- Distortion Map --
H. McCallon reported
that certain aspects of handling the focal-plane distortion maps
were not clear in the existing guidelines covering software interface
and subsystem documentation. The distortion correction method is not yet
fully defined; past discussions have indicated that it is needed in different
forms in different subsystems, but it should nevertheless be maintained in a
single central location.
It was suggested that a relatively low-order polynomial model of the
mapping from pixel space to undistorted true-angle space should suffice. In
PICMAN, the model will need to be implemented in an extremely efficient form,
e.g., a specific polynomial with coefficients read from a data base, since a
general-purpose function called every time distortion correction is needed
would generate far too much CPU usage. In PROPHOT, a separate function call
might be acceptable; this would be desirable, since it would decouple the
details of distortion correction from the inner workings of PROPHOT.
It became clear that the distortion correction process is best designed
with its relationship to the overall mapping of scan coordinates to true-sky
coordinates in mind. Since the three arrays have generally independent scale
and twist-angle properties, each band will have to mapped to an ideal
coordinate system somewhere along the pipeline. This will be
done according to where the needed information is available and when
the mapping is needed. After some discussion, the baseline approach
is as follows:
- POSFRM will use an input distortion map (initially calibrated during
the shakedown period) in registering the different bands' point sources
as part of the frame-offset solution; in the process, POSFRM will
generate discrepancy information that can be used to refine
the mapping. The SIS for the coordinate correction models will be
maintained as part of the POSMAN subsystem (a SIS is necessary,
since 2MAPPS subsystems use the information, even though it is
produced by an offline analysis program (see the next item).
- An offline analysis program will do the calibration of the
distortion map; during the shakedown period, POSFRM will start
off with an identity-matrix model and iterate with the offline
program until the initial calibration is stable; the offline program
will not be documented as part of 2MAPPS, since it is not production
software and may be run only at certain intervals or not at all
during the survey, depending on the discrepancies generated by
POSFRM (and BANDMERGE; see below).
- PICMAN will use the distortion correction model and band-dependent
scan-coordinate correction model to map all pixels to a common true-sky
coordinate system as part of the coadding process; this system will have
the form of scan coordinates that can be associated with inertial
coordinates via translation and rotation only (the definition of
these scan coordinates will involve alignment with J2000 equatorial
meridians); the projection type will be clearly documented via
inclusion in the FITS headers of the coadded image files.
- PROPHOT will use the distortion correction model to represent point
source positions in undistorted band-dependent scan coordinates.
- BANDMERGE will use the band-dependent scan-coordinate correction
model to map all point sources to a common system for band merging; it
will output band-merged point sources in this system and generate
discrepancy information that can be used to refine the coordinate
correction models.
- SDS Review/Mail Schedule --
The schedule for
the technical-subsystem SDS's previously identified as required for the
upcoming November review is as follows. Draft versions are to
be delivered to /proj/2mass/doc/design/sds, printed, and hardcopy distributed
to IPAC 2MASS personnel by Monday, October 16. Reviews and rewrites should be
finished in time for a mailing to UMASS on Thursday, October 19.
- SDS Glossaries --
C. Beichman requested that all
SDS's include a glossary. All 2MASS/IPAC-peculiar jargon used in the SDS
should be defined in the glossary, as should any new terminology
developed for the subsystem described in the SDS. The glossary may be
placed at the end of the document without a section or appendix identifier.
- SDS Online Storage --
The online repository
for SDS's (before being put into the web area) is
/proj/2mass/doc/design/sds/XXXX, where XXXX represents the name of a subsystem.
In a previous memo, J. Fowler indicated the contents desired, namely the
SDS in native wordprocessor format, an ASCII format version (if different from
the native wordprocessor version), and a postscript version. The new aspects
are that separate subdirectories for each subsystem are to be used, and any
linked or embedded files are also to be delivered (e.g., RoboChart native-
format files).