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Hook, R., Durand, D., Simard, L., Schade, D., Koekemoer, A., Corbin, M., & Micol, A. 2003, in ASP Conf. Ser., Vol. 314 Astronomical Data
Analysis Software and Systems XIII, eds. F. Ochsenbein, M. Allen, & D. Egret (San Francisco: ASP), 62
HST/ACS Associations: the Next Step after WFPC2
Richard N. Hook1, Alberto Micol2
Space Telescope European Coordinating Facility,
Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
Daniel Durand, Luc Simard, David Schade
Canadian Astronomy Data Centre,
Herzberg Institute of Astrophysics, National Research Council Canada,
5071 West Saanich Rd., Victoria, BC, V9E 2E7, Canada
Anton M. Koekemoer, Michael Corbin
Space Telescope Science Institute,
3700 San Martin Drive,
Baltimore, MD 21218, USA
Abstract:
After the release of the successful WFPC2 associations, the CADC, ST-ECF and
STScI are now working on joint pipeline software to produce associations of
images from the HST's Advanced Camera for Surveys instrument.
Although the basic approach is very similar to the WFPC2 associations (Durand
et al., 2004) there are some fundamental differences because of the
high level of geometric distortion of the ACS optics. The core of the ACS
association pipeline will perform image combination using the Drizzle method
and hence there will be no need to constrain the position angle of associated
observations as was done with WFPC2. Our goals are the production
of high quality products for the HST archive users and eventual `publication' of
these products within the Virtual Observatory.
Associations are groups of images taken of the same region of the sky and
with compatible instrument modes which can be combined to create a useful
static high-level science data product for access through an archive interface.
The CADC and ST-ECF have already collaborated on the production of
associations of images from the Hubble Wide Field Planetary Camera 2 (WFPC2)
(Micol et al. 2000) and are now working on defining similar products from
the Advanced Camera for Surveys (ACS). This paper outlines how these
associations will be defined, combined and designed for future access
through the Virtual Observatory.
Associations greatly facilitate archive browsing and are intended for
immediate science usage as great care is taken to ensure faithful astrometric
and photometric products. Associations are also uniform and well described
and include supplementary products such as weight maps and appropriate PSF
images.
For ACS the definition of associations is more relaxed than that for
WFPC2 as more sophisticated software is available for the image combination
stage. We require only that observations were made with the same filter and that
they are within a specified radius of each other on the sky (currently 480
arcsecs). They may come from different programs and there are no roll-angle
restrictions. This definition, which may be extended, includes as subsets
the standard STScI associations which are defined on the basic of observation plans
made at Phase II, but also allows for many other, more extensive, data groupings.
A pipeline is being assembled to automate the preparation of ACS
associations. The first step is the construction of the associations
from the observing log. This step also involves the deconstruction of the
STScI associations which group datasets from the same proposal and visit.
Data files for the association are then run through the standard CALACS
pipeline, using the best reference files, and drizzled (Fruchter & Hook
2002) to remove geometrical distortion. Shifts between images are then
determined, either through catalog-based approaches or by using cross
correlation. The cosmic-rays and other defects are then detected and flagged
and the images stacked into clean combined data products, either using
the MultiDrizzle script (Koekemoer et al. 2002) or using the artificial
skepticism method developed by Stetson.
Once a clean combined image has been produced by the pipeline the image
contents will be characterized and source catalogs created. It is also
intended to create appropriate PSFs for objects in the image using Tiny Tim
(Krist 1995) to simulate point objects in the input frames and to
combine them to create appropriate output PSFs by repeating the drizzle
commands with the PSF images.
The final stages of association processing are the saving of all output
products and the ingestion into the CADC storage system and subsequent
publication within the Canadian Virtual Observatory and others.
As of July 2003 there were more than 3000 ACS associations defined,
having a total of more than 22000 members. Table 1 shows the histogram
of how many associations have a certain number of members. A very
complex association is shown graphically in Figure 1. At present the
full processing of a three member association takes about 40 minutes on
a 1.8GHz Linux machine with 2GB of memory. This includes the conversion
of POD files to RAW files and processing through CALACS and MultiDrizzle.
Figure 1:
An Example of a Complex Association of ACS Images. Observations
of fields close to the globular cluster NGC104
 |
Table 1:
ACS Association Statistics
| Images |
Assocs |
|
Images |
Assocs |
|
Images |
Assocs |
|
Images |
Assocs |
|
2 |
1211 |
|
23 |
7 |
|
45 |
2 |
|
86 |
2 |
|
3 |
378 |
|
24 |
18 |
|
46 |
1 |
|
92 |
3 |
|
4 |
378 |
|
25 |
6 |
|
48 |
4 |
|
103 |
1 |
|
5 |
132 |
|
26 |
5 |
|
50 |
6 |
|
116 |
1 |
|
6 |
190 |
|
27 |
6 |
|
51 |
2 |
|
120 |
2 |
|
7 |
46 |
|
28 |
5 |
|
52 |
4 |
|
134 |
1 |
|
8 |
189 |
|
29 |
4 |
|
53 |
1 |
|
140 |
1 |
|
9 |
39 |
|
30 |
7 |
|
54 |
1 |
|
149 |
1 |
|
10 |
77 |
|
31 |
2 |
|
56 |
2 |
|
172 |
1 |
|
11 |
26 |
|
32 |
27 |
|
58 |
1 |
|
211 |
1 |
|
12 |
50 |
|
33 |
4 |
|
63 |
1 |
|
278 |
1 |
|
13 |
12 |
|
34 |
4 |
|
64 |
7 |
|
584 |
1 |
|
14 |
21 |
|
35 |
5 |
|
66 |
1 |
|
|
|
|
15 |
12 |
|
36 |
6 |
|
67 |
1 |
|
|
|
|
16 |
52 |
|
37 |
5 |
|
68 |
1 |
|
|
|
|
17 |
12 |
|
38 |
5 |
|
70 |
2 |
|
|
|
|
18 |
18 |
|
39 |
1 |
|
72 |
2 |
|
|
|
|
19 |
3 |
|
40 |
6 |
|
73 |
1 |
|
|
|
|
20 |
16 |
|
41 |
2 |
|
76 |
1 |
|
|
|
|
21 |
9 |
|
42 |
2 |
|
78 |
1 |
|
|
|
|
22 |
6 |
|
44 |
3 |
|
82 |
1 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Quality assurance is a very important step. Photometry obtained from
catalogs of association data products will be compared to external
catalogs available in published results. Absolute astrometry will be limited to
the precision of available catalogs (e.g., GSC2) but relative astrometry will
be much more precise. Detailed comparisons of the relative merits of
MultiDrizzle and Stetson's `artificial skepticism' methods will be made.
Once the ACS associations are made available, probably in Summer 2004, the
pipeline will allow us to make available deeper and more uniform data products,
offer a faster turn-around time for delivery and make ACS products
available through the Canadian and other virtual observatory access points.
References
Durand, D., et al. 2004, this volume, 209
Fruchter, A.S. & Hook, R.N. 2002, PASP, 114, 144
Koekemoer, A.M., Fruchter A.S., Hook, R.N., & Hack, W. 2002,
``MultiDrizzle: An Integrated Pyraf Script for Registering, Cleaning and Combining Images"
in proceedings of The 2002 HST Calibration Workshop, Baltimore, Maryland, 339
Krist, J. 1995, ``Simulation of HST PSFs using Tiny Tim'',
in ASP Conf. Ser., Vol. 77, Astronomical Data Analysis
Software and Systems IV, ed. R. A. Shaw, H. E. Payne, & J. J. E. Hayes
(San Francisco: ASP), 349
Micol, A., Durand, D., Schade, D., Gaudet, S., Stetson, P.,
Pirenne, B., & Benvenuti, P., Malloci, G. & Raviv, G. 2000, in ASP Conf. Ser., Vol. 216,
Astronomical Data Analysis Software and Systems
IX, ed. N. Manset,
C. Veillet, & D. Crabtree (San Francisco: ASP), 223
Footnotes
- ... Hook1
- Current address: STScI, 3700 San Martin Drive,
Baltimore, MD 21218, USA
- ... Micol2
- Affiliated to the RSSD Division of the European Space
Agency
© Copyright 2004 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: Distributed Data Storage
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