Date: Wed, 1 Oct 1997 17:50:00 -0700 (PDT) From: firstname.lastname@example.org To: email@example.com Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: 2MASS WG Mtg #131 Minutes IPAC 2MASS Working Group Meeting #131 Minutes 9/30/97 Attendees: R. Beck, T. Chester, R. Cutri, D. Engler, J. Fowler, L. Fullmer, T. Jarrett, D. Kirkpatrick, G. Kopan, H. McCallon, B. Nelson, S. Wheelock, J. White AGENDA 1.) Optical-Catalog Associations 2.) Telescope Control Errors 3.) Early Data Release 4.) Policy Change Regarding Darks/Flats 5.) 14-Bit Tests DISCUSSION 1.) Optical-Catalog Associations As discussed in last week's minutes, the optical catalog association capability has been implemented in the POSMAN module POSPTS, complete with final tweaking of the 2MASS point-source positions (image position coordinate updating will be implemented in the near future). Analysis of early results, however, has shown that many 2MASS point sources are not associated with optical catalog objects (currently USNO-A and Tycho catalogs), and that two different reasons are obvious. First, many of the 2MASS objects are very faint and probably fairly red, thereby not showing up in the POSS plates from which the USNO-A catalog was made or in the Tycho survey. Second, many of the USNO-A objects have accumulated significant proper motion, e.g, in excess of one arcsecond; this is enough to cause them to fail the matching test with its initial thresholds, which were set to compromise completeness somewhat for the sake of reliability. The proper-motion effect illuminates the question of how we really want the thresholds to be set. On the one hand, it would be nice to make the associations for most of the objects that have moved, since this would facilitate proper motion studies. But in order to achieve that, the thresholds would have to be opened up enough to allow some spurious associations, especially in dense regions. Another question is how much the term "association" implies about the objects being related; clearly it implies less than "identification", but perhaps more than "happens to lie nearby". Some users of the association information will not be interested in proper motion at all. Since the associations supply five-channel photometric information when the associated measurements apply to the same object, extremely useful criteria are available for selecting specific classes of objects for further study. This usefulness would be significantly diluted if a large fraction of the associations were spurious because of wide-open thresholds meant to capture proper-motion information. R. Cutri will conduct a survey of the science team members who have special interests in the associations and gather a refined set of requirements which will be used to set the operational thresholds. It is likely that the data structure will change also, and the matching algorithm may require some recoding. 2.) Telescope Control Errors R. Cutri reported that telescope pointing control errors have been observed to increase significantly recently. What appears to be a hysteresis effect on the order of 20 arseconds is seen in right ascension in the data of 970927. The source of this problem is currently being sought. [NOTE ADDED IN PROOF: R. Stiening distributed a report on 971001 in which it is stated that errors up to 50 arcsec in RA and 16 arcsec in Dec occurred on a long slew across the zenith; after considerable investigation and hardware adjustments (tightenings and realignments), slews were achieved which returned to their starting point to within less than 2 arcsec on both axes, so this problem seems to be solved.] 3.) Early Data Release R. Cutri reported on a telecon in which early data release had been discussed. The request for early data release came from NASA Headquarters. The project concern is the one usually associated with early data release: assuring the quality of the data without diverting resources needed elsewhere to conduct the survey efficiently. It appears that an acceptable plan has been developed that involves phased releases. In the near term, the 2MASS image gallery already on the web will be made available to the scientific community; this image gallery has been around long enough for any concerns about its quality to have dissipated. In two to three months, several carefully selected coadded images and corresponding source lists will be added. Sometime around next June, about half a dozen survey scans will be made available on line, accompanied by real access tools. 4.) Policy Change Regarding Darks/Flats R. Cutri and J. Fowler reported on a telecon with M. Skrutskie in which darks/flats acceptance thresholds and data utilization policy were discussed. Early in the design stage of the DARKS subsystem, the project expectations were that the hardware would be fairly stable in terms of dark response and pixel responsivity; this led to a policy involving testing each night's dark and flat sequences against "canonical" values with fairly tight acceptance criteria. If the night's darks/flats deviated significantly from the canonicals, then they were not to be trusted, and the canonical darks/flats were to be used. If the night's results were within threshold, then they were to be used instead of the canonicals, in order to track small variations. The assignment of any set of results to "canonical" status was to be done only via careful offline analysis, not in an automated fashion. In practice, the nightly results almost always have failed the acceptance tests. Every time any change to the hardware has been made, the behavior has changed enough to cause rejection when compared to canonical darks/flats obtained from data taken prior to the hardware change. Even the amount by which the tests are failed drifts significantly from night to night. The problem is paradoxical in that the more the night's results differ from the canonicals, the less they can be trusted, but the more inapplicable the canonicals may be. Since most nights processed so far have used darks and flats that were probably sub-optimal, one might wonder how the products could be coming out of the pipeline with such apparently high quality. This is most likely due to the following facts: (a.) the responsivities do not seem to suffer as much from variation as the darks; (b.) the flattening method in PIXCAL/DFLAT depends on additive pixel corrections (not multiplicative) which apparently are able to absorb practically all of the dark model error. So the good news is that the processing seems to be rather robust. It was decided that we want to track slowly varying changes in hardware behavior, and so two changes will be made: (a.) the acceptance thresholds will be opened up enough to allow more nightly-versus-canonical tests to be passed; (b.) the concept of "canonical" darks and flats will be modified - for purposes of the DARKS subsystem, the canonical data will be those used in the most recent previous observation night, independently of whether those were computed from that night's data. This allows each night's darks/flats to become the new canonicals automatically if they pass the tests relative to the previous canonicals, thereby tracking real slowly varying changes. At the same time, a "grand canonical" set of darks/flats/masks will be maintained under human judgment for most quality analysis; these will be the first acceptable set after known hardware configuration. For now, these will be the set obtained from the night of 970903. 5.) 14-Bit Tests Most subsystem cognizant engineers have now reported to R. Cutri on the effects of the 14-bit simulation (see last week's minutes). The consensus was that we should avoid going to 14-bit A/D converters if feasible. J. Fowler reported degadations in the output of BANDMERGE at the 1% to 3% level. L. Fullmer reported that direct source comparison of the same scans processed both ways showed small mean position offsets, but position-discrepancy standard deviations well above 0.1 arcsec for all S/N bins.