WEBT Data Reduction

WEBT campaigns depend on accurate differential photometry with charge coupled device (CCD) cameras using multiple reference stars.

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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 is 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.

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University of Michigan database of CCD images which exhibit problems one might encounter during observing runs.