24 inch Ultra Light

		Designing & Building the 24 inch f/4 “ultra light” Dobsonian Telescope by Greg Babcock

		First Light June 15th, 2000, 8:25pm
		...Sort of. The mirror was not yet aluminized and there remained a lot of adjustments to the OTA and Stand. The telescope was officially completed and collimated on August 10th, 2000, 2:00pm.

		World's lightest 24 inch?
		At first glance, it should be abundantly evident what the objective was when I designed this telescope. Make it minimal and light. Is this the world's lightest 24 inch Telescope? I do not know for sure, but at 105 pounds, I am sure it is amongst the lightest. It is 30 pounds lighter than the 8 inch Telescope built in 1974 and it has 9 times the Light Gathering Power.

		Getting Started
		One day in October 1999, while visiting Steve Swayze and Swayze Optical to exchange Solar Eclipse pictures, Steve asked me if I would be interested in selling my 18 inch Telescope? I told him that I would trade him the 18 inch for a 24 inch mirror. Eventually a similar deal was struck. I liked the 18 inch a lot and was reluctant to part with it, but I was also anxious to take the next step with a new design.

		Design Criteria
		This telescope did not take as long to design as the 18. Once the 18 was completed, I was working on new designs and sketching renderings. The criteria for the design were similar to that of the 18 inch. 1.) I required compactness and portability. It had to fit comfortably into the Ford Explorer and still leave plenty of room for camping gear, 2.) It had to be light and manageable by one person, 3.) I required quick set up and take down, 4.) I wanted the lines of the Telescope to be smooth, simple and even artistic. The 24 inch was the logical next size up from an 18 inch. It has 80% more light gathering power, but is still manageable by one person by using a thinner (1-5/8 inch) mirror. In addition, the thinner mirror’s has a faster cool down time

		The Overall Design - 6 Trusses & a Small Foot Print
		The design criteria called for 6 trusses instead of 8. But it also called for the mounting of the alt-bearings to the bottom of the "Astro System" Baltic Birch Mirror Cell instead of the sides. This was to reduce the foot print, reduce the span between the bearings, and to increase strength. The 6 truss and small footprint design criteria were somewhat contradicting in that 6 trusses cannot be spread out far on a small square footprint. This problem was solved by pivoting the 4 “rear” trusses at the cage connection so that they could be spread out farther. The front trusses are mounted on the cross bar between and next to the alt–bearings. A brace underneath the Alt Bearing "cross bar" transfers the load of the front trusses down the bearing and to where the “side” trusses are mounted. Not perfect, but this seemed to be the best overall truss configuration. Other configurations considered, didn't seem to have as good of leverage, or interfered with the light path, or placed the trusses on weaker center of the cross bar. Another consideration was mounting the front trusses on the end of the bearing. The cross bar could be mounted on the top or end of the bearing. The side trusses could then be mounted nearby as well. I liked the look a lot, but my concern about the design was that it would create a lot of load on a weak part of the bearing, possibly creating flexing as the telescope is rotated. The purpose of the crossbar being located in the middle with reinforcement between it and the mirror cell, is to prevent "racking". As the telescope is rotated to the horizon, one can observe that there are two 3/4 inch alt-bearings supporting a 72 pound mirror and cell. And there is a lot of daylight between them and the "carrier", sometimes referred to as a "flex board". The 6 Trusses permitted the the Mirror Cell to be cut in a more rounded shape at the rear of the scope. This reduced additional weight. In addition to the large cutouts made in the Mirror Cell, holes were drilled in the “triangle” mirror support pieces as well, to reduce weight and increase ventilation. To stiffen the mirror cell, a 1 pound 1/8 inch thick aluminum strap is screwed to the rear perimeter of the cell. Additionally, 2 angled aluminum stringers are attached to the bottom of the mirror cell. 6 Trusses give 3 good contact points on the cage and because it is 6 vs. 8, it aids in assembling the Telesocpe more quickly. The speed of set up is further aided by the tops of the trusses being joined together to make 3 connecting points to the cage.

		Trusses
		The frail looking ¾ inch diameter, .032 inch thick carbon graphite trusses are strong and light (2 pounds total). Because 6 feet was the maximum length I could obtain, aluminum extensions were added to the rear trusses. The trusses are attached to ¼ inch thick aluminum brackets. They are attached to the telescope using thumb screws. Counter sunk threaded inserts guide the trusses into place. This is particularly handy in that the cage can be set on top of the trusses and left until I can make my way to the thumb screws to tighten them. The thumb screws are custom made with a bolt "hex head" so they may be tightened to the cage and mirror box using a wrench. This is especially important to reduce vibration. Where did the material for the Trusses come from? Aerospace Composite Products
		Picture shows connecting knobs.

		Altitude Bearings
		The telescope was designed using a simple CAD program and 18 inch radius was determined to be the smallest radius I could make the bearings and still have clearances as the telescope is rotated to the horizon. Also, the calculated center of balance was a factor in choosing the 18 inch radius. The bearings are lined with 1/8” thick anodized aluminum.

		Alt Bearing


		Base & Azimuth Bearing
		The base resembles a “Lazy Susan". The bottom base ring is 2 inches thick and 32 inches in diameter. It weighs 21 pounds. It is the one component that I pulled the bull by the horns. The telescope itself rides on a “carrier” or "Flex Board" that contains azimuth and altitude bearings. Azimuth rotation is achieved using 4 one inch diameter "Magic Slider" pads placed under the altitude bearings. These were chosen after experimentation with many materials including "transfer ball bearings which continually bound. The alt bearings ride on 4 ¾” “cam-followers” placed in aluminum housings. These were chosen for their smoothness in case the telescope is motorized. In the meantime, to provide friction to the Alt-bearings, felt shims are placed on the Flex Board, between the bearing and the Board. By placing the cam-followers somewhat close together, the telescope could be rotated to the horizon without having long (tall) Alt-bearings. This means the vehicle in which the telescope is being transported could have little interior height (22 inches). This also meant that the shorter bearing would be more stout The telescope’s weight is transferred to the transfer bearings and on to the Magic Sliders and on to the heavy base. There is no center post (hub-less) to provide additional clearance for the mirror as it rotates around the alt axis. This permitted the face of the mirror to be placed just 10 inches above the ground. The carrier rotates around 9 closet bearings (see design updates) that hold it in place. .

		Base & Cam Follower Bearing

		Cage & Spider
		The Spider is pyramid shaped to make it an integral part of the tube length. This allowed me to place the focuser bracket above the ring, allowing the single ring cage to nest on the mirror box for travel. An aluminum strap was wrapped around the cage ring to give it rigidity and to add ½ pound of weight for balancing. A second aluminum strap is wrapped on the inside of the cage on the span where the focuser sits to give the ring additional rigidity where needed. Diffraction was an issue in that the 3 vanes would be 2.5 inches (each) tall and have the potential to create 6 diffraction spikes, even though the spikes are not as intense as the 4 spikes from a 4 vane spider. A composite material was chosen for it’s strength and thermal properties to reduce diffraction. The focuser is a motorized JMI model. The baffling on this telescope is currently limited to a baffle inside the focuser. This consists of a 4-1/2 inch square kydex with a 1-1/2 inch hole drilled through the center. Flock paper lines the inside. Not shown in the photos is a plastic background piece which can be attached to clips. This shield light behind the secondary mirror.


		Picture shows Cage nesting on the "Mirror Box". The spider is assemble by bolting the carbon Vanes to one side of the short aluminum Spider Vanes. Sorbothane is sandwiched in between the aluminum and carbon Vanes. The Vanes are connected to "Cage Ring Posts" in the same manner. The tabs sticking up from the Ring ( top of the picture) is what the Light Shield attaches to. The Shield slides over the tabs. 2 of the 3 Spider alignment knobs can be seen. The Spider's shaft is spring loaded with the brass knob on top of the shaft used for adjusting tension. The picture shows the mirrored plexi-glass Mirror Cover in place over the Mirror. The picture in the upper right shows the Focuser Baffle.

		Portability & Assembly
		The total weight of the telescope is approximately 104 pounds. The heaviest portion being the “mirror box." It weighs 72 pounds. The cage weighs 8 pounds. It is marginally manageable by one person... that one person being me. The telescope can be assembled by one person in approximately 5 minutes from the time you open the tailgate of the vehicle to begin unloading the telescope. The step ladder isn't necessary to place the cage on the trusses, but it is recommended. When pointed at the zenith, the telescope is 98 inches tall and 87 inches to the center of the eyepiece. The purpose of "Ultra Light" Telescopes seems to get lost in the debate of which is better, Light or the large heavy Telescope for the more stable platform. The heavy scope means weight which mean motion and surface area catches the wind. So is there really a stability increase? For me. if the Telescope is large and cumbersome, it does not get used. This is what drives the design. They say that the small the Telescope, the more frequently it is used. This is certainly the case for me as the TeleVue 85 is often grabbed while the 24 remains disassembled and stored. The idea of "Ultra Light" and portability is to reduce that gap. This telescope occupies a very small portion of the cargo area of my Ford Explorer. There is enough room left over for a passenger, 150 square foot awning, tables, chairs, sleeping bags and enough provisions for a week. I still have 360 degree visibility out my windows because the cargo does not impede my visibility. That is what a minimalist design permits. I admire the French Amateur Vincent Le Guern who built their 30 inch telescope so compact that it can fit into his small hatch back. His telescope accommodates him, not the other way around. Other telescopes which I feel were inspiring and influential in the design were built by Mel Bartels, Dan Gray and Bruce Sayre.

		Plans?
		This is the most asked question. Sorry, there are no plans for say available. The original plans were drawn using CAD. These plans were printed and atlered using an ancient device called a pencil. In reality, the design is quite simple once you know the overall dimensions.



		Design Updates:

		Base and Azimuth Bearing
		March 26th, 2005, approximately 1 inch of inside radius was removed from the Base. This was part of the overall remodel of the Azimuth Bearing which included removing the "cheesy" closet bearings that the Carrier or "flex board" rotated around. The bearings were replaced by "cam followers" that now rotate around the outside of the base.

		Modified Base... compare to picture above

		New Light & Dew Shields
		In June of 2004, new light shields at the cage and a new Dew shields were installed. The designs were based on the experience at the 2002 Mt. Bachelor Star Party. Dew, which froze into frost, formed on only a corner of the secondary. A spider vane apparently provided protection. A dew shield was added to the cage to provide protection over the rest of the Secondary. While the Primary stayed dew free, the weight of the frost caused the Dew Shield to slump on to the trusses and blocked the light path. The new shield is smaller and stiffer and has flock paper install. The Light Shield on the Spider is new as well. It is "permanently" attached to the Cage so that it is not part of the assembly of the Telescope. It is smaller and contours with the light blocking Vanes and with the Mirror Cell for nesting. Additional shields can be Velcro-ed onto the lower portion of the shield if necessary.

		*Before...*

		The morning after at the Mt. Bachelor, Oregon Star Party 2002. Note that the Primary stayed dry, but the weight of the frost caused the shield to slump onto the trusses, blocking some of the light path. Note the frost on the Secondary Mirror. The Spider Vane protected most of the Mirror.

		After...



		New Shields... first picture shows the new Cage and Primary Mirror Shields. The small rectangular shield blocks dew while the larger shield has been trimmed to contour with the Spider. The second picture shows the Cage "nesting" on the "Rocker/Mirror Cell" on it's cart. Visible is the newly added rectangle shaped Secondary Mirror Dew Shield.


		The TeleVue 85mm Refractor as a Finder
		July 23rd, 2004
		The 85mm Televue Refractor (TV85) has become a regular companion to the 24 inch. It is used as a stand alone viewfinder. It is fitted with an identical "Red Dot" viewfinder as the 24 inch. I experimented with this a year prior using the TeleVue 76mm. The 85mm the became the "official" viewfinder with the July 23rd, 2004 Star Party held at Mt. St. Helens. It works extremely well. Most of the objects viewed can be seen to some degree in the the 85mm. The primary eyepiece to be used is the newly acquired 20mm Pentax XW. It gives 30x and a 2.3 degree field of view. This is about 6.5 times more sky then the 24 inch with a 26mm Nagler. It is enough power to see the smaller objects. The 26mm can also be used on the TV85. It gives a 3.6 degree field of view. This is enough field to take in the entire Veil Nebula. A 32mm Plossl providing 19x and approximately 2.8 degrees can also be used if I don't want to share the 24 incher's eyepieces with it. The night of the St. Helens Star Party, the TV85 doubled as a "Rich Field Telescope". It was used to view the Moon early on, the Andromeda Galaxy, the Veil Nebula and various Star Clusters.



		Appearances:
		from most recent, back:

		February 25th, 2004, Was guest speaker at the Friend's of Galileo Astronomy Club of Southwest Washington in Longview, Washington's Astronomical Society.


		August 18th, 2001, Oregon Star Party (OSP) walk-about.

		August 16-18th, 2001, Oregon Star Party (OSP) Joint observations with "Sky Camping Group".


		April 2001 Displayed at the "Telescope Optics Workshop", Bellingham, Washington (left) & how it got there (right) demonstrating it's portibility by carrying it in Dave Dansky's Honda Accord.

		Rose City Astronomical Meeting - "Equipment Day", January 15th, 2001


		Amateur Astronomy Magazine - Issue No.: 28 (Winter 2000)

		September 2nd, 2000, at the canceled Oregon Star Party (OSP) walk-about.

		August 19th, 2000, at the Northwest Astronomer's Association

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Where did the idea come from?
The idea originated from NEED. The need to be able to transport a large telescope in a small vehicle to a favorite observing site.


It was Canadian Michael Taylor's 15 inch Telescope (see black & white photo below) featured in the June 1979 issue of Sky & Telescope that initiated my interest in a minimalist design. In reality, Michael's Telescope is massive by today's standards, but it could be broken down and placed in a small car. Dan Grey graciously gives me credit for coming up with the large bearing design. Thank you Dan, but I did not. Scott Beard of Tacoma, Washington rebuilt a Coulter 17.5 inch Telescope using a large bearing design. I first saw the design at the 1997 Table Mountain Star Party. It and Mel Bartels 20 inch, also revealed at the same Table Mountain Star Party, helped inspire the 18 inch Telescope. Mel's design was driven by the need to be able to transport his telescope in a small pickup truck. My 24's design originated in August of 1999 and saw first light in June of 2000.

. Evolution of design

Scott Beard's 17.5 inch Dobsonian	18 inch f/4.55	24 inch f/4
July 1997	June 1998	June 2000
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.		Michael Taylor's 15 inch Telescope on the left.
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Minimalist designs (left) have been on my drawing board since the 1970's. Keep in mind that this was the late 1970's. The lack of affordable technology discouraged the "XC-24". The Dobsonian like "X-1" was do able, but never done.

Right, is Cave Optical's massive 16 inch Telescope. Photo was featured in their 1970 catalog. The image of the 24 inch shows the comparative sizes. The 24 inch gets 80% more light gathering power and can travel in a mid size car.
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After building the 18 inch and just prior to building the 24, I sketched several design ideas. The 25 inch was based on an available mirror ground by Mel Bartels. Note that it and the 30 inch were based on a "folded" optical design as is used in Dan Grey's 28 inch Telescope. Not show was a short lived 40 inch concept. This was based on my 3/4 inch thick x 40 inch diameter coffee table. It too would have been folded. The design was dropped due to the technical difficulty to produce and support the mirror and because of the Telescope's enormous size. It was being designed to travel in a conventional mid size SUV.

30 inch f/3.75 - 25 inch f/6.3 - 24 inch f'/4 with 8 trusses



	Pictures: Upper left Shortly after completion, Upper right, dissembled to show break-down of components To show how it would be transported in Ford Explorer, Lower left: August 2007, next to Carlos from Argentina's GAMA group and the 10 inch Traveler