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: http://hea-www.harvard.edu/RD/saotng/event_spec.html
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SAOtng supports display of photon event data contained in raw files, similar to its support for FITS Binary Table extensions (e.g., X-ray event lists). This is useful in environments such as X-ray detector development, where raw data is being taken from an instrument and you want to process it without first having to convert to FITS. In order for SAOtng to recognize an event file as such, it is necessary to add the file template to the binary event list entry in the fileaccess.cmds file (since there is no automatic way to recognize such files). This file is in the saord bin directory. The binary event list entry looks like this:
binary event lists *.ev *.nepr EVENTS fitsextract fits='$filename' xcen=$xcen ycen=$ycen xdim=$xdim ydim=$ydim block=$block option=$blocking_preference checkNaN=$donan ofits=stdout type=events mode=h 2>$errorlog fitsheader fits='$filename' ofits=stdout type=events mode=hSimply add your file template to the templates in the second line so that SAOtng will know to process files matching the template as event lists.
Because event files do not usually contain enough information to generate an image, we have adopted some conventions for specifying how to interpret the data in these files and for how to bin rows of events into images.
To image an event file, it is first of all necessary to define the structure of the events in the event file. (In a FITS binary table, this is done with the TTYPE and TFORMnnn and TTYPEnnn header values). The record structure is specified using the "events=" keyword on the command line or the EVENTS environment variable. The format of string specifying the event record is:
setenv EVENTS "name1:[n]type1[:dim1],name2:type2[:dim2]..." or setenv EVENTS "name1:[n]type1[lodim1:hidim1]..."where:
For example, an AXAF HRC event record is defined as:
setenv EVENTS "(major:J,time:J,dsec:J,aday:I,dmsec:I,amps:6I,uraw:U:4096,vraw:U:49152,upos:U:4096,vpos:U:49152,fupos:U:4096,fvpos:U:49152,minor:B,ev:B,cp:2B,pha:B,scale:B,veto:B,stat:B,smu:B,smv:B,fmu:B,fmv:B,sum:B,pstat:B,dummy:I)"This definition defines several columns as possible imaging axes. Note that the total number of bytes defined in this event definition must correspond exactly to the number of bytes in the data record, including any padding added by that the program that wrote the data.
In addition to knowing the data structure, it is necessary to know the endian ordering of the data, i.e., whether or not the data is in bigendian format, so that we can convert to the native format for this platform. This issue does not arise for FITS Binary Tables because all FITS files use big-endian ordering, regardless of platform. For example, big-endian data produced on a Sun workstation but imaged on a Linux PC needs to be byte-swapped, since PCs use little-endian ordering. To specify an ordering, use the bigendian= keyword on the command-line or the EVENTS_BIGENDIAN environment variable. The value of this variable should be "true" or "false". Thus, for example, for a PC to access data produced by a Sun, use:
hrc.nepr[bigendian=true] or setenv EVENTS_BIGENDIAN trueIf BIGENDIAN is not specified, the data are assumed to follow the format for that platform and no byte-swapping is performed.
Once the structure of the event list is known, it is necessary to specify the two columns to be used for the 2D binning. SAOtng first looks for a "binkey=([xname[:xdim]],[yname[:ydim]])" specifier in bracket syntax, and uses the column names thus specified. For example, to bin the above-mentioned HRC event list columns "VPOS" and "UPOS", use:
hrc.nepr[binkey=(VPOS,UPOS)]or
hrc.nepr[binkey=(VPOS:49152,UPOS:4096)]Note that you can optionally specify the dimensions of these columns to cover cases where neither TLMAX nor TLALEN keywords are defined in the header. If either dimension is specifed, then both must be specified.
You can set the EVENTS_BINKEY environment variable as an alternative to adding the "binkey=" specifier to each file name. (Note that this is not the same as the FITS_BINKEY environment variable used for FITS binary tables -- experience has indicated that these must be separate so that users can more easily display both kinds of files.)
If no keys are specified or the specified columns are not in the binary table, then SAOtng looks for columns named "X" and "Y". If these are not found, it looks for columns containing the characters "X" and "Y". Thus, you can bin on "DETX" and "DETX" columns if these are the only column names containing the "X" and "Y" characters.
Finally, the data type of the resulting 2D image must be specified. This can be done with the "bitpix=[n]" keyword in the bracket specification. For example:
bintab.fits[key=(VPOS,UPOS),bitpix=-32]will create a floating point image binned on columns VPOS and UPOS. If no bitpix keyword is specified, bitpix=32 is assumed. You also can use the EVENTS_BITPIX environment variable to set this value.
Event files can have event filtering applied to the events during image extraction. This includes filtering on columns and spatial region filtering.