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Astro-Physics 92mm f7 "Stowaway" Telescope

Astro-Physics 92mmf7
Above: Astro-Physics 92mmf7 "Stowaway" Apochromat Telescope with retracting Lens Shade extended, Lens Cover removed.
Telescope shown attached to optional TeleVue "Telepod" Head, with Astro-Physics 2 inch Maxbright Mirror Diagonal attached (209,055 bytes).

Released in July 2002 as a very limited production item the Astro-Physics 92mm f7 "Stowaway" telescope evolved from an 90mm f7 doublet concept, developed by Roland Christen through the original "Stowaway" a limited production 90mm f5 triplet Apochromat telescope in 1999, into the current 19 inch long, 6.5 lb. (3 kg) 92mm aperture 604mm f6.6 triplet Apochromat telescope. It is very likely that not many people will ever see both of these 90mm telescopes displayed at any other location than our Laurel, Maryland showroom or at our Internet site since both of these first production runs are extremely limited in numbers made, and neither is 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.

This 90mm f7 telescope is probably the culmination of years of optical research by Roland Christen of Astro-Physics in an 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.

EVOLUTION:

In 1999 Roland Christen explained the development and performance of the first 90mm f5 Stowaway models:

    "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 daytime instrument, but this lens works extremely well on nighttime 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 Bootes, Hercules and Cygnus down to 1.2 arc seconds, and elongate one at 0.9 arc seconds. The two unequal doubles of Epsilon Bootes 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."

Astro-Physics 90mm Stowaway attached as a Finderscope onto Astro-Physics 206mm EDF Martin Cohen of Company Seven (aside from the factory, C7 is the only retailer of Astro-Physics telescopes in the Americas) had seen the prototype 90mm f7 Doublet some years ago; he had been asking Al Nagler at TeleVue and Roland at Astro-Physics to consider making such a telescope. By 1998 TeleVue went on to develop the highly successful TeleVue 85 telescope. But Astro-Physics continued it's research and development, almost out of curiosity just to see how far they could "push the envelope"; this effort resulted in the July 1999 release of the ultra compact 90mm f5 Stowaway. The public had never before seen such a high performance telescope of this aperture, and all who saw the Stowaway were quite pleased by the very short physical length (just 14 inches/36cm) and stunned by the performance all across a wide range of magnifications. Because of its capability to provide phenomenal wide fields of view as well as good high magnification capability, Martin actually decided to use his compact f5 Stowaway configured as a Finder/Wide Field of view telescope on his larger telescopes - this was in part a slap at the "scalpers" who were within months of the release of these $2,400 gems commanding prices for the scarce Stowaways of $6,000 or more.

Right: Astro-Physics 90mm Stowaway attached onto Astro-Physics 206mm EDF f8 Apochromat 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! (57,871 bytes).

But if the f5 Stowaway telescope could be redesigned and made a bit longer to an f7 or so, then it might be easier to balance the rear heavy tube on alt-azimuth mounts, it would remain very compact, and one could still attain a 7mm exit pupil and larger with very wide fields of view with suitable available eyepieces. And at about f7 the telescope objective lens would be easier to manufacture (only by a company already so adept at triplet fabrication as is Astro-Physics), and the telescope would be less demanding of the eyepiece. By the year 2001, Roland became convinced that a variation of the 90mm concept might prove to be marketable. He commented on the 92mm f7:

    "You asked for this scope. For some, the original f5 Stowaway was too short for an all-purpose visual scope. This telescope sports the same lens as our legendary Traveler, but with a bit less aperture (92mm vs 105mm) and a little over 1/2 the weight. While the Traveler was intended for astrophotography, with formats as large as 6 x 7 cm, the 90 f7 Stowaway has a field size of 2" making it suitable for 35mm and CCD imaging. The scope sets up quickly on even the smallest mounts and gives excellent high contrast images on any object from large deep sky objects, to lunar and planetary views. As the name suggests, this tiny scope can come along with you in your travels, take up very little room and its light weight is a real advantage when you have to traverse half a mile of airport concourse."

FEATURES:

The "Stowaway" 90mm and 92mm multipurpose telescopes resembles the Astro-Physics 105mm "Traveler" EDF telescope with its compact frame, and collapsing Lens Shade to insure compact storage. But the Stowaway differs from the 105 in having:

    • the 90mm Optical Tube Barrel, Front Cell, and Lens Shade feature the durable 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 slipping particularly when wet with Dew, and it will not show reflections (in sunlight, etc.) if used as a terrestrial nature watching 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 to permit very fine focusing.

    • The Stowaway telescopes are furnished with one pair of mounting rings (similar to the "Traveler") with an seven inch long Slide Bar. The custom milled aluminum Slide Bar fits onto the optional Astro-Physics DOVE08 Dovetail Plate. This Bar is threaded with several 1/4 inch 20 TPI and 3/8 holes to facilitate using a Stowaway onto a variety of suitable equatorial mounts, or camera tripods, or popular lightweight Alt-azimuth heads made by TeleVue Optics. 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 design and manufacturing process of the 90mm and 92mm Stowaway telescopes ended up costing about as much as the larger 105mm Traveler telescope since each is about as labor intensive, was about the same in materials (or a bit more to "tool up"), and the new focuser 2 inch is actually more complicated than the traditional Astro-Physics 2.7 inch focuser.

The lens is a very transparent non air spaced design, only the objective's front and rear glass surfaces are exposed to air, and these two surfaces incorporate a multilayer antireflection coatings that improves overall light transmission to greater than 97% in the visual wavelengths. So, the system has about 167 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 subfreezing conditions it rarely requires more than 25 minutes to acclimate from typical room temperature.

The lens is so well figured that with optional eyepiece lenses it is capable of operating at 300X or more cleanly; 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 with the optional Baader Binocular Viewer take a walk on an apparently three dimensional Moon! Or, follow its terminator throughout the moons phases (we suggest optional neutral density filters to attenuate its brightness). The Stowaway will resolve objects on the moon as small as 4.9 km in diameter. It will see the four largest moons of the gas giant Jupiter and their shadows cast as a distinct round disc on the milky white, cream, beige and tan colored bands of Jupiter when they transit past the planet!

With a matched low magnification ocular the "Stowaway" provides an extraordinary wide field of view showing up to 4.4 degrees actual field of view at 11X. It can reveal the entire "Messier Catalog" of deep sky wonders. Or it may be employed for panoramic viewing of a seashore or countryside with a breath taking wide angle eyepiece such as the TeleVue 27mm or 35mm Panoptic, or 22mm Nagler Type 4. 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 92mm "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 about 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".

Astro-Physics 90mm f7 Stowaway in furnished carrying case, with Dew Shield retracted for transport (170,923 bytes). Click on image for enlarged view (226,048 bytes). Right: Astro-Physics 90mm f7 Stowaway in furnished carrying case, with Dew Shield retracted for transport. Case includes five adjustable space partitions for accessories; furnished Carrying Strap not shown (170,923 bytes). Click on image for enlarged view (226,048 bytes).

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 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 diameter 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.

MECHANICAL CONSTRUCTION:

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 enlarged 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 92mm "Stowaway" is the best performing telescope of its size.

"STOWAWAY" 92mm f7 TELESCOPE:

    SPECIFICATIONS
    Color correction: Less than 0.01% focus variation from 706nm to 405nm
    Clear aperture: 91.90mm (3.62")
    Focal length: 604mm (23.8") (actually f6.6)
    Resolution: 1.26 arc seconds
    Coatings: Multi-layer, overall transmission greater than 97% in peak visual wavelengths
    Magnification range: 11x 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: 48.3cm (19") with dew cap fully retracted
    Weight with Lens Shade/Dew cap: 6.5 lbs. (3 kg)
    Carrying case type: Custom padded bag
    Case outside dimensions: 22.5"x 11" x 6.5" (57cm x 28cm x 17cm)
    Case Weight: 4.5 lbs. (2 kg)
    35mm prime-focus field: 3.3 x 2.3 x 4 degrees @ f6.6
    35mm telecompressor field: 4.1 x 2.8 x 5 degrees @ f5.3
    35mm field with 2x Barlow: 1.6 x 1.1 x 2 degrees @ f13.1
    * Specifications are subject to change without notice.

FEATURES OF THE OPTICS

  • Very high correction of spherical and chromatic aberrations made possible by the triplet Apo design.
  • Clear, high purity color free glass types result in brighter, more contrasting 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 optional Field Flattener)
  • High-resolution optics are a good match for fine-grained Technical Pan emulsions
  • High magnification capability makes the 92mm suitable for use (with an optional negative lens) as a photoguide telescope

FEATURES OF THE TUBE ASSEMBLY AND ACCESSORIES

  • Durable, fully machined, corrosion resistant construction
  • Low profile and lightweight mounting rings (pair)
    Mounting rings include a custom milled lightweight Slide Bar
  • Lens Shade/Dew cap retracts (sliding back over front cell) for compact storage
  • Fully baffled tube and focuser assures high contrast
  • Precise geared 2 inch focuser facilitates exact photo visual 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.

SUGGESTIONS

  • 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.

  • 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 antireflection 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.

  • Solar Filter: Astro-Physics 92mmf7 This Solar Filter permits the observing and imaging of details on the Sun's disk including sunspots, and Solar Eclipse events. These filters transmit a broad bandwidth from our Sun while rejecting the harmful infrared and ultraviolet radiation. The Sun is presented as a whitish disk contrasted against the blackness of space.

    Right: Astro-Physics 92mmf7 "Stowaway" Apochromat Telescope equipped with optional Baader Astro-Solar Filter in fitted slip on Cell, and the Baader Binocular Viewer attached (184,912 bytes).

    As stated by Beck, Hilbrecht, Reinsch, and Volker the authors of one of the finest books on the subject "Solar Astronomy Handbook": "Compared with other areas of astronomy solar observing has a number of advantages:

    • Observation can be carried out during the day. You do not have to stay up all night and arrive at work the next day exhausted!
    • There is an abundance of light. Unlike "Deep Sky" astronomy you actually have to discard much of the light reaching your telescope.
    • You can set up your observatory in your own backyard - even in the city -there is no need to escape light pollution at remote locations.
    • Observations can be made practically every clear day and some simple programs like sunspot counts can be done in just a few minutes.
    • You do not need a monster telescope, even a small telescope will show an amazing amount of detail.
    • The view is constantly changing, the Sun's appearance has never been, nor will it ever be exactly the same as today."

  • Binocular Viewer Astro-Physics Stowaway Telescope with optional Baader Binocular Viewer (17,560 bytes)
    This accessory accepts two optional identical focal length 1.25 inch diameter oculars to permit comfortable 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 quite three dimensional. Such viewers are particularly desirable for views of the planets, moon, and sun, and on some of the brighter deep sky objects.

    Right: Astro-Physics "Stowaway" Telescope with optional Baader Binocular Viewer (17,560 bytes).

    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) and furnished with 2 inch male nosepiece fittings to permit straight through observing, although a prism diagonal is included for convenient right angle viewing.

  • 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: At f6.6 good quality Plossl, Orthoscopic, traditional Erfle and some wide angle eyepieces can provide sharp images across most of the field of view 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 across to 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 "Stowaway" (and many other) telescope.

    Below is a brief outline of performance with some selected optional eyepieces. Please refer to the test report generated by Company Seven as we check through the telescopes (enclosed with those telescopes we deliver) for descriptions of these items and many additional photo-visual accessories.

    Eyepiece Magnification Actual field of view Exit pupil
    55mm Plossl 11x 4.36 degrees 8.37mm
    35mm Panoptic 17x 3.67 degrees 5.33mm
    27mm Panoptic 12x 2.89 degrees 3.35mm
    22mm Nagler Type 4 27x 2.95 degrees 3.35mm
    22mm Panoptic 27x 2.36 degrees 3.35mm
    12mm Nagler Type 4 50x 1.62 degrees 1.83mm
    6mm Radian 101x 0.60 degrees 0.91mm
    3mm Radian 201x 0.31 degrees 0.46mm


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