Protocamera Run Point Source Photometry Update

Date: 24 July 1995

To: 2MASS Science Team

From: Roc Cutri

Re: Protocamera Run Point Source Photometry Update


Observations were carried out during the April/May 2MASS Protocamera Run to enable testing of several open issues concerning point and extended source processing and photometry. Analysis of the processed Protocamera data from that run has now been underway for the last several weeks. The purpose of this memo is to provide Team members with a brief overview of preliminary analysis results for point source photometry in preparation for the upcoming Science Team meeting on Aug. 2-3, 1995, and to solicit feedback on the various issues. Please refer to the Galaxy Photometry Results memo issued by Chester and Jarrett on 18 July 1995 for a discussion of relevant extended source issues. The major issues identified during the last 2MASS Science Team meeting relate to the overall uniformity of point source photometry, and the ability to meet the Level 1 Specifications. They include:


Initial processing of the April/May 1995 Protocamera run data with the operational (ops) pipeline was completed at 2:15am on 20 July 1995. Coadded images, point and extended source lists with photometry and ancillary files are available on-line to Team members. The location of data for each scan is listed on the WWW 2MASS Protocamera Processing Status page which can be reached via the IPAC/2MASS information page.

All Team members are welcome and encouraged to review the data products. You will need an active ipac computer account to view or retrieve files, so please contact Jack Lampley at IPAC (818-397-9551) to set up an account if necessary.

Point source extraction, position reconstruction, aperture and psf-fit (KAMPHOT) photometry, extended source identification and photometry, and source list cleaning and merging was carried out separately out for each scan. All scans that displayed difficulty in any stage of processing are flagged in the Bad Scan Log for each night. The most common processing problem was position reconstruction; reconstruction failed for 5-7% of the scans, primarily in very high source density and/or high extinction regions because of difficulties in matching visual wavelength guide stars with highly confused near infrared star fields. No effort will be made at this time to complete position reconstruction for these fields. Fields that lack proper position reconstruction still have source extractions, photometry and coadded images in most cases. New tools that were developed for the position reconstruction of IRTS data have been tested on selected confused 2MASS regions and successful position reconstruction has been achieved. These tools will be further tested and incorporated in the final version of 2MAPPS.


A number of special observations were made during the April/May run to examine how the photometric response varies across the 2MASS Protocamera detector focal plane. The observations consisted of 10 repeated one degree scans across M67, with the telescope stepped 40" (20 camera pixels) in the cross-scan direction (east) between each scan. Since each point on the sky is sampled six times during a scan, a star could fall on up to 60 different positions on the array during the 10 scans. The photometric response around the focal plane is mapped by measuring how the instrumental brightness of a star varies with position on the array.

Factors other than detector response can contribute to brightness variations in a source, such as pixelization, seeing and atmospheric transparency. To minimize the impact of these other effects, and to improve the statistical accuracy of the measurements, we make use of large numbers of stars observed in the 10 scans. In addition, we evaluate how the brightness of a star varies in each apparition relative to its average brightness for all apparitions.

The steps used to create a photometric response "map" are as follows:

The relative photometric response map is the 10x6 array of average brightness ratios. In practice, a brightness threshold is imposed on the sources used to generate the map to ensure that the ratios calculated are not dominated by measurement errors for individual stars. Response maps have been generated for J, H and Ks data obtained on 95-04-22, and they may be viewed from the IPAC/2MASS information page.

The Prototype Camera was cooled with liquid argon during the April/May run, which resulted in a slightly warmer array temperature than previously (approx. 80K versus 72K). Laboratory tests suggested that this could help suppress the detector reset decay pattern observed in the bias frames that seemed to dominate the earlier responsivity tests. The new responsivity pattern is similar to that seen in the 1-dimensional ROUND analyses of the 94-06-01 repeated scans of M92. The largest variations occur in the in-scan direction with response change up to 10% peak-to-peak in a pattern that follows approximately the reset decay pattern. The amplitude of "step" that occurs at mid-array is similar to the earlier data, although the full peak-to-peak scatter appears to be slightly lower. The general shapes of the response patterns in J, H and Ks are similar, but there are response "features" that differ slightly between each that appear to be statistically significant.

While the in-scan response variations appear drastic, the 2MASS technique of obtaining six samples during a scan helps to smooth them out. However, if one or more of the six samples for a source is missed for any reason (i.e. bad pixels, cosmic-ray hits, etc...) the net response along a column will be altered contributing to overall photometric non-uniformity. Since a sample is likely to be missed for a non-negligible percentage of sources (e.g. 3% of sources for 0.5% bad pixels on the array), there is good motivation to smooth the in-scan response function. This affects aperture photometry primarily since it is derived from the coadded data. KAMPHOT is not as sensitive to missed samples because it operates on individual frames and can exclude "bad" samples.

The cross-scan response variations are smaller than we had feared a few months ago, running about 5% peak-to-peak. Even these small variations affect the photometric uniformity, though, as can be seen when comparing repeatibility tests of stars measured during the response tests with those measured in normal calibration scans that used very little cross-scan stepping (M67 was also a calibration field used throughout the run). Photometry for brighter stars (Ks<12) in the normal calibration scans have characteristic dispersions of 1-2% in multiple measurements, which approaches the theoretical limits imposed by the large pixels and pixelization errors. The dispersion in the photometry of high SNR stars from the cross-stepped scans is 3-4%, which should be more representative of the true photometric uniformity of the survey.

If the repeatibility of the photometry from non-cross-stepped scans represents the best that can be achieved, then the goal is to determine how to carry out or correct the cross-stepped photometry to bring it to the same accuracy. New analysis tasks to investigate this are now underway. These include:


  1. Adaptive PSF -- As proposed at the February 1995 Science Team meeting, two refinements to the photometry algorithms were included in the processing of the April/May Protocamera data. The first was to use an "adaptive" psf in KAMPHOT instead of the single psf used to process the June 1994 data. Rather than determine the local "true" psf for every scan, which would require prohibitive amounts of cpu time for the survey and would be difficult in highly confused or very sparse regions, the psf used to extract photometry from a scan was selected from a pre-defined grid of psf's. Selection of the psf was based on a single seeing estimator for the scan, PFRAC. PFRAC is a measure of the ratio of the energy containing in the peak brightness pixel on a source to the total energy for an ensemble of sources, and it correlates well with image FWHM. Thus, for low values of PFRAC, the broadest psf was used, and sharper psf's were selected for larger PFRAC's. Because of time limitations, a sparse grid of psf's containing only three per band was generated for the initial processing of the Protocamera data.
  2. Hybrid Magnitude -- Repeatibility studies of photometry from the June 1994 Protocamera data indicated that aperture photometry yields slightly lower dispersion for bright point sources and KAMPHOT performed better for fainter sources. The cross-over point occurs near Ks=12.5. Therefore, to exploit the strengths of each, an algorithm to derive a hybrid magnitude that is a simple linear combination of the aperture and KAMPHOT magnitudes was developed for use in the current Protocamera processing. This algorithm defaults to pure aperture photometry for Ks < 11.5 and pure KAMPHOT magnitude for Ks > 13.5. The crossover and limits occur approximately 0.5 magnitudes fainter for H and 1.0 magnitudes for J.

  3. Performance - Internal Repeatibility -- During the April/May Protocamera run, calibration fields normally were observed by making six one-degree scans in rapid succession at each wavelength. Thus, we have data to monitor the repeatibility of photometry on timescales ranging from a few minutes to several nights. The internal (short timescale) repeatibility of point source photometry produced with both aperture and KAMPHOT algorithms from the April/May run is excellent, as good if not better than that observed in the June 1994 data. The new data show that the J and H repeatibility is as good as that at Ks. Stars brighter than J=13, H=12.5 and Ks=12.0 show characteristic dispersions of only 1-2% in their photometry. As in the June 1994 data, aperture photometry provides slightly better repeatibility for brighter stars, while KAMPHOT is better for faint stars. However, KAMPHOT with its adaptive psf grid containing only 3 psf's per band, is nearly as good as aperture photometry for bright stars, having characteristic dispersions of 2-3%. Note this is as good as measured for the twilight-sky flattened KAMPHOT results from June 1994. Using a more extensive grid of psf's and optimum flat-fielding may further improve KAMPHOT to the point where it can be used at all brightness levels, eliminating the need for the hybrid magnitude. The hybrid magnitude tracks aperture KAMPHOT photometry precisely as designed, following the minimum of the two. The dispersion is less than or equal to 10% (i.e. measurement at SNR=10) out to J=15.8, H=15.2 and Ks=14.3, in excellent agreement with the Level 1 Specification.

We also conclude that cooling the detector using LAr has had no adverse impact on the photometric uniformity. Analyses are currently underway to determine the effects of the warmer operating temperature on net completeness and reliability.

PHOTOMETRIC CALIBRATION - External Repeatibility - Seeing Effects

Testing the photometric uniformity of data acquired over longer time scales requires calibration to common photometric zero points at each band. The first attempts at this using the April/May Protocamera data consisted of deriving simple photometric transformations for the standard stars within each calibration field. The transformations have the traditional form:
            Mtrue = Minst + k1*X
where k1 is the slope of the extinction correction, X is airmass and Minst is the instrumental magnitude of the star. Note that we currently ignore color terms. It was immediately apparent that the transformations using both aperture, KAMPHOT and hybrid photometry were unphysical, exhibiting much scatter and frequently having extinction terms much larger than the canonical 0.1 mag/airmass typical for near-infrared wavelengths.

The cause of this problem is apparently linked to seeing variations. For example, comparison of measurements of stars in the P221-C calibration field observed at different times on 95-04-22, but at the same airmass, shows a systematic zero point offset of nearly 0.1mag. The seeing estimator, PFRAC, differs between the two indicating a seeing variation of nearly 0.5 arc-seconds FWHM. Examination of the point source curves of growth derived from multiaperture photometry in these fields reveals that the 8" diameter aperture used in the current pipeline misses considerable light, up to 15-18%, in the rather poor seeing we encountered for much of the early part of the run. Moreover, the fraction of light falling outside of the aperture varies considerably with seeing, leading to apparent photometric offsets.

  1. Aperture Photometry Corrections -- To correct for the effects of seeing in the aperture photometry, we derived mean aperture corrections (the difference between the 8" diameter aperture magnitude and the "infinite" aperture magnitude) using all of the brighter stars in each scan, and applied them to the photometry. The corrected aperture photometry for the two P221-C scans shows no net offset. Similarly, aperture corrections were derived for most of the calibrations scans made during the run, and the corrected standard star aperture magnitudes produce much more reasonable photometric transformations for each night.

  2. KAMPHOT Corrections -- The variations in the KAMPHOT photometry are more difficult to understand. One can imagine that psf-fitting, if performed properly, should be "seeing-independent". One deficiency in the current implementation of KAMPHOT is that only 3 psf's per band are used, and the selection of psf is seeing driven (based on PFRAC). If the psf that is utilized for a given measurement does not match the true psf there will be some systematic offset in the photometry. Note that in the final version of 2MAPPS the KAMPHOT psf grid will be much better populated. However, we find photometric inconsistencies even in tests where KAMPHOT is allowed to measure the "true" local psf within scans.

    Currently, KAMPHOT uses only a 2x2 pixel (4"x4") "kernel" to fit the image profile. If the seeing degrades considerably, there may not be sufficient information to achieve proper fitting and that may result in an incorrect brightness estimate. Furthermore, as currently implemented, KAMPHOT normalizes its psf-fit magnitudes to a constant internal zero-point rather than to internal aperture photometry. That zero-point is the proper value gauged from earlier Protocamera data obtained under fairly unform seeing conditions. However, the psf zero-point normalization should really be measured locally if significant seeing variations occur.

    We are currently testing the effect of enlarging the KAMPHOT input array size and its impact on processing time and signal-to-noise. In addition, we propose that the most direct method to ensure proper normalization of the KAMPHOT photometry will be to adopt the traditional CCD photometry technique of normalizing to aperture photometry. To implement this, both aperture and KAMPHOT photometry would be extracted for a scan. KAMPHOT magnitudes would then be adjusted by the zero-point correction necessary to drive the median KAMPHOT-Aperture magnitude difference to be zero for a given scan. This solution assumes that aperture photometry produces the right answer, as we believe it will do when aperture corrections are measured and applied. This will be difficult if not impossible in crowded fields. Consequently, it may be preferable to deduce aperture corrections from a more robust seeing estimator, such as PFRAC, that can be trusted in crowded fields, or interpolated from nearby scans.

  3. Ongoing Analysis Tasks -- The following analysis tasks related to photometric transformation and external uniformity are underway or planned for the near future:
    • Derive aperture corrections and apply to aperture photometry for a large number of scans to examine the uniformity of photometrically calibrated data from different nights.
    • Use all stars in calibration fields observed at different airmasses to determine extinction terms for different nights and note variations with time.
    • Normalize KAMPHOT magnitudes to aperture-corrected aperture magnitudes for repeated calibrations scans, and examine both internal and external repeatibility of corrected KAMPHOT photometry. Determine whether the hybrid magnitude is still necessary.
    • Test performance of KAMPHOT with better populated psf grids.
    • Test KAMPHOT using larger psf-fitting kernels and understand impact on processing time and SNR.


Preliminary analysis of the point source photometry from the April/May Protocamera has shown that the refined algorithms are producing results with excellent internal uniformity. For the first time we have data to study the effects of focal plane response variations and seeing variations on global photometric uniformity. The preliminary analysis indicates that these factors degrade the uniformity in reasonably well understood ways, and we are further characterizing the effects of seeing on psf-fit photometry. Efforts are now underway to develop and test refinements to the flat-fielding procedures and photometry algorithms that will minimize these effects.

We welcome any suggestions you might have regarding the observed effects and planned corrections and analysis.