2.0  FIT

2.1  INTRODUCTION

This software package is designed to interpret and analyze photometric data originating from an ATIS 
controlled facility.  The package consists of two FORTRAN programs that extract and process 
photometric counts of selected stars and areas of the sky.  The ATIS user must organize photometric 
measurements into groups.  Each group defines a set of star and sky observations.  The processing 
programs use only the relevant information within groups to correct and reduce star data.  The programs 
are FIT, which does the initial data reduction, and TAB which generates ASCII text data tables.

2.2  COMPILING AND LINKING

This software should work on all PC, XT, and compatible hardware.  The programs were compiled with 
MircoSoft (MS) FORTRAN 4.0.  The modular structure of the codes require linking to an ATIS library in 
addition to the standard FORTRAN modules.

2.3  INPUT FILES

This table indicates all files used and generated by the ATIS software.  Identifiers with the dots refer to 
the filename extensions.  The nuemonic identifiers refer to specific files.

	Input:
		Catalog.tbl		!  Star catalog
		Refstar.tbl		!  List of reference stars for each group
		A0*.arc			!  The ATIS data files compress in an archive
	Output:
		Sum.dat			!  Summery of the data, stars, and coeff derived.
		Dat.dat			!  Output file required by TAB to calculate the extinction-	
				!  corrected brightness.



	Generally, all programs need to know where to find their input files, what nights to process, 
color filter number and what files to create.

2.4  PROGRAM FIT

	This   program is the workhorse of the package.  Its function is to compute nightly extinction 
coefficients using corrected photometric data of selected stars.  In addition, all counts are corrected for 
dead time and the sky component is subtracted from all star counts.

	The program input requirements are,

		The data file directory
		The range of nights to process (JD)
		Color filter number
		Accepted upper limit on first order extinctions
		Accepted upper limit on first order extinction errors
		Accepted lower limit on airmass differences
		The output file name

	The algorithm has been designed for robustness and efficiency.  Data is grouped in 
chronologically ordered time zones.  The data collected in each time zone is reduced as follows,

	1)  Airmasses are calculated at all times and counts are corrected for dead time.

	2)  Sky counts are subtracted from Star counts.  Sky counts at times between sky events are 	   
	     found by linear interpolation.

	3)  Corrected star counts are scanned for outliers by computing a mean count and sigma and 	 
	     searching for counts greater than two sigma from the mean.

	4)  A two parameter linear extinction model is used to fit data from each extinction star and 	  
	     estimate parameter values and their errors.  Outliers are not used.  Maximum and minimum 	  
	     airmasses are found.

	    The model, 	MO = MT + E0*X , where

			MO is the measured extinction star magnitudes converted from counts as

			MO = log10(-2.5*counts)

			MT and E0 are the two parameters estimated

			E0error is the estimated relative error of E0

			X is the airmass
			DX is the airmass difference (maximum - minimum)

	5)  A weighted average of the star E0 values is determined.  The weight function is the airmass 	  
	     difference.

			EM = sum ( E0*DX ) / sum ( DX )

			sum is over all stars whose E0, E0error, DX values fall inside their user 	
		defined bounds.

After steps 1) to 5) are performed for each time zone , the set of EM values are used to model the 
temporal behavior of the extinction.

	6)  These EM data are used to fit a three parameter model,

			E0 + E1 * time + E2 * time**2

			time is the relative JD time measured from the JD time associated with the first 
			star event of the night.

			The parameters E0, E1, and E2 are called the nightly extinction coefficients.

	Program FIT saves the reduced nightly star and sky data and extinction coefficients as fixed 
length records in a direct access binary file.  An ASCII summary file is also generated.


2.5  FIT - More Notes

I. Purpose:  This program calculates the extinction coeff vs time for each  day using the data available 
from all the stars observed during that night.  The only data that are used are those for which the 
extinction coeff meets the following criteria:
	1) 0 .LE. ext. coeff. .LE. MAX (input value)
	2) delta air mass change .GE. 0.05
	3) relative error .LE. 0.20
	4) points in a  _____ interval that are ___ are removed.      (<- unreadable)
	5) if sky .GE. star; toss out the point
	6) will not use any star designated as "variable" in the calculation of the extinction coeff.
	7) airmass .LE. 2.0
	8) 0 .LE. extinction coeff .LE. 1.0

2.6  REFERENCES

This material was taken from a readme file from a FIT program disk and notes made by William J. 
Borucki.  It is not necessarily up to date with the latest version of TAB.