Within the limited purpose of the present discussion it is enough to know that the arrival times of the photons listed in the input Photon Event Table are contained in a double precision column labeled TIME and are expressed in seconds (spacecraft clock). The values listed in this column can be considered as a first approximation in linear relation to the UT (see the Timing Analysis chapter).
Let's assume that the input Photon Event Table is events.tbl and that a reference sky image were already produced and loaded into the image-display window (see sections 4.2.1 and 4.1.1 for details about producing sky images).
In the case of PSPC data, Projection should be set in correction mode as a first thing:
Midas 001> SET/PROJECTION CORRECT
If you don't want to use the default calibration files for the correction, you can specify new calibration files as in the following:
Midas 001> SET/PROJECTION CORRECT=my_effective_area,my_eventrates
If you don't want to apply dead time correction, skip the second file name.
By setting Projection in correction mode light curves corrected for the vignetting effects and for the dead time can be created. The light curves are corrected to the instrument optical axis.
For WFC and HRI data the correction mode is not applicable (data correction is performed afterwards -- see section 4.2.6).
The standard way of deriving a light curve from a point-like source is to select all the photons coming from a limited circle around it, and to bin them in time:
Midas 010> SELECT/RING cursor BIN TIME 10 in=events output=rate1
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 (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).
The result of time binning, here with a binsize of 10 seconds, is written into the output table rate1.tbl at the column labeled by the default COUNTS_1, and it is expressed in photon counts/bin.
The MIDAS commands SHOW/TABLE and READ/TABLE can be used to display the structure of the output table:
Midas 011> SHOW/TABLE rate1 Midas 012> READ/TABLE rate1
In the column labeled S_TI the start times for each time bin are written in double precision. The first bin is always assigned a start time zero. The time offset, i.e. the actual spacecraft clock time of the first bin, is written to the header descriptor BIN_START. Possible time gaps are physically suppressed, and are indicated by the discontinuities in the S_TI values sequence.
In case Projection was run in correction mode, the extra columns :CORR_1 and :ERR_1, containing respectively the corrected light curve and the corresponding statistical errors, would be added.
Further about light curve correction and background subtraction, see section 4.2.6.