altaz_to_focalplane

specsim.transform.altaz_to_focalplane(alt, az, alt0, az0, platescale=1)

Convert local (alt,az) coordinates to focal plane (x,y) coordinates.

A plate coordinate system is defined by its boresight altitude and azimuth, corresponding to the (x,y) origin, and the conventions that +x increases eastwards along the azimuth axis and +y increases towards the zenith along the altitude axis:

>>> scale = 200 * u.mm / u.deg
>>> alt0, az0 = 45 * u.deg, 0 * u.deg
>>> x, y = altaz_to_focalplane(alt0 + 1 * u.deg, az0, alt0, az0, scale)
>>> print('X = %.2f mm, Y = %.2f mm' % (x.to(u.mm).value, y.to(u.mm).value))
X = 0.00 mm, Y = 199.99 mm
>>> x, y = altaz_to_focalplane(alt0, az0 + 1 * u.deg, alt0, az0, scale)
>>> print('X = %.2f mm, Y = %.2f mm' % (x.to(u.mm).value, y.to(u.mm).value))
X = 141.41 mm, Y = 0.87 mm

This function implements a purely mathematical coordinate transform and does not invoke any atmospheric refraction physics. Use sky_to_altaz() to convert global sky coordinates (ra,dec) into local (alt,az) coordinates, which does involve refraction.

The output shape is determined by the usual numpy broadcasting rules applied to all of the inputs, , so this function can be used to tabulate (x,y) coordinates on a user-specified grid covering different targets and boresights.

Parameters:

altastropy.coordinates.Angle

Target altitude angle(s) above the horizon.

azastropy.coordinates.Angle

Target azimuthal angle(s) east of north.

alt0astropy.coordinates.Angle

Boresight altitude angle(s) above the horizon.

az0astropy.coordinates.Angle

Boresight azimuthal angle(s) east of north.

platescaleastropy.units.Quantity

Conversion from angular separation relative to the boresight to the output focal plane coordinates.

Returns:

tuple

Pair x,y of focal-plane coordinates expressed as astropy.units.Quantity objects, with +x along the azimuth direction (increasing eastwards) and +y along the altitude direction (increasing towards zenith). The output arrays have the same shapes, given by np.broadcast(alt, az, alt0, az0). The output units are determined by the input platescale and will be u.rad if the platescale is dimensionless, or otherwise the SI units of platescale * u.rad.