Survey Strategy


Each brick is observed as two 3x3 “half bricks”, with observations taken ~6 months apart.  In each observing season, a 3x3 half brick of WFC3 pointings is completed in primary, while parallel observations produce a highly overlapping 3x3 tile of ACS observations in the adjacent half brick.  After 6 months, the telescope is rotated by 180 degrees from the original orientation, such that the primary WFC3 pointings cover the area that was tiled by ACS in the first season, and vice versa.  Each pointing is observed for 2 orbits, using the 2 WFC3 cameras for one orbit each.  Pointings are dithered to produce Nyquist sampling in the optical and NIR images, and to fill the chip gap in UVIS.

“Bricks” and “Fields”

PHAT observations are grouped into 23 “bricks”.  Each brick consists of a 3x6 array of HST pointings with complete coverage in the UV, optical, and NIR.  Each brick is assigned a unique number (B??), and is listed under a distinct proposal ID.  Odd numbered bricks extend from the bulge (B01) along the major axis out to the edge of the star forming disk (B23).  Even numbers form an adjacent strip that covers the minor axis, the star forming ring, and much of the outer spiral arm.





Astrometrically aligned photometric catalogs and interactive image explorers are available on the Multimission Archive at Space Telescope (MAST), through the following link: Click here for data

Data Products

The optical and NIR data in PHAT is “crowding limited”, meaning that the limiting brightness of detected stars is set by the magnitude where the local stellar density reaches the maximum number of stars that can be individually resolved in a given camera.  As a result, the depth of the photometric catalogs is a strong function of radius within the galaxy, varying by >5 magnitudes from the high density bulge to the diffuse outer disk in the redder filters.

Photometric Depth

The PHAT survey has observed the northeast quadrant of M31, which shows the least extinction and has the least contamination from M32.   The survey covers the bulge, the inner bar, the main star forming ring at 10 kpc, and the more diffuse warped outer arm.  Details of the survey design can be found in Dalcanton et al 2012.

Within each brick, each of the 18 pointing centers is assigned a field number (F??), starting with “01” in the north east corner, and increasing to “18” in the south west.  The field tiling is set by the WFC3/IR footprint, and observations in other cameras are labelled with the field number of the WFC3/IR field they most closely overlap. 

Stellar Photometry

The primary output of PHAT is 6-filter photometry for all stars in the survey footprint. The photometry is generated by forward modeling the position and flux of every detectable star in all overlapping images, after astrometrically aligning the images to better than 10 milliarcseconds.  The latest data release (Williams et al 2014) includes photometric measurements for approximately 117 million stars.


PHAT takes images in 6 different filters, 2 per HST camera.  The UV is covered by F275W and F336W with WFC3/UVIS, providing excellent leverage on extinction and on stellar temperature for the hottest stars.  The optical is covered by F475W and F814 with ACS/WFC, which offers the best depth and temperature separation for typical stars. The NIR is covered by F110W and F160 with WFC3/IR, spanning wavelengths that are least affected by dust, and that are dominated by the coolest stars.