.help hdshift Feb87 imred.dtoi
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NAME
hdshift - calculate and subtract zero point to align HD curves.
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USAGE
hdshift database
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PARAMETERS
.ls database
Input list of databases containing density, exposure and fit information.
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DESCRIPTION
For each file in \fBdatabase\fR, procedure \fBhdshift\fR calculates and 
subtracts a zero point shift to bring several related HD curves into
alignment.  The individual shifts are calculated by elimination of the 
first coefficient (Bevington, eqn 9-3):
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                _      _      _               _
           a0 = y - a1*X - a2*X**2 - ... - an*X**n

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Here, the averages over y and X refer to individual \fBdatabase\fR averages; 
the coefficients a1, ... an were previously calculated using data from all 
\fBdatabase\fRs, in task \fIhdfit\fR, and stored in the database.  The
a0 term is calculated individually for each database; this term represents
the zero point shift in log exposure and will be different for each database.

On output, the log exposure values in each \fBdatabase\fR have been 
shifted to the zero point shift of the first database in the list.  The
log exposure records are now aligned and it would be appropriate
to run task \fIhdfit\fR on the modified \fBdatabase\fR list and
determine the common solution.
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EXAMPLES
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Shift the curves in four databases to a common zero point.  

	cl> hdshift db1,db2,db3,db4
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SEE ALSO
hdfit, hdtoi
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"Averaging Photographic Characteristic Curves", John Kormendy, from
"ESO Workshop on Two Dimensional Photometry", Edited by P. Crane and
K.Kjar, p 69, (1980), an ESO Publication.
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