Jack-in-the-box by Louis Viljoen - May 2011
Background: We all need a community to live in. Without people around that share the same values, hobbies or interests, life would be ‘boring’. There is such a community in the bottom tip of Africa, and once a year we hang out and have a big ‘get-together’, to show off our achievements of the year. This community is made up of a variety of different characters, and we come from varied backgrounds, but the one thing we all have in common is our love for telescopes. There are a lot of areas in astronomy, but our particular interest is in building telescopes, and we pride ourselves on our collective ability. There are of course some of us that are more competent, but that just strengthens us as a society. We also advocate making the mirrors ourselves, and we can claim a variety of different telescopes, from classical Dobsonians to Dall-Kirkams and Shiefs.
The internet is a wonderful thing; there is so much information there. After completing a standard F8 6” Dobsonian, I knew I wanted to build another telescope. When surfing the net, my eye caught a specific design of telescope, the “coffer” scope, translated from Dutch meaning “suitcase” scope. This particular telescope can be packed into a box, where the box also acts as part of the telescope. There are not many of these telescopes around, but from my research on the internet, the design seemed not to complex, and do-able for me. One of the principal criteria for me is that the telescope can be made in on a kitchen counter, with hand tools. The only power tool available to me is a handheld power drill.
Design / Prototype: The parameters for a fold-up type telescope are pretty straight forward: try to put as much aperture in as small a package as you can fit it into, without sacrificing any structural integrity. Fate played a role in selecting the aperture. While re-packing a cupboard in our telescope building ‘clubhouse’, I found an orphaned 9” piece of recycled glass. This glass blank has lived in the cupboard for a couple of years, as it was not 100% round shape, and also had a couple of air bubbles in. I took a liking to it immediately. (An aside: most of the mirrors we make as a society are from recycled glass, that is melted, and poured into 6” or 8” moulds. I am sure that my mirror has at least a molecule or two of a beer bottle of our favourite South-African beer, Castle Lager.) After settling on the 9” aperture, I decided to make a fast telescope so that it can be as short as possible. Not knowing any better, a F4.5 sounded fine to me, blissfully unaware of the difficulties lying ahead in the mirror making. Like my previous telescope building, I enjoyed making the mirror and the telescope at the same time.
Mirror: How difficult is it to make a mirror? I have made a 6” F8 mirror before, so I was not daunted by a bigger faster mirror at all. Well, the experienced I had previously with the smaller mirror was invaluable. One always read that to make a good 12” mirror, it would be quicker to make a 6”, 8” and 12”, compared to only a 12”. I found this to be true. The art in mirror making is the last stages of figuring - and there is so much written about it, that it could be confusing. Fortunately, there are many experts who guided me with advice. Like most of my telescope, it was also made in the kitchen. In our household, I am responsible for dinner, and it is quite an achievement to make spaghetti bolognaise whilst fine grinding at the same time (I just want to apologise to my family for the occasional grit in the pasta sauce). The mirror was finished eventually finished, with the usual suspects popping up, like turned-down-edges and oblate spheroids. I must say, that one of the most rewarding aspects of making a telescope is that one can produce a mirror by hand to sub-wavelength tolerances.
Telescope: It was not difficult to decide what material to use in the building of the telescope body, MDF (also referred to a chip-board) is cheap, and easy to work with. For the trusses, focuser and mirror cell, aluminium is used. The result is not the lightest telescope, but it does not flex at all. I did do a design of the telescope upfront, but most of the plans evolved as the project progressed. A lot of the design is based on pictures I found on the internet. I borrowed elements from a variety of the different telescope.
For this design to be successful, the suitcase is the telescope, and vica-versa. The other aspect of a “coffer” Dobsonian, is that all the parts that make up the body, must be able to be packed securely (and safely for transport) in the box. It took some ingenuity to make everything fit, but the end result is that only the trusses are external to the box.
I love the fact that unused item (called junk by others) can be used, and 2 LP records werepressed into service: one for the azimuth bearing, and the other for a light shield. Also, old Meccano wheels are employed for the focuser. Old plastic milk bottles are used (as a replacement for Teflon) on all the bearing surfaces.
Final product: Jack (the telescope) was born on the 1st of May 2011. He is a bit of a shy child, and lives most of his life in a box. So I normally refer to him as Jack-in-the-box. Maybe he is related to will-i-am from the Black Eyed Peas (thanks to my 16 year old daughter for that fact). He shares a room next to my other self build telescope, Bubble-no-Hubble.
Like most self-build projects, one accumulates knowledge and experience. This is a good thing, but it also forces you to evaluate the original decisions that were taken. There will always be some aspects that could have been done in a better / faster / more optimal way. These lessons needs to be leaned, and then shared amongst friends, so that we keep on improving and setting new and higher standards in building telescopes.