The keywords EXPRTIME and ZEROMAGN set the zero point for magnitudes. An independent calibration is necessary for adjusting them properly.
The keywords STMETRIC and ETAFUNCT should be set whenever one wants to use the corresponding aperture or Petrosian magnitudes.
Handling the keywords SPROFIL1 -- SPROFIL5 and PRFLCTRL requires some care. The program automatically determines a Point Spread Function when it gets proper initial values from the keywords SPROFIL. Initial values may be wrong by 0.1 each, and the program still can converge to the right Point Spread Function. In the case of rich stellar fields the automatic Point Spread Function determination usually gives excellent results even with drastically wrong initial values. However in frames which contain more galaxies than stars, the automatic procedure tends to return an average profile of galaxies rather than the Point Spread Function. The same may happen when the initial values are too small for fields with a moderate number of galaxies.
In order to check whether the Point Spread Function is correct, it is possible to use the MIDAS PLOT/TABLE commands to plot the dependence of the relative gradient on isophotal magnitude. When the Point Spread Function is correct, the stars will cluster around a (relative gradient = 0) line. If not, then the stellar sequence is shifted, usually upwards, and most often it is not longer linear. The value of the average shift of the stellar sequence should then be added to the first few points of obtained Point Spread Function, which is displayed on the terminal screen and written into the image frame descriptor DPROFILE.
If there are difficulties in finding the initial Point Spread Function one can use the manual mode for choosing objects to determine the Point Spread Function. The keyword PRFLCTRL should then be set to a negative value, and an input frame should contain the descriptor STARS with standard star coordinates. The descriptor STARS with positions for up to 200 stars is produced as follows:
Get an input image frame onto the image display screen.
off, 
on.
.
once more. Now the frame is ready for the ANALYSE/INV command
with keyword PRFLCTRL set to a negative value.
When selecting stars with the cursor, you should also use very bright saturated stars. The program can handle them properly, and they are very useful in extending the range of the Point Spread Function determination. However, in the case of CCD--frames the program does not yet know how to deal with saturated vertical columns that spread out from the central regions of bright stars. Therefore, in the case of CCD--frames, only stars without saturated vertical columns should be used.
Note No method of determining the Point Spread Function can be successful when the data has not been properly transformed into intensity units.
The used point spread function is written as a descriptor DPROFILE into the input image frame. This descriptor is modified whenever ANALYSE/INV is run with keyword PRFLCTRL not equal to zero. The program looks for an initial point spread function into that descriptor at first and it uses data from the keywords SPROFILx when the descriptor DPROFILE is absent.