IPAC 2MASS Working Group Meeting Minutes, #57

IPAC 2MASS Working Group Meeting #57 Minutes, 3/07/95

ATTENDEES

C.Beichman, T.Chester, T.Conrow, R.Cutri, D.Engler, T.Evans, J.Fowler, L.Fullmer, G.Kopan, B.Light, C.Lonsdale, H.McCallon, S.Wheelock, J.White

AGENDA

Implementation Plan
Effect of Seeing on Galaxy Processing
Simulation Including Dead Pixels
Protocam Pipeline Changes for April Data
- Diffraction Artifacts
- Quick Look Turnaround
- Time-Dependent PSF
- Perl-Script Multi-CPU Usage

DISCUSSION

Implementation Plan -- C. Beichman reported that the 2MASS Implementation Plan has been sent to NASA HQ and UMASS, where its warm reception is a source of optimism. The plan involves about half of the IPAC employees and covers a period of time from the present through the year 2002.
Effect of Seeing on Galaxy Processing -- T. Chester reported that studies of the effect of seeing on galaxy processing have shown that for total PSFs between 1 and 3 arcsec FWHM, no significant effect in star/galaxy discrimination is found, provided a reasonably accurate PSF model is employed. The loss of sensitivity with a 3-arcsec PSF is negligible for galaxies, and shape-parameter calculations are still dominated by the undersampling due to pixel size.
Simulation Including Dead Pixels -- B. Light reported that tests involving the inclusion of dead pixels in the Simulator (first discussed in last week's minutes) have now been extended to use the mask generated by CFlat (previously only the input mask had been used; CFlat turns off additional pixels based on responsivity anomalies). The CFlat output mask has more dead pixels indicated around the periphery of existing islands of masked pixels, increasing the local systematic effects. In addition, artificial masks have been used. One artificial mask was obtained by rotating the CFlat output mask through 90 degrees and ORing the result to the unrotated mask. Another artificial mask was obtained by turning off randomly selected pixels until 2% were masked off.
These tests showed that aperture photometry was significantly affected by the higher fraction of dead pixels, but KAMPhot photometry was not, for typical scans and the current operating parameters (e.g., aperture size, thresholds, etc.). The slight degradation of KAMPhot results is still overwhelmed by the effects of interpixel response structure (modelled as a central hole of 0.075 arcsec radius and a square border of 0.075 arcsec width, within which the responsivity is zero).
Protocam Pipeline Changes for April Data -- Changes to the protocam pipeline for the April data were discussed. Several different areas were involved and are described in separate subsections below.
- Diffraction Artifacts -- C. Beichman reported on a study of artifacts created by the diffraction spikes caused by the telescope secondary mirror supports. The rate at which such false point sources are produced is significant, and automated removal of them within the pipeline is desirable. He will provide guidelines for code changes to J. White, who will include them in the code that removes ghosts and persistence objects.
- Quick Look Turnaround -- R. Cutri pointed out that although the data-taking period is only a few weeks and the pipeline is not optimized for fast turnaround, we should nevertheless not overlook the possibility of getting information regarding any anomalies found early on at IPAC to the observatory in time to take corrective steps. This should be possible if the observations to be taken and analyzed first are carefully planned so that the types of observations most likely to profit from quick response are obtained near the beginning of the observing period.
- Time-Dependent PSF -- C. Beichman requested that some effort be expended to provide better tracking of PSF variations than has been done up to now. The guidelines offered were that PSF time variations should be modelled with at least a frequency of once per 24-hour period, certainly no more than once per scan, and not including spatial variations over the focal plane. Several ways to do this were discussed, and each involved difficulties.
  T. Chester reported that T. Jarrett has a program that examines coadded images and (among other things) determines an effective PSF. This could, in principle, be used to generate time-tagged PSF FWHM values, but would require special code and modifications of the pipeline to fit it in. This effort would not produce code portable into the 2MAPPS pipeline, which goes against a guideline urged by J. Fowler with respect to new software developed at this time.
  G. Kopan and J. Fowler had already looked at the possibility of getting PSF FWHM values for KAMPhot to use from the existing PSF and PSFGRID programs. These can run on the aperture-photometry results from DAOPhot that are used in the frame-offsets code, but best results have been obtained in the past by using KAMPhot output for its significantly better centroid results, without which the DAOPhot input would effectively convolve the PSFs with a centroiding error distribution that appears too difficult to deconvolve. In addition, PSF and PSFGRID have been used only on scans ideal for PSF determination, i.e., high density of point sources but unconfused. It is not clear that results from sparse or confused scans would be usable by KAMPhot, and double-passing the data would be required.
  Since KAMPhot has code to compute its own PSF from the scan data it is processing, it could conceivably be made to provide time-dependent PSF modelling. The disadvantages to this approach are that it requires very high CPU usage to do this, and it cannot be trusted to produce stable estimates. Code to modify its PSF computation to make it more stable would not be usable in 2MAPPS, wherein a separate method is planned.
  The 2MAPPS design for PSF estimation could be attempted, but this means rushing the development of that capability, which is not currently scheduled to be underway at this time. The method involves the STATS subsystem estimating the PSF FWHM from the same data it is using for frame-offset computation. Since some statistically useful number of point sources must be examined to get a stable estimate, the plan is to output a time-tagged estimate once every N (TBD) point sources, since this allows higher frequency time tracking of the PSF in dense regions, and while the tracking frequency will be lower in sparser regions, a smaller number of point sources will be affected. What STATS actually is to estimate is the value for a one-parameter seeing model that describes the optical PSF convolved with a seeing disk; the parameter is expected to be derivable from the FWHM estimate, and the whole algorithm will be constrained to make it stable. This, however, is a lot to develop and test before May.
  KAMPhot obtains its PSF model in a sufficiently modular way that patching in various algorithms' results does not appear to involve significant resources or risk. G. Kopan is currently working on the STATS subsystem, but not the PSF-estimation part, only the frame-offset determination part. Nevertheless, it was decided that he and B. Light will pursue modifying his code to include the computation of a 90%-encircled light value to be passed to KAMPhot for use in setting up its PSF model. If this does not run into significantly threatening obstacles, it will be used to obtain the desired PSF time dependence; otherwise, we will have to rethink how to handle this concern.
  J. Fowler pointed out that some form of PSF RTB (Regression Test Baseline) will need to be set up so that we will know what the effects of various code changes are. A subset of existing protocam scans will be identified for this purpose. These will cover the variations in density and confusion needed but will be kept small enough not to introduce disk storage problems.
- Perl-Script Multi-CPU Usage -- The April data will be processed on karloff, which has just had its CPUs upgraded from 50 MHz to 75MHz (G. Kopan has run benchmarks that show the throughput has indeed scaled upwards by 50%), and it is expected that a second similar machine will be available also. In order to make use of the multiple CPUs, it has been suggested that T. Conrow's perl-script protocode be expanded to handle the running of entire individual scans as client/server tasks (see last week's minutes). This involves significant modifications to the pipeline and the perl-script code, but a cursory inspection by T. Conrow, G. Kopan, and J. White of what will be needed has indicated that it should be possible with reasonable expenditure of effort. If successful, it would constitute a major step forward in 2MAPPS pipeline design, and so it will be pursued. If unforeseen obstacles arise to make it impractical in the short term, then techniques developed by R. Beck for use on separate sparcstations will be employed.