The Swift UVOT images are multi-extension FITS files and should be readable
with most FITS readers. The header of each image extension contains the
The Swift Ultraviolet/Optical Telescope data available directly through MAST
consists of several types of files. In the descriptions below [obsid]
refers to the 11-digit observation ID, and [filter] refers to the
filter acronym defined as
wh, uu, bb, vv, w1, w2 or m2 for
the white, u, b, v, uvw1, uvw2 or uvm2 filter
respectively; or, for the 2 grism filters:
gv for vgrism and gu for ugrism. Note not all files
File names are in format: sw[obsid]u[filter]_ex.img.gz
These are level 2 files
describing exposure times per pixel for each
observation and stored as FITS files with one or more image extensions.
These files are generally not produced for grism images which use a clocked
observing mode. (Note MAST uses these files for calculating footprints.)
Names have format: sw[obsid]u[filter]_rw.img.gz These are level 1 images
in raw units (pixels), Fnd stored as FITS files with one or more image extensions.
Sky coordinate Images:
File name have format: sw[obsid]u[filter]_sk.img.gz These are level 2 images
in same format as Raw Images but with aded sky coordinates (RA/Dec).
Most users would probably start with these image files.
It should be noted that small positional shifts may occur between exposures.
Deliberate shifts are done to avoid a hot column on the XRT detector so
that repeated observations of targets do not create artifacts in the BAT
There is also a 0-2' uncertainty in pointing the
spacecraft as a result of it fast slewing capability.
The listed pointing RA and Dec values however do not reflect these
changes between exposures.
There are also a few cases (~20) where the same exposure appears in
2 extensions. These are believed due to garbled telemetry.
sw[obsid]u.cat.gz A level 3 catalog of detected sources (sorted by filter)
stored as a FITS file with one or more binary table extensions. Each extension contains
extracted source information (e.g., position, magnitude, flux, et)
for one filter.
Co-added Exposure Map files:
sw[obsid]u_ex.img.gz coadded exposure maps (all filters). Level 3 FITS file
with one or more image extensions, one for each coadded image for a particular filter.
Co-added Sky Coordinate Images:
sw[obsid]u_sk.img coadded sky images (all filters). Level 3 FITS file is
with one or more image extensions containing coadded images for each filter used.
Co-added Sky Coordinate GIF Images:
Names are in format: sw[obsid]u[filter]skim.gif a level 3 co-added gif sky image .
Note also that a source catalog is currently being produced and will be made available