1-Dimensional Images (``Spectra'')

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, not to any extremity.

CREATE/SPC1

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.

CREATE/SPC2

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.)

Two options are available for inputting these numbers: either the keyword RIPVALS is defined and initialised by the user prior to executing CREATE/SPC2; or by default a period of 10 is assumed, together with the values 4, 5, 6, 3, 2, 7, 1, 4, 5, 0.

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 mod(period,20), respectively, is employed.

Three additional parameters are user-specifiable: the slope of the wave pattern generated (default 0.0); the phase of the pattern (an integer number n $\leq$ period meaning that the first pixel of the image will have the flux of the nth element of RIPVAL; defaulted to 1); and the dimension of the 1-dimensional image (default 128 pixels).

CREATE/SPC3

produces an artificial spectrum with lines whose location are defined by a MIDAS table. The table LINES (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).

The table LINES.TBL 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.

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).

Should a line extend beyond the boundary of any of the 220-wide segments, then it is truncated at the boundary.

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 peak value of the PSF 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 1/100^th 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.