The width measurement algorithm is model independent and measures the radial enclosed flux profile. It provides a more continuous measure of the light profile, and special techniques are used to provide accurate measures for marginally sampled or even undersampled data. It also accumulates measurements with a possible focus value assigned. The results can then be displayed and interpolated as a function of focus and space, to characterize the widths for either instrumental analysis or simply as an accurate and quick way to make a seeing measurement. A reference to the algorithms and user interface is given below.
The width measurement algorithm, display input, and data accumulation and display functions are packaged in three tasks which are simply specialized versions of the same program. The general task for measuring psf widths of stars in images is called psfmeasure. The options for interpolating focus sequences are not used here. Instead the task starfocus is used for focus sequences which are either sequences of images with different focuses or multiple shifted images at different focuses recorded in a single image. Finally, there is a specialized version of this for Kitt Peak National Observatory that uses information about the focus sequence recorded in the header to greatly automate the focus determination by simply pointing at one stellar image and having the task find, measure, interpolate and display the best focus. This simple script task is called kpnofocus. Except for the last task, the software is quite general and should be valuable at other observatories and for use with any imaging data.
These tasks are available in the external package nmisc (NOAO miscellaneous package) which can be obtained from /iraf/ftp/iraf/extern. Papers describing the algorithms and user interface were published in the ADASS III Proceedings (ASP Conf. Series, Vol. 61), and an electronic version is available from /iraf/ftp/iraf/docs/psfmeasure.ps.Z.
Frank Valdes
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