[time-nuts] Sidereal time

[David Forbes]

Lux, Jim (337C) wrote:
> 
> 
> On 1/15/10 9:13 AM, "J. Forster" <jfor at quik.com> 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