From email@example.comTue Mar 25 09:38:34 1997 Date: Thu, 13 Mar 1997 17:33:54 -0800 From: firstname.lastname@example.org To: email@example.com Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: WG Mtg #117 Minutes IPAC 2MASS Working Group Meeting #117 Minutes 3/11/97 Attendees: R. Beck, T. Chester, R. Cutri, T. Evans, J. Fowler, L. Fullmer, T. Jarrett, G. Kopan, B. Light, S. Wheelock, J. White AGENDA 1.) Single-Frame Extraction Position Statistics 2.) Telescope/Camera Status 3.) Effects Due to Band-Dependent Plate Scale 4.) Source Merging Splinter Session Report DISCUSSION 1.) Single-Frame Extraction Position Statistics G. Kopan described a study he has been working on regarding position statistics for the single-frame point-source extractions. He has found some significant dependences of position errors on seeing, especially in the differences between Read1 sources and Read2-Read1 sources. The latter differences appear to be largely due to "freezing" of the seeing over the short integration time for Read1, with coherent seeing displacements over the focal plane. This causes a significant correlated shift between the positions of the two read types. Investigations including several scans with poor-to-bad seeing have indicated that the "pfrac" parameter is probably usable as a position-uncertainty model parameter, possibly in a form as simple as making the position sigma a linear function of the point-spread function's FWHM, with the latter being a TBD function of pfrac. Care is needed in defining the position uncertainty, however, since simply assigning a seeing-dependent sigma to Read1 sources would not be appropriate both to dispersions in Read1 positions about mean position (i.e., averaged over six apparitions in scan coordinates) and dispersions in position discrepancies between the two read types (because of the highly correlated Read1 position shifts due to freezing the seeing). Thus the use to which the uncertainties will be put must be clearly agreed upon before an exact model can be established. Some discussion points that arose were as follows. A.) Since dense scans involve clipping FREXAS input to PFPREP, there can be more Read1 sources input than Read2-Read1 sources, hence Read1 could dominate the frame-offset solutions; this should not be allowed. B.) If inverse-variance weighting is used in the frame-offset solution, then giving the Read1 sources position uncertainties including the component due to freezing the seeing might suffice as a way to prevent the Read1 sources from corrupting the solution. C.) Saturated Read2-Read1 sources should not be allowed to contribute any position information in PFPREP or POSFRM. 2.) Telescope/Camera Status R. Cutri reported that first light was achieved last week at the northern observatory. Optical alignment is on hold while polar alignment problems are being worked out. Pointing reproducibility over short time intervals appears to be good (about 5 arcsec at some high confidence level), based on tests involving slewing away from a given position and returning to it. The dates for receipt of three-channel data at IPAC and start of survey are uncertain. 3.) Effects Due to Band-Dependent Plate Scale Possible software impacts due to band-dependent plate scale were discussed. The intersection of all three arrays mapped onto the sky is expected to be such that excess area of some arrays may be as large as a pixel or more. Some sources detected in or near this region that should be multi-band will be missing some bands, and the different edge tolerances in the detection and extraction phases may yield anomalies. In addition, plate scale differences will make it very difficult to achieve optimal scan dithering in all three bands. The fracturing of multi-band sources near the cross-scan edges will probably have to be fixed during catalog preparation time, since it appears inadvisable to limit detection activity there, and in any case, multiple records of sources in the scan overlap regions will require sorting out, as in the Faint Source Survey. Fractured sources will have been scanned better on adjacent scans, except at coverage gaps. The point was made that one large gap was preferable to an equal area distributed over many small gaps, because the latter would involve more edge artifacts. It was suggested that a flag indicating the proximity of a source to the edge of the scan might be desirable. The tentative decision was that the U-Scan X coordinate would serve this purpose; if its absolute value is more than about 250, then the source came within an arcsec or so of the edge. The exact value will depend on more detailed information regarding plate scale and array alignment. 4.) Source Merging Splinter Session Report After the previous meeting (two weeks ago), a splinter meeting concerned with source merging problems was held. This splinter meeting continued the following week and replaced the regular meeting. The issues involved and decisions made in that splinter session will be reported herein. Issues A.) Read1 sources occasionally appear replicated or fractured into multiple sources. B.) When confronted with too many Read1 sources around Read2-Read1 sources, BANDMERGE often joins the two read types (e.g., Read2-Read1 J detection with a Read1 H detection, although a Read2-Read1 H detection is also available). C.) Sometimes BANDMERGE joins a detection with good photometry with a detection that has no photometric information (i.e., both the PSF magnitude and the aperture magnitude are 99.999). Issue A was found to result from the PFPREP search box being too small. PFPREP computes an initial estimate of the frame offsets using only J Read2-Read1 sources; for this a search window half-width of 3 arcsec in preliminary U-Scan coordinates is used. After the improved U-Scan coordinates are available, the other bands and read types are merged; for this the half-width has been 1.5 arcsec. A typical fractured Read1 source was traced to this point and found to have components separated by as much as 2.55 arcsec; the problem occurred because the "seed" for merging (the first detection in frame order) was the farthest from the mean position, leaving other pieces outside the window to form their own merged source later. Examination revealed that the full-width separation of 2.55 arcsec between Read1 pieces was not unreasonable and corresponded to about a 5-sigma event, which can be expected to occur because of the large number of sources that must be processed. Furthermore, this is just over one pixel in size, and the chances of producing two distinct Read1 detections this close is negligible, so opening up the window to 3 arcsec appears reasonable. This was the decision for resolving this issue. Issue B involves the fact that BANDMERGE uses a position chi-square test to judge the acceptability of different-band detections for merging. This is probably about as good a test as possible when the candidates have position errors drawn from approximately the same Gaussian random population. Since the Read1 and Read2-Read1 position errors apparently constitute different populations, the type of test may need to be changed. A goodness-of-match parameter such as the one used in the IRAS point-source processing was suggested; this involves the cross-correlation of the two position density functions evaluated at the observed separation. This has the advantage that, all else equal, it is biased against measurements with larger uncertainties, since the corresponding density functions are broader, hence lower, hence more inclined to fall below an acceptance threshold. It has the disadvantage that thresholds cannot be set purely on the basis of the acceptable fraction of real cases to sacrifice in the attempt to avoid false matches, unless the actual distributions of position errors are known in advance. For IRAS, Monte Carlo simulations were used to set thresholds. But for IRAS, the density functions were highly non-Gaussian, which is one reason the particular method was used instead of something like chi-square tests. This dilemma was alleviated somewhat by the realization that the Read1 position uncertainties have been artificially high during all the tests so far. This is a result of the temporary substitute POSFRM, which was inserted solely to pass semi-realistic data downstream so that the rest of the pipeline could be tested. This version of POSFRM (-0.9) did not attempt to reduce the Read1 uncertainties as it averaged their single-frame positions; when this is fixed, the Read1 uncertainties will not be so different from the Read2-Read1 uncertainties (this may change when the impact of Agenda item 1 above is fully absorbed), and the Read1 position error model has been preliminary anyway. For now, the decisions made by BANDMERGE with reduced-uncertainty Read1 input will be studied, and no fundamental change to the decision algorithm will be made unless it becomes clear that it is still necessary. Improved Read1 merging in PFPREP should also alleviate this problem. For the remaining cases of merging the two read types, and for the sake of Issue C, some augmentation of the BANDMERGE goodness-of-fit parameter in cases of diagnosed confusion were considered. The method adopted is as follows. Once at least two candidates pass the position test for merging with a seed, the chi-square parameter will be expanded by adding what is most easily described as a dot product of two vectors. The components of the first type of vector are the MAPCOR flag elements describing merge/purge/persistence and a yes/no component describing whether the detection has a non-99.999 magnitude. Each detection being considered for merging has such a vector, and the difference vector will be computed. Each component of the difference vector will be made positive, and then a dot product with the second type of vector will be taken. The second type of vector has components expressing the weight of the corresponding components of the first type of vector. The dot product will be added to the position chi-square, and the smallest result will determine the candidate to be merged with the seed. This much will be tested. If it works acceptably, other more complicated measures will not be taken; for the record, these involved the following suggestions. A.) The search window in PFPREP could be a function of the read-type combination, or a function of the position uncertainties encountered, or a function of the seeing, or a function of the detection magnitudes, or some combination of these (the position uncertainties themselves could be functions of seeing in addition to signal/noise ratio; see Agenda item 1 above). B.) Another pass could be added to PFPREP in which mean merge-group positions serve as seeds (this would avoid the probem of selecting the worst outlier of a group as a seed; it would be rather expensive and risky to implement, however). C.) A new program could be inserted between POSFRM and PFPOST to massage the merge-group file before Read1 sources are taken from it to pass on to MAPCOR; this could fix fractured groups. D.) The order in which PFPREP processes band/read-type could be changed; currently, after the preliminary J Read2 frame-offset computation, the processing is simply J/1, J/2, H/1, H/2, K/1, K/2. It woud probably be better to use all the Read2-Read1 detections as seeds before going to Read1 seeds.