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hdshift imred.dtoi



hdshift - calculate and subtract zero point to align HD curves.


hdshift database


Input list of databases containing density, exposure and fit information.


For each file in database , procedure hdshift 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):

                _      _      _               _
           a0 = y - a1*X - a2*X**2 - ... - an*X**n

Here, the averages over y and X refer to individual database averages; the coefficients a1, ... an were previously calculated using data from all database s, in task hdfit , 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 database 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 hdfit on the modified database list and determine the common solution.


Shift the curves in four databases to a common zero point.  

	cl> hdshift db1,db2,db3,db4


hdfit, hdtoi
"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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