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<H1><A NAME="SECTION001430000000000000000">
1-Dimensional Images (``Spectra'')</A>
</H1>
As with the previous commands, the following are implementable within
the GEOTEST context. Again, all frames are of STEP 1 and START 1; and
the coordinates of a pixel pertain to its center, <EM>not</EM> to any
extremity.
<P>
<DL>
<DT><STRONG>CREATE/SPC1</STRONG>
<DD>generates a 1-dimensional image with a sinusoidal
pattern of mean flux 100 units and with an optional slope. (This is
analogous to the WAVE 2-dimensional case.) Input parameters are: slope
(default = 0.0), period of sinewave (default is 8.5 pixel units),
amplitude of sinewave (default, half peak-to-peak, is 10.0), and the
phase of sinewave (default is 0.0). Additionally, the number of
pixels in the artificial image can be specified, and is defaulted to
128.
<P>
<DT><STRONG>CREATE/SPC2</STRONG>
<DD>produces a 1-dimensional image with an
arbitrary periodic pattern whose period (an integer number between 1
and 20, both included) and the flux values of each pixel can be
specified by the user. (Thus, for a user-specified period
of 13, 13 values are obtained which define the 1-dimensional image
pixel values.)
<P>
Two options are available for inputting these numbers:
either the keyword <TT>RIPVALS</TT> is defined and initialised by the
user prior to executing <TT>CREATE/SPC2</TT>; or by default a period of
10 is assumed, together with the values 4, 5, 6, 3, 2, 7, 1, 4, 5, 0.
<P>
If a negative period or a period of value larger than 20 is
specified, a message indicates to the user that a period of 10 or a
period of value <I>mod</I>(period,20), respectively, is employed.
<P>
Three additional parameters are user-specifiable: the slope
of the wave pattern generated (default 0.0); the phase of the pattern
(an integer number <I>n</I> <IMG
WIDTH="46" HEIGHT="70" ALIGN="MIDDLE" BORDER="0"
SRC="img418.gif"
ALT="$\leq$">
period meaning that the first pixel
of the image will have the flux of the <I>n</I>th element of <TT>RIPVAL</TT>;
defaulted to 1); and the dimension of the 1-dimensional image
(default 128 pixels).
<P>
<DT><STRONG>CREATE/SPC3</STRONG>
<DD>produces an artificial spectrum with lines whose
location are defined by a MIDAS table. The table <TT>LINES</TT>
(used as default) provides an example of approximately randomly
distributed lines with some relatively broad gaps and also some close
pairs or groups. Note that the locations must be given as integer
numbers between 1 and 220 (both included).
<P>
The table <TT>LINES.TBL</TT> must be copied into one's own
directory for use. It is available in the MIDAS directory containing
the source code of the GEOTEST context.
<P>
The spectrum generated consists of 660 pixels and repeats the
pattern defined by the line positions three times. In the first 220
pixels, the line locations are used as read from the table. In the
second 220 pixels, the lines are relocated on the edges of pixels (by
subtracting 0.5 pixel units from the initial line positions). In the
third set of 220 pixels, a user-specifiable offset is added in order
to reposition the lines relative to pixel centers (the default is
0.645).
<P>
Should a line extend beyond the boundary of any of the
220-wide segments, then it is truncated at the boundary.
<P>
Two types of point spread functions (PSF) are available, namely
boxes (default) and Gaussians. The full width of a box (default 1.0)
or the full width at half maximum (FWHM) of a Gaussian (default 1.0)
are user suppliable. If, for a box-like PSF, the width given is less
than 0.001, then a delta function is used instead. For Gaussians, a
FWHM below 0.2 pixel is not acceptable. In all cases, the <EM>peak
value of the PSF</EM> is normalised to 1 unit. (Note that this implies
that the actual appearance of the PSF may be quite different due to
the effects of discretisation due to a finite step size.) Gaussians
are evaluated every
<!-- MATH: ${1/100^{th}}$ -->
1/100<SUP><I>th</I></SUP> of a pixel step size, (this is
necessary in the case when most of the Gaussian is within a pixel),
and truncated at 5 standard deviations from the mean.
</DL>
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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