| 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-bears will not have to be replaced regularly, as is the case with the Teflon pads of many Dobsonians as windborne 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".
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