The Infrared Spectrograph (IRS) will be one of the three instruments onborad NASA's Space Infrared Telescope Facility (SIRTF). Four instrument modules of IRS are designed and built to observe the mid-infrared (5 to 40) spectra of astronomical sources in four overlapping wavelength channels with low- and medium-resolution dispersion optics and As:Si and As:Sb BIB detectors. The IRS Basic Calibrated Data (BCD) pipelines are designed to remove instrument artifacts introduced by a combination of optics and detector effects. The end-products are two-dimensional images containing spectra. Post-BCD processing pipelines will enable further reduction of the BCD frames to extract the one-dimensional spectra of observed sources.
The spectra of astronomical sources are taken by the two-dimensional BIB arrays operated at a temperature of 5K. There are four readout channels, and the analog-to-digital convertor is saturated at a level below the pixel full-well. The nominal data-taking mode for observing spectra is the so-called sample-up-the-ramp, in which pixels are read non-destructively during one Data Collection Event (DCE), and all readings are downlinked to the ground. So the pipeline typically deals with a data cube with multiple sampled layers in one DCE. A two-dimensional IRS spectral image has the following instrument artifacts:
Since the IRS has no shutter it will direct its slit to blank sky areas to take
reference dark frames for calibration of dark current. The non-linearity
effect is not expected to be significant since the A/D converter saturates
at a level below full pixel well, but will be corrected. Although the detector
has pixel-dependent ramp nonlinearity at short time-scale, the same behavior is
persistent in different integration ramps with the same sample duration,
so this can be corrected by layer-by-layer subtraction of a reference data cube
such as the reference dark cube.
Radhits can be identified and removed by examining the discontinuities
in the charge ramp. A segmented fit of the ramp is performed for each pixel
and the probability and strength of the radhits are estimated using a Bayesian
approach. The "droop" effect can be caused by both illumination and radhits
so it should be accounted for before radhits are removed. Saturation at
the A/D converter needs to be corrected since electrons are still accumulating
in pixel wells and contribute to the "droop" level. Any remaining "droop",
such as that caused by radhits in saturated pixels, can be removed by examining
and subtracting the remaining median levels in unilluminated regions.
Such unilluminated regions, where spectra orders are well-separated and
order cross-talk effects are small, exist in all IRS arrays. Any channel-dependent
effects can be similarly removed by examining and correcting remaining
cross-channel median levels.
Flatfielding corrects for both optical dispersion function and pixel
response variation. To estimate flatfield, a number of calibration sources
with known continuum and spectral lines will be observed. The spectral
lines in different sources will be masked, and a slit profile will be
incorporated into the continuum spectrum which is removed from data.
Figure 1 shows the current design of the BCD science and two
calibration pipeline threads. They are shown as flowcharts illustrating
the data reduction sequence and the software components in boxes
that perform specific tasks. In the reference dark-current calibration,
the amplifier drift is removed for consecutive dark exposures. This
is for correctly identifing small radhits during the subsequent
median-filtering of the same data layers of many such exposures.
The flatfield calibration follows nearly the same reduction steps
as in the science pipeline, since flatfielding is performed at the
end of the science pipeline. A result from the science pipeline
reduction using stimulator data is shown in Figure 2.
Further reduction of the BCD is needed to generate spectra.
The purpose is to make it straightforward to extract
one-dimensional spectra from the two-dimensional BCD. This basically
involves "straightening" of the spectra in both two-dimensional
pixel space and one-dimensional wavelength space. We have implemented post-BCD
pipelines that take into account the curvature of
spectra and wavelength resolution elements to perform the
"straightening". The pipeline uses a calibration file containing
elements optimally sampled in wavelength space and their locations
in pixel space in order to estimate the average profile of spectra orders
and perform spectra extraction based on the profile.
The components in a pipeline are all stand-alone modules. Operationally the calling of a software module is communicated via wrapper scripts of the modules. A wrapper script can have extra capabilities such as communicating with calibration servers, checking input files, directing output files, setting database flags, etc., in addition to executing the corresponding module.