Stellar Photometry

VISTA has a package of programs designed to measure stellar brightnesses on CCD pictures. Many of these are taken from the DAOPHOT stellar photometry package originally written by Peter Stetson. Some additional routines are included for interaction with the video display and some modifications have been made to the DAOPHOT routines. The various DAOPHOT programs estimate the total counts in each star on a frame - no attempt is made to convert these counts to magnitudes on some standard system. Some additional routines are included to help with this transformation.

The version of DAOPHOT which is included in VISTA is an adaptation of Stetson's 1987 version of the code. Subsequently, Stetson has made modifications to the standard distribution of DAOPHOT which are NOT included in the VISTA package. In addition, some modification of the original DAOPHOT code was necessary to make it work inside of VISTA. Several new options were added as well by Jon Holtzman. Because of these facts, Stetson can NOT be help responsible for any results that come out of the VISTA/DAOPHOT routines. From extensive use of the code, we feel the routines are reliable, but this cannot be absolutely guaranteed. We are very grateful to Peter Stetson for allowing us to incorporate his code.

If you wish to use a Stetson's up-to-date version of DAOPHOT, contact him for the source code, and simply use VISTA to write out the data in some format which the standalone DAOPHOT can read.

Any questions about the VISTA/DAOPHOT routines should be addressed to the VISTA distributors and not to Stetson. Publications using the VISTA/DAOPHOT routines should give credit both to VISTA and to Stetson but remember, he cannot be help responsible for the results.

The inclusion of DAOPHOT is new to VISTA Version 4. Some of the old VISTA commands have been discarded because they are less accurate and rather difficult to make portable. Gone are FITSTAR, PSF, and BFITSTAR, the old VISTA routines used for point spread function fitting (although note there is now a new command FITSTAR which does something entirely different, namely fits for extinction and transformation coefficients given observed and standard photometry).

The DAOPHOT routines are discussed below (or CTRL-C out of this and type HELP DAOPHOT if you are on a terminal and want to skip straight to the DAOPHOT information). Generally, extensive information on the DAOPHOT routines is NOT given in this manual. Consult the DAOPHOT manual for more details.

Besides the DAOPHOT routines, VISTA has two routines to find stellar objects in a given field, MARKSTAR and AUTOMARK. MARKSTAR is extremely useful, as it allows the user to interactively enter stars, display stars in a photometry list, and access information about a star specified with the cursor on the display. Positions are found by interactively marking stars. The stored position is computed by centroiding around the cursor location. AUTOMARK finds objects automatically in a similar way, without any help from the user. It requires a range of peak values as input which is uses to decide what are stars.

Aperture brightnesses in circular apertures can be measured quickly using APERSTAR, which total counts in an aperture of specified size around all stars on a photometry list. More sophisticated aperture photometry is available in DAOPHOT using the PHOTOMETRY command.

VISTA includes several commands for the processing of instrumental magnitudes: COMBINE lets one combine lists of stars from several frames, MAGAVER computes the weighted mean and other statistics from output from COMBINE, and FITSTAR allows one to get extinction and transformation coefficients. The routines REGISTER and OFFSET allow one to determine the linear plate coefficients from the positions of stars on different frames and to correct the DAOPHOT files for these plate transformations.

Most of the programs use a photometry file. This is a series of records which contain information about the location, brightness, and type of observation for each star. There is one record per star. The photometry file is not currently used by the DAOPHOT routines.

The VISTA photometry file is stored internally in the VISTA program as a common block. It is NOT automatically written to the disk. You have to save the results you make with the photometry routines using the SAVE command. Similarly, you can connect a photometry file to the program with the GET command. The photometry files are stored in the data directory (See PRINT DIRECTORIES) with the extension .PHO, unless you specify otherwise (DAOPHOT files have default extension .COO). If you are using the DAOPHOT routines, all results are stored in disk files just as in standalone DAOPHOT.

Some additional commands:

The operation and limitations of each commands is explained in the section of the help file for that command.