Tom Van Baak wrote: >> Accuracy still won't be much better than1% of the solar diameter or >> about 1 second of time nowhere near the o.1 sec or better hoped for. >> Bruce >> > > Bruce, > > Can you show us how to derive the accuracy number? > > I would have guessed that with fractional degree Al-El > steering, a rotary encoder, 12 hours of sampling, and > curve fitting that one could calibrate solar time against > a local UTC standard to a bit better than that. > > /tvb > > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Tom
First I'll deal with some of the problems of the various proposed transit methods: For a time error of 100 millisec or less the corresponding error in the sun's azimuth is about 1.5 arcseconds. When viewed from the bottom of the atmosphere the sun and stars appears to move about randomly due to the effects of atmospheric seeing. The amplitude of this random image displacement in the visible depends on the altitude of the object being observed it can be as large as several arc seconds when observing stars with an altitude of 45 degrees at night. This random image motion due to atmospheric turbulence is even greater during the day (as much as 10 arc sec rms on Mauna Kea). http://adsabs.harvard.edu/abs/1990ursi.symp...75C The solar image is not uniformly bright across the disk, there is a noticeable darkening towards the limb. There is no sharply defined solar limb, prominences and other outbursts can extend arc minutes from the edge. Local seeing effects such as turbulent air over structures warmed by the sun such as buildings, concrete pads, roads etc will also have significant effects. Internal instrument turbulence due to solar heating can also be problematic. Scintillation in the solar irradiance will also have an effect on the noise in solar meridian transit measurements. At solar meridian transit solar heating of the ground will produce worse seeing than earlier in the day when the suns altitude is lower. The accuracy of a transit method that uses a pair of photodetectors to compare the light from two small sections of the solar limb will be adversely affected by atmospheric seeing and bright prominences. The angular size and declination of the sun vary throughout the year. Thus the position of the limb sensing detectors have to be adjusted to accommodate the variation in the sun's altitude at meridian transit. Making such adjustments without changing the effective azimuth defined by the detectors and associated optics, gnomon or slit is difficult especially when the maximum variation in azimuth due to such adjustments has to be no more than 1 arcsecond or so. It has also been implicitly assumed that azimuth of the transit instrument remains fixed over time and temperature variations. it is actually difficult to ensure the azimuth remains constant to an arcsecond or so over time without taking heroic measures to ensure the instrument mounting plinth is sufficiently stable. Uneven solar heating of the instrument plinth can cause it to the instrument to tilt and twist its azimuth through several arc seconds. Joints between dissimilar materials are prone to cause the instrument azimuth to rotate as it warms up. A suitable kinematic joint such as a Maxwell clamp can reduce the thermal rotation significantly but even then it is difficult to ensure an azimuth stability of an arcsecond or so. The pointing stability of an instrument using plastic (fibers, etc) parts can be problematic especially when it is subject to temperature cycling. Bruce _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
