Keeble Observatory
February 2007 Sky from the Keeble Observatory
Two months ago we bade farewell to the Mars Global Surveyor. This
month we say our goodbyes to another premium piece of astronomical
observing equipment.
Professional photographers can spend a lot of money on their cameras,
much of it on the lenses alone. Without quality lenses, there’s
nothing the film (or the CCD in modern electronic cameras) can do
to overcome bad image quality. Terms like astigmatism, coma, spherical
aberration describe various distortions which can be introduced
by an imperfectly ground lens. But, the quality of the film, or
the number, density, and quality of the pixels on the camera chip
also affect the outcome of a particular photo shoot.
Telescopes are the “lenses” of astronomy, though virtually
all astronomical telescopes capture light with mirrors instead of
lenses. The expense of grinding a mirror to precisely the right
shape is one reason that observatory telescopes cost millions of
dollars. Hubble Space Telescope (HST) was launched with an imperfectly
ground mirror – more accurately, it was ground with high precision
to the wrong shape. The outer edge of the 2.6 meter primary mirror
was too high by a little less than the thickness of a human hair.
Corrective optics were installed by the first shuttle servicing
mission, and the telescope has performed impressively ever since,
with three additional visits by astronauts to repair and upgrade
the orbiting observatory. (Another reason for the cost is size,
since larger and larger telescopes are used to observer fainter
and fainter objects, probing farther and farther into the universe.)
The “film” of the astronomer is the instrumentation
deployed on the telescope to capture and analyze the light from
the distant stars and galaxies. One of the instruments on HST, the
Advanced Camera for Surveys (ACS), has now stopped working. On 27
January controllers noted that HST had gone into automatic safe
mode – a pre-programmed response to a wide range of anomalies,
designed to protect the telescope and its instruments. The particular
anomaly that triggered this mode (“safing” in NASA speak)
was a sudden rise in pressure in the instrument compartment (known
as the Aft Shroud) on the telescope. Note that the telescope orbits
in vacuum, and that it is not designed to be air tight – so
where does this sudden burst of gas come from? It turns out that
the ACS developed a short circuit and blew a fuse! The short circuit
had sufficient energy to vaporize the fuse!
The good news is that there are redundant electronics, known as
Side 1 and Side 2. The blown fuse is on Side 2. The bad news is
that the short circuit seems to have permanently damaged most of
the camera itself. If it is determined that Side 1 can be safely
restarted, some limited capability may be restored. The scheduled
2008 servicing mission, which will be Hubble’s last before
it is retired in the next decade, will probably replace the ACS
with a new camera.
Lunar phases for February: Full Moon at 12:45 am, on the 1st; Last
Quarter on the 10th, at 4:51 am; New Moon on the 17th, at 11:14
am, and First Quarter on the 24th, at 2:56 am.
If you are an early morning sky watcher, you’ll note that
Jupiter is about 30 degrees off the south-southeast horizon as the
Sun rises. That red star about 5 degrees below and to the right
is Antares - meaning “rival of Mars.” The real Mars
is below and to the left of Jupiter, about 16 degrees above the
southeast horizon. Mercury will return to predawn visibility at
month’s end. Saturn sets at sunrise.
As the Sun sets, you’ll see Mercury and Venus both to the
west-southwest, emerging from the twilight. Mercury will move lower
as the month advances before disappearing into the solar glare and
shifting to predawn rising by the end of the month. Shortly after
sunset, Saturn rises to the east and is visible all night as it
swings across the southern sky.
At mid-month, an overhead view about three hours after sunset finds
the Milky Way dividing the sky northwest to southeast. Directly
overhead is the constellation Auriga. Its brightest star is Capella,
about 10 degrees north of zenith. Elnath, about the same displacement
to the southeast, is sometimes associated with Auriga. However,
it’s actually part of Taurus. Notice that Elnath is an Arabic
name, meaning “Butting One” – appropriate for
the tip of the Bull’s horn. Elnath is a blue giant star, about
300 times brighter than the Sun and about 130 light years distant.
Castor and Polluz, in Gemini, lie about 30 degrees east of zenith,
above Leo rising. That’s Saturn just above Leo.
To the south is Orion’s familiar asterism. Last month we
noted the red star on the upper left of this familiar constellation,
Betelgeuse, and promised to look in detail at some of the other
bright stars in Orion. Perhaps the most familiar feature of Orion
are the three aligned stars forming the Hunter’s belt. These
are blue super-giant stars, all about 1500 light years distance,
and each more than 20,000 time brighter than the Sun. The western
most star (to the right) is Mintaka, from an Arabic word meaning
“belt.” The eastern most star is Alnitak, another Arabic
word meaning “girdle.” Alnilam, the middle star is named
for a “belt of pearls.” Below and to the right of the
belt region is the bright supergiant Rigel. It’s about the
same distance and brightness. Current thinking is that these stars
all formed about 10 million years ago on the visible edge of the
Orion Molecular Cloud Complex. We get a peek into this molecular
cloud when we look at the Orion Nebula – in the Hunter’s
sword hanging below the belt. This is an active star-forming region,
with the young Trapezium Cluster only a few million years old.
For your own monthly star chart, you can direct your web browser
to http://www.skymaps.com.
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.
Copyright 2007
George Spagna