Canada-France-Hawaii Telescope

A User's Manual for the CFHT Visible Imager: FOCAM

Data Reduction

The basic data processing of direct CCD images is described in the following sequence:

• Computation of average BIAS image.
• Computation of average, normalized FLAT frames.
• Computation of average DARK frames.
• Computation of the master "fringe" frame (depends on CCD)
• Subtraction of the master DARK from the object frames after proper multiplicative scaling depending on the exposure time.
• Subtraction of the master BIAS from the object frames after proper additive correction of each based on the overscan mean value.
• Division by the master FLAT for each object frame (proper filter).
• Subtraction of the master fringe frame after scaling (sometimes division works better).
• Identification and removal of cosmic ray events.

For detailed data reduction procedures we recommend the reading of the following:

These User's guides describe in detail the use of the CCDRED package within the IRAF environment, a very nice and efficient package.

If your "religion" or local software forbids the use of IRAF at your institution, these two manuals contain enough general information and gives enough details on each processing step, that you may find them very useful.

CFHT offers you several workstations for your data reduction. You can reduce data at the summit on a SPARC 10 workstation (WIKI2, 32 Mbytes memory, 100 Mbytes swap space, 1 Gbytes hard disk) using iraf. There is also KAI located in CFHT's office at Hale Pohaku, and numerous stations at our headquarters in Waimea. For data reduction during observation, use the obs accounts (obs1, obs2...obs6). A similar number of data reduction accounts in Waimea (dr1...dr6) are also available. The CFHT computer network is undergoing an upgrade during the summer 1996, and most figures here (computer speed, disk area, etc) are likely to be improved. See your support astronomers for the appropriate account and password. For extended period of data reduction in Waimea, the dr accounts (dr1, dr2... dr4) are available. Please contact your support astronomer or the system manager B. Link for availability of these accounts. If you plan using any special programs or any modifications to the normal CFHT data reduction or observing accounts, contact CFHT staff in advance.