WEBT Data Reduction
WEBT campaigns depend on accurate differential photometry
with charge coupled device (CCD) cameras using multiple reference stars.
Step 1 - Reduced Frames
Each CCD exposure (or frame) from the telescope is saved
digitally. Later, the observer corrects for the CCD read-out bias, and
the dark current (if needed). The resulting frame is divided by a flat-field
frame to correct for spatially varying response. The resulting frame is
a reduced frame. Cosmic-ray strikes and other defects are usually not corrected
for blazar monitoring.
Step 2 - Instrumental Magnitudes
Photometry is done on reduced frames to produce instrumental
magnitudes for the program object (blazar) and reference stars. Image Reduction
and Analysis Facility (IRAF) is one
means to obtain instrumental magnitudes. CCDPHOT
is also used.
Our format for instrumental magnitudes is evolving. Here
our initial format for instrumental
magnitudes. This format will be replaced with the W2000 instrumental
magnitude format. Here is the our
current draft of the W2000 instrumental magnitude format.
Step 3 - Standard magnitudes and errors
For each frame, an estimate of the standard magnitude, and
the 1 sigma error of the estimate is generated. The color response of the
filter and CCD is used for this in conjunction with magnitudes of the reference
stars in appropriate filter bands, the air mass, and other extinction information.
Often, an explicit color correction is not required. Here is a
draft of the WEBT format for standard magnitudes.
University of Michigan database of CCD images which exhibit problems one
might encounter during observing runs.