WFC3 UVIS 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 20 million PSFs, and include all WFC3/UVIS observations taken during 2009 to 2015. Users can use these PSFs to generate their own PSFs appropriate for their specific observations (filter/detector/focus-value etc.). Or they can use these PSFs to build delta-PSFs which they can 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.


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 Central X value of the PSF.
PSF Y Center Central Y value of the PSF
PSF Flux Total flux (in electrons) within the PSF.
qfit Quality of the PSF fit. Quality worsens as the qfit parameter increases: 0 is good, 1 is bad.
Exposure Time in seconds
Aperture UVIS1, UVIS2, etc (see handbook for details of the apertures)
Rootname Root name of the image
Chip CCD chip: Chip 1 or Chip 2 (Chip 1 corresponds to extension 4, or [sci,2], and Chip 2 corresponds to extension 1, or [sci,1]
Sky Sky level per pixel in electrons around the pSF
Pixc Value of the flux in the central pixel
Midexp Time of the middle of the exposure in MJD
RA_TARG RA of the observed target
DEC_TARG DEc of the observed target
SUN Angle Angle of the target with respect to the sun in degrees
FGS lock Status of the FGS lock during the observations: Fine, or combination of fine and gyros, or gyros.
Shutter position A or B (B has extra jitter in the first 0.3 sec)


The PSFs can be visually inspected by clicking the “album” button. 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 later.

LAST UPDATED: 04/21/2020

Please Contact the HST Help Desk with any Questions