| In previous classes, we encouraged people to
exercise their individuality and ingenuity to the fullest. While this resulted in a nice
variety of interesting approaches, we noticed that they had the greatest problems with
certain mechanical aspects of their telescopes. Accordingly, this time around we decided
to structure things somewhat differently. So now we have three distinct groups within the
class: "beginners" (those making their first instrument), who are building
'scopes of a standardised design; "stragglers" (from previous classes), who are
now more than beginners and who are finishing a variety of instruments; and the more
"advanced" (those who have previously finished a telescope), who are engaged in
a number of projects and who provide tuition and assistance in running the class. The
standardised beginners' telescope has a 6" aperture, the mirror being made of
1"-thick glass cast from recycled material. This has the disadvantage of having
bubbles in it. However, unlike Pyrex, it is very economical and also soft enough to work
quickly - ideal for a first mirror. (We will probably explore the viability of having
larger blanks cast for us, sometime.) The tube will be PVC drainpipe, which is easy to
work, readily available and has good thermal properties. A simple but effective Dobsonian
mounting of Superwood completes the package. By using a standard design, we can make bulk
materials purchases, saving on costs. At the moment, although not as far down the road as
we had hoped to be at this point, I am pleased to report that 70% of the class are in the
polishing and figuring stages. So far, this class has had a smaller dropout rate than
previously experienced. We wish Maureen Chitters well as she embarks on her Masters'
degree, and hope that she will be able to afford the time to continue with her 'scope, if
only on a sporadic basis.
On the "stragglers" front, Eric Brindeau (who interrupted his project to
visit Canada for almost a year ... ask him to tell you about doing astronomy in the middle
of a Canadian winter!) is well on his way to finishing a 6" short-focus mirror. This
is destined to be a combination ultra-portable rich-field telescope and super-finder for
his 12" (which itself is approaching the end of a major refurbishment). Mary McKinnon
has finished a beautiful 8" mirror and is busy fabricating parts for the optical tube
assembly. Wim van Steenderen is tackling an ambitious project: giant binoculars. He has
successfully crossed the major hurdle: his two 6" mirrors are within about one
millimetre of each other in focal length! He is now beavering away on mechanical design
issues.
And what about the "advanced" guys? Following the success of his 6"
mechanical masterpiece, Walter Bacchio has a slew of instruments in various stages of
completion, which I believe are destined for family members. Peter Baxter's 10"
mirror is in the final stages of figuring. This will live in a portable wooden telescope
mounted on a collapsible split-ring equatorial mounting. If his previous masterpiece is
anything to go by, this should be a magnificent piece of work. Bert van Winsen produced a
nice portable 'scope for his son Gavin; this is mounted on an innovative lightweight
collapsible alt-azimuth mounting. Des Fourie played a major role in getting the Society's
aluminising plant up and running, and has been doing a sterling job running it. By
fastidious attention to detail, he has been producing superb results when aluminising
member's mirrors. Andrew Leigh is dividing his time between finishing off his 8"
(which sports a lovely veneered cardboard tube and an equatorial mounting), building his
second telescope, and producing moulds from which components of a Crayford-style focuser
may be cast. There are talks of building a number of larger (12" to 20")
instruments, plans to tackle more ambitious optics (such as Scheifspieglers, solar
telescopes and Cassegrains) and more ... but all in good time.
So what else is going on? (You mean what has been described above is not enough!?)
Well, you will just have to join us to find out! And, to all those out there who started
building a telescope sometime and then let the project lapse, why not join us and
resurrect it?
An accurate determination of the size and age of the universe are essential in
understanding the evolution of the universe. Ever since measurements from the Hubble Space
Telescope in 1994 indicated a very young universe, considerable controversy has resulted.
The main reason for this was the apparent contradiction that the stars in the universe
seem to be older than the universe itself!
In order to resolve this contradiction there are three possibilities:
1. The measurements of the age of the universe are incorrect.
2. The measurements of the age of the oldest stars are incorrect.
3. The model of the universe is incorrect.
There is a great belief that the model of the universe is fairly accurate so that
either 1 or 2 (or both) above are the causes of the problem. Over the past couple of years
scientists have indeed revised their estimates so that the age of the universe and that of
the oldest stars are closer, but not close enough.
The results announced by Prof Michael Feast of the University of Cape Town recently
seems to have resolved that anomaly.
"I hope we've cured a nonsensical contradiction that was a headache for
cosmologists," Michael Feast says. "We judge the Universe to be a little bigger
and therefore a little older, by about a billion years. The oldest stars seem to be much
younger than supposed, by about 4 billion years. If we can settle on an age of the
Universe at, say, 12 billion years then everything will fit nicely."
Why South Africans?
Asked why South Africans should be involved with a satellite launched by the European
Space Agency Prof Feast responded: ``A number of astronomers in this country are
internationally acknowledged as experts on the distances of stars. It is a field in which
we have been able to make important contributions and the Europeans are pleased to work
with us." He went on to point out that ``Although the Hipparcos results are important
on their own, the major breakthrough in understanding only arises when the measurements of
the distances to the stars obtained with Hipparcos are combined with measurements of the
brightness of the same stars. Most of the stellar brightness measurements used in the
analysis were obtained at the South African Astronomical Observatory (SAAO)''.
The observable Universe may be about 10 per cent larger than astronomers have supposed,
according to early results from the European Space Agency's Hipparcos mission.
Investigators claim that the measuring ruler used since 1912 to gauge distances in the
cosmos was wrongly marked. This ruler relies on the brightnesses of winking stars called
Cepheids, but the distances of the nearest examples, which calibrate the ruler, could only
be estimated. Direct measurements by Hipparcos imply that the Cepheids are more luminous
and more distant than previously imagined.
The brightnesses of Cepheids seen in other galaxies are used as a guide to their
distances. All of these galaxies may now be judged to lie farther away. At the same time
the Hipparcos Cepheid scale drastically reduces the ages of the oldest stars, to about 11
billion years. By a tentative interpretation the Universe is perhaps 12 billion years old.
European teams of scientists and engineers conceived and launched the unique Hipparcos
satellite, which operated from 1989 to 1993. Hipparcos fixed precise positions in the sky
of 120,000 stars (Hipparcos Catalogue) and logged a million more with a little less
accuracy (Tycho Catalogue). Since 1993 the largest computations in the history of
astronomy have reconciled the observations, to achieve a hundred fold improvement in the
accuracy of star positions compared with previous surveys.
Slight seasonal shifts in stellar positions as the Earth orbits the Sun, called
parallaxes, give the first direct measurements of the distances of large numbers of stars.
With the overall calculations completed, the harvest of scientific discoveries has begun.
Among those delighted with the immediate irruption into cosmology, from this spacecraft
made in Europe, is ESA's director of science, Roger Bonnet.
"When supporters of the Hipparcos project argued their case," Bonnet recalls,
"they were competing with astrophysical missions with more obvious glamour. But they
promised remarkable consequences for all branches of astronomy. And already we see that
even the teams using the Hubble Space Telescope will benefit from a verdict from Hipparcos
on the distance scale that underpins all their reckonings of the expansion of the
Universe."
The pulse-rates of the stars
Cepheid stars alternately squeeze themselves and relax, like a beating heart. They wax
and wane rhythmically in brightness, every few days or weeks, at a rate that depends on
their luminosity. Henrietta Leavitt at the Harvard College Observatory discovered in the
early years of this century that bigger and more brilliant Cepheids vary with a longer
period, according to a strict rule. It allows astronomers to gauge relative distances
simply by taking the pulse-rates of the Cepheids and measuring their apparent
brightnesses.
Nearby Cepheids are typically 1000-2000 light-years away. They are too far for even
Hipparcos to obtain very exact distance measurements, but by taking twenty-six examples
and comparing them, Michael Feast and his colleague Robin Catchpole of RGO Cambridge
arrive at consistent statistics. These define the relationship between the period and the
luminosity, needed to judge the distances of Cepheids. The zero point is for an imaginary
Cepheid pulsating once a day. This would be a star 300 times more luminous than the Sun,
according to the Hipparcos data. The slowest Cepheid in the sample, l Carinae, has a
period of 36 days and is equivalent to 18,000 suns.
Applied to existing data on Cepheids seen in nearby galaxies, the Hipparcos result
increases their distances. It pushes the Large Magellanic Cloud away, from 163,000
light-years, the previously accepted value, to 179,000 light-years with the Hipparcos
Cepheid corrections, an increase of 10 per cent. Feast and Catchpole feed this result back
to our own Milky Way Galaxy, and into calculations of the age of globular clusters, which
harbour some of the oldest stars of the Universe.
The reckoning involves another kind of variable star, the RR Lyraes, and the Hipparcos
investigators arrive at an age of 11 billion years for the oldest stars. Other estimates
of the oldest stars assigned to them an age of 14.6 billion years. This seemed, absurdly,
to leave them older than the Universe. A team of astronomers using the Hubble Space
Telescope recently declared the Universe to be only 9-12 billion years old. The Hipparcos
Cepheid result increases that Hubble-inferred cosmic lifespan to 10-13 billion years.
Feast and Catchpole have also cleared up a mystery about the nearest and most familiar
Cepheid variable. This is Polaris, the Pole Star. Imperceptibly to the human eye, its
brightness varies at a relatively high rate, every 3 days. That should make it, by the
Cepheid rule, a feebler star than it appears to be.
Hipparcos fixes the distance of Polaris at 430 light-years, and the researchers
conclude that Polaris pulsates with an overtone, at a rate 40 per cent faster than
expected for a Cepheid of its size and luminosity. Several other Cepheids gauged by
Hipparcos also exhibit overtones. Were these not recognized as fast pulsators they would
give false impressions in the Cepheid distance scale. |
Monthly Meeting
