Convenient utilities for constructing an Astropy SIP model from a FITS header.

sip_utilities.py

from astropy.modeling.polynomial import SIP
import itertools
from astropy.io.fits.header import Header

def get_sip_coeffs(hdr: Header  , kind='A') -> dict:
    """Return a SIP polynomial coefficients for the selected
     coefficient type: (A, B, AP, BP).

    Parameters
    ----------
    hdr : Header
        header containing all polynomial coefficients up to the order.
    kind : str, optional
        'A', 'B', 'AP', or 'BP', by default 'A'

    Returns
    -------
    sip: SIP
        An astropy model for the sip polynomail 
    """
    sip_dict = {}
    a_order = hdr[f'{kind}_ORDER']
    for i, j in itertools.product(range(a_order+1), range(a_order+1)):
        if 1 < i + j <= a_order:
            key = f'{kind}_{i}_{j}'
            sip_dict[key] = hdr[key]
    return sip_dict

def get_sip_model(hdr: Header) -> SIP:
    """Construct a Astropy SIP model from the header

    Parameters
    ----------
    hdr : Header
        Header containing SIP coefficients

    Returns
    -------
    sip: SIP
        A polynomial 
    """
    a_coeff = get_sip_coeffs(hdr, kind='A')
    b_coeff = get_sip_coeffs(hdr, kind='B')

    sip = SIP(
        crpix=[hdr['crpix1'], hdr['crpix2']],
        a_order=hdr['A_ORDER'],
        b_order=hdr['B_ORDER'],
        a_coeff=a_coeff,
        b_coeff=b_coeff,
        ap_order=None,
        bp_order=None,
        ap_coeff={},
        bp_coeff={})
    
    return sip