notes on the construction of a portable 8" dobsonian

Setting the Stage

In 1991, Bob Scholtz (my brother-in-law) and I took a trip to Australia to spend some time observing with the members of the Astronomical Society of New South Wales, and to trek through Australia's Northern Territory... the fabled "Outback."

Bob and I are both amateur telescope makers (ATM's) from way back, and both of us have multiple instruments under our belts. So it was only natural that we decided to build a special telescope for the trek Down Under. Our plan was to put together a telescope that could be transported to Australia without taking up our entire luggage allowance.

I had recently come across glass blanks for making a 6" telescope mirror, and planned to ask Bob to grind and polish the optics for a portable telescope I would build. I knew that Bill Herbert, another Columbus ATM, had a nice focuser he wasn't using, so I asked if I could borrow it for the trip. Bill agreed, and the next Saturday morning I found myself in his basement collecting the focuser and asking him why he had it in the first place.

Apparently he had planned to build a rich-field 8" telescope but never finished it. However, he still had the completed mirror there in his basement. When Bill learned I was planning to build an ultra-portable telescope to take to Australia, he gave me the 8" f/4.3 mirror and the 2" focuser. (So much for having Bob make a 6" mirror!) Bill then spent an entire month refiguring the mirror and then had it coated with enhanced aluminum as his contribution to our trip.

I set about making the rest of the instrument. My goal was to design and build a telescope whose optical assembly could be transported as airline carry-on baggage. I selected a two truss tube design with a lengthy pedigree; the new telescope drew ideas from telescopic creations of other ATM's, including Thane Bopp, Tom Burns, Dick Suiter and Bob Bunge.

Dave Kriege of Obsession Telescopes contributed suggestions for finish and materials. I used stained Baltic birch overcoated with polyurethane for the structure, PVC pipe for the altitude bearings, Wilsonart's "Ebony Star" laminate for the azimuth bearing, and etched virgin Teflon for bearing pads. Decorative wood inlay strips on the corners were added to cover the screws that were used during construction.

The finished mirror box contains all the optics and focuser, and stores in the overhead compartment on most airlines. (It also "fits" under the seat in front if you don't mind having some of it stick out and keeping your feet around it so the flight attendant doesn't notice.) When stored, the truss tubes attach to the inside of the tripod legs. Thus, there are only the two pieces to the whole instrument. The tripod was designed to travel in a duffel bag along with sleeping bags and other soft materials, and can be checked as normal baggage on the plane.

As a finishing touch, I added to the mirror box cover a raised-relief map of Australia cut from contrasting wood. The telescope was then named Alice for Alice Springs, one of the Outback destinations on our planned journey.

Alice was started in November 1990 and finished the following April, less than a week before we headed south.

Now for the Details

Dobsonian telescopes work. My plan with Alice was to make a telescope that would be easy to use and interesting in appearance, but that would not vary significantly from the basic principles of Dobsonian design. Some of the specific problems encountered and solutions developed in that process are described below. I can claim little originality in my telescope, as most design solutions are hardly more then variations of ideas developed by other amateurs.

I designed Alice with the following parameters in mind:

High Quality Optics

Even though the telescope was technically a "Rich Field Telescope" (RFT) and supposedly good for lower magnification views only, I wanted Alice to be as high optical quality as possible. I have little tolerance for fast telescopes with blur circles instead of pin-point star images.

Optimal Performance

I decided to do everything possible to make the telescope's performance everything it could be. That meant evaluating the entire system, including optical supports, the diagonal, the focuser and anything else that came to mind.

Stability

Despite its portability, I wanted my telescope to perform like a classical Dobsonian, providing the smooth, backlash-free motion for which the design is known.

Portability

Alice not only had to be an excellent performer, but also had to be extremely portable. The mirror box and focuser needed to fit in the overhead compartments on commercial airliners, and the rocker, truss tubes and tripod needed to fit in a duffel bag that could be checked as baggage.

Ease of Use

The telescope had to be easy to set up and use. Because assembly would be required whenever I moved the telescope, I wanted a design that would go together quickly, would not need extensive collimation in the dark and would not have any loose knobs, bolts or wing nuts to fumble with cold fingers and drop onto the ground (or the mirror!).

Appearance

As Louis Sullivan said a century ago, "Form ... follows function." In other words, make a telescope that works first, and then make it pretty. Thus, once the above five criteria were addressed I would be free to design a telescope that would also look good. Alice was to be the first telescope I had built that would not be painted. No hiding behind wood filler and an extra coat of paint this time!

The Track & Truss Solution

After mulling over numerous telescope designs that would meet the criteria I established, I finally decided to go with a two-truss tube Dobsonian design based upon telescopes designed by Missouri ATM Thane Bopp and further developed by Dick Suiter, Tom Burns and Bob Bunge in Ohio.

In short, the telescope would be little more than a box-like enclosure for the mirror attached to a flat focuser board with two short truss tubes. The tubes would connect to both the mirror box and the focuser board by being drawn down into short channels by non-removable bolts. Thus, the alignment of the focuser board and diagonal would be fixed above the primary and assembly would produce nearly perfect alignment every time.

Because the telescope would be quite short (under 35" focal length), I decided to build a short collapsible tripod to support the Dobsonian rocker at a height that would place the eyepiece at a comfortable sitting height when the telescope was pointed toward the zenith. Likewise, when the telescope was pointed toward the horizon, the eyepiece could still be reached from the same seated position, although I would have to lean over a bit.

A little further figuring (and some midnight oil one November evening) refined the design so that the focuser board and the diagonal assembly would store inside the mirror box, making the entire optical assembly a single portable package. The initial design seemed complete.

I was ready to begin work.

Construction Materials & Tools

Alice is built of Baltic birch plywood, chosen because of its appearance, number of ply and lack of voids. It was certainly not chosen because of its price...this stuff is like gold! I understand that since the breakup of the Soviet Union good Baltic birch has become harder to get. Apparently the "in" material is now Finnish birch. I'm told that the Finns sold their old mills to the Soviets and now produce their birch plywood with new equipment.

I added inlay strips to key places on Alice. The inlay accents not only look nice, but they also cover the many screws and other fasteners that help hold Alice together. Contrary to what most people expect, the inlay is easy to apply. Simply rout a shallow groove, put some wood glue on the prepared inlay strip, and set it in place. Later you can sand, stain and overcoat the inlay along with the rest of the wood. Inlay strips in a variety of patterns and, like the wood handles and knobs used elsewhere on the telescope, are available from well-equipped woodworking outlets.

I didn't compromise on the strength or permanence of the telescope; all connections were glued and screwed. All plywood was stained with an oil-based golden oak stain and covered with several coats of gloss polyurethane.

Every time I build a telescope I learn how to use a new tool. In the case of Alice the tool was a router with a router table. Every circle, arc, groove and channel was done with the router. I had to use carbide tipped tools, as the Baltic birch plywood is full of glue and cuts very hot.

Besides the router, construction required tools no more sophisticated than an electric drill, a belt sander, a fine sander and a paint brush. Such simplicity of construction was important because I am not an accomplished builder, and I don't have a well-equipped shop.

Others helped me by providing me with access to a table saw (for starting out with square corners), a drill press (for installing threaded inserts), a band saw (for cutting the tripod pieces), and a scroll saw (for that map of Australia).

Mirror Box

The "heart" of the telescope is the mirror box; it contains (and stores) all of the optical components, and must be built with enough care to position the primary and secondary mirrors precisely and keep them aligned.

Box

The box is made of 1/2" (seven-ply) Baltic birch plywood. Its outside dimensions are 10" wide by 9.5" long by 11.5" tall. The choice of the 9.5" by 10" cross-section was not arbitrary. That size allowed a 1/4" clearance on three sides of the mirror and a 3/4" clearance on the side toward the focuser. The larger clearance on the focuser side was necessary for the optical path to clear the blocks that support the secondary vanes (described below).

The choice of dimensions also allowed the focuser board to be the same dimension as the side of the mirror box (9.5"), which in turn made it possible to center the truss tubes in the box corners. The centering was necessary for two reasons: (1) to allow the plane of the focuser board to be the same as that of the mirror box side, thus allowing the focuser to be as close to the telescope's center line as possible, and (2) to allow the channels for the focuser board to attach to the truss tubes from the side and the mirror box channels to attach from the top and bottom.

Finally, the 10" width also allowed me to cut grooves on the inside of the box that would support the 9.5" focuser board during storage.

The bottom of the box fits into a groove routed in the four sides and is recessed an inch to allow clearance of the mirror cell adjustment knobs. All joints are glued and screwed for extra strength, and inlay strips cover the corners.

Cover

I also built a plywood cover to fit over the front of the mirror box. The cover is held in place by three cupboard magnets and pops off easily when the cover is pulled by its two attached handles. The cover attaches to three additional magnets on the back of the mirror box when the telescope is in use, where it both protects the mirror adjustment knobs and helps balance the telescope.

I also added a contrasting raised-relief wood map of Australia to the mirror cover. No real reason ... just thought it looked neat.

Centering Pads

To keep the box centered in the rocker and to minimize any friction and abrasion when moving the telescope, small pads are attached on each side of the mirror box and inside the front of the rocker.

Mirror Cell

Cell Design

The primary mirror cell is modeled after the design used by Richard Berry in his excellent book Build Your Own Telescope. Basically, the mirror sits atop a square of 1/2" Baltic birch plywood and rests on three 1" diameter pads of silicon rubber. At each corner of the square cell a short section of 3/4" dowel rises to the height of the front surface of the mirror. The dowels are split in half with the flat side adjacent to the mirror. The mirror is separated form the dowels by pads of silicone rubber squeezed through holes drilled in each dowel. The result is a mirror cell that holds the mirror securely, without any strain or diffraction-creating clips over the front surface.

Cell Adjustment

The cell rests on compression springs that surround three attached bolts that extend out through the back of the mirror box. Adjustment is done with phenolic knobs at the back of the telescope.

Mirror Cover

I made a Plexiglas cover that attaches to the primary mirror cell and protects the mirror from dust, dropped items and prying fingers. The clear cover has four Velcro spots that secure the Kydex light baffle during storage. Although it is removed for observing, the cover is usually in place when people see Alice in daylight. Many times I have been asked if the four black spots on the primary are some new type of optical "thing."

Transporting the Mirror

The cell is left in the box at all times; the mirror is not removed when the telescope is transported. The mirror box assembly may be heavier that way, but the mirror is as safe as could be and never gets handled during normal use.

Focuser Board

General Design

The focuser board supports both the secondary mirror and focuser, and stores in grooves cut into the inside surfaces of the mirror box. Basically, focuser board itself is very simple. It measures 9.5" wide by 7" tall and is cut from 1/2" plywood. One side supports the focuser and the other holds truss channels (described in the next section) and the secondary mirror assembly.

I cut a small hole in the top of the focuser board that acts as a handle for pulling the focuser board out of its storage position inside the mirror box. As a final touch, I mounted a small phenolic knob on the focuser board to serve as a handle for moving the telescope.

Support Vanes

The tapered secondary support vane was cut from 1/16" aluminum with a saber saw. I then filed the burrs off the edges and bent the piece to shape in a vice, clamped between two pieces of wood. (The wood keeps the aluminum rigid except at the point where you want to bend it.) A small hardwood block glued and screwed to the vanes supports the Novak secondary holder. Adjustment is achieved with thumb screws.

The support vane is bent so that the vanes intersect at a 90° angle, thus producing a traditional star image with four diffraction spikes at right angles. To achieve the correct angle in the space available, I had to mount the spider vane assembly on wood blocks attached to the focuser board. The blocks also act as stops for the truss tubes and allow me to place the focuser board in the correct position for tightening the attachment bolts.

Light Baffling

Alice has two features that baffle light and allow her to perform with a simple open truss: 1) a reducing plug inside the focuser; and 2) a light shield attached to the secondary holder.

All of the eyepieces I use are 1.25" diameter, including the 9mm Nagler that gets used almost exclusively (although it has a 2" barrel). I placed an aperture reduction plug in the end of the focuser tube closest to the diagonal, thus limiting the effective diameter of the 2" Tectron focuser to 1.25". There is no vignetting (except when I borrow someone's 2" eyepiece with a large field lens), and the area around the diagonal that can be seen through the focuser is drastically reduced, thus minimizing the size of the more important light shield.

I cut a 6.5" diameter circle out of thin, black Kydex plastic and placed two strips of Velcro on the matte finish side. The strips attach to matching strips on the back side of the secondary holder. When Alice is assembled, the Kydex shield is removed from its storage position on the mirror cover (where it attaches to small Velcro dots) and stuck on the back of the secondary support. When the shield is in place, you cannot see past it through the focuser; the view is the same as if you had a complete telescope tube.

Is it as dark as it would be with a complete tube? No. But it's pretty good. I use the telescope in all conditions, and unless a direct light is shining past the shield and onto the inside end of the focuser, you would never know that the Alice is tubeless.

I am frequently asked if the light baffle causes much diffraction in the image, given that it is located directly in the light path. In fact, it does introduce an additional diffraction spike as a 45o angle to the four-vane cross caused by the secondary support vanes. In addition, the resolution on planets and double stars with the baffle in place is slightly less than with it removed, but I have never found it to be objectionable.

I have considered adding a "cage" that would create a light baffle outside the light path, but it always seemed like too much trouble. The current system, with its Kydex plastic disk attached with Velcro, is so simple that it would be hard to improve. Therefore, I have never pursued the addition of the extra assembly that would be required to baffle without the extra diffraction. Even if a method were found, it wouldn't do anything about the diffraction caused by the secondary support vanes, the primary diffraction cause.

Twin Truss Tubes & Channels

If Alice were to be portable, she couldn't have a traditional telescope tube; she would have to be a truss design. In keeping with my goal of making the telescope as easy to use as possible, I had several other objectives in mind:

1. No Loose Parts - As much as possible, I wanted to avoid loose parts during assembly (they invariably get lost). That ruled out extra bolts, washers, wing nuts or special tools.
2. Minimal Recollimation - I didn't want to have to do a major recollimation on the telescope each time I put it together. Once assembled, I wanted no more than a required "tweak" or two on the collimation knobs.
3. Quick and Easy Assembly - Finally, I wanted a telescope that would be rigid before the focuser board was attached. I most certainly did not want a the truss tubes waving this way and that while I tried to secure a focuser board on top of them.

When I built my 17.5" telescope nearly a decade ago, I used eight tubes in a Serrurier truss. The tubes fit into split wood blocks where they were clamped with bolts. While the design worked extremely well (and has since been adopted by telescope manufacturers such as Obsession, Tectron, Jupiter, Starsplitter and AstroSystems), it is rather bulky for small telescopes. I wanted something more elegant for Alice.

Back to the physics books. Objects can move through space in six directions - up and down, back and forth, and from side to side. Each of these directions is called a "degree of freedom." To keep each truss tube in place, I needed reduce its degrees of freedom to zero. This could be achieved by pulling the truss tube into a groove and clamping it in place.

Because Alice was relatively small, I figured I could get by with just two parallel truss tubes. I placed them on the side of the mirror box for two reasons: 1) a side mounted focuser could be used while sitting down; and 2) placing both tubes in a vertical plane would increase the instruments rigidity by letting the tube work together to offset the effects of gravity. (If you don't believe this, grab a piece of paper at one end and hold it vertically - it stays upright. Now take the same paper and hold it horizontally - it flops downward. The assembled truss acts the same as the sheet.)

Truss Tube Channels

I placed pairs of channels inside two corners of the mirror box and against the inside of the focuser board. The mirror box channels are attached to the top and bottom of the box (looking down into the telescope), and not the sides where the attaching knobs would hit the sides of the rocker.

To make the channels, I started with pieces of plywood 1/2" thick and 2" wide, and routed a groove 1" wide and just over 1/4" deep down the center. The channels were then cut to 6" lengths for the mirror box and 4.75" lengths for the focuser board. I then added a small shelf at the lower end of the two 6" channels to provide a secure seat for the truss tube before tightening. Each channel was then drilled near its center to allow passage of the securing bolt.

Attaching the Truss Tubes

The truss tubes are held in place by carriage bolts that thread into inserts. The inserts are set into short pieces of dowel rod fitted inside the truss tubes. Tightening the bolts draws the truss tube down into the channels set in the mirror box and on the focuser board. The carriage bolts are permanently held in place by recessed lock nuts tightened so that they keep the bolts from wobbling, yet turn freely without binding.

To make assembly a "no-tool" operation, I attached knobs made out of wooden toy wheels to the carriage bolts and stained them to match the telescope. The knob diameters are 2" on the mirror box and 1.5" on the focuser board. Because there are no small parts, all assembly can be done while wearing gloves, an important point during Ohio winters.

I also placed two channel and knob assemblies on the inner surfaces of two of the tripod legs to secure the truss tubes when the telescope is disassembled.

Truss Tubes

The truss tubes are 1.25" diameter aluminum tubing with a .058" wall thickness. A single 72" section from a local hardware store provided just enough for both truss tubes and the center "slider" tube for the tripod.

Final Assembly

The most critical operation was attaching the channels with wood glue; any inaccuracies would result in permanent "decollimation" of the telescope. My solution was to make sure all the pieces were cut square from the beginning and to simply snug the channels against the inside corners of the mirror box for approximate placement.

For final positioning, I glued the mirror box channels in place first (remembering to recess the channels slightly from the opening of the box to accommodate the mirror box cover that nests in the opening). While the glue was still wet, I loosely attached the truss tubes and adjusted them until they measured even and square. Then I tightened the lower assembly which clamped the lower channels in place. The next step was to glue the channels to the focuser board, tighten the upper assembly and wait for the glue to dry.

The Result

The telescope maintains collimation very well. I have disassembled the telescope, transported it several thousand miles and reassembled it without having to change a thing; the center spot on the mirror remains concentric and the finder crosshairs stay right on target. The most the telescope has ever required upon reassembly is a slight adjustment of the top two mirror cell adjusting bolts and a slight realignment of the finder.

Rocker

The side and bottom boards of the rocker that supports the telescope are made of 3/4" (13-ply) Baltic birch plywood. The front board is a single thickness of 3/8" plywood. All surfaces have circular holes cut out of them to reduce weight and provide handles.

The finished rocker is 11.75" wide and 10.25" deep, including the front board. The pivot point for the altitude bearings is 29" above the ground. The rocker's inside dimensions are just adequate to nest the mirror box for compact transportation.

Tripod

Traditional Dobsonian literature recommends building a ground board with three feet. Such a design would have made Alice a "knee-biter" telescope, and required the observer to crouch uncomfortably to use it. I'm too old for such contortions on cold (or warm) nights. I wanted a "sit-down" telescope, and that meant that Alice needed a tripod.

Stability was a must, and that dictated a short, stubby design with cross-bracing. When I built my first telescope over 30 years ago, I used a rigid tripod with a cross-braced center post. I own a very rigid photo tripod with a cross-braced center post. The excellent tripod design Richard Berry used for his refractor in his book Build Your Own Telescope used a cross-braced center post. Guess what I built for Alice?

Tripod Head & Slider Block

The tripod head is made of two layers of 3/4" plywood and has a circular center block that supports the slider tube (the piece left over after the truss tubes were cut). The slider block is also dual-thickness 3/4" plywood and is split to allow it to be clamped in place on the tube, thus locking the entire tripod assembly into a single, rigid assembly. A 1.5" wooden toy wheel serves as the clamping knob. The cross braces are made of 1/8" birch plywood strips obtained at a local hobby shop.

Tripod Legs

The tripod legs have an "I-beam" cross-section. The main plate of each leg is made of 1/2" plywood and the side pieces of 3/8" plywood. The legs are 19" tall and taper from a width of 4.5" to less than 1/2". Each leg is tipped with aluminum to prevent splitting and resist moisture absorption.

I routed a groove in each of the side pieces into which I inserted the main leg plate. The leg was then glued, screwed and clamped. The screw heads were then covered by an inlay strip. As mentioned above, I added channel and knob assemblies to the inner surfaces of two of the tripod legs to secure the truss tubes during storage. The third leg sports a wooden carrying handle.

Eyepiece Rack

As a final touch, I added a "Lazy-Susan" eyepiece rack to the slider tube. The rack has holes for seven 1.25" oculars and cutouts for two 2" accessories. The cutouts serve double duty, as they are necessary to fit around the attached truss tubes when the tripod is folded for storage.

Altitude Bearings

One of the main "secrets" of the Dobsonian design is the careful selection of bearing materials, the combinations I used being Teflon-PVC for altitude and Teflon-"Ebony Star" for azimuth. The 3/32" thick etched Teflon I used came from scraps Dave Kriege sent me (although Ken Novak sells the same material), and the "Ebony Star" laminate was found at a flooring firm.

The side bearings are fairly large (6.5") to provide proper friction for comfortable observing. They are made of PVC gas transmission pipe donated by a friend. The side bearings are attached to the mirror box with screws on the inside of the mirror box.

I cheated a bit when it came to finding the balance point for the attaching the altitude bearings; I counterweighted the inside of the mirror box with some thin steel plates. So sue me! It looks better with the side bearings flush with the top of the mirror box.

Azimuth Bearing

Azimuth bearings on Dobsonian telescopes traditionally have been made of Formica or similar laminate sheets gliding on Teflon pads. Although references in telescope making literature have indicated that smooth, shiny Formica would not give the best performance, until recently no one had seriously tried to find the best materials available.

Then Wisconsin amateur telescope maker Peter Smitka tested various Teflon and laminate combinations for friction, and found that virgin Teflon and Wilsonart's laminate 4552-50 "Ebony Star" offered significantly less friction than other combinations. As a result, both the altitude and azimuth axes of his 200 pound telescope move with approximately two pounds of force.

Teflon comes in two varieties - virgin and reprocessed. Virgin Teflon is the first casting of the resin and contains no impurities. Reprocessed Teflon is made of remelted resins from previous uses and may contain impurities. Either will work for telescopes, but virgin Teflon will ensure the smoothest possible motion.

The azimuth bearing in Alice consists of three 2" by 1/2" by 3/32" thick virgin Teflon pads epoxied to the top of the ground board at 120 degree angles around the center. The pads are mounted 5" from the centering bolt, giving a bearing outside diameter of 10", roughly comparable to the diameter of the mirror (a good rule of thumb). By moving the pads in or out, friction can be increased or decreased slightly to "fine tune" the azimuth motion.

I attached a sheet of "Ebony Star" laminate to the bottom board with contact cement. A 1/2" centering bolt passes down through the bottom of the rocker and through the tripod head where it is secured with a nylon thread lock nut. Pointing the bolt down keeps the inside of the rocker clear, and allows minimum clearance and a shorter rocker.

No provision for changing the bearing friction is necessary in a telescope this size. The motion of the "Ebony Star" moving over the Teflon is smooth, vibration-free and without backlash.

Miscellaneous Points

Some of the other considerations in my telescope are set forth below:

1. Focuser - Even though I would not be using 2" eyepieces (the 9mm Nagler is technically a 1.25" eyepiece with a 2" barrel.), I decided on a 2" focuser. I also wanted a short focuser to keep the telescope's diagonal size to a minimum. Because of the critical focusing demands of a short focus Newtonian (and because Bill Herbert gave it to me), I used Tectron's low profile focuser. The larger focuser also lets me insert a 48mm Lumicon Deep Sky or UHC filter into a 2" adapter and leave the filter in the focuser while I change eyepieces. (I also have a separate adapter without a filter, so I never have to screw and unscrew the filter in the dark.)

2. Secondary Mirror - Size of the secondary mirror is a key factor in the performance of a Newtonian telescope. I decided to make mine as small as possible without adversely affecting performance. Unfortunately, in a telescope as "fast" as Alice, it just isn't possible to have a truly "small" secondary.

   After poring over numerous articles on secondary size, quality, offset, obstruction and just about anything else, I sat down at a personal computer and developed a Lotus 1-2-3 spreadsheet to do the tedious calculations for me.

   The model lets you enter the size of the primary mirror, its focal length, the outside diameter of the telescope tube, focuser height and desired fully illuminated field diameter. It then gives back the major and minor axis measurements of an "ideal" secondary. You can then enter the minor axis of the nearest standard size secondary (or the size you would like to evaluate) and the model will give you the actual fully illuminated field, offset from the optical axis, measured offset along the secondary's surface and the percentage of the primary's aperture and area obstructed. You can also view graphs of magnitude loss and overall field illumination as a function of distance from the optical axis.

   I finally decided on a 2.0" minor axis 1/20 wave quartz secondary from E&W Optical. The secondary (actually 1.88" clear aperture in its Novak holder) creates a 0.26" fully-illuminated field and produces primary mirror obstructions of 25% (diameter) and 6% (area). The light loss at the edge of a one inch diameter field (the largest field stop on my Brandons) is 0.24 magnitude. With the 9mm Nagler vignetting is virtually nonexistent.

3. Finder - The telescope is equipped with a Celestron 10x40 finder. The higher magnification makes it a better choice for locating objects in the "less than perfect" skies of my Ohio observing sites than the more frequently encountered 8x50 finders.

   The finder is mounted in twin alignment rings that rotate around the upper truss tube. The rotating assembly was necessary so I could move the finder out of the way when storing the truss tube in the rocker.

4. Collimation - Proper collimation is critical in a fast focal ratio telescope. I use a set of Tectron collimating tools to check the optics each time I assemble Alice. In keeping with my "no-tools" philosophy, I replaced all collimating bolts and nuts with large knobs and thumbscrews. A slight turn of a mirror cell knob and a minor finder adjustment is all that's required to collimate the instrument.

5. Dew Prevention - Dewing of optical surfaces is the enemy of any telescope with exposed optics - especially in the American Midwest. Because Alice has no tube to protect the optics, I sometimes encountered nights when the telescope was dewed up before she was completely assembled.

   I solved the problem by installing the Kendrick Dew Remover system, developed by Canadian amateur astronomer Jim Kendrick. The system consists of heating elements that fit behind the secondary mirror, wrap around both ends of the finder, and fit around the eyepiece. The heaters are powered by a 12 amp-hour rechargeable battery, adjusted by a small electronic control box, and provide just enough heat to prevent dew from forming on the optics.

   Now on bad nights the mirror box may be dripping wet, but the optics are completely dry.

6. Fan - A telescope will not perform well unless it is near the ambient temperature. Alice's open structure facilitates cooling, but she can still take a half hour to completely cool. I accelerate the process by placing a small battery powered fan inside the mirror box and pointing it toward the covered primary mirror for a few minutes before each observing session.

7. Desert Storm Shield - One of the slickest telescope accessories to come along in recent years is the Desert Storm Shield, an aluminized Mylar telescope cover manufactured by Columbus amateur Ralph Hoover. The aluminum coating reflects the sun's heat during the day, and the Mylar prevents moisture from reaching the telescope. Ralph graciously made a custom cover for Alice which travels with her everywhere.

   One night at the South Pacific Star Party in Australia, an unexpected weather front brought a drenching rain that caught many telescopes on the observing field and lasted throughout the night. I awoke to the drumming rain on my tent many times that night, wondering how Alice was doing, covered only with a thin Mylar bag. No worries, mate. The next morning she was as dry as the Outback.

8. Traveling Cases - Alice saw over 60,000 miles of travel in her first three and a half years, and she always traveled without extra protection - mirror box as hand luggage and the tripod in a large duffel bag stuffed with clothes. Although Alice was never damaged, by mid-1994 I figured that I had pushed my luck and tempted the luggage gorillas too often. I had custom shipping containers made that hold the telescope and accessories.

   Now Alice travels first class; safe in her cases. Now if the gorillas can just keep from losing her...

The Final Result

Well, how did everything turn out? I started out with the criteria of high quality optics, excellent performance, stability, portability, ease of use and appearance.

In fact, Alice is an excellent performer. When the telescope is temperature stabilized, images are sharp across a magnification range of 27x-349x, although almost all observing is done under 200x. Nagler eyepieces provide the best views, and I have added three to my collection (16mm, 9mm, 4.8mm). In fact, the 97x view through a 9mm Nagler is so good that I seldom use other eyepieces. The telescope is very stable and moves smoothly with fingertip pressure. The mirror box fits neatly in the overhead compartment of an airplane, and the tripod packs easily in a duffel bag, although I will use the shipping cases in the future. The telescope assembles in under two minutes and doesn't lose collimation from set-up to set-up. Finally, Alice is good-looking (at least to me).

I originally designed Alice as a special purpose telescope ... just to get some aperture on a plane. (I didn't design for maximum portability with resulting sacrifices in stability, as I knew from long experience that despite the murky skies in town I would use a telescope near home many times for each time I would transport it to a dark site.) However, Alice works so well that I use her almost exclusively. In the three years since her completion, we have accumulated over 65,000 miles of travel, including two treks through the Australian Outback and numerous trips to observing sites and star parties around the United States.

Between observing sessions, Alice stands unobtrusively in a corner of my den, waiting for the skies to clear. That's when her appearance helps. With Ohio weather, one spends more time looking at a telescope than through it.

Since finishing Alice in 1991, I have continued to pursue the twin-truss design, and recently completed six 10" f/5.6 instruments. The 10" version occupies a volume of two cubic feet plus the two truss tubes - overall, smaller than its 8" predecessor. Other ATM's have scaled the design up as far as 12.5" f/4.8 and it still works well.

A Final Note on Design

To the newcomer to telescope making, Alice may appear to be very different than most other Dobsonians. Such is not the case. For the most part, I restricted my design changes to cosmetic and non-critical areas of the telescope. A close examination will show that the main principles of the Dobsonian design such as matched bearing materials, large bearing diameters, short and rigid construction and no cantilevered components still hold for my telescope.

I have seen and used a number of Dobsonian telescopes in the past few years. Unfortunately, few of them demonstrated the stability that the design is capable of delivering. It seems that some telescope makers think that if they simply place a telescope on a plywood alt-azimuth mounting and throw in some Teflon and Formica at appropriate points, they have a Dobsonian telescope that will perform like the ones in the magazines. Not so. All of the things Russell W. Porter used to preach about overhang, small bearing diameters and flimsy construction are as true with Dobsonians as with any other style of telescope.

The Dobsonian approach is successful because it works. Materials, dimensions, construction techniques, loading and forces all play important parts in the design; the telescope builder who adheres to the basic "rules" of the Dobsonian can end up with a telescope that performs wonderfully.

Building Your Own

I am often asked two questions about Alice:

1. Can I copy your design?
2. Do you have plans?

My answers are: 1) yes, you can copy it; and 2) no, I don't have any plans (other than this article). However, it's not that simple. As I mentioned before, in designing Alice I drew on the ideas and techniques of a number of other amateur telescope makers. But I didn't "copy" anyone. Nor would I want someone else simply to "copy" Alice. The ideas are free for the sharing. After all, that's where I got most of my inspiration; other than the truss and track assembly almost every other design idea came from someone else.

Feel free to look over the telescope in whatever detail you choose. Use any idea that catches your fancy. But don't just copy what you see. Figure out why I did what I did and come up with a new solution that is better. Then take some pictures and write down what you did to help the next person. Above all, have fun!

"It is not ... unsportsmanlike to study closely the details of telescopes made by others and to 'lift' this or that feature from them, provided one improves these features."
- Albert G. Ingalls

Note: Additional photos of Ron's scopes are available in the gallery.