Amateur Observatories:  Design and Construction

  THE EDENVALE OBSERVATORY

THE TELESCOPE

This chapter is meant to encourage those amateur astronomers who do not have the space, means or time to build an observatory like some of the splendid examples described in this book.  If you are an amateur and are burning to get down to some serious work, you probably feel a need of something better than a site on the back lawn where street lights have to be dodged and other hazards of portable telescope observing coped with.  Remember, all you need from an observatory is protection for the telescope from the weather and a handy observing base where you can become operational at very short notice.  The rest is vanity, as Hamlet should have said.

In the early 1950's, after making a thousand or two variable star and occultation observations from a sometimes windy and dusty back lawn, I decided that serious amateur observing deserves better conditions.  At that time I was using a completely homemade 6" Newtonian - even the eyepiece elements had been ground and mounted by an amateur.  The telescope was convenient to use but a number of faint variables were beyond its light grasp.  Here was a good case for a larger instrument, protected from the elements.  The telescope had to have a reasonable light grasp, be highly manoeuverable and had to be affordable.  Large off the shelf catadioptrics lay some years in the future and so my instrument had to be home made.

I duly set out to build a 12.5"  Dall-Kirkham Cassegrain telescope and about a year later the instrument was in use.  The optics proved to be straightforward to make.  The final Foucalt tests of the ellipsoidal primary employed a small pinhole source at the near conjugate focus and a knife edge at the far conjugate focus.  The geometry of this arrangement made it a true null test. Then I partly polished and figured the concave tool of the secondary to a spherical shape and used it as a test plate for Newton's fringe testing of the secondary.

The optics were mounted in a very rugged reinforced square Masonite hardboard tube, just 48 inches long.  I gave the optical tube an adequate equatorial mounting with a clock drive, large, easy to read setting circles and smooth slow motion controls.   The instrument has a small finder and a 3" auxilliary refractor.  A parfocal turret gives a choice of 167 or 267 diameters through the main telescope, and the refractors have magnifications of 6 and 48 diameters.  As the various eyepieces and controls are all within a radius of about 250mm [10"] I can switch between these four powers within a second or two.  It takes but little longer to change from one familiar variable star  field to the next.

TWO OBSERVATORIES

Before going into details, I would like to air a few thoughts on amateur observatories:  If you plot the amateur observing stations around the world on a graph with total cost in material and labour along the horizontal axis and annual output of scientific data along the vertical, you will find a strong negative correlation between the two.  At the one extreme is the world's most prolific and accomplished variable star observer, Albert Jones OBE of Nelson, New Zealand.  Albert's 12.5 inch home made Newtonian is kept in a toolshed from which he wheels it for a night's observing.  His instrument is strongly reminiscent of the principal industry of his country, which is agriculture. At the other extreme are any number of beautifully built and equipped observatories, whose builders are too busy perfecting their facilities to get down to serious observing.  Aspiring serious observers will do well to ask themselves what they really want from their observatories.

My own interest is in so called serious amateur work, viz the making of observations which are reported formally and used by the professional astronomical community, in this case variable star observing and occultation timing.  I have no quarrel with recreational astronomers who observe the heavens for the sheer pleasure of it or those who take astronomical photographs which are afterwards admired but not used for scientific purposes.  These good folks pursue different goals and I am not qualified to speak for their needs.   Amateurs who are dedicated to the bringing of astronomy to the public are in a class of their own and I cannot really speak for their needs either.  It is probably better not to confront newcomers with beautiful, expensive facilities but on the other hand a beginner who is asked to look through a telescope which keeps on losing the object, on a windy and light-polluted back lawn is also likely to be discouraged.

THE FIRST OBSERVATORY

The building in which my own 12.5" reflector was first housed, was adapted for that purpose at no cost apart from three 50X100X190mm (2"x4"x6') pine studs and a handful of hardware.   I had designed the telescope and mounting to fit into the end of my garage which has a gently sloping corrugated iron roof. It was a simple matter to remove about 2 square metres (20 sq.ft) of sheeting and to make a panel by fastening the removed sheets to two of the studs.  The studs straddled the hole left by the removal of the sheets and I attached wooden runners made from the third stud to their four ends.  These runners lay in the troughs of the undisturbed sheets and allowed the panel to slide easily.  Two hardware store pulley sheaves enabled me to open and close the panel by pulling on a rope.  The alterations weakened the roof structure slightly and probably violated any number of clauses in the municipal building code but then I never consulted the powers that be.

If you do want to consult someone then it is a good idea to let your neighbours know what you are planning.  If they have enjoyed a look through your telescope, they will probably be supportive when it comes to yard lights on their side of the fence, and electronic noises from your side in the small hours.  In this connection I must digress for a moment to tell of an interview with a house owner while we were house hunting. I asked him about street lights and he replied:  "Don't worry, it is as bright as day here all night".

To return to my first observatory:  The whole job took a Saturday and was completely successful.  The horizon was about 25 degrees all round. The telescope was used for many occultation timing and variable star observations as well as for obtaining thousands of photographs of Mars during the favourable 1956 opposition.
 

THE PRESENT OBSERVATORY

As we learn to our sorrow, the good things of life do not always last. Domestic circumstances dictated a move to another locality and I had to start again without the benefit of a garage with a corrugated iron roof.  Having had to move  repeatedly, I was not keen to build a permanent observatory and opted for a temporary, transportable one.  The decision proved to be sound. The building described below has in fact been moved three times since it was completed.

It  often happens that temporary measures have to serve permanently and this observatory after 23 years is a case in point.  The photograph shows that it is no longer the trim structure that was built in 1972.  Rough handling during the moves and  failure on my part to do maintenance have taken their toll.  The footing has been badly damaged by rainwater because I omitted to erect it on a course of bricks after its last move.  Nevertheless, the building is still weatherproof and it continues to serve its purpose.

The building consists essentially of four panels of Masonite hardboard, 3mm (.125") thick and stiffened with bracing pieces at 600mm (2') intervals.  The panels are bolted together and are capped by a hinged roof of the same material. The roof has a radius which imparts a surprising amount of stiffness to it.  So far it has withstood the onslaught of the large hailstones which are a feature of our South African Highveld summer. The roof is counterweighted and is easily operated with one hand. The telescope is mounted on a 200mm ( 8") steel pipe with a 600mm (2') diameter flange which rests on the flat concrete roof of an outbuilding.  Vibration is not a problem.

The observatory door is only five horizontal, and ten vertical steps from my kitchen door.  It takes less than a minute to leave the kitchen, open the observatory door and roof, uncover the telescope and switch on the drive and chart light.  Reversing the procedure also takes less than a minute. The importance of having an observing station which is convenient in every respect cannot be overstressed.  Amateurs are not paid to spend nights observing but they can produce valuable results by spending short sessions at the eyepiece, sometimes between clouds and sometimes between domestic activities. A super-convenient observing facility is a great incentive to go out and do useful work.  In my opinion a modest but convenient-to-use telescope is far superior to an expensive but inconvenient facility for producing  significant results.  The large, old fashioned setting circles on my instrument are a case in point.  My telescope can be pointed at the desired field more rapidly than expensive computer assisted instruments.

OTHER EQUIPMENT

I use a home made 1P21 photoelectric head for observing occultations of bright stars by the moon or minor planets.  The recording is done on a high speed strip recorder.  It takes only minutes to convert the telescope to the PEP mode.

A Julian Day clock based on a 1951 SKY AND TELESCOPE article by Frank Bradshaw Wood gives the last two integers and first four decimals of the day. By using this clock at the telescope, I eliminate tedious conversions when making up reports.

A two channel homemade seismograph records powerful earthquakes from all over the world, as well as earth tremors which are triggered by mining operations up to several hundred kilometres away.  A solar flare detector which utilises the sudden ionospheric disturbance phenomenon, and a magnetograph for detecting flare-related magnetic disturbances complete the list of scientific equipment. The recording from these three devices is not done in the observatory building.

SECOND THOUGHTS

The aluminium coating of the telescope's primary mirror deteriorates rapidly in the polluted atmosphere where I work.  The fact that the instrument is pointed at the sky for many hundreds of hours a year does not help. If I had realised just how extensively the telescope would be used, I would have taken a deep breath and invested in a 14 inch  Schmidt-Cassegrain optical tube when these became affordable.

If I had known that the observatory would still be in use 23 years after it was built, I would have done more to protect it from the elements.  I would also have made it just a few inches longer because  it is difficult to observe a few northern hemisphere stars, which were added to my repertoire after the observatory was built.

SMALL OBSERVATORIES

Readers should be aware by now that I have a penchant for small, easily affordable observatories.  My observatory, although small, dwarfs the one used by variable star observer Eric Harries Harris of Adelaide, South Australia.  Eric's structure just manages to enclose his 6" Newtonian. It is so compact that he could not find room to mount the observatory clock which is now perched on the end of the telescope like a kookaburra bird.

Finally, an anonymous contributor to the late Carolyn Hurless's cottage publication VARIABLE VIEWS supplied the design at the foot of this chapter. As far as I know this idea has not actually been tried.
 

This article was taken from “Small Astronomical Observatories” by Patrick Moore.

MD Overbeek