90mm f5 Stowaway

The 90mm f5 Stowaway is not available to purchase and we do not have any plans to produce additional scopes with these specifications. This information is presented for information purposes for present owners and those who are curious about its history.

90mm f5 Stowaway - Collection of Photos

Notes on the 90mm f5 by R. 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" 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" focuser with coarse and fine focus adjustment to allow critical focusing at high power.

How does the lens work on astronomical objects? Its 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" eyepiece, it will show almost 6 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 Boötes, Hercules and Cygnus down to 1.2 arc seconds, and elongate one at 0.9 arc seconds. The two unequal doubles of Epsilon Boötes and Beta Cygnus showed beautiful, clean separations with pearl-like star points.

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.


Color correction

0.01% from 650nm to 430nm (red to violet)

Clear aperture

92mm (3.62")

Focal length

450mm (17.7") which is f 4.9


1.24 arc seconds


Multi-layer, 97% transmission


11x to 330x

Tube Assembly

3.61" diameter, textured cream white finish, 14", aluminum machined, fully baffled, flat black interior, permanently aligned, sliding dewcap

Focuser type

2" I.D. Rack and pinion, 2.2" travel, coarse and fine focus knobs with reduction gear

Telescope length

14" with the dewcap fully retracted

Accessories included

Tube rings, 5.5" dovetail bar (fits our DOVE08 - 8" Dovetail Plate), and carrying case

Weight with dovetail and tube rings

3 kg (6.5 lb.)

Carrying Case

Custom padded soft case

Case dimensions

43 x 28 x 13 cm (17" x 11" x 5")

Photographic Field

35mm field

3.0 x 4.3 degrees at prime focus

35mm field w/ 2x Barlow

1.5 x 2.2 degrees @ f10


We have prepared some pertinent support documents relating to this scope and many of our other products on our Technical Support page. Please check there first to find the information that you need.

Upgrades and Spare Parts

Check out our website to find accessories, replacement parts or learn about possible upgrades for your scope.

Copyright 2004-2019 Astro-Physics Inc. All rights reserved.
Last modified: March 11, 2019

Astro-Physics, Inc.
11250 Forest Hills Road, Rockford, IL  61115, U.S.A.
Phone: 815-282-1513   Fax: 815-282-9847