As a first step in the handling of a Photon Event Table it is usually quite useful to produce a sky image from the photon event list. This operation provides a first quick look at the quality of the available data, and it makes it easy to identify interesting -- and uninteresting -- sources by eye. In addition, to load such a ``reference image'' into the image-display window allows the usage of the graphic cursor in defining the sky portions to be selected for further processing, which is much easier (though less accurate) than the definition of selections via a set of numerical parameters (the actual numerical parameters used in the selection are stored anyway to the descriptor header of the output tables, and they can be easily retrieved via the command READ/RANGES -- see section 4.3.6).
There are few basic things which must be known when
binning the input photon list into a sky image. In the input Photon Event
Table the photon sky coordinates are listed in the table columns
labeled XPIX and YPIX. They are integer values
indicating the photon positions in an arbitrary sky pixel system.
The size of a sky pixel is written to the Photon
Event Table header descriptor SKY_PIX_SIZE, in the units
given in the descriptor UNIT_PIX_SIZE. In the case of a
ROSAT-PSPC Photon Event Table, one sky pixel is 0.5 arcsecond
0.5 arcsecond.
When binning a photon list into an image, either the dimensions or the binsize of the output image must be defined. The dimensions of the image are the number of image pixels along the X and the Y directions. The binsize is the number of original sky pixels along the X and the Y directions of the output image pixel.
Let's consider the input photon list contained in the table events.tbl. The safest way to bin it into an image is via the following command:
Midas 002> BIN/IMAGE #512,#512 input=events output=image1
With this command it is possible to control the size of the output image,
which in this case is fixed to 512 
512 pixels
independently on the extension
of the spatial photon distribution on the sky: the system itself will
calculate the appropriate binsize, in order to include all of
the listed photons into the output image image1.bdf.
The resulting binsize is written to the header descriptor BIN_SIZE.
The command line shown in the example introduces some disadvantages:
in fact, a request for a 512 
512 image implies
that a 512 
512 image
will be at any rate created, even if a smaller (rectangular) image
would have been possible -- assuming the same binsize.
This is the case when the photon distribution on the sky it is
not squared or circular, but it is elongated
in one direction (as for instance in a ROSAT survey strip). Only a small
rectangular portion of the squared
output image would be actually used in this case: all the rest would be
padded with zeroes.
A possible solution to that may be define an appropriate binsize, chosen such that the output image is not too big and not too small. The output would be actually the smallest rectangular image containing all the photons, as can be seen in the following:
Midas 003> BIN/IMAGE 30 in=events out=image2
In the case of the initial ROD Photon Event Table for
a ROSAT-PSPC pointed observation (with a 2 degrees wide field of view
corresponding to 15360 sky pixels), a binsize 30
as in the example command line
would define a 512 
512 image.
Generally it is not quite necessary to create bigger images. It is instead
much more feasible to create higher resolution images (i.e.
to choose smaller binsizes) for smaller portions of the sky. The
previously created image, image2.bdf, can be loaded
into an image-display window in order to be
used as a reference image for further spatial selections.
A default 512 
512 image-display window is created and the image is
displayed through an appropriate LUT with the command:
Midas 001> LOAD image2 cuts=0,5 Midas 002> LOAD/LUT rainbow3
the following command can be used in order to select with the box cursor a smaller sky portion from the input photon event lists, and then to bin the filtered photons into a higher resolution image (here with binsize = 5):
Midas 008> SELECT/BOX cursor BIN/IMA 5 *events *smalimage
The arrow keys must be used in order to modify the box-cursor size.
Note that when a spatial selection is defined via the cursor as in the previous example, the user is prompted with the actual command line which is going to be executed -- i.e. the numerical parameters driven command line will always be displayed before execution.
It is also worth to note that the output image itself can be used as well as a reference image for further spatial selections. Working with the cursor would be even more accurate because of the image higher spatial resolution.
Any image can be used as a reference image, including the hardness ratio images, the ``energy'' images produced via the SET/PROJECTION ENERGY option, and the true color images (see sections 4.1.1 and 4.1.6 for details on how to produce such images).
Notice that by setting Projection in correction mode via the command SET/PROJECTION, images corrected for the vignetting effects and for the dead time can be created. See section 4.3.4 for details.
Keep in mind that it is a mistake to run source detection tasks on corrected images, energy images and true color images!