MIRA 57
Spring 2001




Did they go to the Moon ?
by Ivor Clarke, Editor

The other day I read of yet another report of a book trying to debunk the Apollo Moon program as a hoax.  This is not an isolated case, in the last few years there have been several programs on TV and stories in magazines trying to put forward the idea that the world was conned by the Americans into believing  they had been to the Moon.  How many times will this sort of nonsense be tried?  How many times does it take to prove to one and all it did happen?  I know it was real because I stayed up all that Friday night and watched it on an old black and white (remember them?) television!!  So I know it was real!!!!
The ridiculous idea of ”flaked Moon landings" makes me very angry with the stupidity, unscientific half-truths and lack of imagination of the folk who try to push their daft ideas.  NASA‘s Apollo Moon missions could not have been flaked, and were not faked because proof lies in the vaults at the Lunar Receiving Laboratory.  The Moon rocks returned to Earth by the 12 astronauts who walked on the Moon and collected samples where very different in their chemical characteristics from all the terrestrial rocks of Earth.  All of the Moon rocks show no sign of water, almost all of Earth‘s rocks do.  And what about the two million folk who turned up in Florida to watch the Saturn 5 take off.  Where did it go?  And the thousands of ground control staff, technicians, engineers, designers, craftsmen, scientists who worked on the project.  Don‘t you think they would stand up and say something by now?
Few people have seen Moon rocks close up as even today they are studied only in laboratories which can give a very good reason why they need to have a sample.
But it wasn‘t the Moon rocks which had the public enthralled.  It was the fantastic images of the day, the closeup shots of the Moon and the  LEM and Command Module taken by the astronauts.  The fantastic Earth rise shots over the edge of the Moon, first taken by the crew of Apollo 8 during Christmas 1968, which no one had thought of taking until one of the astronauts saw the shot of his life.  Could these have been flaked?
The most important piece of evidence that Man reached the Moon in the late 1960‘s lies in the photographic evidence from and on the Moon.  Even today, with advanced computer image manipulation, it is impossible to make it look real enough.  I have never seen any images, in any SF film, magazine, book or video to compare with the real Apollo images on the Moon.  No one, as yet, has managed to reproduce that slow bouncing effect of the astronauts walk in the low lunar gravity and the way the dust flowed with every step in the dusty vacuum.  No clouds of dust like on Earth but an arc of dust which dropped back to the surface.
I think one forgets how quickly technology moves in the film world.  Moves today have endless sequences re-edited and manipulated on powerful computers.  In films such as the excellent Gladiator the opening battle scene was enlarged about four or five times what was actually filmed in the wood at Fareham, Hampshire.  So films are not what they appear to be, well not today there not.
But 30 years ago, who remembers the films currently on show then?  One was of cause, the classic 2001: A Space Odyssey which came out in 1967.  That was as truthful and as realistic as it could be made with the technology of the day and don‘t forget that Stanley Kubrick had as much money has he needed to recreate the Moon‘s surface.  He did an excellent job, but it still doesn‘t look as good as the real thing.
No film set, on Earth, can realistically show the harsh lighting from the Sun in a hard vacuum with no light scatter from dust particles in the air and hard black shadows with no atmosphere to reflect light into them are almost impossible to achieve in a studio on such a grand scale.  So to say that all the wonderful pictures taken on the Moon are flakes, is an insult to the astronauts and all who worked to put them there.
Today with all the incredible special effects available with a little imagination and a click of a mouse button, it is easy to forget how much we see in films is impossible to achieve in real life.  With the power of todays computers available at the studios it may just be possible to flake it, but 30 years ago such computer power was only a dream.  A 16k ZX Spectrum was more powerful than the Lunar LEM lander computer.  It would have been cheaper to send men to the Moon in the late 1960‘s than try to fake it with the then current film technology.
 







George Alcock (1912 — 2000) Remembered
by Ivor Clarke, Editor

”We will not see his like again";  we often hear this when certain men die, but they could not be more appropriate when the man was George Alcock.  He was simply one of the greatest visual discoverer who ever lived under our cloudy British skies.  His ten discoveries of five comets and five novae surpass even the achievements of Caroline Herschel, who discovered eight comets from Britain.  His extraordinary success in this area was made possible by memorizing thousands of star patterns, containing more than 30,000 stars, as seen through his binoculars.

George Alcock was born in Peterborough on August 28th 1912 during the time of the great East Anglian flood and died on December 15th 2000, 88 years later, with the River Nene once again at dangerously high levels.  Excluding the war years, George spent his whole life in the Peterborough region.  As an eight year old he saw the large partial eclipse (86%) of the Sun on April 8th 1921, through a smoked glass.  On December 30th 1930, he saw a bright meteor ”as bright as Venus".  This single event spurred him to contact the BAA meteor section director, JPM Prentice, with his first serious observation.  Prentice invited George to join the BAA‘s Meteor Section which seemed to inspire George and marked the start of an extremely fruitful observing partnership between himself and Prentice.
But meteors were not George‘s only interest, he also enjoyed observing and sketching the planets through a small refractor.  He independently discovered the 1933 white spot on Saturn, while using a friend‘s 4" refractor, some 3 days after it‘s official discovery by Will Hay in August.  However, he missed the spectacular Giacobinid meteor shower of October 9th 1933; remarkably his mother observed it whilst waiting for a bus!  Up to 450 meteors per hour were visible from dark sites.
On December 12th 1934, he nearly discovered his first nova.  He and Prentice had been out observing at their respective locations, but George had turned in at 1.30am (he was started a new teaching job the next day), whereas Prentice stayed out a while longer and spotted the 1st mag Nova Herculis.  Some 57 years later, George would himself discover a nova in Hercules!
George met his future wife, Mary Green in 1936, like George she was a teacher and they married in 1941.  He continued his mammoth meteor watches until he was called up in December 1940 for war service, but he still managed some meteor watches during his RAF service!  He returned to civilian life in March 1946 and continued his meteor work with Prentice.  He was hoped to see the next big Giacobinid shower, following his mother‘s chance observation.  But he was clouded out; while Prentice enjoyed clear skies on October 9th 1946 and saw a meteor storm.
In November 1947 George and Mary moved house into the Peterborough countryside from there he ultimately notched up 5 comet and 5 nova discoveries.  During the early 1950‘s it became  obvious that radar work being carried out at Jodrell Bank made meteor watches redundant.  This didn‘t stop George completing a record breaking Quadrantid watch on the night of Jan 3rd/4th 1951, of 10 hours and 48 minutes!  This was his last major meteor watch. He had been the country‘s most dedicated meteor observer for almost twenty years!
He wanted his observations to be VERY useful.  But, ”What could I, a single observer, with not much money, do?" he asked.  The answer would involve a mammoth effort: discover a comet!  So in 1953 he embarked on a 5 year comet search with his 4" refractor.  From his meteor work of the past twenty years, George could recognise about a thousand stars in patterns, this does not mean he could name them all, or that he could even draw the constellations down to mag 5 or so.  It simply meant that he could tell if a new star was breaking up the old pattern.  Nevertheless, the idea of committing the Milky Way, as seen through binoculars, to memory was ”preposterous" even to George‡ it implied memorising perhaps 20,000 or 30,000 stars!!  In the summer of  55, he decided he would search for nova too.  In 1958, without a discovery, he decided to carry on for another 5 years spending hundreds of hours per year sweeping the sky for comets and novae.  Other comet hunters showed George that a comet could be discovered; it just needed infinite dedication and infinite patience!
There‘s little doubt that George had the wrong equipment for comet hunting.  His 4" refractor with a 1° degree field was not a comet sweepers dream instrument.  In 1957 he borrowed Prentice‘s dilapidated 25x105 binoculars and immediately realised that he needed something similar.  His brother spotted a pair for sale at the 1959 Boat Show in London.  George was delighted with the astronomical performance of the binoculars and £150 changed hands!  Not only did the binoculars have wide field eyepieces giving a 3-degree field, the eyepieces were inclined at 45°.  At a stroke, comet sweeping became comfortable and the field of view increased by 9 fold!  History was about to be made.
After only seven months, on the 25th August 1959, three days prior to his 47th birthday, George spotted an intruder in Corona Borealis.  The next night he checked the field again, the suspect had moved one degree, he had made his first discovery! Comet Alcock 1959e, the first comet discovered from Britain since in 1894.  After more than six years of fruitless sweeping, the second Comet Alcock was discovered only five days later on August 30th. Comet 1959f was discovered in the morning sky in Cancer.  After a 65-year gap in British comet discoveries (and not for the lack of people trying) the discovery of two British comets in a week was a fairy-tale event.  George was a well-known observer prior to August 1959, but he would always be a legend after that.  What might have happened if August 1959 had been a cloudy month? How long would George have carried on without a discovery?
His third comet was discovered in March 1963, his fourth in September 1965, but his fifth took another 18 years!  After 1965, George placed more emphasis on nova hunting; after all, he had a unique talent in that field, he had memorised the northern Milky Way!  During the 1970s he became increasingly frustrated by the encroaching skyglow from Peterborough, making nova hunting, not comet hunting, the natural direction in which to continue.  From 1967 to 1976 George used his memory of the northern Milky Way to full advantage, spotting four novae in a 11 year period.  He had rivals in this field too, most notably the Japanese photographic patrollers.  But he was the only successful observer  who was searching visually, on clear nights he had a huge advantage over the photographers.  Bright novae would be spotted almost instantly by him; there was no hassle of developing films, mounting negatives and stereo blinking.  He could also observe in an instant and between cloud banks, his visual approach had much more flexibility.
His first nova success came on July 8th 1967 when he swept up Nova Delphini rising through 6th mag.  At last, 12 years of memorising the Milky Way through binoculars had paid off; it must have been a huge relief.  George now had five discoveries to his credit: four comets and a nova.  Nova Delphini 1967, is still the only nova to have been discovered in Delphinus.  It was the first British Nova to be discovered since Prentice discovered DQ Her in 1934. The nova rose to a peak of 3.5 in December 1967
George‘s second nova was discovered a mere nine months after the first in Vulpecula.  It was discovered on April 14th 1968, rising to a peak of mag. 4.8 a week later, with HR Del on the rise to it‘s final fourth mag. peak, there were two British naked eye novae, only 15° apart, in the April 1968 dawn sky!!  George often said the sight of those two novae toget-her was ”the greatest thrill of my observing career".
Two years later, George notched up his third and faintest nova in Scutum.  Another six years would elapse before he bagged his fourth, on October 21st 1976, NQ Vul, a nova right next to the famous 'Coathanger‘ asterism.  This was an important discovery for George as his morale was somewhat dented by 'missing‘ the 1st mag. nova V1500 Cyg on August 29th 1975, the day after his 63rd birthday by a few hours and the loss nearly made him give up.
One can only marvel at the mental stamina of the man, notching up literally thousands of fruitless hours of searching between discoveries as well as caring for his bedridden wife and teaching by day (up to 1977). His last two discoveries, a fifth comet and fifth nova must have stretched even his patience, occurring after gaps of seven and eight years.  George swept up Comet 1983d on May 3rd 1983 in Draco, already 6th mag. and 12‘ in diameter.  It transpired that it had been spotted fractionally earlier and the Infra-Red Astronomy Satellite (IRAS) had secured images of it on April 25th.  This was the only time George had to share a comet discovery, IRAS-Araki-A!cock became the third closest comet flyby of all time, passing within 3 million miles of Earth on May 11th!  It was the brightest comet he discovered and he had done it from indoors, aged 70, with hand-held 15x80 binoculars.  Two years later, on Jan 30th 1985, George made the observation which he personally considered was his tenth discovery, he spotted an outburst of the recurrent Nova RS Ophiuchi, again while observing from indoors.
His final discovery, on March 25th 1991, when George was 78 years old, was a remarkable one in many ways.  Firstly, he had a strong feeling that he was going to be lucky that night, so he was not at all surprised when he spotted the 5th magnitude intruder.  Secondly, he was, once again, observing from indoors, through a downstairs double glazed window, using 10x50 binoculars.  Thirdly, the confirmation was itself, equally remarkable.  At the time of the discovery, 0435 UT, nautical twilight had already arrived.  He phoned Denis Buczynski in Lancashire with the details, the ONLY star visible in the twilight sky was Deneb!  Astoundingly, Denis photographed the object and secured a position.  An independent discovery was made in Japan and the new object (V838 Her) was one of the fastest fading novae of all time, dropping 3 mag. in 2.8 days!
Although V838 Her was the last discovery for George he continued to search the sky from indoors and to sketch comets, like Hyakutake and Hale-Bopp, that came along.  Asked how he would like to be remembered, George replied ”As an observer".

Adapted from an article in The Astronomer, Jan 2000 by M. Mobberley and others.






Northern Exposure
By Mike Frost

For my summer holidays this year, I went to Chena Hot Springs, sixty miles east of Fairbanks in central Alaska - at the end of October.  I did not go to acquire a suntan.  Chena Springs is only a degree and a half of latitude beneath the Arctic Circle, and after the equinox the length of the day decreases rapidly, so that our length of day in mid-Alaska was not much longer than mid-winter in England.  Bathing in the springs‘ steamy hot pools was very pleasurable, but the return trip to the changing rooms was usually swift as the daytime temperature rarely rose above -10 deg C.  Night-time temperatures dropped down to a minimum of -22 deg C (-8 deg F), and although there was rarely any breeze, Chena Springs being in a steep valley, it was impossible to spend more than about half an hour outdoors at one go after the Sun had set.  Which was a pity, as the reason for our trip to Chena was to observe the aurora borealis, or northern lights.
Why go all the way to Alaska to do this?  The answer lay in the reasons for formation of the aurora.  The northern lights form as the result of the interaction of the solar wind (charged particles ejected at speeds of 100 - 400 km per second from the surface of the Sun), with the Earth‘s magnetic field.  Earth‘s magnetic field is like a bar magnet, with the field lines running between the magnetic poles, which are offset by a few hundred miles from the geographic poles. Mostly, the solar wind is deflected around the Earth‘s magnetic field, but some particles are trapped by it and spiral down the field lines toward the magnetic poles.  As the particles encounter the Earth‘s atmosphere they begin to hit the constituent gases, mostly nitrogen and oxygen, knocking either atoms or molecules of these gases into higher energy ”excited" states. The atoms or molecules then return to their original low-energy states, emitting as they do so characteristic wavelengths of a particular wavelength or colour.  Molecular oxygen, for example emits a wavelength of green light that gives the principal colour of aurorae, whereas an oxygen atom will emit a red photon, and molecular nitrogen a purple wavelength.
The upshot is that an oval of aurora forms, usually at around 20 degrees away from the Earth‘s magnetic poles, with the oval‘s major axis fixed relative to the Sun.  During the course of a night, therefore, the Earth will rotate beneath the oval.  Observers at a given location, sufficiently north, will see the aurora, first of all as a simple circular arc, lying toward the north.  As midnight approaches, the arc begins to widen, and then rays begin to appear, streaming up towards the zenith.  The arc develops into the familiar curtain and sometimes two or more curtains appear.  The aurora goes higher and higher into the sky as the auroral oval passes overhead, with more curtains appearing from the north if you are lucky.  Then into the early hours the aurora recedes, as the Earth rotates back out of the oval.  After about three am the best of the show is usually over and the observers can retire to bed for the rest of the evening (planning a lie-in, of course).
So the plan was to observe aurorae from Chena Springs.  There were some problems in the way, of course.  First of all, the latitude of the auroral oval was not guaranteed.  At times of coronal mass ejection and other intense solar activity, the oval can expand, effectively moving aurorae to the south.  This is good news for observers in Britain, say, but would not have been welcomed by those of us who had just turned up in Alaska!  Fortunately, although there was a coronal mass ejection, it occurred right at the end of our stay, so the ejected particles didn‘t have time to travel to Earth before we left town to return home.
The second problem was more familiar — the weather.  The second night of our stay was almost completely cloudy, although we did make several trips outside to see if patchy peepholes through the clouds were revealing any activity.  Our final day was most disappointing of all.  After a crystal clear frosty morning, temperatures suddenly warmed by a few degrees, and clouds descended on the valley.  This was particularly frustrating to me as I had signed up to take a scenic flight north to Fort Yukon, within the Arctic Circle.  However the destination was clouded out and I missed my best opportunity to enter the circle for the first time.  The cloud persisted all night and eventually snow set in.
However, the other two nights offered great aurora viewing.  On our first night we saw a fair display.  I had never seen the northern lights before, so you can imagine my thrill as the northern horizon began to brighten.  It looked for all the world like a skyglow from the next town to the north — except that north of Chena Springs there was nothing at all, all the way to the North Pole!  The glow became a green arc, which gradually rose overhead.  Some people claimed to be able to see a pinkish glow at the top of the arc; I thought that maybe I could detect it with averted vision, but I suspected this was more suggestion than a genuine sighting (for me, at least).
The first night‘s aurora was satisfying, but we had a better show two nights later.  Now the auroral arc widened into a curtain of light and beams of light, pointing toward the zenith, began to glide from right to left across the curtain.  As the curtain moved overhead, a second arc appeared beneath it.  The sight was spectacular, and our photographers, sprinkled along the resort runway (no flights at night!) set to work recording the event.  However, they encountered an unexpected problem.  One member of staff, rather the worse for wear from drink, decided that he was going to view the lights, from the comfort of his car!  Several long exposures were ruined as the sozzled aurora-gazer drove his auto two hundred yards from the staff block to the runway, and then up and down the access road by the runway‘s side, blaring the Eagles‘ Greatest Hits in the process.  Eventually, he decided to leave the air-conditioned comfort of his car; but that meant that we now had a drunk local giving us the benefit of his wisdom on the aurora.  Unfortunately, he picked on me.  I can still remember him telling me earnestly that the prevailing winds from Fairbanks meant that you usually saw the aurora first in the north, and then overhead; I didn’t have the heart to tell him that it was actually the prevailing spin of the Earth that did the trick!
Mind you, the weather did provide us with some interesting phenomena.  The combination of very cold and very dry air led to the formation of ”diamond snow", where the snowflakes have almost no opportunity to grow.  When you stand in a daytime fall of diamond snow it‘s almost impossible to see any flakes unless you turn to face the sun and then they glitter like dust in the sunshine as they fall.  And of course there were a variety of effects due to ice crystals in the atmosphere — sundogs and halos around the sun, glories around the shadow of the plane on our flights.  The phenomenon I was most intrigued by was what I first called an icebow, a white ring in the blue sky, circling the anti-solar point as a rainbow would.  Reading up afterwards, I reckon I saw an ”anthelic arc" but I‘m not certain.
So that was our stay at Chena Springs, a very comfortable resort and a good place to spend a few days holiday.  The aurora watching season lasts for a few weeks around the equinoxes; although you can see the northern lights at any time of the year, the displays are best when the alignment of Earth and Sun is favourable (and, as with the tides, I don‘t really understand this properly).  This is fortunate for resorts like Chena Springs, because it fills a hole in the schedule between the summer hiking season and the winter snow-sports season.  We were not the only aurora party in the resort — it is also very popular with Japanese visitors and there was a large group staying at the springs.  I am told, however, that the Japanese believe that any child conceived beneath the northern lights will grow up to be unusually intelligent.  So we didn‘t see much of the Japanese.
Our return flights to England were particularly memorable.  Alaskan Airways, who ferried us from Fairbanks to Anchorage and on to Seattle, pride themselves on being a ”small-town" airline, with a laid-back approach to schedules. (”Holy Smoke!" announced the pilot as we arrived at Anchorage, ”Alaskan Airways is on time!").  On the outward journey we had a particularly giggly flight attendant who primed the captain.  He welcomed us to Alaska, then informed us that he had consulted Anchorage weather station, which had guaranteed there would be no aurorae all week! We were delighted to prove him wrong.  On the return journey we had a superb view of the Alaskan Rockies and the spectacular coastline, split by mountains and fjords and glaciers.  We stopped overnight in Seattle, where I was given a guided tour of the city by an old friend of mine from university, who these days runs an Internet software house doing very well on the coat tails of Bill Gates and co.  Half our party split away at Seattle, flying on to Hawaii for the second, warmer half of their holiday.  Those of us who had to go back to work flew to Minneapolis and then changed planes for the final overnight leg back to England.
An hour out of Minneapolis, flying over the Canadian Prairies, the astronomer in the seat behind me covered his head with a blanket, pressed his face to the window, and announced, ”There‘s an aurora out there!".  Was I glad I had a window seat!  The coronal mass ejection from the Sun had arrived at the Earth and had pushed the auroral oval south almost down to the American border.  And from thirty two thousand feet above ground and several thousand feet above the clouds, we had a birds-eye view.  It was superb! There were two sets of curtains, with beams rippling their way along, twisting the arcs back on themselves.  Once again I could see no pink tinge at the top of the curtains, but there was an unmistakable redder glow at the bottom of the lower curtain, which our tour leader Dr John Mason, who was sat next to me, explained was the oxygen atom transition, which only occurred for particularly energetic displays of the Lights.  The tour party proceeded to cause havoc on board the plane as we commandeered every free window and every free blanket, swapping seats to let everybody have a view of the show.  The cabin crew were understanding but firm — ”you can only sit there until the meal is served", one told me, as I parked myself in the jump seat while someone borrowed my window seat.  For a period of about an hour we had ringside seats, then as the plane flew beneath the oval the aurora moved above us and out of sight.
What a finish to the holiday!  We speculated that, given a reasonably clear night, the auroral oval was easily far enough south to give a great display of the northern lights from England. Unfortunately, the weather had definitely not co-operated.  As we flew over the south of England, we could see that every river was flooded, every major road either jammed solid or completely empty.  Worst for me, there was an ominous lack of activity on the railways.  The great Halloween storm had struck England even as we were in flight, and the country was only just beginning to recover.  It took me five and a half hours to travel from Gatwick to Rugby on the train, and I was fortunate that the only trains running north out of London terminated at Northampton, within a bus journey of home.  It was exhausting end to an exhilarating holiday.
Three days later I was back across the Atlantic, in Pittsburgh.  But that‘s another story. . .







A question-and-answer style interview with George Pace, of NASA‘s Jet Propulsion Laboratory 
George Pace is the Project Manager of JPL‘s next interplanetary mission, the 2001 Mars Odyssey

Q/ What is the 2001 Mars Odyssey mission?
The Odyssey mission is the next mission to Mars.  It is an orbiter and it will do observations of the surface looking for elements and minerals.  It will also measure the radiation background at Mars, radiation that would be harmful to humans.

Q/  What‘s been the most challenging aspect of this mission for you so far?
The most challenging aspect for me has been to keep the team focused because this project has been through a lot of changes.  We started off with an orbiter and a lander and it got re-scoped several times.  Due to the loss of two spacecraft in 1999, NASA decided to forego the lander and only use an orbiter for this Mars mission.  The project has been through several JPL organiza-tions so the most difficult thing for me has been keeping the team focused on getting the job done.

Q/  How did you become interested in space  exploration?
I always loved airplanes and I went to college  intending to be an aeronautical engineer.  But Sputnik changed my whole perspective of what I  wanted to do.  Instead of airplanes, it turned out to be aerospace.

Q/  What advice would you have for young people starting out today wanting to do what you do?
Clearly the math and science are important, but don‘t overlook the other classes like English and things like that, because communicating is very important on jobs like this.  Being able to express yourself and work with other people, that‘s as important as any technical knowledge you might bring to the job.

Q/  Why do you think the public is so fascinated with Mars in particular?
I think of all the planets Mars is most like Earth.  It is close to Earth.  There is a possibility that water might have existed there, or might even exist there now.  It is the planet most likely to support life of some sort ˜ if life does exist elsewhere in the solar system ˜ and that‘s why we are looking for the water and hopefully looking for the life. So that‘s what‘s fascinating.






THE VERY LARGE TELESCOPE
by Paritosh Maulik


In late 1980‘s, the European Southern Observatory decided to build a cluster of large telescopes, named the Very Large Telescope in Paranal, Chile.  This facility is in addition to the existing observatory (New Technology Telescope) in La Silla, Chile.  The new telescope complex, VLT is an updated version of NTT and is to incorporate the experience gained from NTT.   The new telescopes at Paranal can operate either as stand alone or in combination as in an interferometric mode.  In this article we shall see why interferometric telescope and the basic workings of the VLT.  In some cases I have quoted numbers, this not to drown the readers with numbers or to say size matters, but only to show the extreme ranges the parameters for such an instrument, for example a 45,000kg mirror with 8m diameter has a surface finish of about 25nm (25x109m)

Why Interference Mode?
For an optical instrument, the resolution is the smallest diameter of the object we can see in the image.  When we look through an optical microscope, say at tissue cells, we illuminate the object with a visible light of a wavelength of say 5000Å ~ 500nm.  This is adequate for most routine analysis with a typical resolution of 2000Å.  But for certain special cases, for example a bacteria, the features are so fine that optical light is not adequate.  So we have to illuminate the specimen with an electron beam (wavelength  0.05Å ~ 0.5nm) in a specially built instrument called an electron microscope; typical resolution is 10Å.
Astronomical objects on the other hand can emit in the radio, infrared, optical, X-ray or gamma-ray range.  In the optical range, to see the details of these objects, we have to increase the resolving power of the instruments.  This is achieved by increasing the numerical aperture of the instrument, that is to make the mirror bigger.
In radio telescopes, the wavelength is of the order of 10‘s of mm, so we need a larger diameter aerial to have any useful resolution.  However this does not increase the resolution that much.  For example, the resolving power of 250 foot (76m) Jodrel Bank radio telescope is about 3000 times less than that of an observatory telescope.  One of the ways to improve the resolving power of radio telescopes is to combine the signal from two or more telescopes, separated by a known distance, to form an interference pattern.  When the beams are in phase, the angular resolution is  the wavelength ÷ base line separation  between the telescopes.
Thus the larger the separation of the aerials the smaller is the angular resolution, i.e. we can see finer details.  The same interferometric principle now has been applied to optical telescopes as well.  The logic goes something like this: use ground based telescopes, this permits larger mirrors, combine the signals from two telescopes, and the image quality is comparable to that of the Hubble Space Telescope.  Since Hubble‘s mirror operates above the atmospheric turbulence, Hubble can achieve unprecedented image quality with a relatively small 2.4m mirror.  Larger space mirrors are restricted by the space craft payload. The two Keck telescopes at Mauna Kea in Hawaii were the first such instruments to be built to employ the interferometric mode.
In this article we shall concentrate on the working of the VLT and in a later article we shall discuss the instruments used, what the astronomers are aiming to study and the next generation telescope.

VLT at Paranal, Chile
The Very Large Telescope Interferometric facility of the European Southern Observatory is being built in stages in Paranal, Chile.  The aim is to install four similar telescopes of 8.2m diameter each (Unit Telescopes, UT) and three Auxiliary Telescopes (AT) telescopes of 1.8m diameter.  These telescopes can work independently or in interferometric mode.  When working in the interferometric mode the equivalent angular resolution will be equal that of a 200m telescope.  This facility is expected to have a life span of 25 years.
The Paranal Observatory is in the Atacam desert in northern Chile.  To build the observatory some 300,000 cubic metres of rock was removed from the Paranal peak, above 2600m.  The peak height was lowered by 28m.  This site contains the telescopes and the interferometric laboratory; also there is a base camp at the bottom of the mountain to house infrastructure like Mirror Maintenance, administration, dormitories, power plants etc.
The variation in temperature between minimum and maximum, over a 5 five year period was recorded to be between -8° to +25°C, with maximum rainfall of about 100mm year.  The pollution levels are very low but the site is in a seismic active area.  So, damping and shock-absorbing elements have been incorporated into the structure.  During the construction seismic activities did take place and choice of these elements were vindicated.

Basic Concept of the Project
The four 8.2m UT telescopes and the three AT telescopes will be available for interferometric observation for part of the time, but the AT‘s can be positioned in 30 locations and will operate only in interferometric mode.  The maximum possible baseline is 202m.
These telescopes will operate between the wavelengths of 0.3 to 25nm (3000 — 250,000Å, from near ultraviolet to infrared).  In the Independent Telescope Mode each UT can be used in one of the following foci, one Cassegrain, two Nasmyth, and one coudé optic train.
In Combined Coherent Mode or VLT Interferometric mode: two or more UT‘s, two or more AT‘s, and  UT‘s and AT‘s combined can operate in interferometric mode.  In this mode the angular resolution can be expected to be that from a telescope of 200m diameter.  In Combined Incoherent Mode, light from 4 UT‘s can be combined to give a 16m collecting area.
  

The Unit Telescopes
Fig 2 shows the basic light path of the telescope using Ritchey-Cretien optics.  In this mode both the primary and the secondary mirrors are hyperboloidal, the advantage being image free from coma and spherical aberration over a large field.  These instruments can operate in either Cassegrain or Nasmyth mode; there is coudé (elbow in French) foci set up as well.  This arrangement sends the beam via a collection of mirrors to another area.  This section is fixed, so that heavy instruments like spectrographs can be included.  The UT‘s are all altazimuth mounted.  The telescope tube rotates up or down in altitude on a horizontal axis supported by bearings on a fork.  This fork in tern can rotate on a vertical axis called azimuth axis.
The names for the telescopes in Mapuche language were chosen by a local school student:


ANTU       (UT1) pronounced an-too,                       The Sun,
KUEYEN   (UT2) pronounced qua-yen, like in quake, The Moon,
MELIPAL  (UT3) pronounced me-li-pal,                    The Southern Cross,
YEPUN     (UT4) pronounced ye-poon,                     Sirius.

The Mirrors
The Primary Mirror blank is made from a glass ceramic material with very low thermal expansion called Zorodur.  About 45,000kg (45tons) of molten ceramic material was poured into a concave mould, which was kept spinning until the temperature dropped to around 800°C.  Due to the high viscosity of the material, the hot blank can retain its shape at this temperature and the blank is cooled to around room temperature over a three month period.  This took place in Germany.  It was then sent to France for polishing.  At the early stages of polishing, the geometry is measured by infrared interferometery and at the final stages with visible light of 633nm.  The mirror characteristics actually achieved are listed in Box 1; the results are very close to expectations.  Since the mirror is thin, it can distort as it tilts.  So the mirror is supported on 150 actuators for shape correction.
The Secondary Mirrors are made from a light weight metal called beryllium.  Starting with beryllium metal powder, it is hot pressed to a density of about 99.7%.  The reason for starting with powder rather than a forged stock is that if beryllium metal is shaped by forging, it shows anisotropic properties, whereas such a problem  does not arise in a blank made from powder.  After the blank has been made, it is ground into shape, a convex hyperbolic mirror.  During grinding, the mirror blank is repeatedly annealed for stress relieving, if this is not done, the mirror may distort during use.  The mirror blank is then electro-nickel plated.  The thickness of coating on the back face is thinner than the front face, this is done to reduce the weight of the mirror.  The coefficient of thermal expansion of nickel and beryllium are different, hence if the coating thickness is not properly controlled, differential thermal expansion will cause distortion, as happens in bi-metallic strips in thermostats.
The plated surface is then polished.  During polishing, stress relieving annealing, as discussed earlier, is done.  All these long winded procedure guarantees long term stability of the mirror.

Nasmyth Mirror, M3
This mirror reflects the light from the secondary mirror to instruments set at Nasmyth configuration.  When the telescope is operating in Cassegrain mode, this mirror is removed from the optical path along the telescope axis.  This flat mirror is elliptical in shape and is made from Zorodur material in Germany.  The mirror is controlled by an  active optic system.
The Wave Font Sensor, an unit samples the  quality‘ of light and this signal is used for required correction, it is situated off the axis of the  telescope.  It collects the signal from different areas of mirror 3 and hence the image quality depends on whether the beam is from on or off axis.  The mirror is very flat, the surface roughness is less than 1nm.

Coudé Train Mirrors
This is a cluster of mirrors which relays the image to the Coudé.  Mirror M4 sends the Nasmyth beam to the Coudé focus at correct magnification.  The other mirrors in the train are  shown in Fig 2.  The cylindrical mirrors corrects the astigmatism introduced by the spherical mirrors.  Cylindrical mirror M6 causes differential magnification along the x and y axes.  This is corrected by the shape of the mirror M4.
All ceramic mirrors, M1, M3 to M8 are coated with aluminium (99.995% pure).  The coating is done by a process called physical vapour deposition.  The mirror to be coated is put in a vacuum chamber and a plasma of aluminium is created; this plasma then is attracted to the negatively charged mirror.  An UK company designed and built the coating unit.  The advantage of this process is that, it is a very clean process and during the coating process the temperature of the mirrors  remains virtually unchanged.  This minimises the stress build up.  The onsite coating unit at the base camp, is about 2km from the telescopes.

Some other Additional Units and Their Roles
At the focus of the telescope, an annular portion of the sky is visible around the mirror M2. Unwanted radiation from this region contributes to the collected signal leading to decrease in the signal to noise ratio.  A sky baffle is employed for partial  compensation of this effect.  It is essentially a hood of 1.5mm diameter around mirror M2 (mirror diameter 1.1m).  Segments of the sky baffle is selectively coated on the visible side (the side facing the  primary mirror).  The characteristics of the coating is such that it is black in the visible optical range and of low emissions in the infrared range.  In order to prevent local hot spots developing on the baffle, its temperature is maintained to that of the telescope ambient.

Nasmyth Adapter Rotator
The functions of the adaptor is as follows
i)   Field acquisition; location of the desired object
ii)  Guide the telescope drive; follow the object
iii) Field stabilisation; adjustment of M2 and also the telescope drive for faster response time.
iv)  Wave front sensing for the active optic system
v)   Deflection of light to coudé,
vi)  Feeding the light from the reference star to coudé field.
The role of the rotator is to locate the focal plane and rotate the instruments to follow the optical field.
 
Wave Front Sensor (WFS)
The role of this unit is to maintain the image quality.  The primary mirror is relatively thin.  As it moves during observations, it can distort under its own weight.  It can also distort due to thermal effects.  All these lead to degradation of image quality.  A CCD sensor measures the wave front at the focal plane.  This sensor also detects the image of a reference star used for guiding the telescope.  This reference star image is compared against the image of the object.  The same optical system is used to carry the signals both from the reference star and that of the object.  Any error is then due to the incorrect shape or position of the mirror.  A corrective signal is sent to the telescope control system for guiding error.  Measurement frequency for field stabilisation <70Hz (cycle/sec) and about 1Hz for guiding.
Retaining the Shape of the Primary Mirror
There are 150 hydraulic actuators supporting the primary mirror M1.  These actuators receive a signal from the WFS unit and control the shape of the mirror.
 
Sodium Laser Guide Star
The unit telescope UT1 is fitted with a sodium laser guide star.  This is a sodium laser beam pointing to the sky.  Part of the beam is reŸected from the dust molecules in the mesosphere at about 90km altitude and is used as a reference beam.  The image from the telescope is compared to this reflected laser beam.  This information is used to correct the shape the primary mirror.  The choice of reference star or laser star?  Both has its own pros and cons.  A bright reference star may not be available all the time, but it suffers distortion only during as an incoming beam.  Laser beams on the other hand, although available all the time, suffers from distortion twice, during both the outward and return journey.
 
Cooling of the Primary Mirror
The heat generated by the electronic system and the hydraulic support system can introduce air turbulence; this can affect the image quality.  The electronic cabinets are kept cold by cooling fluid.  During the day a fan circulates the air, but during the night as the observation continues the telescope ambient temperature increases, however the fans are switched off to avoid vibration.  By all these elaborate procedures it is expected that the mirror temperature can be maintained within -0.1° to + 0.2° C of the ambient during the 80% of the observation period.
 
Pupil Alignment
At the centre of the M2 mirror there is a facility to accommodate an alignment target or a light source (via LED and fibre optics).  The light source is used primarily for the pupil alignment when the larger unit telescopes are working in interference mode.

M2 Unit
This unit controls the position of the secondary mirror M2.  WFS also sends signal for the optimum  positioning of the secondary mirror M2.  This mirror can be adjusted for focusing, centring and pointing.  The position of M2 is not absolutely fixed, it can be repositioned
i)    to alter the focus between the Cassegrain and Nasmyth configuration
ii)   to align the centre of the primary and the secondary mirrors
iii)  wind buffeting movements and atmospheric disturbances can be adjusted
iv)   during the infrared observations, the mirror is oscillated between two predetermined angular positions to subtract the background radiation.  A detector in the focal plane sees two images of the nearby sky, one with the desired object and other without the object.  These two signals are subtracted to eliminate the background radiation.  The background radiation may be considerable stronger than the object.

Delay Line
We have discussed earlier that the position of the
UT‘s are fixed, while that of the ATS can be moved to one of the 30 predetermined locations. During the interferornetery mode the light from the telescopes is guided underground to the central interferometric tunnel.  The beam is combined in the beam combination laboratory and the fringes thus produced are analysed.  But the arriving beams may not be in correct phase.  In order to bring the beam in the correct phase an additional path is introduced to the beam by a mechanical - optical unit called delay line.  The resolution of the combined beam is so good that it should be possible to locate an astronaut on the surface of the Moon.  This unit was built in the Netherlands.
The reasons for path difference between the individual beams are due to path differences due to the physical location of the telescopes, the movement of the astronomical objects with respect to the rotation of the Earth and atmospheric disturbances and or mechanical vibrations which can lead to rapid path variations.
The delay line works like a telescope; the incoming beam hits a mirror of variable curvature at the focus and it is reflected back in a parallel direction.  The mirror assembly is mounted on a carriage, driven by a linear motor on a 60m long track.  The second or the fine adjustment is done via a piezo electric sensor at the back of the mirror.  A fringe sensing unit detects the path difference and sends the corrective signals.

The Auxiliary Telescopes
The AT‘s are of same design as the UT‘s, but use a 1.8 m diameter main mirror.  These are also altitude - azimuth mounted and the light is sent to the bottom of the telescope via Coudé Optical train.  Then the Coudé relay optics sends the beam to an underground tunnel and the delay line.  The first two AT‘s are expected to be operational at Paranal, by the middle of 2002. The third one will be delivered at a later date.
Since these are somewhat lighter in weight they can be moved to 30 stations.  The mechanical arrangement of the instruments are so precise that realignment may not be needed.  The location to new a new position may involve not only travelling on straight line, but also turning 90°. The role of the AT‘s in combination with the 4 UT‘s, will be able to provide good quality images in the interferometric mode.  The longest possible base line is 202m. The AT‘s are solely dedicated to VLTI with the UT‘s; the latter may work independently.  AT‘s will be used for Narrow Angle Astrometry mode of VLTI.  This is accurate mapping of astronomical objects.  This requires long base line, regular and long-term monitoring, which is not possible with larger telescopes.
Testing and commissioning of second generation of instruments for VLTI will be carried out with these small AT‘s, leaving the UT‘s free to carry out own dedicated tasks.

Software
Users station Local Area Network (LAN) is used by the operator to connect each UT to instrument receiving light.  This makes the switch over simple.  Then there is Paranal Area LAN which connects other modules.  Control information requires a band width of about 1M bit/sec.  The local control units reply in short messages and only when interrogated.  Data such as viewing cameras, guide camera, wave front sensors require about 5M bit/sec for shorter periods. Astronomical data from Instruments and infrared detector data may require about 30 - 40M bit/sec.
Data for Stabilisation and Guiding may need only a small amount of data, but these are handled by another LAN.

Time Keeping
A dedicated GPS receiver maintains the Universal Time, however there is an electronic clock as well, if no satellite is available.  The time is distributed to the workstations and Local Control Units (LCU).  But some of the LCU‘s receive signals from the GPS directly.  These units also have back up clock in case of GPS failure.

I hope this article gives an idea about the complexities involved in such a project.  Starting with the basic principles of physics, and converting these principles into working instruments calls for precision instrumentation and engineering on a large scale.  AII these have to be achieved within the time scale and budget. Project planning becomes vital to keep any eye on the progress.  These instrument are just as much achievements for astronomy, as also for multidisciplinary engineering.
 
 

Acknowledgements
Most of the information here is from the ESO web site.  This gives details of all aspects of the VLTI facility.  Useful web addresses www.eso.org/outreach/press-rel/ www.eso.org/outreach/info-events/ut 1ft/whitebook Since this article has been written all four unit telescopes have seen the first light and images have been released.  Slowly instruments are becoming live and the project seems to be on course.