gtmktime Help FileSynopsis:
gtmktime is used to update the Good Time Intervals (GTI) extension and make cuts based on spacecraft parameters contained in the Pointing and Livetime History (i.e., FT2 spacecraft) FITS file. A Good Time Interval is a time range when the data can be considered valid. The GTI extension contains a list of these GTIs for the file. The sum of the entries in the GTI extension of a file corresponds to the time when the data in the file is "good". In the case of the LAT data, the GTIs are the list of times that the LAT was collecting data over the time range you selected. The Science Analysis Environment (SAE) tools use the GTIs for the exposure calculation. Notes:
The gtmktime tool reads the spacecraft data file and, based on specified cuts, creates a set of GTIs. These are then combined (logical and) with the existing GTIs in the event file, and all events outside this new set of GTI are removed from the file. New GTIs are then written to the GTI extension of the new file. The gtmktime tool is most commonly applied to event data; and, by default, the event data will be filtered according to the GTIs that are created. This Cuts can be made on any field in the spacecraft data file. The default is to select times when the spacecraft is not in the Southern Atlantic Anomaly (SAA), i.e. "IN_SAA!=T". Cuts are made using C-style relational syntax; for example:
Several of the cuts made above with gtselect will directly affect the exposure. gtmktime will select the correct GTIs to handle these cuts, including the complex cut on zenith angle. The easy way to implement this cut is to exclude time intervals where the buffer zone defined by the zenith cut intersects the ROI from the list of GTIs. In order to do that, run gtmktime and answer "yes" at the prompt:
Applying this cut is especially important if your ROI is small (< 20 degrees), bringing your source of interest close to the Earth's limb. Note: If you are studing a very broad region (or the whole sky) you would lose most (all) of your data when you implement the zenith angle cut. In this case you can allow all time intervals where the cut intersects the ROI, but the intersection lies outside the FOV. To do this, run gtmktime specifying a filter expression defining your analysis region, and answer "no" to the question regarding the ROI-based zenith angle cut:
Here, RA_of_center_ROI, DEC_of_center_ROI and radius_ROI correspond to the ROI selection made with gtselect, zenith_cut is defined as 105 degrees (as above), and limb_angle_minus_FOV is (zenith angle of horizon - FOV radius) where the zenith angle of the horizon is 113 degrees. gtmktime also provides the ability to exclude periods when some event has negatively affected the quality of the LAT data. To do this, we select good time intervals (GTIs) by using a logical filter for any of the quantities in the spacecraft file. Some possible quantities for filtering data are:
The current gtmktime filter expression recommended by the LAT team is: DATA_QUAL==1 && LAT_CONFIG==1 && ABS(ROCK_ANGLE)<52. Here it is an example of running gtmktime on the 3C 279 filtered events file. It is useful to rename the spacecraft file to something easier. Here, we have renamed L100806104523E0D2F37E54_SC00.fits to spacecraft.fits.
After the data preparation it is advisable to take a look at your data before beginning the detailed analysis. The Explore LAT Data tutorial has suggestions on method of getting a quick preview of your data. Other kinds of cuts on the data could be made using the gtselect tool. See the gtselect documentation for further explanation. Parameters are passed following the FTOOLs model (i.e., they can be passed interactively by: answering a prompt; as a list in a command line; or by editing the parameter file). To be prompted for gtmktime parameter values, enter (at the command line): gtmktime
The output event FITS file is called: _3C279_3C273_back_filtered.fits. The same example can be run in the command line as follows:
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