IPAC 2MASS Working Group Meeting #83 Minutes

IPAC 2MASS Working Group Meeting #83 Minutes, 1/09/96

Attendees: R. Beck, T. Chester, R. Cutri, T. Evans, J. Fowler, L. Fullmer, T. Jarrett, D. Kirkpatrick, G. Kopan, C. Lonsdale, H. McCallon, S. Terebey


  1. AAS Meeting
  2. PS CDR Schedule
  3. Persistence at Scan Endpoints
  4. Artifact Blanking in Images


  1. AAS Meeting -- As part of the cost cutback to help reduce future impact of the budget fiasco, the number of IPAC attendees at the upcoming AAS Meeting will be limited to one. R. Cutri will handle the entire job in a single day. People who have posters for Roc to take to the meeting must have final hardcopy to give him by next Monday afternoon. He departs Tuesday morning.

  2. PS CDR Schedule -- The Point Source Critical Design Review previously scheduled for late January has been postponed for an undetermined length of time (probably about two months). A telecon involving IPAC and science team members resolved some of the position reconstruction issues but left others hanging, and the latter must be resolved via testing to prove that the proposed algorithms work. This cannot be done by the end of January. In addition, some photometric processing tests have been proposed by M. Weinberg, and these too will be pursued in the interim. This is preferred over holding a partial CDR in January with many items left for later cleanup.

  3. Persistence at Scan Endpoints -- Concern has developed recently over the effect of bright stars at the endpoints of scans that could cause persistence effects in those and subsequent scans without the stars themselves being included among the scans' point source extractions because of incomplete coverage (i.e., making apparitions on fewer than six frames). The current persistence removal algorithm uses bright point sources to trigger searches for persistence sources at predictably correlated locations in the same scan. Without the bright sources themselves, no search will be triggered.

    The suggestion was made that the Read1 sources detected by FREXAS be passed on by POSFRM for use in MAPCOR even when they have fewer than 6 apparitions. That way MAPCOR could use them to search for persistence in the scan, although such sources at the end of a scan would have to be carried forward for use in the next scan. Unfortunately, this looks impossible with the current asynchronous scan processing design (i.e., a scan needing such Read1 sources from its predecessor could easily be processed before it).

    It would be possible to handle Read1 sources at the beginning of a scan this way; the effects due to Read1 sources at the end could be left for final catalog preparation time, since some cleanup will be required then anyway. The incompletely covered Read1 sources would be appropriately flagged so that BANDMERGE and other subsystems could take appropriate action (TBD).

    It was pointed out that another approach to persistence could also be implemented that uses the identification of pixels on which the bright source was observed. Currently the search for persistence objects is done in space coordinates rather than pixel coordinates. Since the pixel hit by a bright source would be expected to decay exponentially, a search using a predictable pattern of negative Read2-Read1 sources involving that pixel should be efficient. It is not clear why the Read2-Read1 subtraction does not always produce negative sources, which would eliminate persistence objects, but apparently the persistence behavior is erratic enough to create some positive artifacts. It is also not clear how the poor fit involving a single frame persistence object stacked up with several apparitions of noise still emerges from KAMPHOT as a point source; some chi-square tuning may be needed, and perhaps these objects are systematically biased toward having some empty apparitions rejected because of dead pixels.

    The consensus was that the passing on of incompletely covered Read1 sources at the beginning of scans should be done, with changes to MAPCOR (and maybe PROPHOT) to use (or avoid) the information, and that BANDMERGE must have general purpose flag-dependent processing control. Furthermore, the pixel space search method should be implemented (presumably in MAPCOR), and global (i.e., not scan limited) persistence searches must be planned for final catalog preparation.

    It was pointed out that persistence could be caused by bright objects observed during slews, since there is no shutter. The cost of waiting for persistence to fade before beginning a scan has been investigated already, and it has a surprisingly large cost in survey time (reasonable delay times lead to survey extensions of about four months). Besides hoping that the slew rate will be sufficient to prevent serious persistence problems from slewing over bright objects, no plan for cleaning up such problems came forth.

  4. Artifact Blanking in Images -- The last meeting introduced the topic of whether to blank persistence objects in the Image Atlas. Science team discussions in the interim have led to the tentative decision not to do this, at least not unless the unblanked image can be obtained easily. This issue was widened to include all artifacts, not just persistence.

    The team preference seemed to be to provide the images with warning symbols placed at artifact locations, and to provide information in the FITS headers that could be used by IRAF and Skyview to replace these symbols with the original pixels. Users would thus be reliably made aware of the artifacts without requiring great effort to get at the original pixels. Where the symbol insertion and FITS header expansion would be done was left open.