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STARAST

STARAST

## Purpose

Compute astrometric solution using positions of 2 or 3 reference stars

## Explanation

Computes an exact astrometric solution using the positions and
coordinates from 2 or 3 reference stars and assuming a tangent
(gnomonic) projection. If 2 stars are used, then
the X and Y plate scales are assumed to be identical, and the
axis are assumed to be orthogonal. Use of three stars will
allow a unique determination of each element of the CD matrix.

## Calling Sequence

starast, ra, dec, x, y, cd, [/Righthanded, HDR = h, PROJECTION=]

## Inputs

RA - 2 or 3 element vector containing the Right Ascension in DEGREES
DEC- 2 or 3 element vector containing the Declination in DEGREES
X - 2 or 3 element vector giving the X position of reference stars
Y - 2 or 3 element vector giving the Y position of reference stars

## Outputs

CD - CD (Coordinate Description) matrix (DEGREES/PIXEL) determined
from stellar positions and coordinates.

## Optional Input Keyword

/RightHanded - If only 2 stars are supplied, then there is an ambiguity
in the orientation of the coordinate system. By default,
STARAST assumes the astronomical standard left-handed system
(R.A. increase to the left). If /Right is set then a
righthanded coordinate is assumed. This keyword has no effect
if 3 star positions are supplied.
PROJECTION - Either a 3 letter scalar string giving the projection
type (e.g. 'TAN' or 'SIN') or an integer 1 - 25 specifying the
projection as given in the WCSSPH2XY procedure. If not
specified then a tangent projection is computed.
OPTIONAL INPUT-OUTPUT KEYWORD:
HDR - If a FITS header string array is supplied, then an astrometry
solution is added to the header using the CD matrix and star 0
as the reference pixel (see example). Equinox 2000 is assumed.

## Example

IDL> starast,ra,dec,x,y,cd ;Determine CD matrix
IDL> crval = [ra[0],dec[0]] ;Use Star 0 as reference star
IDL> crpix = [x[0],y[0]] +1 ;FITS is offset 1 pixel from IDL
This is equivalent to the following command:
IDL> STARAST,ra,dec,x,y,hdr=h

## Method

The CD parameters are determined by solving the linear set of equations
relating position to local coordinates (l,m)
For highest accuracy the first star position should be the one closest
to the reference pixel.

## Revision History

Written, W. Landsman January 1988
Converted to IDL V5.0 W. Landsman September 1997
Added /RightHanded and HDR keywords W. Landsman September 2000
Write CTYPE values into header W. Landsman/A. Surkov December 2002
CD matrix was mistakenly transpose in 3 star solution
Added projection keyword W. Landsman September 2003
Test for singular matrix W. Landsman August 2011

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