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Chapter 2: Header Keyword Definitions

Association Keywords

After February 1997, the observation logs changed to reflect the FITS with extensions paradigm used by STIS and NICMOS. As mentioned earlier, the _jit.fits and _jif.fits table and image are multi-extention FITS files and must be worked with as such. A jit or jif file will normally contain a number of extensions corresponding to individual exposures within a science observation if that observation is associated. The data in each of these extensions is accessed within IRAF by specifying for the jitter image say, rootname_jif.fits[n] where n is the extension number, corresponding to the nth exposure in the association.

Generally OMS (the system that creates the Observation Logs, among other things) tries to put all obslogs in an association into one single file with multiple extensions. It will occasionally happen that one or more exposures planned as part of an association will be orphaned. If this happens, the observation log files, at least for NICMOS, will be generated seperately.

Below are briefly listed the header keywords produced after February 1997 concerning associations and extensions:


The unique identifier for an association, which can contain exposures and additional association products (see example below)


Name of the association table produced as part of the science dataset. This table provides the information regarding the observation and how exposures and products were associated. The MEM_TYPE column in this table corresponds to the ASN_MTYPE keyword discussed below.


Complete/Incomplete- Indicates whether or not the association had orphan exposures. This affects whether the observation log files are archived individually or in one multi-extension FITS file. The observation log files will normally only be combined if the ASN_STAT indicates COMPLETE.


T/F- Was the observation an association?


Rootnames of each association product. Within an association there can be multiple products. There exists only one association product for STIS, but NICMOS may have one association product for the target and up to 8 background products. Only one observation log file is created for the target and any background associations. The multiple extensions in the observation log files will contain pointing data for the background product and the target products. The order of the exposures (extensions) in the observation log file will follow the order given in the rootname_asn.fits table, which is one of the calibrated science products, and whose name is given in the ASN_TAB keyword.


Role of the particular exposure in the association.

An example at this point will probably be immensely useful. The idea of all this discussion of course is to relate a given observation log's extention to the correct exposure. Below is an observation's asn table, showing a spiral dither chop, n48o06020_asn.fits.
cl> tedit n48O06020_asn.fits
1	N48O06G9Q	EXP-TARG	yes
2	N48O06GAQ	EXP-TARG	yes
3	N48O06GBQ	EXP-BCK1	yes
4	N48O06GCQ	EXP-BCK1	yes
5	N48O06GDQ	EXP-TARG	yes
6	N48O06GEQ	EXP-TARG	yes
7	N48O06GGQ	EXP-BCK1	yes
8	N48O06GHQ	EXP-BCK1	yes
9	N48O06GIQ	EXP-TARG	yes
10	N48O06GJQ	EXP-TARG	yes
11	N48O06GKQ	EXP-BCK1	yes
12	N48O06GLQ	EXP-BCK1	yes
13	N48O06G20	PROD-TARG	yes
14	N48O06G21	PROD-BCK1	yes
The ASN table tells us the association contains 12 exposures, 6 on target and 6 on sky, grouped into 2 association products (N48O06G20, target and N48O06G21, background), and the rootnames and role of each exposure in the association.

Now looking at our n48O06020_jif.fits image:

cl> catfits n48O06020_JIF.fits
0     n48o06020_jif N48O06020_JIF.FITS                     32          
1       IMAGE       jit                   1     64x64      32        
2       IMAGE       jit                   2     64x64      32          
3       IMAGE       jit                   3     64x64      32        
4       IMAGE       jit                   4     64x64      32        
5       IMAGE       jit                   5     64x64      32          
6       IMAGE       jit                   6     64x64      32          
7       IMAGE       jit                   7     64x64      32           
8       IMAGE       jit                   8     64x64      32           
9       IMAGE       jit                   9     64x64      32        
10      IMAGE       jit                   10    64x64      32     
11      IMAGE       jit                   11    64x64      32         
12      IMAGE       jit                   12    64x64      32   
...we see that the jitter file has 12 extensions (plus the zeroeth group only containing header parameters). These 12 extensions correspond to the 6 exposures for both the target and background association products. Looking at the association keywords in the header, we see...
ASN_ID  = 'N48O06020'          /unique identifier assigned to association
ASN_TAB = 'n48o06020_asn.fits' /name of the association table
ASN_STAT= 'COMPLETE'           /status of association (COMPLETE/INCOMPLETE)
ASN_PROD=                    T /product created (T/F)
PRODNAM0= 'N48O06020'          /rootname of the zeroth association product
PRODNAM1= 'N48O06021'          /rootname of first association product
PRODNAM2= '                  ' /rootname of second association product
PRODNAM3= '                  ' /rootname of third association product
PRODNAM4= '                  ' /rootname of fourth association product
PRODNAM5= '                  ' /rootname of fifth association product
PRODNAM6= '                  ' /rootname of sixth association product
PRODNAM7= '                  ' /rootname of seventh association product
PRODNAM8= '                  ' /rootname of eighth association product
PRODNAM9= '                  ' /rootname of ninth association product
This block tells us that the jif and jit files have extensions with data covering these association products, and that they correspond to the listing of exposures in the table whose name is given in the ANS_TAB keyword, in this case n48o06020_asn.fits. Looking at the ASN_MTYP and EXPNAME keywords for each extension of the n48o06020_jif.fits file, we see...
n48o06020_jif.fits[1], ASN_MTYP = EXP-TARG EXPNAME = N48O06G9J
n48o06020_jif.fits[2], ASN_MTYP = EXP-TARG EXPNAME = N48O06GAJ
n48o06020_jif.fits[3], ASN_MTYP = EXP-BCK1 EXPNAME = N48O06GBJ
n48o06020_jif.fits[4], ASN_MTYP = EXP-BCK1 EXPNAME = N48O06GCJ
n48o06020_jif.fits[5], ASN_MTYP = EXP-TARG EXPNAME = N48O06GDJ
n48o06020_jif.fits[6], ASN_MTYP = EXP-TARG EXPNAME = N48O06GEJ
n48o06020_jif.fits[7], ASN_MTYP = EXP-BCK1 EXPNAME = N48O06GGJ
n48o06020_jif.fits[8], ASN_MTYP = EXP-BCK1 EXPNAME = N48O06GHJ
n48o06020_jif.fits[9], ASN_MTYP = EXP-TARG EXPNAME = N48O06GIJ
n48o06020_jif.fits[10],ASN_MTYP = EXP-TARG EXPNAME = N48O06GJJ
n48o06020_jif.fits[11],ASN_MTYP = EXP-BCK1 EXPNAME = N48O06GKJ
n48o06020_jif.fits[12],ASN_MTYP = EXP-BCK1 EXPNAME = N48O06GLJ

This information will reflect what is seen in the _asn table, and gives positive identification of each extension in the jitter file, relating it to the individual exposures, in associations or otherwise.