Database Description

What is being released

The CATS team has collected Keck Adaptive Optics (AO) images of over 300 distant galaxies. These images are primarily in fields with existing multi-wavelength data from HST, Spitzer, and Chandra. The early observations were obtained with the Keck Natural Guide Star (NGS) facility, and focused on the GEMS and COSMOS galaxy fields. More recently we have used the Keck Laser Guide Star (LGS) facility to image galaxies in GOOD-S, GOODS-N, and EGS.

In this data release, we are providing basic information on all of the CATS observations. Reduced Fits images of the NGS observations are available for download. We are also providing one data set in raw form for researchers to examine. Finally we are providing a library of NGS point-spread-functions (PSFs) to help constrain photometry. More discussion of the AO PSF is provided below.

This is an initial data release. A second release is planned for the summer and will contain FITS images of the LGS data.

CATS Observations

The CATS fields were observed with the "wide" field of NIRC2 (0.04" per pixel) and cover a total of 9.4 square arcmin . The K'-band (2.12 microns) was used for all observations. Observations on each field were divided into individual frames having four 30-second coadds, in order to avoid saturating stars and bright galaxies in the fields. The NGS guide star was always positioned in the same physical corner of the NIRC2 detector to minimize detector persistence effects and maximize coverage of the AO-corrected field. In addition, a 30-position, 7"-diameter circular dither pattern was used to help identify persistent stellar images sometimes present in the reduced data.

CATS Data Reduction

The images were reduced using an automated pipeline developed for the Center for Adaptive Optics Treasury Survey (CATS) (eg. Melbourne et al., 2005). Standard infrared data reduction procedures were used, but special attention is given to sky subtraction and the final co-addition of frames to preserve small, low-surface brightness objects. A supersky frame was created by median-combining all frames of a field from one night. Before combining, deviant pixels and astronomical objects (galaxies and stars) were marked and omitted from the supersky. The supersky was scaled to match each raw frame and then subtracted. Since our fields are mostly blank sky and have sufficient sky flux, the normalized supersky was also used to flat field the sky-subtracted frames. Additional deviant pixels found in the sky-subtracted and flat-fielded frames were added to those previously detected and all were ignored for the remainder of the reduction process. The frames were then warped with a conic polynomial to correct for geometric distortion introduced by the NIRC2 optics (Thompson, Egami, & Sawicki, 2001).

Individual (dithered) frames were aligned using a two-step process. First, alignments were estimated using AO information in the FITS headers. Next, the estimates were used to find one or more manually specified object(s) in all frames, and centroiding was done to determine final alignments.

In preparation for the final combining, any remaining sky offsets between individual frames were subtracted out. The individual frames were shifted into alignment and then combined using a clipped-mean. Deviant pixels and pixels with persistence were not included in the clipped-mean. A weight map (inverse variance) was also produced for each final image. Instrumental zero point magnitudes were determined on most nights from photometric standard stars and used to convert final images to flux units (micro-Jy). World Coordinate System (WCS) information was determined using each frame's guide star. The automated pipeline produces a single product for all individual frames from a particular epoch and guide star offset location.

Point-Spread-Function

Because the atmospheric conditions and AO correction are constantly changing, the AO PSF is time-variable. It also varies spatially with distance and position angle from the guide star. As a result it is dificult to know the PSF at any given location in the AO image. For the NGS data we are providing a library of possible PSFs. The PSFs are NGS images of stars taken over the course of the CATS program. We suggest that a simultanious fit to both a galaxy model and PSF will be valuable for doing photometry of galaxy sub-components.

For the LGS data we were able to better track the realtime AO PSF. We obtained short, un-saturated images of the tip-tilt guide star before each science exposure to track the real-time PSF in one position on the image. After our science exposures, we imaged a star cluster to track the spatial variation across the image. In addition TMT was using a MASS atmospheric profilier during the course of many of the LGS observations. The MASS profiles are a useful tool in determining the atmospheric variability thoughout each night. As a result, the LGS data release will have better documented PSFs.

AO Workshop

Maintained by Dr. Jason Melbourne (Caltech)

Last update May, 2008