Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.adass.org/adass/proceedings/adass94/mackentyj.ps Äàòà èçìåíåíèÿ: Tue Jun 13 20:50:00 1995 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 02:23:10 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 43

Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
MACSQIID: A package for the reduction of data from the
SQIID Infrared Camera
J. W. MacKenty
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore
MD 21218
Abstract. The SQIID camera was developed at NOAO's Kitt Peak
National Observatory to provide multi­band infrared imaging. It is well
suited for galaxy surface photometry as it has a uniform, stable flat field
and a large field of view. We have developed a software package to facil­
itate the reduction of data from this instrument for this purpose. This
package is appropriate for use with observations where the telescope is
moved a small fraction of the field of view between exposures. This pack­
age uses the stability of the camera to construct flat field calibrations
over several nights. The flat field and image summation algorithms avoid
the use of the median and instead rely upon a combination of source de­
tection in the J +H +K summed frames, sigma clipping, and iteration.
The sky illumination pattern is determined from the residuals between
observations of different targets on the sky.
1. Introduction
We have developed a software package to facilitate the reduction of data from
the NOAO SQIID (Simultaneous Quad­color Infrared Imaging Device) camera
(Ellis et al. 1992). Our approach emphasizes the design of a software package
targeted towards a specific instrument and a limited set of observational projects.
Rather than build software for the general problem of reducing infrared images
from a large range of instruments, this package is intended only for use with the
SQIID camera. Therefore, it incorporates, and to some extent conceals from
the user, considerable knowledge about the camera. Further, while the SQIID
camera can be used to map large regions of the sky by constructing mosaics,
this package is appropriate only for use with observations where the telescope is
moved a limited fraction of the field of view between exposures and the entire
set of exposures of a target possesses some degree of overlap.
The SQIID camera employs four 256 \Theta 256 pixel PtSi array detectors which,
although their quantum efficiency is rather modest (6.6% at J to 3.4% at K),
have excellent cosmetic qualities, good linearity, outstanding temporal stability,
and little sensitivity to the effects of previous exposures. SQIID incorporates
dichroic beam splitters and re­imaging optics to simultaneously view the same
field in the J (1.2 ¯m), H (1.6 ¯m), K (2.2 ¯m), and L (3.5 ¯m) passbands.
Due to its very low sensitivity, the L\Gammaband channel is not incorporated in the
MACSQIID software package. SQIID has a scale of 1: 00 36 pixel \Gamma1 and a field
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of view of 5: 0 5 on the KPNO 1.3 m telescope, and 0: 00 43 pixel \Gamma1 and 1: 0 8 on the
KPNO 4 m telescope.
2. A Methodology for the Calibration of Infrared Images
The goal of the MACSQIID package is to produce images from a set of obser­
vations which are free from the various instrumental and atmospheric effects.
An observing run consists of many (typically 5 to 20) exposures (60 to 300 s
each) of a series of targets. We will refer to each set of exposures as a ``dataset''.
Within each dataset the telescope was offset a distance less than the SQIID's
field of view and larger than the extent of the target(s). For larger targets, a
few exposures of equal duration are obtained of a nearby, blank area on the sky.
2.1. Basic Calibration Equation
In each SQIID exposure, we detect the flux distribution of the target field of
view plus the sky and instrumental backgrounds. The detected image, I (x;y;t) ,
is related to the original scene, O x;y , by:
O (x;y)
= [ (I (x;y;t) \Gamma Dark (x;y)
) \Theta F lat (x;y)
] \Gamma Background (t) \Gamma Sky (x;y)
:
The individual pixels in each image have unique sensitivities (``flat field'')
and signal internal to the instrument (``dark current''). The dark current is
directly measured by obtaining many exposures with a cold dark slide inserted
behind the instrument's input aperture. The MACSQIID package makes the
following assumptions. First, that the dark current and flat field are constant
over a significant number of datasets (in practice they are stable over a week long
observing run). Second, that the sky background can be modeled separately
as a uniform level which varies rapidly with time (``background'') and as an
illumination pattern which is constant within a given dataset (``sky''). It is
assumed that the ``sky'' pattern is mainly the result of large scale illumination
effects (e.g., moonlight) and of the telescope structure, therefore it is taken to be
constant with the small motions of the telescope which occur within a dataset.
2.2. Iterative Approach
The basic approach to the solution of the calibration problem is successive iter­
ations which improve the quality of the flat, sky, and background calibrations.
This process is implemented in two loops. The outer loop operates on each in­
dividual dataset. It calibrates (via the Basic Calibration Equation) the image
from each exposure, then shifts and averages the exposures for each target. Pix­
els flagged as ``bad'' in the dark or flat field are excluded from this average. The
averaged images from three passbands are then mapped to a common coordinate
system (usually the K band's due to its lower S/N) and summed. This summed
image is then scanned to locate ``sources'' based on a photon statistics model
of the expected sky noise. Finally, all pixels in the original individual images
which correspond to sources in the summed image are flagged.
The inner loop simultaneously utilizes all observations from one night (or
even an entire observing run). Working on one passband at a time, the back­
ground for each exposure, the average flat field, and the sky for each dataset

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(i.e., target) is determined. To avoid the biases inherent in median filtering, a
combination of the exclusion of pixels flagged as containing sources and iterative
sigma clipping is used for each part of this process. The sky is determined from
the residual of the spatially smoothed average of the exposures of each target.
This gives the sky a S/N comparable to that achieved in the flat field (typically
a few hundred samples per pixel) and is justified by the recognition that the sky
illumination pattern is out of focus (i.e., does not contain significant high spatial
frequency structure). The inner loop is executed a user determined number of
times (typically 3 to 5) in order to achieve convergence for most of the pixels.
Pixels with poor solutions may be excluded from further use in the outer loop.
The outer loop is repeated until the image quality is satisfactory. This
usually requires 3 or 4 iterations. The first pass of the outer loop is provided
with a simple ``bootstrap'' flat field solution constructed from a median of the
images in each dataset. The user must select which exposures are to be included
in the inner loop (e.g., those without bright sources and sky exposures) and
which to use to determine the sky illumination pattern for each dataset.
3. Implementation
We start with many J , H , and K images in IRAF files. IRAF tasks are used to
make appropriate dark calibration frames. Also, the relative offsets, rotations,
and scale changes between the J , H , and K passbands are determined with the
IRAF geometry tasks. The MACSQIID package then provides four tasks:
BUILD creates a ``dsf '' file containing all images, calibration files, and mis­
cellaneous data for a dataset. It is driven by a simple ASCII script file. The use
of a script file for control rather than an interactive interface permits painless
re­execution should it become necessary to start over. Non­linearities in the
detectors at high signal levels are corrected for at this stage.
PROCESS provides an interactive task to calibrate and examine images.
This implements the ``outer loop'' portion of the calibration process defined in
x 2.2. The user is presented with a simple menu of one key commands. These
support the basic calibration (including an initial bootstrap calibration using
a median based flat field generated on the fly), geometric mapping between J ,
H , and K channels, the alignment between exposures (star marking and cross
correlation of source pixels), and source detection and flagging. Considerable
statistical information is displayed during the execution of the various calibra­
tion steps. The user has the option to exclude individual exposures from the
combined image. This task also provides for the interactive examination of
calibration data and raw, calibrated, and flagged images using a named pipe
connection to SAOIMAGE.
CALIB updates the background, flat field, and sky calibrations in a large
number of dataset simultaneously. This implements the ``inner loop'' of the
calibration process defined in x 2.2. This batch program is driven by an ASCII
script file. It provides copious reporting and statistics to assess the quality of
its solutions.

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EXTRACT produces IRAF format images from the internal format ``dsf ''
file. This is generally used at the completion of the calibration process to allow
further display and analysis within the IRAF system.
The MACSQIID package uses several programming methods which reflect
the intention to produce a specific rather than a general purpose software pack­
age. The package embeds considerable knowledge about the SQIID instrument.
This is mostly contained in ``include'' files as defined parameters. These serve
to hide details of SQIID from the user (e.g., the noise model ``knows'' about
the system gain factor). The code uses a large data structure (implemented
within fortran common blocks) which places all of the images in system mem­
ory. This supports fast interactive performance to encourage the user to explore
the data and experiment with alternative calibrations. This data structure is
common to each task and directly maps onto the ``dsf '' file format.
Crucial to the goal of keeping the coding effort limited in scope was the
availability and use of existing interfaces to the external environments. The
IRAF ``imfort'' interface was used to access IRAF format images at the start
and end phases of the calibration process. The ``sao­iis'' 1 package by Jim Wright
(formerly at CFHT) provides simple and efficient means of displaying image data
and interacting with the SAOIMAGE display tool.
4. Conclusions
This package provides a means of achieving near­optimal (i.e., background or
photon noise limited) calibration of data from the SQIID camera. The use of
object detection and iterative sigma clipping results in good flat fields at all
spatial scales. The separation of the sky background into a time variable back­
ground component and a target (dataset) variable illumination pattern permits
the recovery of the calibration accuracy inherent in the flat fields. This depends,
in part, on the recognition that the sky illumination pattern is intrinsically out
of focus in the detector plane.
This package was designed to encourage the user to interactively examine
datasets and to understand the quality of the calibration(s) achieved while plac­
ing the computationally intensive steps into batch programs. It simplifies the
data management task by grouping all of the observations and calibration files
for a dataset into a single large file of a custom design yet provides IRAF format
files for further analysis subsequent to the calibration process. The code devel­
opment effort was kept to reasonable levels with the use of existing interface
packages (``imfort'' and ``sao­iis'').
Acknowledgments. The assistance of Mike Merrill and Ian Gatley at the
NOAO in understanding the SQIID camera and its data is gratefully acknowl­
edged. Gina Jones' skill and patience in the testing of this package is also
appreciated. Copies of this package are available from the author.
1 ftp://ftp.cfht.hawaii.edu/pub/sao­iis

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References
Ellis, T. et al. 1992, Proc. SPIE, 1765, 94