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In general, objects like stars and galaxies are so far
away from us that we can't detect a shift in their positions
due to their motion through space: they appear stationary
with respect to the other stars and galaxies. This is
why the patterns of stars ("constellations")
described by our ancestors hundreds or even thousands
of years ago are recognizable to us today. To the casual
observer, the stars in the nighttime sky do appear stationary.
But if one spends a bit of time and looks carefully, they
will soon notice that stars are rising in the east and
setting in the west... the whole sky appears to be turning.
In fact, it's the *Earth* that is turning, once upon its
axis every single day. For the same reason, we see the
Sun rising in the east and setting in the west. In order
for ground-based telescopes to make detailed observations
of stars and galaxies, we must move them to exactly compensate
for the Earth's rotation. Astronomers refer to this as
moving the telescope at the "sidereal" (stellar)
tracking rate.
For objects much closer to the Earth, like the planets
and moons of our solar system, moving our telescope at
the sidereal rate is generally not enough. These objects
are close enough that we can detect the motion of these
objects as they orbit around the Sun or around one of
the planets. To keep our telescope exactly centered on
one of these objects, we have to move the telescope at
some "non-sidereal" tracking rate. We first
need to calculate where the particular object of interest
will be from moment to moment during the period we will
be observing it, and then we need to precisely move our
telescope to exactly match our calculations. In the beginning
of this year, the staff at Subaru Telescope tested the
telescope's ability to do non-sidereal tracking by observing
Comet LINEAR.
Comet LINEAR (C/1999S4) was discovered on September 27,
1999 by the Lincoln Near Earth Asteroid Research project
(LINEAR) run by the MIT Lincoln Laboratory. Comet LINEAR
passed closest to the Earth on July 23 at a distance of
about 56 million km; it will reach its closest point to
the Sun (perihelion) at a heliocentric distance of 114
million km on July 26. Shortly after its discovery, researchers
thought the comet could become bright enough to see with
the unaided eye (like Comet Hale-Bopp in 1997). Unfortunately,
based on its most recent behavior, Comet LINEAR will only
be visible with binoculars or a telescope.
Subaru telescope first observed the comet on January 8
using CAC (a simple digital camera used mostly during the
initial testing of Subaru Telescope) attached at the Cassegrain
focus, and with CISCO at the Nasmyth focus on June 16. Dust
and gas ejected from the brightest part of the comet, the
central condensation, can be seen in both the images shown
here, giving the inner part of the comet (the "head")
a typical spindle-like shape. Dust and gas particles flowing
towards the anti-solar direction produce the comet's tail.
Based on the sharpness of the central condensation in both
images, these test observations confirm Subaru Telescope's
ability to do non-sidereal tracking. This clears the way
for studies of objects within our solar system.
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Comet LINEAR (C/1999 S4). Left: visible
light image using CAC, showing spatial distribution
of both gas and dust coming from the central condensation.
Right: near-infrared image using CISCO, showing spatial
distribution mainly of dust. Stars appear streaked
because the telescope was moved to follow the comet.
The star-streaks show three colored segments, one
for each of the exposures taken sequentially through
a different color filter to create the final color
composite.
- Row Resolution (145KB)
/ High Resolution (353KB)
Guidelines
for use of Subaru Images
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(July 24, 2000)
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Image of Radio Galaxy 4C+40.36 and its Newly Discovered
Companion.
Spectrum of Radio Galaxy 4C+40.36.
