get_lt

acstools.utils_calib.get_lt(hdr)

Obtain the LTV and LTM keyword values.

Note that this returns the values just as read from the header, which means in particular that the LTV values are for one-indexed pixel coordinates.

LTM keywords are the diagonal elements of MWCS linear transformation matrix, while LTV’s are MWCS linear transformation vector (1-indexed).

Note

Translated from calacs/lib/getlt.c.

Parameters:

hdr (obj) – Extension header.

Returns:

ltm, ltv – (LTM1_1, LTM2_2) and (LTV1, LTV2). Values are (1, 1) and (0, 0) if not found, to accomodate reference files with missing info.

Return type:

tuple of float

Raises:

ValueError – Invalid LTM* values.