Bob's 12.25-inch f/5.4 "Ellie" Low Tech/ Light Weight Suitcase Telescope

Last touched 2006 June 3

For the 2000 Table Mountain Star Party, I had taken a 6-inch f/5 travel scope that I've taken on many trips and fits either in a suitcase or carry on luggage. At the 2001 Winter Star Party, I saw a 14-inch f/6 "suitcase" telescope that had traveled to WSP from Seattle. I had a set of 12-inch f/5.4 optics I had constructed in 1990/1 in a portable dobsonian that hadn't seen much use since I had finished TJ, my 20-inch Dobsonian. Why not build a new scope that would be able to make the 2001 trip to TMSP and any other future trips?

After researching what other ATM's have been doing by looking at various webpages, I was able to form a decision path:

• For the entire scope to be carry-on, it could only be 14-inches wide. After much thought, I decided my 12-inch mirror was too large. A 10-inch, yes, perhaps an 11-inch;
• Build the scope so it folds up to become its own shipping container. I decided against this for weight restrictions; to build strong enough to survive the baggage monkeys, the scope would become heavy and difficult to carry around the airport;
• Carry on a box with the optics; build the rest of the scope to fit in a suitcase. Construction could be light, I'd have the most important parts of the scope with me and an old suitcase from the attic would get to deal with the monkeys.

Ellie would be a very low profile truss tube dobsonian. It would have large altitude bearings, a very low profile mirror box (which needed to fit under an airline seat), and extremely lightweight focuser cage. In fact, I decided to use only a single ring of wood to support both the spider assembly and focuser. Large lightweight scopes have used this in the past, but have never hit the mainstream of TM design.

Ever since I had put DSCs on my 20-inch, almost two years ago, I found that I often never bothered to attach the finder to the 20 in the field. After the normal two star alignment, I just let the DSC's do the finding. With this in mind, I decided I'd take a leap and design Ellie not to mount a finder scope. This is sort of like when John Ericsson, who designed the famous Ironclad USS Monitor, approached the US Navy with a design for a steam ship with no sails. I would just use two points on the telescope to point it to bright stars and be off.

The spider assembly was built from a piece of oak wood, two pieces of sheet metal from a chunk of scrap ductwork and nylon screws. I reused some of the secondary holder from the original 12-inch, including one pulling spring and three pushing collimation screws. Nylon screws attached the spider to the focus ring. I used surplus heat rope to make a secondary anti-dew heater. I removed the heat rope from its protective coating (since I was only going to use 12-volt and not 110ac), wrapped it around the backside of the secondary mirror. Electrical leads were soldered from the heat rope to the spider vanes.

The focuser is my standard low tech push-pull job; 2-inch PVC pipe, fitting through a 1.5-inch thick piece of balsa wood that is sandwiched between two pieces of 1/16-inch aircraft plywood. This focuser plate attached to 1/8-inch piece of aircraft ply that is then attached to a board attached to the focus ring. Screws and thin plastic shims allow me to adjust the focus plate for collimation.

The focus ring itself is a carefully selected piece of 3/4-inch Laun cut on with a router. Unattached, the ring flops and bends alarmingly. Assembled and attached to the trusses and mirror box, the ring is held stiff by the mirror box and trusses. No movement or flexing is visible or felt while moving the scope around.

Focus ring truss tube clamps were an issue. I wanted to make compression style clamps that would not require lose screws or tools. My first set made from Oak didn't work out; the Oak was brittle and broke when I made the sides of the pieces thin enough to meet the required dimensions. I've cast parts from fiberglass before, so I made some test pieces; I make a wood sample of the final piece and use it make a mold with plaster. Auto repair fiberglass works well if you use Vaseline on the mold as a release material.

By chance, I made the sample piece from a chunk of scrap Popular wood. I was impressed by Popular's light weight. It's fine grain allowed the thin walls I needed. I made some more "samples" and scrapped the time-consuming fiberglass path. Once completed and installed, electrical leads were soldered from the ends of two of the spider vanes with wires running into the inside of the truss clamps so that two of the truss tubes would carry current to the secondary dew heater without the need for any plugs.

The truss tubes are 3/4-inch OD aluminum, 0.049 thick. I was surprised not to be able to find these in any of the local hardware stores. MacMaster-Carr carried them, but they were expensive: $5 per foot. An evening of searching on the web found an aircraft parts supplier in the Midwest that had them in stock for $2 per foot. Since the scope had to fit in a suitcase, each truss was cut down into two pieces 23-inches long and one shorter piece. Threaded inserts were installed in the appropriate ends. Assembled, they are held together with 1/4-inch 20 TPI threaded rod. I cut the trusses down with a pipe cutter. In hindsight, this was a mistake, because the pipe cutter does not leave a clean, flat end. With slightly beveled ends, the tubes did not screw together square. This looks ugly, but doesn't seem to affect performance since collimation of the optics doesn't care if the trusses aren't perfectly straight.

To attach them to the mirror box, I elected to use the channel method used by many ATM's, including NOVAC's Craig Tupper and well known ATM Ron Ravneberg of Columbus, Ohio. Pipe clamps and shelving supports did the trick nicely. There was some slippage, so I dipped the pipe clamps in liquid rubber coating; it worked like a champ. The truss channels are inside the box for a more compact design.

The mirror box was the most complex piece to build. It had to be light, as strong and stiff as I could make it, fit under an airline seat and be engineered to support the mirror, fans for cooling, channels for the trusses, and attachment points for the altitude bearings. It also had to be as square as possible in order for the DSC's to work properly. All sides were again made from 1/4-inch Laun. I had learned that Laun is stiffer in one direction, so pieces were carefully selected and cut to provide the best possible strength for their location in the box. The Box has a full size lid that hinges up when the scope is assembled to provide some dew protection of the primary mirror.

The mirror cell was another complex piece. I had ground, polished and figured the mirror while living in Columbus, Ohio in 1990. I had worked on the mirror almost every day and finished it in about three months. Amateur Marshal Holman of Lillian, Alabama, home cast the mirror blank I started with. Marshal had been experimenting with casting his own mirror blanks using technology very similar to that used in the Arizona Mirror Lab operated by Rodger Angel. The blank I acquired from Marshal was even honeycomb. It is 12.5-inches in diameter and is 2-inches thick, but weighs only about 6 lbs. A 5/8-inch thick front plate supports the actual surface of the mirror. The honeycomb walls were designed to meet where a nine-point flotation supports would be located.

In the prior telescope I had these optics in, I had made a 9-point cell. Over time, the mirror rotated and the some of the support points dropped into the honeycombs. I had adjusted for this with collimation and not seen any affect on the quality of the images even though the mirror was no longer supported properly. This experience had convinced me that I would be able to get away with a very simple mirror cell on Ellie; a single, flat piece of wood. Again, I used 1/4-inch Laun, but this time reinforced with some aluminum angle. One bolt in the center of the cell pulled and three screws push from mirror box. Three steel support risers grasp the mirror in similar fashion of the mirror cells of yesteryear. My next concern was environmental; I wanted the honeycombs to be able to breath. I carefully cut out holes in the board so each honeycomb can be vented.

The bearings are 24-inches in diameter, almost as large as the bearings on my 20-inch. They are made from 1/2-inch Fir "AC" plywood scrap. They attach with two knobs. To provide stability, I use a piece of extra truss tubing to connect the front sections of the bearings. This ensures the spacing between the bearings is equal through the entire 180-degree arc.

With this much of the scope complete, I banged together a simple rocker box for testing and balance. I experimented with different weights until I discovered that it would take about 15 lbs of weight on the back of the mirror box to move the center of balance of the OTA to the point between the bearings. This was disappointing; it was much more than I had hoped for. I didn't want to have to carry that much additional weight in my travels.

After reading up, I decided I'd try to use spring counterweights. See an article by Tom Kyajci (http://education.gsu.edu/spehar/FOCUS/Astronomy/krajci/krajci.htm). After doing some tests on springs from Lowes, I was completely surprised how easy and fast it was to set up the spring counter weight system. I thought it was take several hours, but instead, it was done in an hour. And it worked. The springs held the scope in altitude just as the math suggested. But - there was another problem.

The mass of the OTA was still forward of the bearings. When I tossed together my test rocker, I did the conventional thing of putting the azimuth bearing directly under the altitude bearings. When the scope was pushed down in altitude, the mass took over and the ENTIRE scope started to tip forward, with the rear of the ground board lifting first.

I made a new rocker, this time making it longer, with the cut outs for the altitude bearings in back and a small box to hold a battery and other pieces in the front. I cut the hole for the azimuth-bearing forward of the traditional location. I decided to locate it where the scope was balanced when pointed 45 degrees up from horizontal. Now the scope was a little "tippy" when pointed down low, or pointed almost to the zenith.

On site at the 2001 Table Mountain Star Party, the tip became more of a problem when the scope wasn't on level ground. My immediate fix for this was to add weight in the form of tools borrowed from my father-in-laws truck toolbox. You get some interesting comments when you have twenty different wretches in the bottom of your rocker box. "Does it really take all those tools to assemble this telescope?" In any case, this worked like a champ - the increased mass in the bottom of the rocker greatly stabilized the scope and made it more useable. I've toyed with the idea of trying to find some plastic bladders that I could fill with water that would fit in the bottom of the rocker. Water is normally easily available and I would not have to transport the weight. Other options include stones and gravel that might be available locally.

Without this extra "locally supplied" mass, the assembled telescope weighs about 40 lbs.

To transport the telescope, I selected an old 20x25x8 soft suitcase. I acquired foam stuffing from a local sewing store and cut out inserts to hold the ground board, rocker and focus ring (which fits into the rocker) in the case. The cut down trusses fit along the sides of the rocker box. The detached bearings - only 1-inch thick - fit into an inside pocket on the lid of the case. Knobs, the spring counter weights, and DSC encoders fit snugly in the storage box on the front of the rocker. Small places in the suitcase are filled with old t-shirts. Eyepieces and the Sky Commander computer are wrapped in some of these t-shirts. The entire, loaded suitcase weighs in at 45 lbs.

I cut a disc of Laun ply that just fits on the clips that hold the mirror in the mirror box. This acts as a mirror cover to protect the mirror during transport and reduce dust collection on the surface. With the lid closed and an added suitcase handle, the mirror box is a 14x14x5.5-inch box weighing in at about 12 lbs and fits easily under an airline seat.

I provided space on the back of the mirror box for two computer CPU fans. I use these to blow air into the box, with the hope of moving air around the mirror's honeycombs. Since the mirror has no place where the Pyrex is more 5/8-inch thick, with proper ventilation, the mirror cools down very quickly. In the mirror box, wires were soldered to the channels for the two truss tubes that carry the dew heater current. These wires and the fan wires will hopefully, one day in the future, attach to a control box in the rocker that will allow the different systems to be turned on and off with the click of a switch. Currently, I just twist wires together as needed and attach them to a 5 amp/hour 12-volt alarm Radio Shack system battery that stores in the travel suitcase.

Finally, I attached an additional truss channel to the side of the rocker. Using scrap truss pieces, I run up a pole about three feet up to a small platform that carries the Sky Commander computer.

Ellie provides good, clean images. The Sky Commander DSCs work perfectly - better than any other DSC telescope I've built - because I paid careful attention to making the mirror box and rocker as square as possible. The movements of this telescope are not as buttery as I would like and I hope to smooth them out over time with some experimentation. Nonetheless, the scope is completely useable and is a fine general-purpose telescope that can travel almost anywhere.

In 2003, I inserted wood dowels into the base of the truss tubes, drilled holes and worked in threaded inserts. I used these to replace the plastic coated clamps. For one night stands, the clamps worked, but for longer periods, they allowed the truss tubes to move and the entire OTA assembly would loosen up.

With this change, was able to put Ellie out on the back deck for much of 2003 as part of my Mars observing program. I used Ellie when TJ, my 20-inch was either out of action or I thought the seeing wasn't good enough for the bigger scope. For a lightweight travel scope, Ellie performed well on Mars and provided some good views.

Later in 2003, Ellie travelled to Oregon for some observing from private property.

As the travel season approached in 2004, I sandcast two lead weights for the back of Ellie's mirror box. This greatly improved Ellie's stability and how smooth Ellie moves in altitude.

In August 2004, Ellie again travelled to Oregon, this time for the Oregon Star Party. However, on the way, foam padding inside the suitcase was moved around during a TSA inspection and when a large object was dropped on the case, Ellie's focus ring was broken, the side board of the cradle was cracked, and several other minor dings were the result. These were nothing that a bit of glue couldn't fix, and Ellie was able to experience the fine, fine, fine dark skies of the OSP.

For 2005, Ellie traveled to Washington state and observed Herschel objects from private property near Pasco, WA. Again the focus ring was broken at the now weak location of the earlier break. The answer was supplied by my father-in-law; a short strip of fiberglass cloth and some filler. After gluing the break, the cloth was wrapped around the break and the fiberglass worked into the cloth. Now repaired to stronger than original strength, the ring had no problem on the trip back to Maryland.

A move into a new house in 2005, opened up the possiblity of observing from a drive way in a light polluated environment. Simple modifications made to Ellie in 2006 now allow quick observing sessions, mostly of variable stars. Details here.

Ellie and four legged observing partners set up, July, Washington State, 2005.

On July 3, 2005, Ellie was used to make this drawing of the galaxy M-51 and Supernova SN2005cs. The dot surrounded by pointers is the exploding star in the galaxy. The skies were dark, the seeing pretty good. A 12mm eyepiece at 120x was used.

Ellie set up at the 2004 Oregon Star Party. Ellie logged six Arp galaxies at this star party.

Here's our Golden Retriever, Tramp, posing beside the packed suitcase and mirror box, ready to go. Tramp is a great observing partner. The suitcase is checked, the mirrorbox carries on.

The mirror box is open the mirror cover is sitting to one side. It's tight fit to keep the box small. Not your regular "two inches too big" approach.

The Suitcase, opened up, with everything inside and a couple pieces of packing removed. Foam rubber does most of the work, with old t-shirts filling some holes and holding eyepieces. The bearings and baffling go inside the case lid pocket.

A pair of the lower strut clamps. Pieces of shelving supports are used, along with electrical pipe clamps and some cute plastic knobs. On the left hand clamp, you can see the solder joint for the connection to carry power up to the secondary mirror dew heater. Also, the pipe clamps have been dipped in that plastic coating stuff you see advertised on TV. This helps to keep the struts from moving.

The focus ring strut clamps as seen from the inside, showing the dew heater wire. Note the dew heater wire is soldered to the spider vane, so there are no wires on the spider vanes to increase diffraction. Also, nylon screws were used to keep weight down. Finally, note the single ring is made from 1/4-inch plywood!

The focus ring strut clamps as seen from the outside, showing the clamping screw. These clamps are made from poplar wood, very light, pretty strong and very workable.

I make almost all of the components of my telescopes, the spiders, the mirror cell, the diagonal holders, the focuser, etc. For me, this is part of the fun of telescope making. It also saves money. Anyway, here is the diagonal holder for Ellie's 2.1-inch minor axis secondary mirror. The core of the holder is oak. One pulling, spring loaded screw, three pushing 10/24 TPI screws for the adjustments. The spider vanes are two pieces of furnace duct steel, cut with sheet metal sheers. Since the wood isolates the vanes, I use them to act as conductors for the secondary dew heaters. Here, you can see the wires leading to the heat rope. Note the quick dis-connects so I can remove the secondary mirror easily.

The scope has been set up and I'm left with the empty suitcase and stuffing. Normally, I'll close the lid of the case and use it as a table to hold the laptop and any star charts.

Here's the back of the honeycomb mirror. This blank was home cast by Marshal Holman of Lillian, AL. He used the same mold material used by the big Mirror Lab in Tucson. He used scrap Pyrex from United Lense. He carved his initals in the center honeycomb. The honeycomb makes the mirror very strong, very stiff and allows it to cool off quickly.

Here's the front of the mirror cell. The cell is a disk of 1/4-inch plywood. Three simple "L" brackets hold the mirror in place. The brackets are coated with that liquid rubber stuff. The holes are cut in the disk to allow each honeycomb cell to "breath."

And the back of the mirror cell. Three pieces of alum. angle provide a level of stiffness for the plywood disk. In the very center of the cell, you can see the hole for the "pull" screw that holds the mirror the mirror box. Three screws in the mirror box push against the angle to provide alignment adustment.

Did I say that I'm cheap? Here's the most visible sign of it... a PVC pipe push-pull focuser. I get a lot of ribbing about this, but hell, I've just saved $300 bucks! And the views through my scope are just as good... well, you get the drift. Details: the core of the focuser is a piece of balsa wood (very lightweight!), sandwitched between two pieces of aircraft grade plywood. I cut a hole for the pipe very carefully to ensure a very tight fit and then sand it until it moves just right. Please note that I can adjust the tilt of the focuser with four small adjustment screws and thin shims. The attachments to the focuer ring are made from extra truss tubing and threaded inserts. I used nylon screws to save weight.

The lower part of the scope when assembled, front view.

Side view of the lower section, showing the spring counterweights.

Here's the assembled scope, with Tramp in for scale. Note the small table attached to the side of the scope to hold the Sky Commander DSC control unit.

Assembled, upper tube detail. Normally, I don't assemble it with the focuser in the up positionl like this.

In this configuration, Ellie is an ideal instrument for visually observing variable stars. From this location, well inside the dome of the DC Metro sky dome, Ellie can reach about 12.5 magitude on an average night. The laptop table from TJ is used. Our home wireless network allows realtime access to the AAVSO website, where I download variable charts as needed and then enter the observations into the website as they are made using the "Webobs" tool. An occasional look at bright Messiers and some planetary observing is sometimes done.

Also in this mode, Ellie has made a couple of trips to some nearby observing locations, as she will fit into the back of a Ford Escort wagon without any breakdown, making for very quick packing, setup, teardown and unpacking: ideal for quick semi-dark observing sessions.

The 12-inch, pre "Ellie"

Here's Bob's 12-inch Newtonian Telescope
Bob originally built this scope in Columbus, Ohio in 1990. It was a very light weight telescope that fits into a very small package so it doesn't take up much room in the car. It was made using simple wood construction and didn't require any tools, or loose parts to assemble. This telescope was displayed at Stellafane in 1991. While it didn't win any "official" awards, the Goddard Astronomy Club gave it an award. At the 1991 Hidden Hollow Star Party, Bob won first place with this telescope. Bob often used this telescope when there wasn't enough room to bring TJ2.

Here's the back end of the old 12-inch scope
This image shows off the honeycomb structure of the homecast mirror blank made by Marshal Holman. This is the original 12-inch scope.

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