ATM

12" Dobsonian Telescope

How would I feel when I see the southern sky the first time, where most of the constellations were unfamiliar to me or are upside down? Will I get the feeling being displaced to a far away planet in our Galaxy? Do fellow astronomers exaggerate when bragging about Namibian skies? How dark are the skies really? What optical aid will be available to me to see all those great deep sky objects? These were the questions going through my head before the trip.

Design requirements

After a while of consideration the following design goals emerged:

• Carry-on luggage limits on size must be met. Most European carriers define these limits as 55x40x20cm and 8kg. Making the parts small enough to snugly fit a bag seemed to be possible but it was clear that I had no chance to comply with the weight limitation if I was going to design the telescope to optimally fill out the given space.

• The telescope should have the biggest possible light gathering capability.

• Assembly/disassembly should be without tools.

• I was using a wide variety of eyepieces. To reduce the balance problem the altitude wheels should be large.

• I enjoy using Binocular Viewers on deep sky objects, even though I can see deeper single eyed. Binocular viewers consume large amounts of back focus.

• I was not going to watch planets with this scope, so a small secondary was not required.

The closest design to my idealised scope seemed to be a Dobsonian telescope. I would have to pass on a drive and go-to capability; on the other hand it would offer me the biggest aperture for a given size and weight combination.

I had built a 10" Dobsonian telescope almost 20 years ago but this had been a very basic project. Having no serious experience with woodworking, I needed time to workout the details. I bought the book “The Dobsonian Telescope” a router and a pedestal for my drilling machine. The Berry and Kriege book was of great help to me even though I was building a much smaller and lighter telescope than those covered in the text. I used the basic drawing capability of my word processor to generate plans and to visualise how the parts would fit.

The design process took far longer than anticipated. During 3 months, I over and over again returned to reconsider construction details and make changes. Even after building the scope I continued to refine it. So I went from a single armed design to a truss construction because of added stability for a penalty of some extra weight.

Construction

Since the mirror was thin, I was concerned about flexing. Consulting the excellent mirror cell calculator plop I decided to build an 18-point flotation cell.

The mirror cell is designed to be an integral part of the mirror box. The cell was made from an aluminium mason level. If a quality product is used the level provides a beefy structure for little weight. I cut the level to produce 2 hollow profiles. The ends were filled with wooden blocks, so that the profiles could be bolted directly to the mirror box sides. They are kept from turning by extra wooden pins.

I placed small wooden blocks with a nut inside in the profiles and passed the adjustment screws through them. Each adjustment screw head sticks out from the bottom of the cell and is used for collimation. The end of each screw connects directly to an aluminium bar hold in place comfortably with lock nuts. Each bar carries 2 triangles that are made of aluminium sheets and have 3 floor protector pads to contact the mirror. A plastic ring with Velcro pads keeps the triangles in the correct position. Extra wing nuts on adjustment screws at the back of the cell are used to counter the screws against the aluminium profile, so once collimated I fasten the wing nuts to freeze the settings. The collimation is stable through the observing session. I use a laser for collimation. The open truss construction allows to collimate the scope working back at the adjustment screws without moving up to the eyepiece.

Because of transportation requirements it was out of question to fix the mirror with silicone to the cell. This would produce a too big single piece, so I decided to use a sling and the mirror would be removable. But I couldn’t make the sling work satisfactorily. My sling material slacked under the weight of the mirror and despite adjusting the length several times it caused the mirror to shift from its optimal position on the cell. The result was a visible amount of astigmatism. Then I experimented with a cradle like construction to keep the mirror in place. Actually there are 2 cradles fitted with felt that correspond to the mirror diameter and are mounted on a bar of plywood. The bar is mounted paralell to the mirror cell and works very well. All parts are movable. I nudge the mirror into position before collimation and it stays there without any problems.

I selected 18mm Baltic birch over such exotic materials like carbon fibre or foam constructions for the primary structure mainly because of the better dampening feature of plywood and the lack of expertise with other material. I carefully designed the mirror and rocker box parts as well as the secondary cage. The primary objective was to make them stiff but light weight and in such a way that some of the parts fit inside others to reduce the transportation volume. I made the following parts to fit into each other:

• The rocker box sides fit into the mirror box sides

• Altitude bearings can be taken apart and placed into the bag to save space

• The mirror is placed into the disassembled secondary cage with a foam collar used in between

• Hollow spaces are used for small items like focuser, bolts etc.

I cut the parts with a router. To save weight, some of them were hollowed out. I used all metric M6 screws in the project to minimize the impact of losing a screw in the outback. A short search in the net found me a company in my county that produced screws with nice knobs. The knobs are very useful however add some extra weight compared to basic screws.

To fit the parts it was not possible to use wood screws because they had to come apart often. I also didn’t like the idea of using machine screws together with L-brackets because they are not stiff enough. I decided to try out fasteners used for furniture. In a local hardware shop I found small cylinders of 10mm diameter and 15mm length that had a perpendicular M6 nut machined into them. Thus to fit to perpendicular parts I only had to drill the hole for the cylinder and a perpendicular hole for the bolt in the first plywood piece and a 6mm hole for the bolt into the second piece. The parts are then connected with an M6 bolt. The cylinders fit extremely well and the construction is very stiff.

I also used several threaded insets. The cardinal rule for insets is to bevel out the plywood for 2 or 3 plies. Otherwise it is guaranteed that the upper layer of the plywood will break. I also found that it is not very easy to sink them into the exact perpendicular position. Therefore I build a simple and very practical tool based on an eyebolt. I use two threaded spacers with the same 10mm diameter as the hole of the threaded inset. I screw the spacer, the threaded inset and the second spacer on the M6 eyebolt. The spacers ensure the eyebolt stays square to the hole during insertion. Now it was possible to place the threaded inset into the bevelled hole and use a screwdriver as a lever to screw the inset into the plywood. It seems the attainable precision isn’t short of professional workmanship.

I decided to use the legs of photo tripods as poles for the truss structure. In a photo store I found a nice lightweight aluminium tripod and bought 3 of them. I took apart the legs and machined them for proper holes. Tripod legs are not as stable as one piece poles but can be easily shortened to 40cm to fit into the bag. The weakest part of the tripod legs is the plastic clutch, so you may want to buy a model with emphasis on a heavy duty clutch. The tripod legs are screwed to the mirror box and the tips are clamped to the secondary cage. A nice side effect of using tripod legs is the possibility to alter the length of the truss structure for binocular viewing. I marked the poles for the proper lengths, so it is easy to extend them to the required length.

My secondary mirror is a bit on the oversize side, but this is the price to pay for binocular observation. I had to use an all-metal secondary holder. An old X-ray film of one of our cats is used as a light shroud mounted opposite to the focuser. The 2” Crayford focuser weighs 400gr and works adequately. Initially I used a Rigel QuickFinder which I recently replaced with a Telrad.

I made a shroud from black cotton that can be attached by Velcro around the mirror box but don’t really need it. Despite the open construction, the scope rarely dews up, even in the Alps where I observe frequently. However I made a shroud around the secondary mirror which is more inclined to dew.

I decided to use big altitude bearings. To make them fit into the bag, each of them were made out of 2 parts. I was surprised how well they fit. Probably it was a reward for using a router? I covered the bearings with Ebony Star and use Teflon strips on the rocker box.

The rocker box is the heaviest single part after the mirror. To reduce weight I cut out parts, especially from the bottom. Because of the asymmetric load of eyepieces, in order to balance the truss structure the azimuth bearing is offset to the side of the focuser. A ring made of Ebony Star is glued to the bottom and rides on 3mm Teflon strips. A bolt connects the base with the rocker box and allows for adjustment of the force needed to turn the scope in azimuth.

First Light

Well, unfortunately the mirror was late and this meant I had to have first light in Namibia. Despite much heavier than allowed (my scope now tips the scales at 15kg), the nylon bag was visibly smaller than the maximum allowed dimensions, so I guess nobody had the idea to weigh it. Screwing the scope together takes about an hour (there are many bolts and adjustments to be done).

I had several teething problems during my first trip and corrected them after returning to Austria. Among these were the sling problem mentioned above, balancing using springs as used in my early design. Also the transportation dimensions dictated a smallish base and one has to have a careful hand when turning the scope in azimuth. I have since replaced the bottom part with a bigger equatorial platform which gives better stability for use at home. In general a portable scope can probably never be as stable as a solid one scope, on the other side this one allows me to go places where the sky is still as dark as before industrialisation started. Several small changes (often unexpected ones) improved the stability by a big margin.

The skies in Namibia were extraordinary, exceeding all my imaginations by a good margin. I had great views of objects in Centaurus, Carina, Vela and Sagitarius as well as of hundreds of objects in the Magellanic Clouds, so bringing a 12” has been a good thing. However, after returning home, I was sure that the most striking object I had seen was the unaided view of our Milky Way with all the bright stars, clusters and dark clouds that become so apparent in the dry desert air! In the mean time my scope has travelled a second time to Namibia without any problems and I look forward to travel once again to dark skies soon!

Clear skies to you! E-mail tv204@yahoo.com
Tahir Saban, Baden Austria.