screen_time_filter_gti.c and nhrc.flt.h are the source files, and
screen_time_file_gti is the executable.

The program uses a special FITS library, developed here at SAO but
publicly available - funtools. I advise that this library be installed
locally, it can be found as the anonymous FTP site
sao-ftp.harvard.edu, look in /pub/rd/funtools-1.0.

Basically the process consists of several parts:

The input data consists of the Level 1 (evt1) event file, which
contains all of the events telemetered to ground, with their
instrument positions corrected (de-gap), and aspect corrected to the
sky. A second input file is the Level 1 standard filter (std_flt1)
file which contains the good time intervals (gti) for the the
observation. Sometimes the gti records are included in the evt1 file,
and if so that same file could be used as the source for the gti's,
but I usually take the std_flt1 file to be sure.

The output file can be named whatever you want (screened.fits) and it
is a FITS file with the same structure as the evt1 file, but for which
the processing usually done by the pipeline to get to Level 2 is
done. Specifically, this software will do the following:

1. If desired, event times are shifted by 1 event to correct for the wiring 
problem. (This is the default behavior)

2. There is a filter that will only pass events where the time 
difference between an event and its predecessor is less than a specified 
value. This can be useful if the event rate is too high to assure that all 
events times are correct. By requiring two events to be close in time, it 
helps assure that there was no missing intervening event whose time was lost. 
Normally I set this filter to a very large time (100 sec) to
effectively turn it off.

3. Event times are checked against the good time intervals. If they are 
outside these intervals there events are not included in the output file.

4. There is an event screening algorithm that looks at several event 
properties and compares them (sometimes in combinations) to acceptable ranges 
for valid X-ray events. This reduces background and other "bad" events from 
the output data stream

The result is a "screened and filtered" list of events which is analogous to 
standing Level 2 pipeline, but with better times suitable for further analysis.
If the time functions are turned off, the output is essentially the same as 
Level 2 except that the format is Level 1 (which hardly matters in most cases).

I have done some testing of the software, and I believe it is working
properly, but I must admit that I have run only a limited number of
cases, but the reults seem satisfactory. Be aware that it does not
come with any certification.

There are many parameters that can be set by a user, but ordinarily should not 
be needed. The complete list can be seen by running:

	screen_time_filter_gti -h

The parameters needed are:

	scree_time_filter_gti -i[input_evt1_file] -g[input_std_flt1_file]
		 -o[output_file_name] -H[0|1]

	where -H0 -> HRC-I, and -H1 ->HRC-S

Optional parameters of use are;
 the filter time -D[time in seconds], so a value sof 0.005 might not be
 unreasonable (5 ms).

 the flag to turn off time fix -F1 (no time fix)