ACA dark current

The mica.archive.aca_dark package provides modules related to the ACA dark current:

• Dark current calibrations

• `Dark current modeling`_

• `Processing`_

Dark current calibrations

The mica.archive.aca_dark.dark_cal module provides functions for retrieving data for the ACA full-frame dark current calibrations which occur about four times per year (see the ACA dark calibrations TWiki page).

The functions available are documented in the mica.archive.aca_dark section, but the most useful are:

• dark_temp_scale(): get the temperature scaling correction

• get_dark_cal_dirs(): get an ordered dict of dark cal identifer and directory

• get_dark_cal_image(): get a single dark cal image

• get_dark_cal_props(): get properties (e.g. date, temperature) of a dark cal

• get_dark_cal_props_table(): get properties of dark cals over a time range as a table

As an example, let’s plot the raw and corrected warm pixel fraction over the mission. The correction in this case is done to a reference temperature of -15 C:

from mica.archive.aca_dark import dark_cal
from Ska.Matplotlib import plot_cxctime
from Chandra.Time import DateTime

dark_cals = dark_cal.get_dark_cal_dirs()
times = []
n100 = []
n100_m15 = []
npix = 1024. * 1024.

for dark_id in dark_cals:
    print('Reading {}'.format(dark_id))
    props = dark_cal.get_dark_cal_props(dark_id, include_image=True)
    scale = dark_cal.dark_temp_scale(props['ccd_temp'], -15.0)
    image = props['image']
    times.append(props['date'])
    n100.append(np.count_nonzero(image > 100.0) / npix)
    n100_m15.append(np.count_nonzero(image * scale > 100.0) / npix)

times = DateTime(times).secs
figure(figsize=(6, 4))
plot_cxctime(times, n100, 'o-', color='red')
plot_cxctime(times, n100_m15, 's-', color='cyan')
grid(True)
xlim(DateTime('2000:001').plotdate, DateTime().plotdate)
ylim(0, None)
title('Warm pixel fraction')

Note that the temperature assigned to a dark calibration is the mean of the temperature for the invidivual dark replicas (typically 5). These in turn use ACA hdr3 diagnostic telemetry for high-resolution temperature readouts which are available before and after (but not during) each replica.