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STARFIT - Fit Gaussian profiles to stellar images

The prime purpose of this program is to do Gaussian fits to stellar images to provide estimates of FWHM etc.

Of course, the stellar images are not Gaussian, but the program was initially conceived as a way of providing a faster and more objective method of finding the best telescope focus after a FOCUS run. The advantage of the fitting program is that all the data in the image are used to estimate the FWHM, not just the particular row or column you happened to select with the Lexidata cursor.

To invoke the program, type STARFIT. The program works similarly to PHOTOM in many respects, being operated exclusively by the Lexidata switches and joystick. The various switches are described below.

EXIT - operating this simply exits from the program.

RESET - clears the overlays and resets the program to its starting condition.

UP - each time this switch is operated the Lexidata cursor increases in size by 2 pixels.

DOWN - the opposite of UP, it decreases the Lexidata cursor by 2 pixels.

READ - reads the current cursor position and performs an operation that depends entirely on the currently selected MODE.

MODE - each time this switch is operated the program switches to a new mode. There are five modes as described below.

1. LIST - identical to LIST in PHOTOM. i.e. it outputs a 9 9 array of pixel values centred on the currently read cursor position.

2. STATS - performs a fully 2-D linearised least-squares Gaussian fit to the section of the image enclosed by the Lexidata cursor box. The output to the terminal is in the form:

   Example

   Point No 1 Pixel coords 279, 314 Counts = 8416

   

   The initial estimates are just those you would get with PHOTOM if you did profiles of and through the peak pixel values within the box. Similarly, the background, magnitude and sky noise are those you would get if you used the STATS option in PHOTOM. The size of the RMS difference in relation to the sky noise gives an indication if how good the fit is.

3. PROFILE - this is the default mode. It is identical to STATS as described above, but, in addition, after doing the fit, plots the best fit Gaussian and the data along the axes through the peak pixel in the selected area.

4. REMOVE - does STATS as described above, but, in addition, after doing the fit, removes the best fit Gaussian and redisplays the resultant image.

5. CLEAN - similar to REMOVE, but, after doing the fit to the selected object, it uses the default FWHM rather than the fitted FWHM at that position when subtracting off the Gaussian. The default values are picked up from the last Gaussian fitted by STATS, PROFILE, or REMOVE. This enables you to subtract off a standard shaped Gaussian from any position.

There are three points that should be noted.

• Both CLEAN and REMOVE are disabled if the file is protected, i.e. you cannot accidentally modify a run that has not been dumped to tape.

• REMOVE and CLEAN modify the image on the disk if it is not protected. If you want to use this option and keep a modified version on disk, then you should make a copy of the image, display the copy and then do a STARFIT on it.

• Consecutive reads of the same position will bypass the Gaussian fitting and just carry out the operations specific to the currently selected mode. BUG: At present, the program thinks it is in the same position if either the or the remain unaltered, so, to repeat STARFIT on images in a row, e.g. after FOCUS, move cursor up and down erratically from one image to the next.