Orwell Astronomical Society (Ipswich)

Construction Of A 150 mm Reflector

Astronomy On A Shoestring

I think I was about twelve years of age - and just into secondary school - when heavenly matters first crossed my consciousness. I find it quite difficult to remember having had many real learning experiences at school up to that point - being one of a generation of children who suffered from both war-time and immediate post-war education rationing. Scientific and mathematical subjects had provided very little of interest or stimulation for me or any of my peers up to the age of ten and most of us didn't even know the difference between planets and stars (things haven't changed much since then I fear, even amongst the adult population...)

Then, suddenly, like switching on an electric light, along came two teachers who took hold of my imagination. One - in trying to make geometry and arithmetic relevant to a bunch of yawning adolescents - gave us an introduction to air navigation. This was the real stuff, taught the pen, pencil and protractor way! (The teacher had been a navigator in the Royal Air Force during the Second World War). What war? did I hear you say? Yes, I know, it was years and years ago, but you will have to bear with me... Was this what geometry and arithmetic was all about? Wow, how exciting! Then came the second teacher. He gave us an introductory look at the crucial role played by physics and mathematics in the material universe. By then I knew the majority of the group one air navigation stars listed in the Air Almanac and could recite the Greek alphabet. This was real science - and there was no stopping me. I wanted to know more, and quickly.

A few months later, I saw an article in the local newspaper about the Ipswich and District Astronomical Society (IDAS) - and very soon, at age thirteen, I was its youngest member [1]. Two shillings for annual membership (ten pence in modern currency) seemed good value even to me. Under the influence of a few of the Society's "old hands" who took me under their wing and an excellent series of local lectures on Astronomy and Astrophysics run by Cambridge University that first winter, I soaked up knowledge of astronomy like blotting paper.

My observing had up to then been with the aid of a mounted x10 pocket telescope. I remember perching on a step-ladder (on which my little pocket telescope was mounted) in our garden late at night and seeing neighbours peeping through cracks in their bedroom curtains to see what the "strange lad" was up to. Despite the opinions of the neighbours, I was captivated by the sheer beauty of the universe and had soon learned by heart the names of most of the northern constellations and the principle stars in them together with their mythology. I could trot out the majority of the entries in Messier's catalogue. This was, however, not enough and I needed a better instrument to help me explore the heavens.

Buying a telescope in the immediate post war days - and at only thirteen years of age with limited pocket money - was obviously out of the question. The solution was to build one. There was a member of the IDAS - Norman Whatling - who gave me the initial ideas as to how it could he done. Norman had already made at least one 6" mirror and was in the process of grinding an 8" disk. Indeed, he gave me a badly chipped 6" blank disk to experiment with. The back of this disk was soon to become the tool for my own mirror grinding.

When I was not at school or doing homework, I was in the shed at home putting together all the equipment that I would need to grind a mirror. Although my father was an engineer, he watched me with some puzzlement - especially as I began to commandeer his tools and demanded to be shown how to use them. In fact, although my preparations were done in the shed, all my mirror grinding had to be done outside as the shed was simply not big enough and was far too dusty for precision optical work. I had a big oil drum weighted half-full of bricks outside with a mirror grinding jig mounted on top. This permitted me to walk round in a circle while grinding ‑ an essential feature to achieve a spherical mirror - and I could splash water around without annoying my father.

After investigating (and dismissing) all the alternatives of ready shaped but rather thin porthole glass disks from which to grind a mirror, I went to Smyth Brothers' shop in Fore Street, Ipswich (long since closed) and ordered at their counter a 7" square of 1" plate glass. I remember the occasion vividly as the staff quizzed me as to what the glass was for. They pronounced that I, like other members of IDAS, compounded by my tender age of 13 going on 14, was mad to attempt to cut out and grind a disk without very specialised knowledge or equipment. Even my parents were sceptical.

My excursion into producing a 6" telescope mirror was about to begin. The slab of plate glass that I had ordered duly arrived and I clamped it in a jig that had a plywood template on top with a 6" hole cut in it. Into this hole I fitted a notch-edged 6" cake tin. I turned the tin glass cutter using a carpenter's hand brace mounted above it - making it look a little like Caxton's first printing press. I fed a trickle of wet 80 grit carborundum powder in around the edge of the cut. After about three hours of very noisy rotation, out dropped - to the amazement of all the sceptics - a perfectly circular 6" glass disk.

I then visited Smith and Daniel's in Westgate Street (the shop has long since disappeared) to order a range of carborundum powders, from 120 grit through to 400. Yes, you guessed - they thought that I was more than slightly eccentric. The undamaged back of the old mirror that I had been given became the lower tool and grinding began. To help grip the blank disk of the mirror, I fixed an old wooden drawer knob on the back with pitch. I recall using 80 grit for the rough grinding - and trying to guess when the curvature was deep enough. I used to wet the "frosted" surface of the mirror periodically with soapy water and focus the image of the sun on a light coloured wall until I could get a surprisingly sharp image at about five feet - I was aiming to finish with a mirror of around 4' 6" focal length. It seemed to take a long time to work through the successively finer and finer grades of carborundum until I was satisfied that I had completely eliminated from the mirror surface all the pits created by the earlier coarser grits.

I created my first pitch tool for the polishing stage by making a thin rubber gasket mat through which I had punched holes using a football stud punch. I placed the mat on the soapy surface of the upturned mirror and poured melted pitch over it. Onto this hot sticky puddle I bonded the tool. With a quick reversal of tool and mirror before the two stuck together, I slid off the still soapy mirror and peeled the mat away from the pitch leaving a faceted pitch tool on which the polishing operation could begin.

Polishing a glass telescope mirror using red jeweller's rouge was, I discovered, a messy business. The stain seemed to get onto everything I wore and my fingers were permanently coloured - as if with the nicotine of a chain smoker, only red. I cannot recall anything being said at school about them, but I suspect that I got some funny looks. Polishing seemed never ending and got slower and slower until I thought that the clarity of the glass that I had achieved in the centre of the mirror would never reach the edges - but it did eventually.

I remember my schoolboy excitement on producing a classic spheroid shadow on the mirror using the Foucault knife-edge test. We had a long kitchen at home which had a hard tiled floor - essential if you are trying to keep a test lamp, knife edge and mirror absolutely free from vibration and still dare to breathe normally. What was this silly boy up to now? I heard people say. After several more weekends of grunting over my polishing jig, and periodically halting the household as I insisted on setting up my testing equipment in the kitchen, I believed that I had produced a reasonably acceptable parabolic mirror.

I then had to make up the telescope design as I went along, using materials as they came to hand, not having much to go on. In my final school year during a visit to the London Science Museum, I obtained permission to visit Foyle's Bookshop in Tottenham Court Road. It was absolute heaven to gaze on all their scientific books and wish they were mine. Indeed, the trick was to select a book and curl up on the floor at the end of one of the long alleys between Foyle's shelves, read it and then put it back. A bit like a lending library - except that I never found any of these books in my local library.

One book in particular had been recommended to me. It was Amateur Telescope Making - a Scientific American publication edited by Albert Ingalls. Someone had told me that every successful telescope maker had one - this was the "Bible" - and it had just been re-published. I ordered a copy - for the staggering price of £2.25. When it arrived, I read it from cover to cover, and it provided me with all the telescope making techniques and ideas that I needed. I treasure it even today - perhaps through nostalgia - and I added to it Book Two when it came out.

At age 15 I left school and took my first job in local government, which gave me privileged access to a number of helpful craftsmen. They produced for me a beautiful lightweight square cedar telescope tube with dimensions that would exactly fit my completed mirror. Although the mirror was finished, it still required silvering. I read and re-read Amateur Telescope Making and felt that with luck, this was another job that I could undertake myself. Thus it was that I took a trip to Wiggins the chemist, at the bottom of Berners Street (another shop no longer in existence). I gave them precise quantities of all the materials that I needed - silver nitrate, potassium hydroxide, ammonia, nitric acid and distilled water. They looked shocked and I was ushered in to see someone who I believe was Mr Wiggins himself! He quizzed me for a long time as to what I was up to. Eventually he sighed and made me sign the poisons register before handing over the goods that I requested.

Even though I had virtually memorised the instructions and techniques for silvering a mirror, I was well aware that if the process went wrong I could either poison or maim myself. I was therefore particularly careful about this phase of the work. My greatest care was protecting myself from acid burns or producing a dish of silver fulminate - the equivalent of Semtex - that could demolish our kitchen and me with it.

I was astonished at how well the whole operation went. I noted that the books said that a silvered surface would probably only be good for a couple of years, but my original silver coating - though very dull the last time I looked - is still intact today, in early 2003, fifty years on. I suspect, however, that its efficiency is at rock bottom when it comes to light gathering.

Over the remaining months the equatorial fork mounting took shape and I balanced it carefully, using a car stub axle at its centre. I fixed the mirror and made a spider from hacksaw blades to hold an ex-army right-angled secondary prism. Whilst most things were home-made, I had to resort to buying eyepieces. I finally became the proud owner of a fine instrument with eyepieces providing magnifications of x100, x150 and x300.

In the years that followed the completion of my telescope, I spent hours using it to scan the night sky for objects of interest and wonder. lthough I still have all the parts of my telescope, and in that sense it is still operational, I have not used it for several years due to virtually impossible sky conditions where I live. Perhaps some day I will get it out of mothballs - and maybe give it away.

Footnote

John Barbrook

Original: Newsletters January, February, March 2003
Updated: 11 March 2023