skyproj {celestial} R Documentation

## Tan Gnomonic and Sine Orthographic Projection System WCS Solver Functions

### Description

Converts RA/Dec (degrees) to x/y (pixels) position using the Tan Gnomonic or Sine Orthographic projection systems, and vice-versa. Translations adapted from: http://mathworld.wolfram.com/GnomonicProjection.html and http://mathworld.wolfram.com/OrthographicProjection.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, CTYPE1 = 'RA--TAN', CTYPE2 = 'DEC--TAN')
xy2radec(x, y, header, CRVAL1 = 0, CRVAL2 = 0, CRPIX1 = 0, CRPIX2 = 0, CD1_1 = 1,
CD1_2 = 0, CD2_1 = 0, CD2_2 = 1, CTYPE1 = 'RA--TAN', CTYPE2 = 'DEC--TAN')
```

### 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 CTYPE1 projection system. This is the RA in degrees at the location of CRPIX1. `CRVAL2` FITS header CRVAL2 for the CTYPE2 projection system. This is the Dec in degrees at the location of CRPIX2. `CRPIX1` FITS header CRPIX1 for the CTYPE1 projection system. This is the x pixel value at the location of CRVAL1. `CRPIX2` FITS header CRPIX2 for the CTYPE2 projection system. This is the y pixel value at the location of CRVAL2. `CD1_1` FITS header CD1_1 for the CTYPE1 projection system. Change in CTYPE1 in degrees along x-Axis. `CD1_2` FITS header CD1_2 for the CTYPE1 projection system. Change in CTYPE1 in degrees along y-Axis. `CD2_1` FITS header CD2_1 for the CTYPE2 projection system. Change in CTYPE2 in degrees along x-Axis. `CD2_2` FITS header CD2_2 for the CTYPE2 projection system. Change in CTYPE2 in degrees along y-Axis. `CTYPE1` The RA projection system type. Either 'RA–TAN' for Tan Gnomonic (default), or 'RA–SIN' for Sine Orthographic. 'RA–NCP' is approximated by Sine Orthographic with a warning. Over-ridden by the FITS header. `CTYPE2` The DEC projection system type. Either 'DEC–TAN' for Tan Gnomonic (default), or 'DEC–SIN' for Sine Orthographic. 'DEC–NCP' is approximated by Sine Orthographic with a warning. Over-ridden by the FITS header. `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 and Sine Orthographic projection systems for solving an image WCS (covering most moden imaging and radio data). 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).

Aaron Robotham

### References

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

`deg2dms`, `deg2hms`, `dms2deg`, `hms2deg`

### Examples

```#A simple example:

CTYPE1='RA--SIN',CTYPE2='DEC--SIN')

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

exdata_start=expand.grid(1:10,21:30)
plot(exdata_start)
CD1_2=-0.05, CD2_1=0.05, CD2_2=0.1)