Astro-Tech 8" f/4 Imaging Newtonian Optical Tube

A review of three Astro-Tech astrographs (the AT6RC and AT8RC Ritchey-Chrétiens, as well as this AT8IN Imaging Newtonian) in the December 2009 issue of Sky & Telescope said "While all three of the Astro-Tech scopes represent excellent value, the AT8IN, with its 8-inch aperture and $499 price tag, wins the biggest-bang-for-the-buck award . . . there's a quality to this scope that seems contradictory to its relatively low cost."

This Astro-Tech Imaging Newtonian optical tube has:

• very fast f/4 focal ratio 8" Newtonian reflector optics
• contrast-enhancing extended tube length with 15 internal baffles
• built-in cooling fan for faster primary mirror cooldown
• 2" dual-speed Crayford focuser with 2" and 1.25" compression ring accessory adapters
• 8 x 50mm finderscope in a quick-release bracket
• dual split hinged mounting rings

This 8" Astro-Tech AT8IN optical tube is a very fast focal ratio f/4 reflector that has been optimized for wide field deep space imaging. It can be used with 35mm cameras, DSLRs, web-cams, DSI-type cameras, and large format CCD cameras alike.

Astronomy named the Astro-Tech AT8IN a Star Product for 2010, saying "Astronomy Technologies has optimized the AT8IN for astroimaging, but the scope is just as adept for backyard viewing . . . a versatile instrument that will take you into the depths of the universe in style."

The light weight standard equipment mounting rings of the Astro-Tech Imaging Newtonian allow you to use it on a wide variety of dovetail plates and German equatorial mounts - from the most basic to the most elaborate high payload go-to system.

In addition to deep space imaging, the Astro-Tech AT8IN is also usable for solar system imaging as well as deep space and solar system visual observing. Solar system visual contrast will be somewhat reduced compared to a longer focal ratio reflector because of the AT8IN's larger secondary mirror that has been optimized for full-field photographic illumination. However, the visual performance will still be much more than acceptable, and the 800mm focal length of this Astro-Tech makes it quite possible to achieve the high powers needed for detailed lunar and planetary observing. Simply add optional 1.25" or 2" eyepieces, and perhaps an optional Barlow lens, and this Astro-Tech Imaging Newtonian will provide you with crisp and sharply detailed close-up views of the Moon and planets.

This Astro-Tech Telescope's Optical System . . .

Newtonian reflector optics: 8" aperture, 800mm focal length, f/4 focal ratio, parabolic primary mirror; 70mm m.a. elliptical diagonal mirror. The mirrors are 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.
The mirrors are 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. For maximum contrast, the four secondary mirror spider vanes have been optimized to be as thin as possible without losing stability.

No-tool push-pull mirror cell: The die-cast aluminum primary mirror cell has six large hand adjust push-pull collimation knobs. These make it easy to collimate the primary mirror without tools, even while wearing gloves or mittens in cold weather. To further ease collimation, the primary mirror is precisely center-spotted. The Sky & Telescope review praised the "solidly made" primary mirror cell and secondary mirror holder.

Built-in cooling fan: A low-vibration/high CFM fan is mounted on the primary mirror cell. The fan is powered by a supplied battery pack that uses eight user-supplied AA batteries. Alternatively, the fan can be powered by a 12 VDC rechargeable battery if one is being used to power your mount's drive system.

Extended optical tube with 15 internal baffles: To increase the contrast, the optical tube of the Astro-Tech AT8IN is extended 8.75" forward of the focuser centerline to act as a lens shade to keep ambient light from hitting the diagonal mirror. In addition, there are 15 internal knife-edge baffles installed down the interior length of the optical tube to eliminate any stray light that might get past the extended optical tube and lower the contrast. The baffles and tube interior are finished in matte black to further absorb stray light. The result is exceptional contrast, far better than a conventional reflector for both imaging and visual observing.

The white-painted 32" long x 9" diameter (with 9.25" diameter front and rear cells) Astro-Tech optical tube is fabricated of rolled steel, to allow the mirrors to cool to ambient temperature more quickly. While the rolled steel optical tube is a little heavier than a more-costly aluminum tube, the Sky & Telescope review points out that "it also helps make it a very rigid setup, which is a good thing for astrophotographers." The scope's tube end rings are sturdy die-cast aluminum, to protect the tube during transport and provide exceptionally rigid support for the optics, "adding to the scope's overall rigidity," according to the Sky & Telescope review.

For essentially coma-free imaging with the AT8IN, consider adding the Astro-Tech ATCC coma corrector. This imaging accessory essentially eliminates the coma inherent in all fast focal ratio reflector telescope designs, so that the coma-free star images remain point-like all across the field.

2" dual-speed Crayford focuser: The precision-made 2" Crayford focuser drawtube ends in a 2" accessory holder. A 1.25" accessory adapter is standard equipment. Both the drawtube's 2" accessory holder and the 1.25" accessory adapter have non-marring compression ring eyepiece/accessory holders.

The focuser has two coarse focusing knobs. One knob also has a smaller concentric knob with 10:1 ratio reduction gear microfine focusing. This provides exceptionally precise focus control during critical CCD imaging. The focus knobs have ribbed gripping surfaces so they are easy to operate, even while wearing gloves or mittens in cold weather. A drawtube tension knob on the side of the focuser lets you adjust the drag on the focuser drawtube to hold various equipment load weights while focusing. A second knob lets you lock in a precise photographic focus.

The drawtube has 34mm (1.35") of travel. There is 80mm of back focus available from the top of the focuser's 2" accessory holder to the image plane. A 2" diameter x 35mm long extension tube is provided to match the back focus requirement to your particular photographic application. The 35mm extension tube has a non-marring compression ring eyepiece/accessory holder.

Split tube rings: A pair of die-cast aluminum hinged split tube mounting rings are provided. Each ring has a flat boss on its underside with a 1/4"-20 thread mounting hole, flanked by two 4mm metric holes on 30mm centers, for installing the ring on a Vixen-style or Losmandy-style "D-plate" dovetail mounting plate. This lets you mount the scope on virtually any equatorial mount. In addition, there is a flat boss with a 4mm metric hole on the top of each ring. This allows you to install a separate dovetail on top of the optical tube for mounting photoguide rings and a guidescope or similar accessories piggyback on top of the AT8IN. The lightweight optical tube (18 pounds tube only; 21.6 pounds with rings and finderscope) rotates in its felt-lined die cast cradle rings to bring the focuser and finder to the most comfortable viewing position.

Finderscope: 8x 50mm straight-through dark crosshair achromatic design, in a spring-loaded quick-release mounting bracket. The finder has a long and comfortable 13mm eye relief. To focus the finder, loosen the trim ring behind the objective lens cell, screw the lens cell in or out to focus, and tighten the trim ring to lock in the correct focus.

Tech Details: 

Supplied Accessories: 

• 8 x 50mm finderscope in quick-release bracket
• Cooling fan and battery pack
• Hinged split tube rings
• 35mm x 2" compression ring extension tube
• 2" and 1.25" compression ring eyepiece/accessory holders
• Dust cover