Thursday, May 26, 2016



The “Sam Brown” Pipe Mount – A Telescope Mount For All Seasons

First Published in the Astronomical Society of South Australia Bulletin
 March 2006 (www.assa.org.au)

Martin Lewicki
Home-made Classic 6-inch Newtonian Reflector



Way back in 1974 I decided it was time to build a decent telescope. Until then I observed mainly with a pair of 10x50 binoculars and prior to this a home made simple lens 30mm telescope made from whatever was lying about at the time. My new project was to grind and polish a mirror to make a 6-inch Newtonian reflecting telescope.

As I frequently visited my brother’s bookshop in Adelaide it was just a short walk around the corner to Chesser Street to seek advice from John Cole from Cole Precision Optics. John supplied the Pyrex mirror blank and plate glass tool along with the abrasives and polishing formula along with patient advice on what I would be up against in making my first mirror. He also supplied me with a singular book by Sam Brown, the classic All About Telescopes published by Edmund Scientific. Its profusely illustrated pages had all the instructions on making telescopes of all kinds from simple to complex. The instructions covered mirror grinding and polishing, telescope math, mounts, photography and optical theory.
I was so excited and driven that the next weekend, in the space of 48 hours, and virtually no sleep, I had ground and polished the mirror and tested it with a rhonchi set-up in my shed. The following day I handed it over to John Cole for examination. He declared it a “good mirror” — at least for a first timer, and we decided to go ahead with the coating. The optical performance subsequently turned out to be rather good showing diffraction disks and airy rings on stars on nights with steady seeing.

In the mean time I had to think about what kind of mount I would choose for my 6-inch reflector. This is before the days of John Dobson and his famous “no-fuss” dobsonian telescope mount, which I’m sure I would have chosen had it been contemporary. I perused through Brown’s book at the copious choices of mounts that could be made from all manner of materials. While I would have liked to have gone for one of equatorial roller bearing mounts I felt this was a bit too advanced for me. I was instead taken by a simple mount that could be made from plumbing pipes. The illustrations showed how I could use a 45-degree elbow joint and a T-section to make a hand-push turn-on-threads German equatorial mount.
The 45-degree elbow joint would serve to angle the upper part of the mount to point roughly at the south celestial pole at mid latitudes and rotate on its screw thread in right ascension. The T-section would hold a cradle with telescope and rotate on its thread in declination. All this is balanced by an opposing counter-weight. Brown suggests using 1.5-inch galvanised pipe parts for a 6-inch reflector but I went for 2-inch pipe for sturdiness and to possibly handle a future 8 or 10-inch scope.

After a few trips to the hardware store to buy the joints and 2-inch pipe cut to length for the pedestal I followed Browns instructions. While the 45-degree elbow would have been quite suitable for a rough polar alignment near 45 degrees latitude, for Adelaide at 35 degrees this is too far out for comfortable equatorial tracking. It would require too many frequent corrections in declination to keep an object in the field of view as I tracked the scope. So I decide to combine 45-degree, 90-degree and T-elbows that enabled me to twist the first two into a position so that the “polar-shaft” could be set at 35 degrees. I had a friend drill and tap three holes in the lower elbow to insert bolts to lock the polar elevation in place.

The lock bolts could also be loosened to adjust for a different latitude if required. The top of the mount was fitted with a timber cradle for the tube and mated to one end of the T-pipe with a flange to turn in declination. An azimuth-lock bolt was also added to the elbow mated to the pedestal pipe to facilitate aligning the “polar shaft” to south.

Modified pipe arrangement for 35° latitude

The turning screw threads for the right ascension and declination axes were lapped (fine ground) with wet 600-grit carborundum powder left over from the mirror making process. Several twists of the mated threads smoothed their contacts. They were washed then lubricated with a layer of grease, re-mated and finally assembled on to the pipe pedestal which had three legs made of meranti timber bolted to the base.

While a mount that turns on plumbing pipe threads appears to be crude in comparison to a precision roller bearing mount the pipes turned out to work remarkably well. Once the telescope tube was mounted and balanced with a counter weight the movement on the axes was surprisingly smooth. I found the natural clutch action of the 2-inch threads had very little backlash when fine aiming by hand. In fact it is much smoother than any 6-inch dobsonian that I have ever across since!

In addition, when the mount is rough-aligned with the celestial pole within a degree or two I can gently hand push (or pull, depending on which side of the meridian the telescope is pointed) in just the one right ascension axis to track an object for 10 to 20 minutes at low magnification before a declination tweak is required. You give it a gentle push, let go, and it stays put. Even inexperienced visitors get the hang of tracking a planet very quickly by this method because it avoids the more tedious and sometimes confusing step-wise actions required to track an object as with a dobsonian mount.

An important advantage of an equatorial mount is its ability to easily tack an object through zenith where sky transparency and seeing conditions are optimal. The alt-azimuth configuration of a dobsonian however hits a mechanical dead spot as it becomes ungainly to point along its azimuth axis aimed at zenith requiring awkward manipulation of the tube to acquire and track an object. A similar dead spot exists for the equatorial mount when the telescope is aimed at the celestial pole. But here only a small circular section of the sky is involved that is rarely targeted by telescope users.

A German equatorial mount necessarily has a protruding counter weight shaft and the eyepiece will rotate to different orientations depending on where the telescope is pointed. This may require a raised footstep for some people to reach the eyepiece in some viewing orientations. And of course the counterweight adds a necessary, but a dead weight to the entire assembly.

My mount plus pedestal weighs in at 15 kg compared to the wood mass of an equivalent dobsonian at about 12kg. Also my mount has several parts that need to be assembled and disassembled in the event of having to load in a car for transportation. Assembly and disassembly can take 10 or more minutes. This might be a disadvantage for the frequent dark sky sojourner. However I find the smooth push-aiming action and simple equatorial tracking of the Sam Brown pipe mount a sufficient compensation for the drawbacks.

Over the years my pipe mount has served me well. The galvanized iron parts are so sturdy and robust that at the end of an observing session I bring only the telescope tube indoors for storage and permanently leave the mount out in the backyard — exposed to the elements! This saves me a lot of assembly-disassembly and carrying weight. It has survived 32 winters, summers, and seasons in between and has been subjected to all manner of weather onslaught in that time with no loss of performance!

The only regular servicing is cleaning and re-greasing the threads on the turning axes every two years or so and changing the iron lock bolts when they begin to rust. Though in future I intend to use brass bolts for more permanent solution. The meranti timber cradle and tripod base with several coats of outdoor clear enamel lasted most of those 32 years though in the last couple of years had become so weather beaten that I finally replaced them with fresh timbers.

Casual visitors often comment on the“contraption” standing sentinel in my backyard and sometimes venture a guess as to its purpose. Among other things it has been taken for an attempt at postmodern sculpture and a device for stringing up tennis rackets. Others have correctly guessed its purpose and one tradesperson, a plumber, and owner of a 4-inch commercially made equatorial reflector especially commented on the simple design and smooth motion of the axes. Meanwhile my astronomy friends have dubbed it the “plumber-scope”.

Over the years I did consider making a dobsonian mount to replace the pipe mount for my 6-inch reflector and follow the trend of the compact, quick set-up that it affords. But the Sam Brown pipe mount has a quaint elegance about it that I find difficult to relinquish. While these days the cost of making an equatorial pipe mount would exceed that of making a dobsonian mount, a telescope that is polar aligned brings it in line with the way the diurnal sky rotates and has a natural feel as you track a celestial object. My indelible pipe mount as it stands in my backyard and weathers yet another round of seasons reminds me of the continuity of a life of observing. I tip my hat to Sam Brown. ***

1 comment:

  1. Thank You. This how-to about your plumber scope is simple and graceful in its own way . 5 stars

    ReplyDelete