EXPERIENCE OF BUILDING A TELESCOPE
by Graham Low, Member of ASSA Pretoria
I grew up on a farm, and with no television, and weekends were often spent studying the stars with my parents pointing out the stars they knew. When the road department needed to improve accessibility to new areas we were forced to sell the farm, and I found myself living in a flat in town where you could not see the stars.
Later, marriage followed and then children and their education. Then suddenly the kids had left the nest and I decided to watch the stars again. The sale of equipment on the internet encouraged me to buy a telescope but I found it to be unsuitable for astronomy and nothing but rubbish. I then bought another one, I was able to see the Moon but not the stars. Buying a bigger one was not an option as the cost of a good scope was completely out of my price range, leaving me with one option – to build my own scope.
After much thought on the matter, and reading a few books which made it seem simple, off to the ATM class I went. The first day I was given a book of instructions together with two 152mm pieces of glass about 30mm thick. The first decision was to decide on the focal ratio of the scope influenced by the transport of the scope when completed, as well as the type of vehicle I owned. Having a bakkie helped to solve the problem and finally decided on 1219 mm with an f8 focal ratio. This in return affected the depth of the sagitta of the mirror to 1.17 mm.
With the decision made the next item was to bevel the edge of the mirror to at least a 3mm in order to prevent any splintering of the glass edge during the grinding process. I sat in front of the wheel with other new members feeling a little out my depth and carried out as instructed and shortly had completed the task. The two mirrors were closely examined and a decision made on which was to be the tool and which the mirror as bubbles were visible in the glass. The grinding techniques were explained as well as the type of strokes to be used with warnings of incorrect strokes causing a problem later when polishing.
I was given my first grit, with a mesh size of 80, a bucket of water and told to start using the W stroke. Well glass on glass with grit in between made a real scraping noise but I persisted to the end of the day and went off home with the mirrors, grit, and book, to return the next week to continue.
Once at home, I built a board with 3 cleats in order to jam the mirror while grinding proceeded. After reading the notes about three times, with a lump in my throat I placed the mirror in the board added grit and slowly continued to grind away at the mirror adding water and extra grit as required, a procedure that was to last for many weeks and hours. I returned the following week to class. After testing the mirror it was decided that more grinding was required before progressing to a finer grit. I followed the instructions, and Saturday afternoon was spent grinding the mirror as well as a few hours during the week. Saturday soon returned and off to class I went. Tests proved all was correct and I progressed to a finer grit 120.
The process now seemed to move rather fast and after a few more weeks I had completed the finer grit grinding of the mirror with the pitch lap to be made the next Saturday. The lap was warmed up and poured on to the tool but to my disgust it did not form properly and had to be removed and redone. This pour was a success and the polishing started.
Polishing required a very clean area free from grit and dust. The polishing movement was very similar to grinding procedure. The lap was always kept damp with the mirror on top when storing. I was fortunate in not having my mirror and tool sticking to each other like some of my fellow class mates. The polishing came to the end and the final tests began. The mirror was tested using a light source (Foucault test).This was a very complicated test trying to see the faults on the mirror and I was forced to borrow a book which helped me to understand what I was looking at. However, many of the advanced students as well as tutors are extremely helpful and happy to explain the finer details. Soon it was time for the mirror to be tested by Chris Stewart.
The mirror was set up on Chris’ Foucault tester. Chris together with two tutors Johan and Dave took their first look. "Mm,mmm '' was all I heard and off they went, to return later and informed me they were not satisfied and gave me further instructions on what to do. After I completed the instruction I set the mirror up to be tested again. "Mmm, mmm " and the decision ... to let the mirror rest till the next week.
The next week we warmed the lap up and using a homemade tool cut the grooves and reinstated the lap to its former shape. With a few more strokes my hot mirror was set up for another test and 'mmm, mmm’ was the report again. They returned and advised me to try twice round star stroke and set it up again. This was done, and within fifteen minutes tested and then once more the star stroke ... and the result was positive and the mirror at last was approved, with Chris prepared to sign off the mirror.
Before I could get too excited, Rodney was off with my mirror for aluminising. We returned the next week and Rodney handed my mirror back, to find it had like chicken pox marks all over. We were all shocked and the test showed an error in the aluminizing process, so off Rodney went with my mirror for a second try. During the week Rodney phoned to say my mirror was aluminized and I could go and fetch it.
I now had to build a telescope for the mirror. Johan had a card board tube for me. We discussed the construction and built the primary mirror holder and then I went home to continue building my scope.
The following Saturday at class I worked on my secondary mirror. The mirror was attached to a broom stick cut to forty-
The first day I dropped my secondary mirror and had to return to class feeling rather sheepish to grind another one. In order to cut the square and round edges, I had to design various attachments for my power tools which worked out very well.
I managed to get a drawing on the internet and started cutting the wood, slowly the telescope came together and then I assembled all the parts, going to Johan to discuss further problems and assembling the telescope with two steel bearings on the side on 4 pieces of Teflon, giving me a smooth movement when moving the telescope in any direction.
At last D-
The next week in the Pilansburg Game Reserve I was able to find the Seven Sisters as well as Orion's Nebula and other items. Wow! again. Living in Centurion I am fortunate to have good visibility and I am able to see many stars and planets.
If you build your own scope, you do it in your own time, if you are like me you want it working as soon as possible, then you need to put in the hours at home, after work and on weekends or you can take it easy and only work on it at class on Saturday afternoons. You can use bits and pieces you have in your garage which will cut the cost but you will have to buy the some things but it is cheaper than buying a readymade one and the satisfaction you have when the job is done is more than you can imagine.
I have since made changes to the scope to allow for better viewing and found certain items needed to be improved on, I have also built a finder scope to start with the ASSA 100 identification challenge. It took me a little over four months to complete the project and spent less than R2 000.00 for the materials, excluding eye pieces.
The telescope has proved to be a great success and I had great fun learning everything that went into building it. Talking to others who have been doing it for longer is very informative; I learned more than I would have if I had read it in a book.
The North Star party which I went to in Warmbaths in March was unbelievable with sightings of galaxies, nebula, planets Jupiter and Saturn. Seeing the comet Lemmon was a highlight and I was able to track it for two minutes.
My plan is to take my scope with me on my next trip to Namibia where the skies are big and the stars are bright.
I wish to thank my tutors for their endless patience and assistance.