Name

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  GCROT

Purpose

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  This function computes, as a function of galactic coordinates, the
  angle between a vector pointing towards the galactic north pole
  and a vector pointing towards the North Celestial Pole. This can
  be used to orient images aligned to ecliptic and galactic
  coordinate systems.

Category

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  coordinate systems

Calling Sequence

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  result=GCROT(L,B,/DEGREE)

Inputs

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  L: Galactic longitude. Radians are assumed unless /DEGREE is set.
  B: Galactic latitude. Radians are assumed unless /DEGREE is set.

Keyword Parameters

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  DEGREE: If set, the input and output are given in degrees.
 

Outputs

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  The clockwise rotation that aligns the galactic north pole with
  the NCP (in J2000 coordinates). In radians by default.

Restrictions

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  This has not been tested for accuracy higher than roughly .5
  degrees. I wouldn't use this to determine the orientation of spectra
  slits or other high precision tasks without testing first.

Procedure

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  Uses a spherical trig identity taken from wikipedia.org/wiki/Great-circle_distance
 

Modification History

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  Written by: Chris Beaumont, May 2008.
  June 2008 Added /degree keyword. cnb