20.1 AIPS

AIPS, NRAO's Astronomical Image Processing System, is a set of programs for the analysis of continuum and line observations, and is widely used with VLBA and VLBI data. These programs are available for a wide range of computer operating systems, including various flavors of Linux and the Mac-OS/X operating system. Sections 24.1 and 24.3 give contact information. Extensive online internal documentation can be accessed within AIPS. An entire chapter in the AIPS Cookbook (NRAO staff, 2007) provides useful ``how-to'' guidance for those reducing VLBI data, including discussion of VLBA calibration transfer, space VLBI, polarimetry, and phase referencing. Appendix C of the AIPS Cookbook provides a step-by-step guide to calibrating many types of VLBA data sets in AIPS, employing simple VLBA utilities, including calibration modifications for VLBA+VLA datasets. A new ``frozen'' version of AIPS (currently 31DEC08) is produced each year, and a newer version (currently 31DEC09) is updated and made available throughout the calendar year. Observers are encouraged to use a very recent version of AIPS, since only these can properly handle data from the DiFX software correlator, and new capabilities, such as simplified data-reduction procedures and improved astrometric calibration, are implemented frequently.