R: Tan Gnomonic Projection Pystem WCS Polver Punctions

tanproj {celestial}

R Documentation

Tan Gnomonic Projection Pystem WCS Polver Punctions

Description

Converts RA/Dec (degrees) to x/y (pixels) position using the Tan Gnomonic projection system, and vice-versa. Translations adapted from: http://mathworld.wolfram.com/GnomonicProjection.html

Usage

radec2xy(RA, Dec, header, CRVAL1 = 0, CRVAL2 = 0, CRPIX1 = 0, CRPIX2 = 0, CD1_1 = 1,
CD1_2 = 0, CD2_1 = 0, CD2_2 = 1)
xy2radec(x, y, header, CRVAL1 = 0, CRVAL2 = 0, CRPIX1 = 0, CRPIX2 = 0, CD1_1 = 1,
CD1_2 = 0, CD2_1 = 0, CD2_2 = 1)

Arguments

`RA`	Vector or matrix; target right ascension in degrees. If matrix then the first column will be used as RA and the second column as Dec.
`Dec`	Vector; target declination in degrees. Ignored if RA is a matrix.
`x`	Vector or matrix; target x-pixel. If Matrix then the first column will be used as the x-axis and the second column as y-axis.
`y`	Vector; target y-pixel. Ignored if x is a matrix.
`CRVAL1`	FITS header CRVAL1 for the Tan Gnomonic projection system. This is the RA in degrees at the location of CRPIX1.
`CRVAL2`	FITS header CRVAL2 for the Tan Gnomonic projection system. This is the Dec in degrees at the location of CRPIX2.
`CRPIX1`	FITS header CRPIX1 for the Tan Gnomonic projection system. This is the x pixel value at the location of CRVAL1.
`CRPIX2`	FITS header CRPIX2 for the Tan Gnomonic projection system. This is the y pixel value at the location of CRVAL2.
`CD1_1`	FITS header CD1_1 for the Tan Gnomonic projection system. Change in RA-Tan in degrees along x-Axis.
`CD1_2`	FITS header CD1_2 for the Tan Gnomonic projection system. Change in RA-Tan in degrees along y-Axis.
`CD2_1`	FITS header CD2_1 for the Tan Gnomonic projection system. Change in Dec-Tan in degrees along x-Axis.
`CD2_2`	FITS header CD2_2 for the Tan Gnomonic projection system. Change in Dec-Tan in degrees along y-Axis.
`header`	Full FITS header in table or vector format. Legal table format headers are provided by the `read.fitshdr` function or the hdr list output of `read.fits` in the `astro` package). Also the hdr output of `readFITS` in the `FITSio` package provides legal vector format inputs. If a header is provided then key words will be taken from here as a priority. Missing header keywords are printed out and other header option arguments are used in these cases.

Details

These functions encode the standard FITS Tan Gnomonic projection system for solving an image WCS. They do not deal with higher order polynomial distortion terms.

Value

`radec2xy`	Returns a two column matrix with columns x and y.
`xy2radec`	Returns a two column matrix with columns RA and Dec (in degrees).

Author(s)

Aaron Robotham

References

http://mathworld.wolfram.com/GnomonicProjection.html

Examples

#A simple example:

radec2xy(10,20)
xy2radec(radec2xy(10,20))

#A more complicated example, where we transform and rotate large amounts:

exdata_start=expand.grid(1:10,21:30)
plot(exdata_start)
exradec=radec2xy(exdata_start, CRVAL1=20, CRPIX1=100, CRVAL2=30, CRPIX2=130, CD1_1=0.1,
CD1_2=-0.05, CD2_1=0.05, CD2_2=0.1)
plot(exradec)
exdata_end=xy2radec(exradec, CRVAL1=20, CRPIX1=100, CRVAL2=30, CRPIX2=130, CD1_1=0.1,
CD1_2=-0.05, CD2_1=0.05, CD2_2=0.1)
plot(exdata_start,cex=2)
points(exdata_end,col='red')

#The residuals should be very small (in the noice of double precision arithmetic):

plot(density(exdata_start[,1]-exdata_end[,1]))
lines(density(exdata_start[,2]-exdata_end[,2]),col='red')

[Package celestial version 1.4.1 Index]