2MASS Cross-Scan Photometric Responsivity Tests

R. Cutri - IPAC

Introduction

A number of special test scan sets have been carried out at the northern 2MASS facility on Mt. Hopkins to investigate photometric residuals in two dimensions on the 2MASS detector arrays. The test scans consisted of a set of 10 normal scans, usually one degree in length, cross-stepped 1/10 of the detectors field-of-view (~51") between each scan. The combination of the six in-scan samples afforded by the normal 2MASS scanning procedure and the 10 cross-scan samples allows stars to fall on up to 60 locations on the arrays. Comparing the apparent brightness of each source in different positions around the arrays allows us to measure photometric uniformity of the detectors.

Spatial non-uniformity in the photometric response of array detectors can be produced by flat-fielding errors, flexure in the camera, distortion in the optical system, vignetting or a number of other sources. As you may recall, flat-fielding residual tests made with dark-sky flattened data taken the Prototype Camera showed that there could be as much as 15% peak-to-peak variation in the photometric response to point sources across to the array. At least some of the response non-uniformity was linked to the reset decay bias pattern. These results led us to design the unique twilight sky flat-fielding technique used in the survey. Twilight sky flats reduced the photometric residuals to <=5% peak-to-peak in the same ProtoCam data.

Northern Survey Tests

Special cross-scan-stepped scans were obtained on 971026n, 971125n, 980113n, 980125n and 980715n. The 971125n test data were lost due to a bad tar block on the mountain data tape. The table below lists the number of frames in the scans for each test, the position on the sky, and the number of unique J, H and K_s sources detected in each set.

Date    #fr   RA       DEC      nJ     nH    nK_s971026n  48  352.727 +38.803    874    618   582

971125n                         Lost due to tape error

980113n  48   41.461 +38.790    843    822   721

980125n  48   55.209 +38.788   2007   1609  1231

980715n  48  270.220 +10.634   2712   2177   982

Data Analysis

The FREXAS subsystem in 2MAPPS detects sources on individual frames and performs aperture photometry of the detections, using a two camera pixel (4") radius aperture. For this analysis, the R2-R1 frame extractions from FREXAS for the cross-stepped test scans were tagged with equatorial positions, and the sources in all of the scans in each band were positionally cross-correlated. All apparitions of each unique source were identified, tagged with the scan and frame numbers from which it came, the frame coordinates, magnitude, and equatorial position residual. There can be up to 60 apparitions of a given source, if it falls within the central strip covered by the ten scans. In general, most sources will be seen less than 60 times.

For each unique source, the mean brightness of all apparitions was evaluated, and the flux residual with respect to the average was calculated. The 256x256 arrays were divided up into 10x6 bins and the average and RMS residuals for all apparitions falling within each of the 60 bins was then evaluated. The mean residual in each bin gives the relative response of pixels in that ~26x42 pixel region of the array.

To map the true relative response over the full array, we should utilize only sources detected 60 times. In practice, though, this often yields too few sources to produce a statistically useful result. As a compromise, sources with >= 48 or 54 apparitions were used. This still produces a useful measure of the 2d response residual (see the 980113n summary for an example). The accuracy of the residuals measured in each bin will also be governed by the number of samples and the brightness of each sample. A reasonable number of bright sources are better than a large number of faint sources. Thus, the magnitude threshold was "tuned" for each night and band to the brightest level that provided at least ~10 bright sources per bin.

The links below give the summaries for each night. Each summary provides numerical tables showing the mean flux residual in each of the 10x6 bins, the RMS of the residuals in each bin, and the total number of star apparitions going into each bin. Also shown are images constructed from the residual and RMS tables.

Figure 1-3 below show the responsivity residual maps for J, H and Ks, respectively, comparing the three dates. In each Figure, the maps are arranged top-to-bottom in the order 971026n, 980113n, 980125n and 980715n. All responsivity images are shown with a full range of 0.9-1.1, with lighter colors indicating larger flux ratios (i.e. relatively higher responsivity) and darker colors indicating smaller ratios (i.e. relatively lower responsivity).

Figure 1 - J Responsivity Flux Residuals for 971026n, 980113n, 980125n and 980715n

Figure 2 - H Responsivity Flux Residuals for 971026n, 980113n, 980125n and 980715n

Figure 3 - K_sResponsivity Flux Residuals for 971026n, 980113n, 980125n and 980715n

The six in-scan samples made for each star during normal 2MASS data acquisition will effectively smooth out any residuals in the in-scan direction. Therefore, the net cross-scan photometric residuals can be derived by column-averaging the residuals. The Figure 1 shows plots of the column-averaged relative cross-scan response residuals measured on the three nights. The red lines indicate the data from 971025n, green is 980113n, blue is 980125n and cyan is 980715n.

Figure 4 - Column-Averaged Flux Residual

Conclusions

The flux residual maps from the three 1998 tests show only small residuals across the array. There is a hint of residual in the in-scan direction that suggests the bias reset decay pattern, but it is small. There are peak-to-peak variation across the arrays of up to ~5%, but most of this is averaged down by the in-scan sampling. The column-averaged plots suggests that there should be at most 1-2% of photometric bias in the edges of the arrays due to response variations across the arrays. The overall pattern of residuals in the 1998 tests is random, with perhaps a slight spherical surface-like component (primarily in the J-band) suggestive of an illumination pattern.

The residual maps from 971026n appear very different from the 1998 tests. They are dominated by a strong gradient across the array, with a ~10% variation from edge-to-edge. This would be expected to produce large photometric biases when comparing scan-to-scan overlaps on this night. However, the satellite movie from that night shows intermittent clouds. The photometric zero point plot for the night also shows considerable internal scatter within the calibration scan sets, so we cannot rule out that the apparent responsivity gradient could have been produced by a changing transparency due to clouds during the test. The consistency of the 1998 tests suggest that the October 1997 results may be pathological.

The dramatic differences between the 1997 and 1998 responsivity tests indicate that the photometric residuals may vary with time. The reason is not yet understood. I recommend that cross-scan stepped test scans should be repeated frequently, at least once per month, until we understand the origin of the variation, and decide on a corrective course.

R. Cutri - IPAC
Last Update - 30 July 1998