IPAC 2MASS Working Group Meeting #85 Minutes

IPAC 2MASS Working Group Meeting #85 Minutes, 1/30/96

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

  1. Telescope Status
  2. PIXCAL Tests and Timing
  3. Distortion Modeling
  4. Karloff Disks Cross-Mounted to Lugosi
  5. Dependence of (R2-R1)-R1 Magnitudes on Scan Direction


  1. Telescope Status -- R. Cutri and C. Beichman reported that agreements between the project, the telescope manufacturers, and the bonding company have led to the decision that the telescope assembly can be kept on the same schedule and budget as previ- ously planned. The use of the new telescope for the northern-hemisphere sur- vey is now as likely as the use of the Kitt Peak 50-inch telescope. [Note Added in Proof: M. Skrutskie stated later that '...the new telescope on Mt. Hopkins has revived from being a longshot to being a 50/50 competitor with the 50".'] The 2MAPPS schedule remains compatible with reception of data from the new telescope in September 1996.

  2. PIXCAL Tests and Timing -- J. Fowler and G. Kopan reported that the 2MAPPS processor consisting of the integrated subsystems PIXCAL, RDFRAME, DFLAT, and FREXAS has successfully executed in a test environment. The interfaces between all four subsystems (and that between them and the PCP subsystem, which will run this processor) appear to be functioning correctly, and the test environment and input data formats duplicate those of 2MAPPS. Output data appear to be correct. Future input from the observatory should be able to be processed by this program.

    Some anomalies were observed in the timing measurements, however. The variation of CPU and I/O time with processing load was inconsistent, with lighter processing loads sometimes taking more time, and with identical execu- tions taking different times. Apparently the way in which CPU and I/O times are measured by the operating system is not as straightforward as expected, and variations of about 30% are encountered, probably depending on other activities on the machine. Even so, the execution times are close enough to the FDD allocations to eliminate concern. Comparable processing in the proto-pipeline takes about ten times longer; the main speedup is believed due to the greater efficiency in FREXAS relative to DAOPHOT.

  3. Distortion Modeling -- J. Fowler, G. Kopan, and B. Light reported that the baseline 2MAPPS approach to handling optical distortion is currently to retain documentation of a plan for implementing it if needed, to derive certain statistical para- meters in POSFRM that will reveal whether optical distortion effects are present to a significant extent, but otherwise to assume that distortion corrections are not needed unless evidence to the contrary arises. The PROPHOT and PICMAN SDS's will carry liens flagging the fact that distortion correc- tions are not being made.

    The decision to adopt this approach is based on:

    POSFRM will accumulate statistics on the means and variances of the position separations between point source apparitions in each panel and refined position estimates (i.e., based on all apparitions). This will be done in a grid over focal plane position. Significant distortion should produce significant mean separations, and the dependence on focal plane position should allow a distortion correction model to be derived if needed.

    This model would be implemented in PROPHOT as a subroutine that converts apparent pixel coordinates to non-distorted coordinates, so that PROPHOT can stack apparitions optimally. It would not be necessary for PROPHOT to depend in any way on the internal details of the correction subroutine.

    PICMAN would have to implement the correction in a more complicated way. Whereas PROPHOT spends relatively little time doing position dependent computations, PICMAN would need the distortion corrections inside its most deeply nested loops. This would probably require model dependent coding to implement the corrections in the most computationally efficient way.

    No claim is made that the cost of implementing a distortion correction is prohibitive, only that it is too high to undertake without a clear need. Furthermore, the discussion above should not be considered a final description of the issues involved or the methods to be developed. It should also be noted that this discussion does not address differential refraction, effective scale factor variation during scans, or other questions to be investigated by H. McCallon.

  4. Karloff Disks Cross-Mounted to Lugosi -- R. Cutri reported that D. Wittman is in the process of cross-mounting karloff's disks to lugosi. Since karloff's CPUs are being used heavily, use of lugosi's CPUs for processing data on karloff's disks is currently recommended.

  5. Dependence of (R2-R1)-R1 Magnitudes on Scan Direction -- T. Evans reported that the differences in aperture magnitudes for Read1 and Read2-Read1 detections of the same sources show a dependence on scan direction. She displayed graphs of this phenomenon and supplied the following summary which constitutes the remainder of this section.

    The R1 and R2-R1 (.ph) sources in each of the HHE survey scans taken on photometric nights have been matched to each other and compared. Only the sources with both R1 and aperture R2-R1 magnitudes between certain cutoffs were used:

    		J: 9.5 <= mj <= 11.0 
    		H: 9.0 <= mh <= 10.5 
    		K: 8.5 <= mk <= 10.0
    Note that the R1 magnitudes were calibrated to the aperture R2-R1 magnitudes by only one R1 offset per band over the whole May '95 observing run.

    The plots of R1-(R2-R1) magnitudes (DM) vs. X (and, shown in previous meetings, Y) positions show no significant trends with position, but the average DM for ascending scans is slightly more positive [m(R1)>m(R2)] than for descending scans. Here are the approximate values (meaning the fitted line intercept at X=0):

    J: asc: .044 +/- .003         desc: .019 +/- .004 
    H: asc: .014 +/- .001         desc:-.013 +/- .002 
    K: asc: .017 +/- .002         desc:-.009 +/- .003
    The difference offset of from 0 is usually more statistically significant for the ascending scans than the descending scans. The difference between the two averages for each band is about .026 mag. It is assumed that this difference is caused by differences in the chopping secondary action or settling time for ascending and descending scans, or some other telescope dependent effect. Some careful testing of the survey telescopes is needed; but note that the survey R1 photometry will be calibrated to the aperture R2-R1 magnitudes on a scan-by-scan basis. If the problem is only with the R1 data, this calibration scheme should theoretically remove the effects.