From email@example.comWed Sep 10 14:42:27 1997 Date: Fri, 30 May 1997 17:57:02 -0700 (PDT) From: firstname.lastname@example.org To: email@example.com Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: WG Mtg #124 Minutes IPAC 2MASS Working Group Meeting #124 Minutes 5/27/97 Attendees: R. Beck, T. Chester, R. Cutri, T. Evans, J. Fowler, L. Fullmer, T. Jarrett, G. Kopan, B. Light, H. McCallon, B. Nelson, J. White AGENDA 1.) Band-to-Band Position Anomalies 2.) Unset Par File Parameter 3.) New Dithering Pattern 4.) 2MAPPS Part of Next Week's Review DISCUSSION 1.) Band-to-Band Position Anomalies H. McCallon presented evidence that the H array moves slightly relative to the J and K arrays during scans and slews. It is not yet determined that the H array alone is involved, but further tests should reveal whether it is. The latest version of PFPREP computes a 5-parameter band-to-band alignment model for all band pairs on a single-frame basis. For sufficiently dense scans, the parameters are relatively well determined at the frame level. The five parameters for any given band pair are: offsets in X and Y, scale factors in X and Y, and rotation angle. Data from 970514 were used in the study presented. No anomalies have been observed in the scale factors or rotation angle, but the X and Y offsets show statistically significant variations over the night and during most 6-degree scans. Scan 036 in particular showed an unusually large change of almost a full arcsecond in J-H offsets between the beginning and end of the scan. Whereas some band-to-band alignment change due to flexure has been expected, it was not supposed to be large enough to make the 5-parameter model inapplicable to a complete scan, and so the pointing reconstruction does not currently include (e.g.) a linear fit to these parameters over the duration of a scan (in fact, sparse scans could not supply enough data points for reliable linear fits). The changes in offset over scan 036 are clearly nonlinear; a linear correction would remove most of the error, however (a visual estimate based on the plots would be about 80%). As it stands, scan 036 would have J-H and H-K alignment errors of about plus or minus 0.5 arcsec over the scan. Some of this would be reduced by band merging in the case of multi-band sources successfully merged (0.5 arcsecond error would not make a serious impact on band-merging accuracy except in confused cases). It is not known whether scan 036 is as bad as the phenomenon ever gets, however. Plots of excursions over the entire night show what appears to be a random walk covering several arcseconds, and the possibility that such a large excursion could occur during a scan cannot be ruled out. The large effects are seen in J-H and H-K, but J-K also shows a statistically significant effect that is much smaller and appears correlated with the others over the night. Further study of the phenomenon and possible software enhancements will be pursued. NOTE ADDED IN PROOF: H. McCallon later established high correlations between all offset variations (i.e., all band pairs and both axes) and identified the H-pickoff dichroic as a possible source for the effect, since it would be capable of producing the large J-H and H-K variations with the small correlated J-K variations. Information has been set up on the 2MASS web site discussing this study. 2.) Unset Par File Parameter J. Fowler reported that during investigation of the anomalies discussed in section (1.) above, he and H. McCallon discovered that the array temperatures were always 0 deg C as reported in the "par" file sent from the observatory. Since the origin of the anomalies was a mystery at the time, correlation with any and all available parameters was being sought (e.g., zenith angle, time, temperature, etc.). It was obvious that the array temperature parameters were not being set by the output-generating software at the observatory. This prompted a check of other parameters in the par files that might have apparent values that were not actually resulting from measurements, and this yielded the following "group 2" (supposed to be measured at scan start time and again at scan end time) parameters: wind speed, wind direction, and seeing. The barometric pressure, which is needed for the refraction model in POSMAN, was obviously being set on a one-value-per-night basis, which implies dubious usefulness, but at least its value changed from night to night. The working group voted to recommend that the par files not include any parameters that are not actually measured (since they appear to be measured quantities in the downstream data flow, creating confusion and possible false conclusions), that the array temperature and barometric pressure parameters be actually measured and recorded in the par files at the start and end of each scan, and that if corresponding changes to the observatory software are not feasible, then the RDFRAME module will be modified to prevent erroneous parameter specifications from being included in the downstream data flow or at the very least attach FITS header comments that identify such parameters as received from the observatory but not to be used because of not having been actually measured. 3.) New Dithering Pattern T. Chester reported that a new dithering pattern was supposed to be in effect as of 970514, and he requested that analysis be done to verify that the new pattern is being executed and that it yields the desired improvements. The modifications for reducing the electrical noise have also been made, and these have already been found to be successful. 4.) 2MAPPS Part of Next Week's Review R. Cutri reported that although 2MAPPS is not the main subject to be dealt with in the upcoming review, its products will nevertheless play an important role in judging the operational readiness of the observatory. The status of 2MAPPS itself will be discussed in the morning of the second day. Final lien schedules should be sent to J. Fowler for inclusion in the document to be posted on the web for the Science Team to read before coming to the review.