Index: /trunk/archive/scripts/src/instruments.txt
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--- /trunk/archive/scripts/src/instruments.txt	(revision 4387)
+++ /trunk/archive/scripts/src/instruments.txt	(revision 4387)
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+Each instrument has its own configuration file, which specifies how to
+identify the instrument based on FITS headers and how to read the data
+in.  The former is specified in a sub-metadata section labelled
+"RULE", and is fairly straight-forward: you simply specify what FITS
+keywords that instrument can be expected to have, along with the value
+that it should have.  "TELESCOP" and "INSTRUME" are useful, if present,
+for this purpose.
+
+Specifying how the data is to be read in is a bit more complicated.
+This is specified by several keywords.  The two most important are
+"PHU" and "EXTENSIONS".  "PHU" specifies what the FITS file
+represents, and may have one of two values: "FPA" indicates that
+the file contains an entire focal plane array; "CHIP" indicates that
+the file contains a single chip.
+
+"EXTENSIONS" specifies what each of the extensions represents, and may
+be one of the following values, with the proviso that it cannot
+specify a higher level than that specified by "PHU": "CHIP" indicates
+that each extension is a chip; "CELL" indicates that each extension is
+a cell; "NONE" indicates that there are no extensions.
+
+The other two important keywords are "CONTENTS" and "CELLS".  The
+contents of "CONTENTS" varies according to the values of "PHU" and
+"EXTENSIONS", as shown in the below table, but "CELLS" contains
+metadata with values for each cell name or cell type.
+
+PHU	EXTENSIONS	EXAMPLE		CONTENTS
+===	==========	=======		========
+FPA	CHIP		megacam_splice	For each chip, a list of cell types that comprise the chip.
+FPA	CELL		megacam_raw	For each extension, the chip and cell type, separated by a colon.
+FPA	NONE		lris_blue	For each chip, a list of cell types that comprise the chip
+CHIP	CELL		gpc1_raw	For each extension, the cell type
+CHIP	NONE		lris_red	The cells that comprise the chip
+