The application operates in one of three ways:
The mode of operation is defined by the parameters MODE and CURSOR. The settings required for each mode are, respectively:
For the simplest case, files generated by SECTOR, the data to be displayed is brightness (either in the form of magnitudes or relative to sky) versus the distance (transformed using logarithmic, square root or quarter power functions if required) from the galaxy centres. Once this is displayed, you are asked to define (using the keyboard or cursor as appropriate) the part of the profile that is to be employed in calculating the scale length of the galaxy being examined.
The other two applications, ELLPRO amd ELLFOU, generate a great deal more information since they also derive the shape and orientation of the ellipses used to `fit' the galaxy. In addition, they calculate Fourier descriptor values for each isophote out to the fourth order terms. Consequently, they have bigger output files with a slightly different format. The differences in the format are normally unimportant since GRAPHS automatically adjusts to the type of file it is asked to read, but they are described in Section 15. The majority of data stored in the output files can be shown in graphs. The data that may be displayed includes any of the following quantities (specify which using parameter WHATD) versus radius:
Note that the X and Y co-ordinates are always stored in output files in the co-ordinates of the Base frame, not that of the Current frame. The Base (GRID) frame co-ordinates are in units of pixels and always start at (1,1); they remain the same for a given image NDF, whereas Current co-ordinates may change according to which frame is selected as the Current one. The radius value (in arc seconds) used for the graphs may be transformed by:
The transformation required is chosen using the RADISP parameter.
The radius values employed are the semi-major radius size for an isophote when reading an ELLPRO or ELLFOU file, but are the mean distance to the chosen galaxy origin when reading from a file created using SECTOR. The units are arc seconds.
The keyboard only mode of use for GRAPHS asks for a few simple inputs and then reads data from the named text file. It also calculates the scale length (using the radius range requested) and outputs the results to another text file. This automatic mode is particularly useful with ELLPRO and ELLFOU output files that can contain the profiling results for hundreds of galaxies. The output file contains the image co-ordinates read from the ELLPRO/ELLFOU file and then columns containing the scale length (both for spiral and elliptical models) and the resultant extrapolated central surface brightness values.
One useful consequence of the way GRAPHS works is that output files from SECTOR, ELLPRO and ELLFOU may be concatenated as required so that analysis of several output file can be performed in one go.
the example below shows how to obtain a graph of position angle versus linear radius for the profile conatine in text file RESULTS:
% graphs mode=true infile=result whatd=p radisp=r cursor=true
ESP --- Extended Surface Photometry