IPAC 2MASS Working Group Meeting #80 Minutes, 11/28/95

Attendees: T. Chester, R. Cutri, T. Evans, J. Fowler, T. Jarrett, G. Kopan, B. Light, C. Lonsdale, H. McCallon, J. White

AGENDA

1. Hemisphere Classification
2. Photometric and Position Precision
3. Survey Strategy Requirements Document
4. Compression Method for Online Images
5. Blended Point-Source Processing

DISCUSSION

1. Hemisphere Classification -- The hemisphere-classification splinter group decisions were reported by J. Fowler. As discussed last week, this group was tasked to propose procedures for tracking the distinction between the north and south hemisphere with respect to data affected by this distinction. The group was composed of J. Fowler, G. Kopan, and J. White. The recommendations, which were accepted by the members present, were as follows:
   • The directory structure will incorporate the hemisphere distinction at the highest level. Directories named "north" and "south" will have the date directories underneath them, continuing with a directory structure similar to that used for protocamera data analysis. Thus, for example, data taken on observation date 97-06-12 in both hemispheres will be kept separate by placing the data from the northern-hemisphere observatory in and under the directory "north/97-06-12" and the data from the southern hemisphere observatory in and under the directory "south/97-06-12".

   • TAPELOAD will identify the hemisphere from the contents of the observatory tape it is processing, create the directory tree extensions needed, and set up hemisphere-dependent data files for the 2MAPPS subsystems in those parts of the tree where the subsystems will look for them. Thus, for example, POSFRM will obtain the hemisphere-dependent focal-plane distortion model by reading it from a directory accessed via a relative offset from its default directory (e.g., "../../distmodel"), without having to know which hemisphere is actually involved.

   • Subsystems which require explicit knowledge of the hemisphere being processed will obtain it directly from their NAMELIST input, which will be set up for them by TAPELOAD in the same way that other hemisphere dependent data files are set up (i.e., item (B.) above). This information will also be in the headers of all FITS files, so subsystems that read such files can obtain the hemisphere information that way if preferred.

   • Cognizant engineers whose subsystems involve hemisphere dependent data must identify the parameters involved to J. White and arrange for the files to be installed in separate-hemisphere data bases for TAPELOAD to use in setting up and loading the directories for subsequent processing.

2. Photometric and Position Precision -- C. Beichman and R. Cutri have requested that precision requirements for position and photometric information be placed in the FDD. J. Fowler gave a recommendation on how to include these requirements; this resulted afterwards in the following paragraph to be placed at the end of section 2.6 in the FDD, "Subsystem Coding and Documentation Requirements" (this will be in version 3.3 of the FDD).

   "To guarantee precision sufficient to support the accuracy requirements specified in the 2MASS Level 1 Science Objectives and Specifications document (see section 1.1, item 5), the following precision requirements are levied on the 2MAPPS subsystems. Position coordinates and their uncertainties shall be represented in disk files with a precision corre- sponding to at least 0.02 seconds of arc, and computations involving position coordinates and their uncertainties shall be done in double precision as needed to guarantee conservation of this precision. This implies file formats representing position parameters accurate to three decimal places when represented in seconds of time, two decimal places when represented in seconds of arc or camera pixels, and six decimal places when represented in floating-point degrees. Photometric quantities and their uncertainties shall be represented with a minimum precision of 0.001 magnitudes, with similar requirements regarding computational pre- cision needed to guarantee conservation of this precision and correspond- ing precision when other photometric representations are used."

3. Survey Strategy Requirements Document -- C. Lonsdale reported that she has prepared a draft version of the 2MASS Survey Coverage Strategy and Plan document (this is the name used in the FDD, section 1.1 item 4, where it is noted as TBD; the actual name may turn out to be different). The draft will be distributed locally for comment by the end of the week, and a version with revisions incorporated will be sent to the project at the end of next week.

4. Compression Method for Online Images -- T. Jarrett displayed some sample coadded images of several fields compressed to various degrees of file size reduction by the HST compression algorithm. Although the decision of which compression algorithm to use for the online image data base is not final, the HST algorithm is acquiring the reputation of a preferred standard, and it seems to be the front runner for use at IPAC. This is a "lossy" compression algorithm, which is acceptable for the online image data base because of the anticipated use of this data base (i.e., visual field inspection, preparation of finding charts, and the like, as opposed to detailed photometric analysis, for which the full coadded images must be reloaded from tape once they have been moved offline). The algorithm was developed by R. White and has been used for some distributed HST images. The desire is to achieve a compression factor of 100, so that the expected image data base size of 4 terabytes can be reduced to an online data base of 40 gigabytes. The members present were not quite satisfied with any of the examples of this compression level, however, with the qualitative acceptability level appearing to run between the 20 and 50 reduction factors. Further consideration will be given to the compression method and level of reduction to be used.

5. Blended Point-Source Processing -- R. Cutri reported that he is preparing a memo for distribution to the science team describing the problems in handling blended point sources (see last week's minutes) and various approaches to solutions. This memo will also contain a proposal for implementing band-filling.

• 2MASS Project
• IPAC 2MASS page
• IPAC Home Page