cool-down times. The battery-operated fan is powered by a battery pack that uses eight user-supplied AA batteries and plugs into a jack on the mirror cell frame.

  • Diagonal mirror: 47mm m.a. B270 “water white” optical crown glass, coated with 91% reflectivity aluminum and overcoated with a protective layer of silicon dioxide (quartz) for long life.

  • Diagonal mirror support: Fully adjustable diagonal holder mounted on a low-diffraction four-vane thin spring steel spider.

  • Focuser: Dual-speed 2” Crayford type, with 1.25” eyepiece adapter. The machined aluminum focuser has dual coarse focusing knobs for smooth and precise image control with either hand. There is a separate 10:1 reduction ratio microfocus knob for ultra-fine focusing at high magnifications. The ribbed focus knobs are easy to operate, even while wearing gloves or mittens in cold weather. A 35mm extension tube for eyepiece use is standard equipment. The extension tube, and both the 2” eyepiece holder and the 1.25” adapter use non-marring soft brass compression rings to hold eyepieces in place, rather than simple thumbscrews that can scratch your eyepiece barrels.

  • Two eyepieces: One is a fully multicoated 2” 30mm (40x) 68° field SuperView eyepiece with a wide 1.7° actual field of view that’s almost three and a half times the diameter of the full Moon. The second is a fully multicoated high power 1.25” 9mm (133x) 52° field Plössl.

  • Eyepiece tray: A standard equipment eyepiece tray attaches to the side of the scope’s altazimuth base. It has cutouts to hold one 2” eyepiece and three 1.25” eyepieces.

  • Finderscope: 8 x 50mm straight through fully-multicoated dark crosshair achromatic design. The finder has a long and comfortable 13mm eye relief. It focuses by loosening the trim ring behind the objective lens cell, screwing the lens cell in or out to focus, and tightening the trim ring to lock in the correct focus.

  • Finderscope bracket: The 8 x 50mm finderscope mounts in a quick release bracket that slips into a pre-installed metal mounting shoe beside the focuser. The bracket is held in place in the mounting shoe by a single large hand-tighten chrome-plated knob that maintains the finder’s collimation, even if the finder is removed between observing sessions.. The bracket has two hand-adjust collimation screws that work in conjunction with a third spring-loaded post in the bracket. This makes collimation faster than the traditional three- or six-screw collimation methods used with conventional finder brackets.

This Telescope’s Mount . . .

  • Type: Standard rocker box Dobsonian mount.

  • Altitude ball bearings: Instead of the usual large circular trunnion bearings riding on a few Teflon pads in the semi-circular cutouts in the altazimuth base, the 8” Astro-Tech Dob uses innovative sealed metal ball-bearing systems to provide smooth vertical motion of the optical tube. The aluminum bearing housings fit snugly into cutouts in the sides of the altazimuth base. Because the bearings are sealed systems, the elements cannot affect the smoothness of the Astro-Tech’s vertical motion, as it can when dust and grit invariably get trapped between the exposed trunnions and Teflon pads of conventional Dobsonians and scores the surfaces. The vertical motion of the Astro-Tech tube is consistently ball-bearing smooth. And the sealed ball-bearings will not have to be replaced regularly, as is the case with the Teflon pads of many Dobsonians when wind-borne grit degrades the Teflon/trunnion interface.

  • Altitude clutch: Large ribbed clutch knobs are built into the two altitude bearings. A gentle twist on either knob adjusts the amount of friction within the altitude ball-bearing systems. This allows you to keep the focuser from rising or sinking when changing between eyepieces of considerably different weights, as for instance when switching between 1.25” and 2” eyepieces.

  • Tube balance system: The altitude bearings of the AT8D are bolted to slotted plates attached to the optical tube. This allows the optical tube to be moved fore and aft in the slots to balance the optical tube. The altitude bearings have been positioned at the factory to balance a wide range of eyepiece weights with the occasional help of the altitude clutch knobs. However, if you have mostly heavy eyepieces in your observing kit, you may want to adjust the tube balance to compensate for the tube’s resulting nose-heavy balance.
    By tilting the optical tube down until it is parallel with the ground, and loosening the altitude clutch knobs, you can observe whether the eyepiece end of the tube rises or falls when you let the tube go with your heaviest eyepiece installed. You can then loosen the balance lock screws on the altitude bearings and slide the altitude bearings back or forth in the slotted plates as needed to balance the optical tube properly. An engraved scale on each plate makes it easy to move both altitude bearings the same amount. Tighten the balance lock screws and your tube is balanced for the majority of your eyepieces.
    Balancing the tube needs only be done once if you discover that your optical tube is consistently nose- or tail-heavy. The altitude clutch knobs will take care of most occasional minor imbalance situations when changing between eyepieces of different weights.

  • Altazimuth base (rocker box): The altazimuth base that the optical tube rides in is crafted of strong, lightweight, and water-resistant laminated particle board. The base is shipped disassembled, but can be put together in about a half an hour using only a Philips-head screwdriver and the supplied hardware and hex-head wrench. The AT8D moves in azimuth on dozens of steel roller bearings riding between two metal plates. Push the scope lightly in any direction and it starts moving at the touch of a finger – smoothly and with no fuss. Stop pushing and it settles down immediately, with no shudder or vibration to mar your viewing experience. A carrying handle is provided to make carrying the rocker box easier.

The illustration shows the 10” version of the Astro-Tech Dob. Except for a larger tube and rocker box, the 10” is identical in appearance to the 8”.

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.

0.57 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.
54 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:
Very Good
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
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1. david on 3/10/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
I purchased the AT8D to donate to a charity auction. I settled on an 8" dob as the simplest to use and most bang for the buck. After putting it together and taking it for a test spin, I was very impressed. The motions were beautiful, if not perfect with adjustments. The thing has been very well thought-out, with a good system for adjusting the OTA to compensate for eyepiece weight and accessories. Movements in both Alt and Az were superior to many expensive high end dobs.
The optics were very good. Though the night had pretty good seeing, I could not test to see if the mirrors met the Dawes limit. However, I feel certain that the optics were in the 1/4 wave PV or better category. There was virtually no shift in collimation in altitude shifts as detectable with a laser.
The 2" focuser is of good quality.
Though I own a good number of expensive custom and high end scopes, a scope of this quality for this price might call into question purchasing a custom 8" solid tube scope. Commercially produced dobs have certainly come along way.
The telescope is a good combination of easy to move and set-up, while affording excellent views of the moon planets, and wide field deep sky objects.
To be clear, I have no connection to Astronomics or anyone who works there.
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Visual Accessories
Barlow Lenses (1)
2X apochromatic ED Barlow for 2" eyepieces, with 1.25" adapter
by Astro-Tech
Collimating Tools (1)
Newtonian reflector collimating tool
by Celestron
Eyepieces (2)
20mm 1.25" Value Line Plössl
by Astro-Tech
6mm 1.25" Value Line Plössl
by Astro-Tech
Filters (1)
1.25" #ND9 13% Transmission neutral density grey Moon filter
by Astro-Tech
  • 8” f/5.9 diffraction limited BK7-equivalent optics
  • 3-point primary mirror floatation system
  • Dual-speed 2” Crayford focuser with 1.25” adapter
  • Ball-bearing altitude motion with clutch
  • Roller-bearing azimuth motion
  • Tube balance system
  • Battery-operated cooling fan
  • 8 x 50mm finderscope
  • 2” 30mm SuperView eyepiece
  • 1.25” 9mm Plössl
  • Eyepiece tray
Astro Tech 8" & 10" dobsonian manual 1001kb(s)
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Astro-Tech - AT8D 8" F/5.9 Dobsonian reflector

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Astro-Tech - AT8D 8" F/5.9 Dobsonian reflectorClose-up of the altitude bearing assenbly and and tension knob of the Astro-Tech AT8D Dobsonian.Close-up of the AT8D dual-speed Crayford focuser, 2" eyepiece, and finder bracket.
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This value-packed Astro-Tech AT8D 8” Dobsonian reflector offers big performance at a little price. It’s easy to transport, easy to assemble, easy to own, and easy to use . . .

. . . our 36th year