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WFC3 Observed PSF Search through MAST

To facilitate users in building WFC3 PSFs appropriate for their observations, we are making a full dataset of WFC3 observed PSFs available. The database contains more than 30 million PSFs, and includes all WFC3 observations taken from 2009 to 2020. Users can use these PSFs to generate their own PSFs appropriate for their specific observations (filter/detector/focus-value etc.) or to build delta-PSFs, which can be use in conjunction with the library PSFs. These resulting PSFs are expected to be more representative of their observations, and hence may help in accurate photometry/astrometry. All supplemental details are available in ISR 2021-12 (Dauphin et al.).


The search parameters are briefly described below.

Parameter Description
ID Unique ID for identifying the PSF within this particular image. The downloaded file will include this ID in the file name.
Focus Model focus value taken from the webpages The model takes the breathing of the HST within the orbit into account. See the above links for more details about how the focus model.
Filter Name of the filter used.
PSF X Center PSF X center coordinate.
PSF Y Center PSF X center coordinate.
PSF Flux Total flux within the PSF (in electrons for UVIS and electrons/second for IR).
qfit Quality of the PSF fit. Quality worsens as the qfit parameter increases (e.g. 0 is a better fit than 1).
Exposure Time Exposure time of the observation in seconds.
Aperture The active region of the detector to be read out (e.g. UVIS1, UVIS2; see the WFC3 Instrument Handbook for details on apertures).
Rootname Unique image name (see naming conventions in Section 2.2 of the HST Data Handbook).

CCD: Chip 1 or Chip 2 (UVIS only).

Note: Chip 1 corresponds to extension 4, or [sci,2], and Chip 2 corresponds to extension 1, or [sci,1].

Sky Sky background flux per pixel around the PSF (in electrons for UVIS and electrons/second for IR).
Pixc Central pixel flux (in electrons for UVIS and electrons/second for IR).
Midexp Middle exposure time in MJD.
RA_TARG Angle of right ascension of the observed target in degrees.
DEC_TARG Angle of declination of the observed target in degrees.
SUN Angle Angle of the target with respect to the sun in degrees.
FGS lock Status of the FGS lock during the observations: Fine, gyros, or a combination of both.
Shutter position A or B (B has extra jitter in the first 0.3 sec).


Once satisfied with the advanced searching parameters, click "Search". The PSFs can be visually inspected by clicking the “Show Cutout”, or by clicking "Album View" and "Cutout". The download option allows users to download the raw, and the calibrated (flt and flc) files.

More details on the procedures for building PSFs and delta-PSFs will be provided in the future.

Last Updated: 06/02/2023


Please Contact the HST Help Desk with any Questions