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The program sortac

The program sortac sorts the FRFs according to time, pulse height channel, and detector identification over a user specified period. Sortac reads a catalogue file to determine which FRFs contain data from the required period. To create this catalogue file the user needs to run catget, specifying the start and end times of the observation (see section 2.3 ). All FRFs within the specified time interval must be present in the catalogue file and on disk.

Before running sortac it is useful to run the program predgin90 to check the angle between the Sun and the LAC pointing direction. If this angle, [beta], is less than 90° then the data may be contaminated by solar X-rays. If this is the case then consult section 2.6 for advice on how to proceed with the analysis. Predgin90 works as follows:


** PREDGIN90 Version 1.1 Mon Jun 16 17:26:13 1997 **
LIST List file = lp.lis =
YEAR Year required (2 digits) = 87 =91
MONTH Month (1-12) = 1 =7
DAY Day = 1 =

PREDX: Sun angle predictions from terminal 

 Start date : 1991 Jul 01
 List  file : lp.lis
 Visibilities file : temp.vis
 Temporary file : sangl_1.lis
 Temporary file : sangl_2.lis

NOM Enter source name =  =mkn 335
POSSTR Enter source position =  =00 03 45 19 55 29

 SOURCE mkn 335 AT 00 03 45 19 55 29  (  0.938  19.925)


The position of the source, POSSTR, should be in the form hh mm ss dd mm ss. The file lp.lis may be inspected as follows:

qcl>$cat lp.lis

PREDX: sun angle predictions from terminal 


 Source mkn 335
 00 03 45 19 55 29  (  0.938  19.925)

  In optimum beta range  1991 Jul  2  to  1991 Aug 21
  In optimum beta range  1991 Nov 14  to  1992 Jan  1


If the observation date falls in one of the periods of optimum beta range then there will be no solar contamination. The files sangl_1.lis and sangl_2.lis contain the actual [beta] angle for a given day number (1st July = 0 in this example) and MJD respectively.

Sortac allows for the rejection of data when certain house-keeping (HK) parameters fall outside a given range. It is standard practice to apply cuts based on the LAC count rate above 24 keV (known as Surplus above Upper Discriminator or SUD), the energy that a cosmic-ray requires to penetrate the magnetosphere (Cut-Off Rigidity or COR) and the solid state electron monitor count rate (SOL2), in order to eliminate periods of high background (these limits are usually different for target and background observations; recommended values are listed in table 2 ). Contamination from the bright Earth can be prevented by restricting the angle between the satellite pointing direction and the Earth's horizon (YELEV) to greater than 6°. In addition, because of problems caused by the low energy collimator reflection (see section 4; Turner et al. 1989), it is worth restricting the angle between the LAC pointing direction and the source position to less than 0.5°. The high voltage checks should always be used as it is not possible to do this after the data has been sorted.

An example is given below, and the sortac documentation can be consulted if more details are required.


GNG_DIR directory containing data = [] =./
GNG_CAT_FILE name of catalogue file = AC000N.CAT =mkn335_91a.cat
GNG_TARGET target name =Mkn 335
GNG_RA_DEC_CHK target position to be specified? = No =Y
GNG_RA of target in degrees = 0.0 =0.938
GNG_DEC of target in degrees = 0.0 =19.925
GNG_TARG_DIFF Allowed angle between Y axis and target = 1.50000 =0.4
GNG_RAW_TYPE of data required = MPC1 =
GNG_SQUISH Squish idents to ODD and EVEN channels = Yes =Yes
GNG_RAW_COMPAT all compatible data to be used? = Yes =
GNG_FLD do you wish to fold the data = No =
GNG_TIME_BIN in seconds = 64.0000 =128.0
GNG_AUX_INFO Auxiliary information to be outputed = FULL =
GNG_DEAD_CORR Do you want the output data dead time corrected = No
GNG_OS_DEF Apply default Operational Status (OS) checking = Yes =
GNG_NUM_PER number of time periods selected = 1 =
GNG_ST_PER001 start time (? for formats) = START =G91 07 02 05 33 00
GNG_END_PER001 end time (? for formats) = END =G91 07 02 19 30 00
GNG_SYNC modify start time so sorted bins match itegration times = No
GNG_SOL_CHK data accept only at low SOL rate? = Yes =
GNG_SOL_MAX maximum SOL rate = 15.0000 =
GNG_PI_CHK data accept only at low PI rate? = No =
GNG_COR_CHK data accept only at certain rigidity = Yes =
GNG_COR_MIN minimum cut of rigidity = 7.00000 =10
GNG_COR_MAX maximum cut of rigidity = 20.0000 =20
GNG_SUD_CHK data accept only within certain SUD range = Yes =
GNG_SUD_MIN minimum SUD rate = 0.0 =
GNG_SUD_MAX maximum SUD rate = 14.0000 =10.0
GNG_LONG_CHK data accept only within certain Longitude = No =
GNG_HV_DEF Default HV checking applied = Yes =
GNG_POINT_CHK data accept only for sky or earth pointing? = No =
GNG_YELEV_CHK check Yaxis/horizon angle = Yes =
GNG_YELEV_MIN min Yaxis/horizon angle = 6.00000 =
GNG_YELEV_MAX max Yaxis/horizon angle = 120.000 =
 SAC_SFPROC: SFN not in sequence 
   FRF Name      Date         Time        SFN        RA       DEC   Data:accept
  91070204111   91:07:02    5:33:37:672    60        1.10    19.80
 SAC_SFPROC: SFN not in sequence 


In order to reduce the size of data cubes, it is normal for sortac not to preserve all the detector information for MPC1. Instead the data is compressed to only two idents, corresponding to the top- and mid-layers of the LAC (see table  3 and Turner et al. 1989). If the time binning is sufficiently coarse, sortac can compress energy and detector identification of MPC1 data so that it mimics the MPC2 or MPC3 data formats, allowing MPC1 data to be sorted simultaneously with MPC2 or MPC3 data (see table 4). Similarly MPC2 data can be compressed onto MPC3 data.

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