Backnanger Sterngucker

Gerd Huissel's pages
The traveldobs, a "All-in-one-Box"
dobsonian telescope

For several years i have dreamed of a telescope matching the following conditions:

• Aperture suitable for deep sky, about 20 to 25cm
• Compact dimensions, perhaps it is possible to take the scope as hand luggage for air travel. Anyway there is always a place in the car for transportation
• Perhaps I could take it along with my motor bike, it would be nice to travel to the ITT Telescope Meeting with the bike
• Accessories for collimating and observing, including a small star atlas and a red light lamp, must all fit into the carrying case
• Good handling, the scope shouldn't "go with the wind"
• Maximum observing convenience
• Solid construction
• Beautiful design in my opinion

Sure, there are ATM telescopes out there with smaller dimensions and less weight than my construction. But every ATMer chooses the variant that suits best his personal demands, and so I did.

The main mirror of 8 inch aperture is manufactured by Synta from China. As far as I know telescopes with these mirrors are sold in the USA under the brand name Orion. The optics in its cell weights in at 3kg. I thought a 10-inch mirror is to heavy, exceeding the weight limit I had expected. I found a secondary mirror manufactured by Lichtenknecker and a cool 1.25-inch focuser from Astro Techniek of Netherlands on the ATT-fair in Essen. A Rigel Quickfinder completed the list of parts which I had to buy. A 2-inch focuser wasn't considered, because eyepieces with 2-inch barrel are too heavy. The rest of the little telescope was made of 9mm birch plywood, approximately 1.5 square meters. For a few parts like the rings for the altitude bearings and the strips for the teflon pads I used 18mm birch plywood.

The secondary holder is a black plastic pipe cut off under 45 degrees. It was made by a friend of mine on his lathe. The pipe is attached in the secondary cage and hold in position with a curved spider made from a 1.5mm sheet of aluminum bended to a radius of 100mm. The experiment with a 1mm aluminum sheet wasn't successful, the spider just wasn't stiff enough. The offset of the secondary mirror is 2mm which is realised with 2 washers at both ends of the spider's curve.

Some dimensions:

• Secondary cage: Inner diameter 220mm, outer diameter 260mm, height 185mm
• Mirrorbox: Outer dimensions 258x258x165mm (LxWxH), inner dimensions 240x240mm
• Truss tubes: Eight aluminum pipes with 10mm diameter and 1mm wall, length 650mm. Each truss is divided into two pieces of 325mm length
• Overall length of the tube: 1000 mm
• Point of balance at 230 mm above the ground board of the mirror box
• Diameter of altitude bearing: 250mm, made of 18mm birch plywood
• Outer dimensions of the rocker box: 313x313x170mm (LxWxH)

After finishing the scope all parts fit into each other within a precision of approximately 1/10mm. It really helps to plan in advance and to do all saw cuts with identical dimensions in one stroke. The altitude bearings are fixed to the mirror box by screws. Rotating the mirror box by 90 degrees it fits into the rocker box for transport. In each corner of the rocker box there is room of exactly 20x20mm to store the 16 parts of the trusses. The rocker box serves as the bottom of the carrying case. The secondary cage is placed exactly between the altitude bearings and on top of the mirror box. Therefore the inner height of the case is 380mm.

The cover of the carrying case is used as a podest to raise the focuser to an convenient height for observing. The central axis and the teflon pads of the azimut bearing are placed on top of the cover. After putting the parts together to the carrying case ein outer dimensions 313x313x400mm (LxWxH).

Upon the dust cover of the mirror there is room for a Rigel Quickfinder, the laser collimator, a red light lamp, the "Karkoschka" star atlas and five eyepieces with 1.25 inch barrel. A sheet of black cloth with velcros, the "stocking" for the truss tube against stray light, prevents the eyepieces from dangling in the box. Shaking the carrying case with all parts inside you can't hear any noise! Bingo! Now it is time to put all parts together upon a balance — 11kg! Without the eyepieces the desired weight of 10kg is reached.

With pointing the telescope to the zenit the focuser raises to a height of 1.36m, at the horizon the focuser lowers to 0.93m above the ground, just a convenient seating height. This is the main advantage over other lightweight and compact telescope solutions. The first light showed an optic in good condition with brilliant and crisp star images at low and medium magnifications. The test with higher magnifications at approximately 200x or even more will be done if the planets could be seen under better atmospheric conditions.

The maximum field of view is approximately 1.6 degrees using a 32mm Erfle eyepiece with 1.25 inch barrel. Even a Meade 8.8mm UWA eyepiece which weights in at 400g doesn't force the scope to make a headstand. Following David Kriege's suggestion of 15 pounds per square inch surface pressure for the teflon bearings and using EbonyStar on the altitude and azimut bearings really pays off.

Restrained diet and no hunting for world record lightweight resulted in a beautiful little telescope (in my opinion) with good handling characteristics. The setup of the telescope is done in under five minutes, collimating with a laser collimator lasts less than two minutes. After that you only can hope for clear skies and best seeing.

Feel free to copy some details or the whole telescope - for questions please mail to Gerd Huissel.

Comparable Solutions

Ein Nachbau exisitert inzwischen in Kanada, siehe den Artikel im Newsletter Pulsar für den Winter 2007 des Kitchener-Waterloo Centre der Royal Astronomical Society of Canada