PROFILE: Compute Surface Brightness Profile by Elliptical Isophote Fitting

PROFILE computes the radial surface-brightness profile of an object by fitting it with elliptical contours. The center of the object must first be calculated with the AXES command. PROFILE uses this center as the starting point for its calculations. The profile is found by sampling the image with a set of circles with radius 1, 2, 3, ... pixels. The average value of the pixels along a circle is the mean surface brightness of that contour. Low order sine and cosine transforms are taken along the contour to derive its center, position angle, and ellipticity. After these are found for the entire image in the radius specified, the contours are adjusted to more exactly fit the isophotes. The first iteration usually turns the original circles into ellipses with varying position angles and eccentricities as a function of major-axis length.

A slightly different version of this command written by G. Djorgovski uses a slightly modified interpolation scheme, slightly different median filtering method for computing derivatives, and the inclusion of sky subtraction in the photometry. These features may be enabled by using the GPROF keyword, or may be selectively enabled by using the SKY and PSEUDO keywords.

High-accuracy sinc interpolation is used to find the values of the pixels along the inner 15 contours. This number (15) can be modified with the NFINE= keyword. Outside of this, either a lower accuracy (but faster) interpolation can be used, or an even faster bilinear interpolation scheme. The kind of iteration used is set by the ITER keyword; the first number specifies the number of iterations using bilinear interpolation, and the second the number of iterations using a lower accuracy sinc interpolation. The keywords N=n or RMAX sets the number of contours to be computed, to be n with the N=n keyword, or to the value in the variable RMAX (set by command RMARK) if the RMAX keyword is specified.

The result of the profile calculation is stored in a common block for later use. Print the contents of this block with PRINT, or save it in a disk file with SAVE. The results are also written into the specified spectrum. Use PLOT to show that spectrum.

The option FOUR turns on calculations of the profile which have the terms cos(4*theta) and sin(4*theta), where theta is the position angle in the calculation. Normally these are turned off for speed.

Position angles are calculated assuming that the position angle of the 'top' of the image (as seen on the TV) is 0. This can be changed with the PA keyword. Use PA < 0 to indicate that the image has been reversed right-left (the 'normal' arrangement for an image is north at top, east at left).

The INT keyword allows interactive control of the iterations. The user may change the step size from 1.0 to some fraction, and also solve for contour centers before the next iteration. Full output from each iteration can be shown on the terminal if the keyword TTY is specified.

The output of this program can be redirected.

Example:

See Also: ANNULUS, SECTOR, RECON, EGAL, TVPROF, CLPROF, CPROF, SAVE, GET, SNUC