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The 90mm f5 "Stowaway" Telescope
(actually 92.5mm/3.64 inch aperture - but who's counting)

Astro-Physics 90mm Telescope (70748 bytes)
Above: Astro-Physics 90mm Telescope installed onto an optional tripod, with optional "Maxbright" Diagonal and 1.25" Eyepiece (70748 bytes).

Announced in June 1999 as a very limited production item, the long awaited Astro-Physics 90mm "Stowaway" telescope has evolved from an 90mm f7 doublet concept into a 14 inch long, 6 lb., 92.5mm aperture 450mm f4.9 triplet Apochromat telescope. It is very likely that not many people will ever see this 90mm telescope displayed at any other location than our Laurel, Md. showroom or, at our Internet site since this first production run is extremely limited in numbers made to prove the concept, and it is not likely to remain in regular production. If you are interested in a telescope such as this, then you should contact Company Seven for advice about availability or alternative telescopes.

The 90mm f5 telescope is the culmination of years of optical research by Roland Christen of Astro-Physics. This effort aimed at developing a very fast and portable multi purpose telescope that will allow you to enjoy sharp, high-contrast images wherever you go.

Notes on the 90mm f5 by Roland Christen:

    Several years ago a friend asked me for a really well-corrected small telescope that he could use for bird watching. One that would travel well, fit on a small tripod and have reasonably good performance on the night sky since he is also an avid amateur astronomer. He already had a Traveler (which his wife loves for deep-sky wide-field observing), but he wanted something about 1/2 the size. After many design attempts and prototypes, we finally came up with a very short 90mm f5 that weighed just a bit more than half that of the Traveler. I gave it to him at the Winter Star party several years ago and he loved it. He especially appreciated the high-contrast, color-free images it produced. The white plumage of egrets remained white, unlike the views with other short refractors where color fringing is really noticeable and objectionable. He dubbed it the "Stowaway", since it fit easily into his luggage and the tube is only 14 inches long.

    In the production version, we kept the original 3-element APO lens design, added about 2mm to the clear aperture (it is really a 92mm refractor) and developed a brand new 2 inch focuser with coarse and fine-adjustment to allow critical focusing at high power.

    How does the lens work on astronomical objects? It's one thing to be a superb day-time instrument, but this lens works extremely well on night-time objects. It is, after all, a similar design to the famous Traveler EDF lens. With a 40mm wide-field 2 inch eyepiece, it will show almost 6 degrees of sky at 11x. On the other end of the power range, with a 3x Barlow and 4mm eyepiece (337x), I was able to easily split a number of double stars in Bo×tes, Hercules and Cygnus down to 1.2 arc seconds, and elongate one at 0.9 arc seconds. The two unequal doubles of Epsilon Bo×tes and Beta Cygnus showed beautiful, clean separations with pearl-like star points.

    Recently on Mars, I easily made out the North Polar Cap and the dark ring around it. I observed disc detail on the ball of the planet. There were prominent white clouds on the edge of the morning side and near the south polar regions. This observation was well after opposition when the planet was down to 12 arc seconds diameter.

    In short, I am very pleased with how well this small telescope performs with such a short focal ratio. There is nothing else like it on the market today and this limited production run will become an heirloom for sure.

The "Stowaway" 90mm multipurpose telescope resembles the Astro-Physics 105mm "Traveler" EDF telescope with its compact f5 frame, and collapsing Lens Shade to insure compact storage, but differs from the 105 in having:

    • the 90mm optical tube, front cell and Lens Shade feature the textured off white finish common to the 13cm and larger Astro-Physics telescopes; this will "blend in" better if the "Stowaway" is employed as either a wide field Finder telescope or as a photo-guide telescope on top of a larger Astro-Physics telescope. Also the finish is something of a "matte" finish, and so will be less prone to showing reflections (in sunlight, etc.) if used as a terrestrial spotting telescope.

    • A scaled down machined focuser: from the standard 2.7 inch model to a new 2 inch focuser which incorporates a very smooth reduction gearing mechanism.

    • The telescope is furnished with one pair of mounting rings (similar to the "Traveler") with a 5.5 inch long Slide Bar. The custom milled aluminum, lightweight Slide Bar fits onto the optional Astro-Physics DOVE08 Dovetail Plate, and this Bar is threaded with one 1/4 inch 20 TPI holes to mount it onto suitable camera tripods or a common popular Alt-azimuth heads. The Slide Bar can be removed to attach the telescope by its two mounting rings (each with a 1/4 inch 20 tpi threaded hole) onto a variety of other stands and mounts.

The telescope design and manufacturing process ended up costing about as much as the larger 105mm Traveler telescope since the amount of labor was about the same (or a bit more to "tool up"), the new focuser accepts up to 2 inch diameter accessories and is actually more complicated than the traditional Astro-Physics 2.7 inch focuser, and there is not that much difference in the other costs of materials.

The objective's two air to glass surfaces have a multi-layer anti-reflection coatings that improves overall light transmission to greater than 97% in the visual wavelengths. So, the system has about 169 times the light gathering power of the unaided human eye making it through the objective lens. On most nights, the settling down time for the lens is on the order of minutes, and even in sub-freezing conditions it rarely requires more than 30 minutes to acclimate from typical room temperature.

The lens is so well figured that it is capable of operating at 300X or more cleanly with optional eyepiece lenses; this reaches well into that threshold necessary to routinely obtain meaningful views of the planets; showing several bands and their shades of color (beige, tan, browns) on Jupiter, the Cassini division on Saturn, the polar cap on a fiery red Mars. With an eyepiece that shows 1/2 degree field or so, or the optional Binocular Viewer take a walk on an apparently three dimensional Moon! Or, follow its terminator throughout the moons phases (optional neutral density filter suggested to tone down its brightness). The 90mm should resolve objects on the moon as small as 4.8 km in diameter; it will see the four largest moons of Jupiter and their shadows cast as a small round disc on the cream, beige and tan colored surface of Jupiter when they transit past the planet!

With a matched wide angle ocular the "Stowaway" reveals its extraordinary wide field of view ability showing up to 5.7 degrees actual field of view at 8X to reveal the entire "Messier Catalog" of deep sky wonders, or to be employed for panoramic viewing of a seashore or countryside. Imagine a telescope that at a dark sky site has the combination of resolving power and field of view to sweep the Milky Way, see the Double Cluster in Perseus (NGC-869), find the Andromeda galaxy (M-31) and see it's ellipsoid shape, and see the form of large Nebulae such as the Veil and North American Nebulae! With an optional TeleVue Optics 22mm Nagler Type 4 or the 35mm Panoptic ocular the "Stowaway" can reveal all three stars of Orion's belt, closer in it reveals the jewel like stars of the Trapezium - a birthplace of stars, with a sweeping wisps of greenish gas clouds surrounding it. With a higher magnification M13 (the Hercules star cluster) takes on a "salt and pepper" appearance even from suburban skies on a clear night. By the time one finds a large enough aperture catadioptric, reflecting, or achromatic refractor to see similar detail, then one can only see a fraction of their area due to the relatively high focal length of these competing telescopes; imagine trying to observe the night sky through a straw!

The 90mm "Stowaway" compares favorably against many telescopes in field of view:

  • 1.9 degrees maximum at 22X for a common 80mm achromatic refractor,
  • 1.4 degrees at 31X for a common 8"f6 reflecting telescope,
  • 1.3 degrees at 32X for an 8" Schmidt-Cassegrain (even when operating with an optional telecompressor lens)
  • 1.3 degrees at 32X for an 3-1/2" Maksutov-Cassegrain.

In fairness, the cost of the "Stowaway" (equipped with either an optional Alt-Az or Equatorial mount) is positioned at a higher cost than most of the above telescopes. And as a practical matter for some applications in astronomy the nominal match of the "Stowaway's" exit pupil to an average human eye will be obtained at magnifications of between 17X and 13X, while the best performance will be obtained at all magnifications if a wise choice of well corrected oculars is made - this is not attainable with several of the simpler eyepiece designs. Also consider that an optional good quality "Barlow" amplifying (negative) lens, or long eye relief TeleVue "Radian" oculars (also introduced in 1999) will usually be put into use to attain the highest use able magnifications.

Optional camera adapters are available to employ the telescope for film photography, or for CCD imaging operations (such as is possible with our SBIG CCD systems. Techniques and hardware permit imaging of the planets, moon, or deep sky wonders, or of terrestrial wildlife over a wide range of magnifications.

With optional 2 and 3 inch long, 2 inch diameter extension tubes it is possible to use the telescope to observe at at very close distances as a "long distance microscope".

A particular joy of this instrument is that with a suitable mount is at the limit for those who prefer a completely "carry-on luggage" degree of portability. Few larger refractors may be carried on, but then these will require a notably heavier mount that must be packaged and checked in separately. Furthermore, this is a telescope which one can grow with since most of the accessories that one would buy for this telescope are upwardly compatible with any larger telescope that one is likely to buy at Company Seven.

An eyepiece of between 22mm to 35mm focal length can also make the telescope suitable for use as your finder!

Astro-Physics 90mm Stowaway attached as a Finderscope onto Astro-Physics 206mm EDFLeft: Astro-Physics 90mm Stowaway attached as a Finderscope onto Astro-Physics 206mm EDF telescope. Note optional 2 inch "Maxbright" diagonal and TeleVue 31mm Nagler Type 5 ocular providing 15x showing 5.19 degrees field of view - talk about the Tail wagging the Dog! (57871 bytes).

An optional 90 degree mirror diagonal will provide a comfortable viewing position for the observer by diverting the image at the rear of the telescope off axis by 90 degrees to the side, with minimal degradation of image quality. The view when using the telescope as it is provided will appear to be right side up, and is reversed left to right; this is not usually a problem for astronomy or nature observing because the image quality provided by the high quality mirror and 2 inch diameters panoramic oculars is exceptional.

While Company Seven does offer optional 45 degree inclined image erecting prisms, these prisms can only accommodate 1.25 inch oculars. Furthermore, the erecting prisms will not provide image quality to equal that of a good mirror diagonal, this is particularly perceptible at higher magnifications. However, for moderate to medium magnification applications where one needs correct images (such as to read numbers, etc.) then the prism should be suitable.


Astro-Physics 2 Left: Astro-Physics 2" Focuser with coarse (Black knob) and fine (Brass knob) geared reduction. Smaller silver knob at on Focuser Housing is Drawtube Lock Screw, on Drawtube to the left is 2" Accessory Set Screw. The Astro-Physics Maxbright Diagonal shown here is optional. (41,274 bytes). Click on image for englarged view (385,643 bytes)

The mechanical construction of the Stowaway telescope is completely trouble-free and keeps the optics permanently aligned. It's optical tube assembly is precision machined in house with the most modem CNC equipment available; there are no fragile die casts in this telescope. Expert machinists transform solid, aircraft-quality aluminum into a fully baffled tube assembly. Astro-Physics staff has endeavored to achieve the highest absorption of stray light possible to provide the user with maximum contrast. The textured off white optical tube, and black anodized focuser will retain their beauty for many years. You will appreciate the unique design and fine craftsmanship of this telescope.

This new Astro-Physics 2 inch focuser is a very finely crafted unit with several unique features. The components are machined to extremely high tolerances, assuring that there is no wiggle between the drawtube and housing. Knife edge baffles are machined into the walls of the telescope optical tube and of the focuser drawtube, these and painted flat black in order to maximize contrast by essentially eliminating any internal reflections. The inside diameter (I.D.) of the drawtube is 2 inches thus permitting the avid astrophotographer to employ up to a 35mm format film or CCD camera to capture images with minimal vignetting. You can use standard 2 inch accessories, and with the furnished 1.25 inch adapter (threaded for 48mm filters) use common oculars and accessories too. Recessed brass locking rings are installed at each thumbscrew location; as you tighten a thumbscrew a brass locking ring clamps onto the part that has been inserted; consequently the focuser drawtube and any accessories are held securely in place and will not mar the surface of your accessories. This is particularly important considering the heavy and expensive accessories that you may use.

We invite you to compare the optical performance and mechanical construction of any other scope of comparable size on the market today. You will find that the Astro-Physics 90mm "Stowaway" is the best performing telescope of its size.


Color correction: Less than 0.01% focus variation from 650nm to 430nm
Clear aperture: 92.5mm (3.64")
Focal length: 450mm (17.7") (actually f4.9)
Resolution: 1.24 arc seconds
Coatings: Multi-layer, overall transmission greater than 97% in peak visual wavelengths
Magnification range: 8x to 330x
Tube assembly: White/Black finish, aluminum tube; fully baffled, permanently aligned cell construction; engraved focuser
Focuser type: 2.0 inch Astro-Physics rack and pinion focuser, 2.2 inch travel; 2" to 1.25" reducer
Focuser Gearing: Coarse, and Fine Focus with 10:1 Reduction
Telescope length: 35.6cm (14") with dewcap fully retracted
Weight with Lens Shade/Dewcap: 6.5 lbs. (3 kg)
Carrying case type: Custom padded bag
Case outside dimensions: 17"x 11" x 5" (43cm x 28cm x 13cm)
35mm prime-focus field: 4.4 x 3.0 x 5.3 degrees @ f5
35mm telecompressor field: 5.4 x 3.7 degrees @ f4.
35mm field with 2x Barlow: 2.2 x 1.5 degrees @ f9.9
* Specifications are subject to change without notice.


  • Very high correction of spherical and chromatic aberrations made possible by the triplet Apo design.
  • Clear, high purity colorfree glass types result in brighter, more contrasty images
  • Stunning lunar/planetary and deep-sky views - can be used as a finder telescope too!
  • Breath taking terrestrial and panoramic views
  • Well suited for 35mm format deep-sky astrophotography (with opional Field Flattener)
  • High-resolution optics are a good match for fine-grained Technical Pan emulsions
  • High magnification capability makes the 90mm suitable for use (with an optional negative lens) as a photoguide telescope


  • Durable, fully machined, corrosion resistant construction
  • Low profile and lightweight mounting rings (pair)
    Mounting rings include a custom milled lightweight Slide Bar
  • Lens Shade/Dewcap retracts (sliding back over front cell) for compact storage
  • Fully baffled tube and focuser assures high contrast
  • Precise geared 2 inch focuser facilitates exact photovisual focussing
  • 1.25 inch reducing adapter (threaded for 48m filters) with non-marring brass locking ring
  • Beautifully machined parts with lustrous finish
  • Machined aluminum lens cover to protect against dust
  • Sturdy padded carrying case will fit easily in airline overhead storage compartments.


  • 400 German Equatorial Mount with Dual Axis Drive
    This portable mount is perfect for visual and photographic studies in either the Northern or Southern Hemispheres.

  • Binocular Viewer: This accessory accepts two optional matched similar focal length 1.25 inch diameter oculars to permit observing with both eyes simultaneously through a telescope. The resulting images can be breath taking, producing a view that is unrivaled in contrast, clarity, and even distribution of light, and appearing three dimensional. Such viewers are particularly desirable for views of the planets, moon, and sun, and on some of the brighter deep sky objects.

    Astro-Physics 90mm Telescope with Binocular Viewer
    Illustrated Above: Astro-Physics 90mm Telescope with optional Binocular Viewer.

    Incorporating optics made by Carl Zeiss in Germany, these are precisely assembled and aligned into the Zeiss castings (now made by a third party in Germany).

  • Astro-Physics "Maxbright" 2 inch precision Mirror Diagonal. The "Maxbright" mirror features a very high-tech dielectric coating (developed originally for military optics used in hostile environments such as blowing desert sand) which provide optimum performance and years of use. This diagonal has no aluminum or other reflective metal coating. Instead, the reflective surface consists of 52 layers of thin film oxides similar to those used in anti-reflection coatings. The coatings are deposited by an electron beam evaporator at a high temperature. The result is that reflectivity is above 99% over the entire 4000 to 7000 ╗ photo-visual range. Thin film coatings have extremely low surface scatter compared to aluminum or enhanced aluminum coatings; examination with a laser source shows approximately a 5 fold improvement in surface scatter.

  • "Starbeam" Sight/Finder: Beautifully machined and black anodized aluminum, on a quick release machined hardware. Battery powered illuminator is adjustable in brightness. It projects a 10 arc minute diameter red dot onto a transparent 40mm clear aperture window so that the dot appears to be over wherever the scope is pointed. Six arc minute accuracy make "star hopping" easy. Optional mounting hardware is required to employ this, contact us for details as we will support those which telescopes we deliver. Not generally suggested for daytime use.

  • Field Flattener/0.8X Telecompressor: slides into the 2 inch barrel of the "Stowaway" telescope focuser. This makes the 90mm telescope perform as a f4 Flat Field telescope, with only very slight vignetting at the extreme corners of the film negative. Requires only an optional "T-Adapter" ring for Nikon, Minolta, etc.

  • 1.25 Inch Prism Diagonal: 45 degree 1.25 inch diameter precision image erecting prism diagonal with high transmission coating. This is a good choice for those who must have an image erect and correct left to right view, and who do not intend to use the telescope at higher magnifications.

  • Barlow/Negative Lens: Consider using either our Astro-Physics 2" diameter 2x Barlow, or a TeleVue "Powermate" lens (available in 2.5X, 4X, or 5X) to increase your magnification and economize your eyepiece selection.

  • TeleVue Equalizer: A slip fit machined bronze, heavy 2 to 1.25 inch diameter reducer adapter. Helps to keep telescope balance when switching to and from light and heavy accessories and eyepieces. Particularly helpful when using telescope with Alt-Azimuth mounts.

  • Eyepieces: Plossl, Orthoscopic, traditional Erfle and wide angle eyepieces show sharp images only in the center of the field when employed with telescopes such as this. These are acceptable as long as you realize this limitation, and find this unobtrusive. If you object to less than perfect images at the edge of the field, then Company Seven recommends the TeleVue "Nagler", "Radian", or "Panoptic" eyepieces. These oculars have the best flat field images and will bring out the most in your "90mm" (and many other) telescope. Please refer to the photo-visual test report generated by Company Seven as we check through the telescopes (enclosed with those telescopes we deliver) for descriptions of these items and additional accessories.

    Eyepiece Magnification Actual field of view Exit pupil
    35mm Panoptic 13x 4.82 degrees 7.0mm
    27mm Panoptic 17x 3.8 degrees 5.4mm
    22mm Nagler Type 4 21x 3.9 degrees 4.4mm
    22mm Panoptic 21x 3.1 degrees 4.4mm
    12mm Nagler Type 4 38x 2.1 degrees 2.4mm
    6mm Radian 77x 0.62 degrees 1.21mm
    4mm Radian 115x 0.42 degrees 0.8mm


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