Lux, Jim (337C) wrote:
On 1/15/10 9:13 AM, "J. Forster" <[email protected]> wrote:
Heresy:
Why do you really NEED super accurate siderial time? If you are using it
to point a telescope, you only need it accurate enough to get the guide
stars into the field. Remember, the atmosphere refracts kinda randomly.
-John
What if it's a *radio* telescope? Or a DSN dish? Both of those have to track
sidereal motion. At Ka-band, the 70meter dish needs pointing on the order
of 1 millidegree. That's about 250 milliseconds, I guess.
Or better yet, a submillimeter radiotelescope? At the SMT on Mt. Graham, we
measure pointing to 0.1 arcsecond and have typical pointing errors of ~2
arcseconds. These are measured at many points in the sky using 5-point data
(center, N, E, W, S offsets) from a planet. The pointing model gets updated
regularly as needed.
The tracking system receives IRIG time from a GPSDO directly. The time accuracy
needed is ~1 millisecond. LST is calculated using the canonical 10 digit number
cited previously.
--David Forbes, the HHSMT, Arizona
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