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<H2><A NAME="SECTION00736000000000000000">
Helpful Hints</A>
</H2>
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 <TT>PLOT/TABLE commands</TT> 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.
<UL>
<LI>Set cursor box: cursors on,
<!-- MATH: $\fbox{\tt Track}$ -->
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off,
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on.
<LI>Type: <TT>GET/CUR STARS/DESCR</TT>. The cursor appears on the screen.
<LI>Point the cursor to the selected star and press
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<LI>After recording all standard stars, set cursors off and press button
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once more. Now the frame is ready for the <TT>ANALYSE/INV</TT> command
with keyword PRFLCTRL set to a negative value.
</UL>
<P>
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.
<BLOCKQUOTE>
<DIV ALIGN="CENTER">
<B>Note</DIV>
<I>No method of determining the Point Spread Function can be
successful when the data has not been properly transformed into intensity
units.</I></B></BLOCKQUOTE>
<P>
The used point spread function is written as a descriptor DPROFILE into the
input image frame. This descriptor is modified whenever <TT>ANALYSE/INV</TT> 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.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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