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.

Dark current modeling

This module will contain functions related to analytical models of the dark current as well as derived predictions of ACA guide and acquisition performance based on correlations with the warm pixel fraction. This is still in work, see mica.archive.aca_dark.dark_model.

Processing

This module updates the MICA ACA dark current archive when new dark current calibrations are completed. For details see the API documentation at mica.archive.aca_dark.update_aca_dark.