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Astronomical Data Analysis Software and Systems XIII ASP Conference Series, Vol. 314, 2004 F. Ochsenbein, M. Al len, and D. Egret, eds.

HST/ACS Asso ciations: the Next Step after WFPC2
Richard N. Hook1 , Alberto Micol
2

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.

1.

Introduction

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

1 2

Current address: STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA Affiliated to the RSSD Division of the European Space Agency

62 c Copyright 2004 Astronomical Society of the Pacific. All rights reserved.


HST/ACS Associations: the Next Step after WFPC2 will be defined, combined and designed for future access through the Vir Observatory. Associations greatly facilitate archive browsing and are intended for mediate science usage as great care is taken to ensure faithful astrometric photometric products. Associations are also uniform and well described include supplementary products such as weight maps and appropriate PSF ages. 2. Definition of Associations

63 tual imand and i m-

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. 3. Pipeline Processing

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 ob jects in the image using Tiny Tim (Krist 1995) to simulate point ob jects 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. 4. Statistics

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


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Hook, Durand, Simard, Schade, Koekemoer, Micol & Corbin Figure 1. An Example of a Complex Association of ACS Images. Observations of fields close to the globular cluster NGC104

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.

5.

Quality Assessment

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.

6.

Conclusions

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.


HST/ACS Associations: the Next Step after WFPC2 Table 1.
Images 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

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ACS Association Statistics
Images 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 44 Asso cs 7 18 6 5 6 5 4 7 2 27 4 4 5 6 5 5 1 6 2 2 3 Images 45 46 48 50 51 52 53 54 56 58 63 64 66 67 68 70 72 73 76 78 82 Asso cs 2 1 4 6 2 4 1 1 2 1 1 7 1 1 1 2 2 1 1 1 1 Images 86 92 103 116 120 134 140 149 172 211 278 584 Assocs 2 3 1 1 2 1 1 1 1 1 1 1

Asso cs 1211 378 378 132 190 46 189 39 77 26 50 12 21 12 52 12 18 3 16 9 6

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, ADASS 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, ADASS IX, ed. N. Manset, C. Veillet, & D. Crabtree (San Francisco: ASP), 223