Re: [guide-user] Re: Field rotation rate formula

Kevin Cooper Sep 22, 2003

Just a piece of 'useless' information.
When gathering light from a point source, that is not near the zenith,
with a spectrograph , astronomers frequently set the spectrograph slit
to the parallactic angle so as to make sure they gather all the light
that is dispersed by atmospheric refraction

Kevin Cooper..

Bill J Gray wrote:

>Hi Siebren,
>
> (Pause to grab Meeus' _Astronomical Algorithms_ from the
>shelf... set a fixed-size font, if needed, to puzzle through
>the following formula:)
>
> "The parallactic angle q can be calculated by means of
>the formula
>
> sin(H)
>tan q = -----------------------------------
> tan(phi) cos(dec) - sin(dec)cos(H)
>
> where... phi is the geographical latitude of the observer,
>dec the declination of the celestial body, and H its hour
>angle at the given instant."
>
> The parallactic angle is basically that at which Guide's
>charts in "zenith up" mode are rotated relative to those in
>"north up" mode, for a given point in the sky. That is,
>draw a line (great circle) from that point to the north
>celestial pole, and another one to the zenith. The angle
>between those great circles is the parallactic angle.
>
> The above expression is actually not all that ugly to
>differentiate analytically, but I'd recommend the numerical
>approach anyway; it's simpler still.
>
>-- Bill
>
>
>
>
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