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.

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)