AAS 207th Meeting, 8-12 January 2006
Session 130 The HST Survey of the Orion Nebula Cluster
Poster, Wednesday, 9:20am-6:30pm, January 11, 2006, Exhibit Hall

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[130.04] Large Area Mosaics of the HST Orion Treasury Program

L. E. Bergeron, S. Casertano (STScI)

The HST Orion Treasury Program (10632, P.I. M. Robberto) used three imaging cameras aboard HST to map a large area of the Orion Nebula Cluster. ACS was the primary camera and imaged a contiguous area of ~535 square arcminutes in 5 bands. WFPC2 in parallel covered nearly the same area in 4 additional bands, and NICMOS was used with parallel Field-Offset-Mechanism (FOM) dithering to maximize the area covered (~165 square arcminutes) in two bands, although it was not contiguous.

In order to remove the absolute HST pointing uncertainty (typically 1-2") and mosaic the images, the 2MASS catalog was used as an astrometric reference. Sources were identified in the HST images and then a weighted least-squares solution to the shifts and orientations was determined, internally in overlapping image areas and against the known catalog source positions simultaneously. This resulted in excellent relative registration and absolute World Coordinates tied directly to the 2MASS reference. The new Multidrizzle (Koekemoer et al, 2005) package in STSDAS was used to combine all the data into mosaics with appropriate shifting, weighting and CR/bad-pixel masking. The process is described here.

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