[time-nuts] Sideral time

[Colby Gutierrez-Kraybill]

I love following the broad tangents that come up on this list...

I wanted to chime in on observatory use of sidereal time, and guider  
scopes, etc...  At the research class instrument level, as is being  
pointed out (har har), pointing models are heavily used, for both  
first acquiring a source and more importantly to ensure that tracking  
the source is steady across the period the telescope is taking data.

Second, guider scopes in radio astronomy seem to be rare.  The uses  
I'm aware of are usually used for coming up with accurate pointing  
model coefficients, by pointing at dozens of mag 1-6 stars all across  
the sky periodically.  How often that is done depends on how stable  
the mounts of the telescopes are.  For many radio telescopes, having a  
bright enough source in or near the field of view is rare enough to  
put it very low down on a priority list of control elements.  Not to  
mention they're not much use in bad weather conditions that are  
otherwise okay for the frequency ranges a given radio telescope  
operates at, nor during the day, where even though you can see stars  
with proper filters, you're even more limited on how many you can see.

For our day-to-day use, sidereal is only used as a reference for  
knowing approximately what RA is up at that moment, which makes a  
visual sidereal display very useful.