fitsmgmt.header.cmt2hdr

fitsmgmt.header.cmt2hdr(
    header,
    histcomm,
    s,
    precision=3,
    time_fmt='{:.>72s}',
    t_ref=None,
    dt_fmt='(dt = {:.3f} s)',
    set_kw=None,
    verbose=False,
)

Add HISTORY/COMMENT entries, optionally with a timestamp.

Parameters

header : ~astropy.io.fits.Header

The header.

histcomm : str in ['h', 'hist', 'history', 'c', 'comm', 'comment']

Whether to add history or comment.

s : str or list of str

The string to add as history or comment.

precision : int, optional. = 3

The precision of the isot format time. Default: 3.

time_fmt : str, None, optional. = '{:.>72s}'

The Python 3 format string to format the time in the header. If None, the timestamp string will not be added.

Examples::
  • "{:s}": plain time 2020-01-01T01:01:01.23
  • "({:s})": plain time in (). (2020-01-01T01:01:01.23)
  • "{:_^72s}": center align, filling with _. Default: '{:.>72s}'.
t_ref : ~astropy.time.Time, optional. = None

The reference time. If not None, delta time is calculated. Default: None.

dt_fmt : str, optional. = '(dt = {:.3f} s)'

The Python 3 format string to format the delta time in the header. Default: '(dt = {:.3f} s)'.

verbose : bool, optional. = False

Whether to log the same information. Default: False.

set_kw : dict, optional. = None

The keyword arguments added to ~astropy.io.fits.Header.set(). Default is {'after': -1}, i.e., the history or comment will be appended to the very last part of the header.

Notes

The timing benchmark for a reasonably long header (len(ccd.header.cards) = 197) shows dt ~ 0.2-0.3 ms on MBP 15” [2018, macOS 11.6, i7-8850H (2.6 GHz; 6-core), RAM 16 GB (2400MHz DDR4), Radeon Pro 560X (4GB)]:

%timeit ccd.header.copy() 1.67 ms +/- 33.3 µs per loop (mean +/- std. dev. of 7 runs, 1000 loops each) %timeit air.cmt2hdr(ccd.header.copy(), ‘h’, ‘test’) 1.89 ms +/- 141 µs per loop (mean +/- std. dev. of 7 runs, 1000 loops each) %timeit air.cmt2hdr(ccd.header.copy(), ‘hist’, ‘test’) 1.89 ms +/- 144 µs per loop (mean +/- std. dev. of 7 runs, 1000 loops each) %timeit air.cmt2hdr(ccd.header.copy(), ‘histORy’, ‘test’) 1.95 ms +/- 146 µs per loop (mean +/- std. dev. of 7 runs, 100 loops each)

Same benchmark on MBP 14” [2024, macOS 26.4.1, M4Pro(8P+4E/G20c/N16c/48G)], 2026-05-27:

hdr = fits.Header()
hdr["SIMPLE"] = True
hdr["BITPIX"] = 8
hdr["NAXIS"] = 0
hdr["EXPTIME"] = 20
for i in range(193):
    hdr[f"H{i:03d}"] = i

%timeit hdr.copy()
# 224 µs ± 654 ns per loop (7 runs, 500 loops each)
%timeit air.cmt2hdr(hdr.copy(), "h", "test")
# 396 µs ± 10.2 µs per loop (7 runs, 200 loops each)
%timeit air.cmt2hdr(hdr.copy(), "hist", "test")
# 394 µs ± 5.17 µs per loop (7 runs, 200 loops each)
%timeit air.cmt2hdr(hdr.copy(), "histORy", "test")
# 397 µs ± 3.35 µs per loop (7 runs, 200 loops each)