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Fv is a stand alone program developed at the HEASARC for viewing and editing any FITS format data file. It provides a graphical user interface (written in Tcl/Tk) for editing FITS header keywords and for viewing or editing the data in FITS images and tables. (Some sample windows from an fv session are shown on the cover of this issue.) When fv first starts up it displays a file selection window that lists all the FITS files in the current directory. Compressed FITS files (gzip or Unix compress format) can also be directly read and viewed by fv. The user may browse different directories to find the desired file. Once a file has been selected a summary window appears that lists information about all the Header-Data Units (HDUs, also known as extensions) within that FITS file. Individual HDUs may be deleted from a file, or HDUs may be copied from one FITS file to another via an internal 'cut and paste' clipboard. When editing multiple HDUs or files, the user may end up with many different windows opened simultaneously on the desktop. To help manage all these windows, there is a central fv "window manager" which can be used to temporarily hide different windows or to return a particular window to the top of the desktop. There are also options in fv to control whether fv should open a new window whenever a new file is edited or to reuse the same window from the previously opened file. The FITS header keywords in any HDU may be edited in a scrolling text window. The user can search for a particular string of characters in the header, and keywords may be edited, inserted, or deleted. An output ASCII text file listing of the header may also be generated. Selected columns in a FITS table may be viewed and edited in a spreadsheet-type window. This window can be used to display vector columns and variable-length array columns as well as simple scalar columns. The rows can be sorted on one or more columns of the table and the statistics of the values (min, max, mean, standard deviation) in any column may be displayed. The individual values in any cell of the table may be edited, and rows and columns may be inserted or deleted. The user may specify conditional expressions like "(columnA > columnB) && (columnC = 0)" to delete only those rows where the expression is true. Selected rows or columns of data may be copied to the internal clipboard and then copied to other locations in the same or other FITS tables. In all these functions, null or undefined table elements are properly accounted for. The table calculator function may be used to create a new column or overwrite the value in an existing column using an arithmetic expression which may involve the values in other columns of the table, as in "ColumnA = sqrt (ColumnB * ColumnC + 100.)". A column histogramming function is also available for creating 1-D or 2-D FITS arrays by binning the values in 1 or 2 columns of a table. This can be used, for example, to create a 2-D image from the 'X' and 'Y' columns in an event list type table. This array may then be saved to a new FITS file and/or displayed with POW (see below). Sophisticated line graphics and image display functions are provided by the POW plotting tool which is integrated with fv and is also available separately for use by other applications. Multiple plots and images may be displayed simultaneously, including plots showing cuts in intensity along any row or column of an image, plots of the values in any column of a table versus row number, and plots of one column versus another column. Users may interactively change the size of the plot and zoom in on any selected region of interest. There are also numerous options for changing the plotting style and for adding annotations. The plots may be saved and printed in postscript format. FITS 2D images may be displayed in POW with all the usual pan, zoom, and color table manipulation facilities. Blinking between 2 or more images is supported as well as a 'movie' mode for cycling through the planes in a 3-D data cube. POW supports the standard FITS World Coordinate System (WCS) keywords in astronomical images, and will display the R.A. and Dec coordinate of the cursor position. POW will optionally display a grid of R.A. and Dec contour lines on top of the POW can also be used to generate or edit 'regions files' which define various geometric areas (circle, ellipse, rectangle, polygon, etc.) within an image which are typically used to mark the locations of particular stars, galaxies or blank sky areas. The cursor is used to draw new elements and to reposition and resize existing elements in the region file. In the future we plan to continue to maintain fv and add new enhancements as time and resources permit. One particularly useful new feature which should be available by the end of 1998 will enable fv to directly open and view any FITS file over the network, simply by entering the ftp or http URL to the file. We also hope to add more data analysis type functions to fv now that most of the basic file editing functions have been implemented. Fv currently runs on most Unix platforms, including SUN O/S, SUN Solaris, Dec OSF, HP-UX, and Linux PCs. It will be ported to run under Windows NT/95/98 and Mac O/S just as soon as one of the underlying Tcl/Tk libraries (iTcl 3.0) is released for those platforms, hopefully by mid-1998. The fv home page on the Web is located at: http://heasarc.gsfc.nasa.gov/ftools/fv.htmlfrom which one can see examples of some of the fv capabilities and download the latest version of the software. Any questions about fv should be sent to: ftoolshelp@athena.gsfc.nasa.gov.
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