Date: Fri, 6 Mar 1998 12:46:09 -0800 (PST)
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To: 2mass
Subject: IPAC 2MASS Working Group Mtg #145 Minutes
Cc: chas,, stiening, bgreen
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           IPAC 2MASS Working Group Meeting #145 Minutes

Attendees: R. Beck, C. Beichman, T. Chester, R. Cutri, D. Engler,
           T. Evans, J. Fowler, L. Fullmer, T. Jarrett,
           D. Kirkpatrick, G. Kopan, H. McCallon, B. Nelson,
           B. Wheaton, S. Wheelock, J. White


1.) Obsolete Saturation Parameter
2.) Science Team Meeting
3.) Photometric Error Estimates
4.) Southern Observatory Status
5.) Northern Observatory Status
6.) New 2MASS Hires


1.) Obsolete Saturation Parameter

    B. Wheaton reported that in working on replacing the single-
number soft-saturation values with individual-pixel values, he
ran across a parameter named "hibad" that is used in DAOPHOT-
derived aperture magnitude routines to characterize what is being
replaced with the soft-saturation values (i.e., pixel values too
high to be trusted). Code that references the hibad parameter is
to be found in FREXAS, PROPHOT, and GALWORKS. Rather than change
the code, the team decided to set the value of hibad to an
impossibly high number (99999.9 was chosen) so that any tests of
whether a pixel's value is greater than hibad will fail. All
subsystem cognizant engineers are requested to verify that either
their code does not use this parameter or else that its value is
set to 99999.9.
    The full-image soft-saturation data are not quite ready, but
the software modifications are ready for test using a full-image
composed of a single value for all pixels. The first tests will
use the value that has been used as the one-size-fits all value,

2.) Science Team Meeting

    R. Cutri reported that the Science Team Meeting on the
previous day, 2 March 1998, had resulted in excellent reviews
overall for 2MAPPS 2.0, with only a few relatively small liens to
be removed in the next few weeks before production could begin.
The only significant lien involves further tuning of MAPCOR
parameters, since some concern remains about how bright-star
artifacts are handled. It was also decided that a correction
algorithm for J bias jumps will be implemented, but that will not
be part of 2MAPPS 2.0; the risk of attempting to fix these bias
jumps by hardware replacement was judged too high. Tests of the
effectiveness of such algorithms are required by May 15, and if
all goes well, inclusion in 2MAPPS should occur sometime this
summer. In the short term, the goal is to freeze 2MAPPS 2.0 in
two or three weeks. J. Fowler pointed out that a waiver may be
needed for MPCAT, for which there is still no automated selection
of the appropriate orbital element set for the observation epoch;
adding this later should not impact data products, but it would
make operations much easier and more reliable.

3.) Photometric Error Estimates

    One additional item that came out of the Science Team review
was concern about the photometric error estimates are computed
for aperture magnitudes. Single-frame aperture magnitudes have to
be averaged over the multiple apparitions, and in the process the
single-frame photometric uncertainties are use to compute the
"refined" uncertainty of the averaged magnitude. Currently the
uncertainty sent downstream is the larger of two that are
computed differently. Code that performs this averaging exists in
PFPOST, which does it for Read1 sources, and PROPHOT, which does
it for Read2-Read1 sources. The goal has been for the two to
operate as similarly as possible. Both currently compute the mean
magnitude by inverse-variance-weighted averaging of the single-
frame magnitudes in the flux domain (to accomodate negative
contributions to the average). For such an average magnitude, the
formally correct error variance is the inverse of the sum of the
inverse of the input variances. But the code also computes the
observed dispersion in magnitude values, estimates the population
variance as N/N-1 times this dispersion, and then computes the
reduced variance (divides by N), referred to as the "reduced
population variance". This would be an appropriate error variance
for an unweighted mean, but is formally not correct as an error
variance for the inverse-variance-weighted mean. Nevertheless,
the code passes on the larger of the two variances as a sort of
conservative estimate.
    The concern, expressed primarily by M. Weinberg, is that
testing of statistical significance is impaired by carrying an
error variance that is not appropriate for the way in which the
mean is computed, and also that the error model in the aperture
photometry routine is apparently not considered correct, since
its output is not trusted when the observed dispersion is larger.
At the least, an unweighted mean should be used when the reduced
population variance is used as the error estimate. A splinter
group composed of R. Cutri, J. Fowler, and B. Wheaton will take
the action item to resolve this issue.

4.) Southern Observatory Status

    R. Cutri reported that first-light data from the southern
observatory are expected as early as Wednesday night. The front
end of the pipeline has been tested as well as it can be to
prepare for fast-turnaround processing of these data. Subsystem
cognizant engineers are requested to deliver their southern-
hemisphere NAMELIST files and any other relevant data files to
/2massc/del/datas by the end of the day tomorrow (Wednesday).

5.) Northern Observatory Status

    Replacement of invar rods and recollimation of the northern
telescope was discussed at the Science Team meeting. The working
group expressed concern about whether this would make the PSF
tables obsolete, necessitating another recomputation (a time-
consuming activity, since many different seeing conditions must
be processed). This issue was not entirely resolved, but since
one outcome of the Science Team meeting was a caution by J. Elias
to allow schedule time for retuning and recalibrating after
hardware changes, and since some hardware changes may prove to be
inevitable, it seems likely that the capability to recompute PSFs
in a timely fashion would be prudent, and a need for it should be
expected in the near term.

6.) New 2MASS Hires

    T. Chester reported that two people were being hired for the
2MASS project, a 2MASS Analyst and a 2MASS Science Liaison