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Milsom's terrific travelscope: extreme portability doesn't get more elegant than this.

IT'S BECOME A FACT of astronomical life: To enjoy really good, dark skies, most of us have to travel. And this reality is reflected in the kinds of telescopes emerging from home workshops all over the world. The travelscope, once a charming novelty, has become one of the most popular ATM projects going. We've featured portable scopes in the past (including my own airline-portable 8-inch f/4 in the December 2001 issue, page 120), but I think Francis Milsom's 6-inch f/4 reflector is really something special.

"If you can drive to an observing site, your scope only has to fit into your car," Milsom notes. "But if you're flying to dark skies, then you need something that really- breaks down." Not only has this UK-based telescope maker designed a very compact scope, but it also disassembles into a package that can easily be stored in the overhead compartment of a commercial airliner.

Milsom's travelscope has several interesting and unusual features. Most obvious are its single-strut construction and what he describes as a "pseudo-Dobsonian" mount. Less eye-catching, but perhaps the cleverest feature of all, is the nifty focusing mechanism. To focus the scope, you slide the eyepiece, secondary mirror, curved-vane spider, and light shield back and forth as a single unit.


Although the basics of such "sled" focusers date back to early 18th-century reflectors, they're not often seen in modern amateur scopes. Milsom's focuser achieves its back-and-forth motion by means of pulleys and a stainless-steel cable. In his own words:
I made a two-piece sliding carriage block. One piece is faced
with PTFF [Teflon] and fits inside a 3/a x 3/4-inch aluminum
channel, while the other piece rides on  a pair of PTFF clips that
fit over the outside of the channel. The two pieces are linked
by a screw passing through a slot milled in the channel. By
adjusting the screw's tension, I can regulate the focuser's friction
so that it slides along the channel easily, yet with enough
resistance that it doesn't slip. Turning the focus knob makes the
carriage block slide tip or down the channel.

Also crucial to the scope's functionality and good looks is the back end of the instrument. Milsom's sketch below shows most of the details. In spite of its unusual configuration, the mount has bearings faced with Teflon and Ebony Star (a Formica-type laminate), as is typically found in Dobsonians. The scope itself attaches to the mount with three screws--two passing through holes in the brass angle strip and one through the mount arm directly into the mirror box.


One of the potential "gotchas" that telescope makers often encounter with a scope that breaks down into many parts is the extensive collimation needed each time the instrument is set up. Milsom saw this coming. "After assembling the scope for the first time, I tweaked the collimation with the star test until I could slide the focus carriage back and forth without changing the alignment. The next day, I cut small reference lines across any joints that might rotate--mainly the strut sections. Now when I reassemble the scope, I just make sure the marks line tip, and the only collimation required is a minor touchup."


All of Milsom's effort would amount to little more than an attractive engineering exercise if the scope didn't work well or took forever to assemble in the field. "Although there appear to be lots of components, this scope sets up very easily," he notes. Milsom begins by inserting the three legs into sockets in the ground board and attaching the mirror housing to the side arm of the mount. After assembling the three strut segments into a single piece, he affixes the secondary mirror assembly to the focuser mechanism. Finally the cover comes off the primary mirror, and Milsom is ready- to check the collimation and observe. The whole process takes only five minutes or so.

"I've used this little scope during two trips to the United States and an excursion to northern Spain," Milsom says. "Happily, I've found it works beautifully. Indeed, the biggest problem I've had so far was persuading a US customs official that the odd-looking contraption was really a telescope. I managed to convince him by putting it together. That even made him smile!"


Contributing editor Gary Seronik is a veteran telescope maker who journeys to Costa Rica each year with his 12 3/4-inch travelscope.
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Title Annotation:Telescope Workshop; Francis Milsom
Author:Seronik, Gary
Publication:Sky & Telescope
Geographic Code:1USA
Date:Aug 1, 2008
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