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BPRECESS

BPRECESS

## Purpose

Precess positions from J2000.0 (FK5) to B1950.0 (FK4)

## Explanation

Calculates the mean place of a star at B1950.0 on the FK4 system from
the mean place at J2000.0 on the FK5 system.

## Calling Sequence

bprecess, ra, dec, ra_1950, dec_1950, [ MU_RADEC = , PARALLAX =

## Inputs

RA,DEC - Input J2000 right ascension and declination in *degrees*.
Scalar or N element vector

## Outputs

RA_1950, DEC_1950 - The corresponding B1950 right ascension and
declination in *degrees*. Same number of elements as
RA,DEC but always double precision.
OPTIONAL INPUT-OUTPUT KEYWORDS
MU_RADEC - 2xN element double precision vector containing the proper
motion in seconds of arc per tropical *century* in right
ascension and declination.
PARALLAX - N_element vector giving stellar parallax (seconds of arc)
upon output to contain the values of these quantities in the
B1950 system. The parallax and radial velocity will have a very
minor influence on the B1950 position.
EPOCH - scalar giving epoch of original observations, default 2000.0d
This keyword value is only used if the MU_RADEC keyword is not set.

## Notes

The algorithm is taken from the Explanatory Supplement to the
Astronomical Almanac 1992, page 186.
Also see Aoki et al (1983), A&A, 128,263
BPRECESS distinguishes between the following two cases:
(1) The proper motion is known and non-zero
(2) the proper motion is unknown or known to be exactly zero (i.e.
extragalactic radio sources). In this case, the reverse of
the algorithm in Appendix 2 of Aoki et al. (1983) is used to
ensure that the output proper motion is exactly zero. Better
precision can be achieved in this case by inputting the EPOCH
of the original observations.
The error in using the IDL procedure PRECESS for converting between
B1950 and J1950 can be up to 12", mainly in right ascension. If
better accuracy than this is needed then BPRECESS should be used.
An unsystematic comparison of BPRECESS with the IPAC precession
routine (http://nedwww.ipac.caltech.edu/forms/calculator.html) always
gives differences less than 0.15".

## Example

The SAO2000 catalogue gives the J2000 position and proper motion for
the star HD 119288. Find the B1950 position.
RA(2000) = 13h 42m 12.740s Dec(2000) = 8d 23' 17.69''
Mu(RA) = -.0257 s/yr Mu(Dec) = -.090 ''/yr
IDL> mu_radec = 100D* [ -15D*.0257, -0.090 ]
IDL> ra = ten(13, 42, 12.740)*15.D
IDL> dec = ten(8, 23, 17.69)
===> 13h 39m 44.526s +08d 38' 28.63"

## Revision History

Written, W. Landsman October, 1992
Vectorized, W. Landsman February, 1994
Treat case where proper motion not known or exactly zero November 1994
Handling of arrays larger than 32767 Lars L. Christensen, march, 1995
Fixed bug where A term not initialized for vector input
W. Landsman February 2000
Use V6.0 notation W. Landsman Mar 2011

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