AstropyWCS

class snappl.wcs.AstropyWCS(apwcs=None)

Bases: BaseWCS

A WCS that is defined by an astropy.wcs.WCS.

Methods Summary

from_header(header)	Create an AstropyWCS from a FITS header.
get_astropy_wcs([readonly])	Return an astropy.wcs.WCS object, if possible.
get_galsim_wcs()	Return a glasim.AstropyWCS object, if possible.
pixel_to_world(x, y)	Go from (x, y) coordinates to ICRS (ra, dec)
to_fits_header()	Return an astropy.io.fits.Header object, if possible, with the WCS in it.
world_to_pixel(ra, dec)	Go from (ra, dec) coordinates to (x, y)

Methods Documentation

classmethod from_header(header)

Create an AstropyWCS from a FITS header.

NOTE: if the header claims that the transformation type is “TAN” (i.e. CTYPE1 is “RA—TAN”), but the header also has a “PV1_0” keyword, this function will assume that the transformation is actually TPV.

See: https://github.com/thomasvrussell/sfft/blob/45efa77452f020b8832a14c8682b87c5ffee4a93/sfft/utils/ReadWCS.py

Parameters:

header (duckish astropy.io.fits.header.Header) – Something that behaves like a FITS header, in that it can be accessed as a dictionary, has the copy() method, and canbe fead to astropy.wcs.WCS().

Return type:

AstropyWCS

get_astropy_wcs(readonly=True)

Return an astropy.wcs.WCS object, if possible.

Parameters:

• readonly (bool, default True) – If True, you are promising not to modify the WCS you get back! If you’re going to modify it, set readonly to False. (For some subclasses, this doesn’t actually change behavior.)

• degree (int) – The degree of the astropy WCS used to approximate the WCS in the object. The default is subclass-dependent. Ignored by some subclasses.

For some subclasses, this astropy.wcs.WCS may only be an approximation of the true WCS represented by the object.

get_galsim_wcs()

Return a glasim.AstropyWCS object, if possible.

pixel_to_world(x, y)

Go from (x, y) coordinates to ICRS (ra, dec)

Parameters:

• x (float or sequence of float) – The x position on the image. The center of the lower-left pixel is at x=0.0

• y (float or sequence of float) – The y position on the image. The center of the lower-left pixle is y=0.0

Returns:

• ra, dec (floats or arrays of floats, decimal degrees)

• You will get back two floats if x an y were floats. If x and

• y were lists (or other sequences), you will get back two numpy

• arrays of floats.

to_fits_header()

Return an astropy.io.fits.Header object, if possible, with the WCS in it.

world_to_pixel(ra, dec)

Go from (ra, dec) coordinates to (x, y)

Parameters:

• ra (float or sequence of float) – RA in decimal degrees

• dec (float or sequence of float) – Dec in decimal degrees

Returns:

• x, y (floats or arrays of floats)

• Pixel position on the image; the center of the lower-left pixel is (0.0, 0.0).

• If ra and dec were floats, x and y are floats. If ra and dec

• were sequences of floats, x and y will be numpy arrays of floats.