Include polar motion in (ra,dec,mjd) to (alt,az) transforms about desisurvey

[dkirkby]

To illustrate the effects of neglecting polar motions, the plots below compare (Alt, Az) over the nominal DESI 5-year survey for targets at dec=+30deg observed at HA=0:

The blue points are calculated with astropy and include polar motion and the red line is based on this IDL code, which should be equivalent to utils.radec2altaz(). Note that there is a systematic offset of 7 deg in Az during the DESI survey, while the effects on Alt are much smaller ~ 0.1deg.

For reference, the plot was created with:

kpno = EarthLocation.from_geodetic(
        lat='31d57m50.30s', lon='-111d35m57.61s', height=2120.*u.m)
t_start = Time(datetime(2019,8,28,19,0,0))
t_stop = Time(datetime(2024,7,13,19,0,0))

def astropy_test(mjd1=t_start.mjd, mjd2=t_stop.mjd, dec=30. * u.deg, n=1023, ha=0.*u.deg):

    mjd = np.linspace(mjd1, mjd2, n + 1)
    tvec = Time(mjd, format='mjd', location=kpno)

    lst = tvec.sidereal_time('apparent')
    # Assign RA based on HA
    ra = lst - ha
    # Convert to AltAz
    altaz = SkyCoord(ra=ra, dec=dec).transform_to(
        AltAz(obstime=tvec, location=kpno, pressure=0))
    
    fig, ax = plt.subplots(2, 1, sharex=True, figsize=(8, 8))
    
    # Compare altitudes with RA=LST expectation
    ax[0].plot(mjd - mjd[0], altaz.alt.to(u.deg).value, '.')
    expected = (np.arcsin(np.cos(dec - kpno.latitude))).to(u.deg).value
    ax[0].axhline(expected, color='r', ls='--', lw=2)
    ax[0].set_ylabel('HA=0 Altitude [deg]')
    
    # Compare azimuths with RA=LST expectation
    ax[1].plot(mjd - mjd[0], altaz.az.to(u.deg).value, '.')
    expected = np.arctan2(0, np.sin(dec - kpno.latitude)).to(u.deg).value
    ax[1].axhline(expected, color='r', ls='--', lw=2)
    ax[1].set_ylim(None, 180.5)
    ax[1].set_ylabel('HA=0 Azimuth [deg]')
    ax[1].set_xlabel('Elapsed Days')
    
    plt.subplots_adjust(hspace=0.05)
    plt.savefig('polar.png')

[dkirkby]

Polar motions are now included consistently throughout desisurvey and surveysim using functions in desisurvey.utils.