Preliminary specs for MOS Output Data Products
The tangible products with scientific value from an astronomical
instrument are the data. These data return home with the investigator
for further analysis. In keeping with astronomical convention, all
data products from the MOS shall be stored within FITS files.
Required Data Products
The primary data product from the MOS is a CCD image containing
spectra from the fibers. This image is stored in FITS format. MOS
uses the newer Keck Data Acquisition System (DAS) rather than the
older one long in use at Lick Observatory. The Keck DAS permits such
FITS files to be stored on disk and/or tape.
The identities of the object in each spectrum on the CCD image cannot
be documented without including auxiliary information. This
information is based upon the observer's original input catalog and
upon the fiber configuration which was used for the observation. It
shall provide a complete description of the mapping between spectra,
fibers, objects and their celestial coordinate locations. This
auxiliary information shall be stored in a FITS ASCII table extension.
The archivally correct scheme to keep the FITS table extension
properly associated with the FITS image would be to append the table
extension to the FITS header and data unit (HDU) for the primary
image. Unfortunately most current FITS image readers ignore anything
beyond the primary HDU (PHDU); when reprocessing such an image in place on
disk they will typically destroy any extension HDUs. Also, the Keck
DAS is not currently designed to append arbitrary FITS extensions to
the image HDUs. For these reasons we choose to create the table
extension for MOS in a separate FITS file.
In order to document the relationships between these separate files
FITS Grouping Convention shall be used.
The grouping convention requires that an additional FITS table be
'GROUPING' table contains a list of all
the related HDUs. In the case of the current MOS data the list of related
HDUs would be
Other tables could also be included; e.g., it could be useful to
include a table that documents approximately where on the CCD each
fiber spectrum falls.
- The FITS
GROUPING table (which lists the others)
- The primary HDU containing the CCD image
- The ASCII table containing fiber/object configuration
During an observing session the Keck DAS produces FITS files on disk with
names such as /full/path/prefixNNNN.fits.
The full directory path (/full/path/) can be modified and is
stored in the FITS header as the string value of the keyword
The file name (prefix) can also be modified and is stored in the
FITS header as the string value of the keyword
The sequence number (NNNN) is always zero-padded to 4
digits and is stored in the FITS header as the integer value of the
FRAMENO (and for compatibility with the Lick DAS
The sequence number typically increments by one after each saved CCD
readout, but the observer is allowed to modify this sequence number
such that more than one FITS file has the same value of
There is thus no guarantee of a single unique identifier for each
image, but in practice it is extremely uncommon for an observer to so
modify the sequence number.
A point of difficulty
The FITS Grouping Convention works well when all HDUs are stored
within the same FITS file. It also works well when HDUs are stored
in separate named files on a directory-structured file system with
Uniform Resource Identifier (URI)-like characteristics.
Because FITS tapes lack a universal scheme for associating files
with file names the grouping convention can only work well if the
tape reader uses the
OUTDIR keywords to reconstruct a file name.
One possible solution would be to adopt the NOAO convention
of storing the file system name in the PHDU using the FITS keyword
IRAFNAME. Other solutions are in use by other projects.
For MOS it shall be that the FITS file containing the grouping table and
the fiber/object table extensions will be written to tape
immediately following the FITS file with the CCD image PHDU.
This seems to be the best solution for the initial
implementation. In the long run the lack of a single FITS keyword
which can serve as a unique, file system-oriented identifier in the
Keck DAS needs to be addressed.
The DAS must acquire the fiber/object configuration information from
the LLNL SCRAP code. The SCRAP code
is responsible for configuring the MOS fibers in accordance with
observer requests; it is best suited to provide the information
to the DAS.
During the time that these images are stored on disk the file names
can also be used to indicate their association. The tables can be
stored in a file whose name is clearly associated with the image.
E.g., for image
d0001.fits the file containing the
grouping table and the fiber/object table shall be
Changes to the CCD image HDU
In accordance with the guidelines of the FITS grouping convention
the FITS file containing the CCD image is not required to change.
It is strongly recommended, however, that two additional FITS keywords
be added to the image header. These keywords provide an indication
that the primary HDU is a member of a group by pointing to the
grouping table that contains the PHDU.
A FITS HDU may indicate its membership in as many as 999 groups. The
MOS PHDU belongs to only one. There is no need for GRPID2 through
GRPID1 = -1
This FITS grouping keyword requires an integer value.
This indicates that the HDU is a member of a group.
The negative sign indicates that the grouping table
resides in a different FITS file; there must therefore also be
GRPLC1 keyword. Taking
indicates that the grouping table to which this HDU belongs
EXTVER = 1.
GRPLC1 = 'prefixNNNN.mos'
This FITS grouping keyword gives a partial URI (in this case
the name of another file in the same directory) in which
the grouping table resides.
Structure of the FITS file with the tables
The newly-added FITS file must consist of five different HDUs.
The detailed structure of each HDU is indicated within the links.
There is a data dictionary that describes
the MOS-specific keywords in the above HDUs. It is essential reading
for understanding of the MOS and its data products.
A primary HDU with zero-size image
FITS requires that a PHDU be at the beginning of all files.
A grouping table
Documents the relationship of the CCD image and the
other FITS HDU(s).
The input catalog table
This reiterates the total content of all input catalogs
provided by the user. If the assignment procedure deploys
fibers to positions not contained in the input catalogs
(e.g., ``sky fibers'') then the celestial coordinates of
those positions are also included.
Assigned objects point into the fiber table to identify their fiber.
The fiber configuration table
For each object in the input catalog table this contains the
refracted position in celestial coordinates and the distorted
MOS micro-positioner coordinates. The complete list of
distorted MOS coordinates can be used to drive the sky-viewing
camera to all objects in a cluster for the purposes of
refining the distortion map. The refracted celestial
coordinates are provided in order to permit post-run analysis
of modeling errors in the astrometric software. (``Sky
fiber'' positions are originally specified in one of these two
coordinate systems; they are then transformed back into the
The MOS hardware table
Documents the relationship of fibers, pivots, and ``slits''.
Documents the MOS model used in the software including the
locations of the pivots and the parking spots for the fibers.
Assigned fibers point into the catalog tables to identify
Back to the MOS home page.
Steve Allen <firstname.lastname@example.org>