[time-nuts] Sideral time

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 15 23:02:43 UTC 2010


Colby Gutierrez-Kraybill wrote:
>
> 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.
>
Minimising the error that an autoguider or similar device has to deal 
with is conducive to better tracking.
> 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.
Most large optical telescopes (eg MMT) don't seem to use classical 
finder or guide scopes.
A classical guide scope doesnt work well with 20th magnitude objects as 
its aperture just isn't large enough.
Using an eyepiece with a large telescope is problematic at best.
Usually an acquistion camera with a limited field of view is used instead.

Bruce





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