990915n 2MASS Cross-Scan Photometric Responsivity Tests

R. Cutri - IPAC


Introduction

A special cross-scan responsivity test was carried out on 990915 UT at the northern 2MASS facility on Mt. Hopkins to investigate flat-fielding efficiency of the new H-band array that was installed during the summer downtime. The new array exhibits more spatial structure in its response than the old array, as well as the J and Ks arrays in the same camera. The objective of these tests is to quantify any residual photometric non-uniformity that might result from the response variations across the focal plane.

The cross-scan test was similar to those described in the report on 2MASS Cross-Scan Photometric Responsivity Tests. This test consisted of a set of 12 one degree scans, cross-stepped 1/10 of the detector's field-of-view (~51") between each scan. The combination of the six in-scan samples afforded by the normal 2MASS scanning procedure and the 12 cross-scan samples allows stars to fall on up to 60 locations on the arrays. Twelve cross-scan steps were used to increase the number of stars that have 10 samples across the array. Comparing the apparent brightness of each source in different positions around the arrays allows us to measure photometric uniformity of the detectors.

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, instrumental 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 twelve 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.

The 990915n tests covered a region of moderate density, so that it was possible to limit the analysis to sources that were actually sampled 60 times across the array. 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. The density of these scans made possible sampling at least 20 stars per bin.

The link below gives the summary for the analysis of the cross-stepped data for the night. This 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.

990915n

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. Figure 1 shows plots of the column-averaged relative cross-scan response residuals measured in each band. The blue, green and red lines indicated J, H and Ks, respectively.

Figure 1 - Column-Averaged Flux Residual
 

Conclusions

The flux residual maps from the 990915n test show that the photometric residuals across the arrays are comparable in all three bands. There is perhaps a 1% systemmatic radial bias in the residuals, with the responsivity increasing radially outward from the centers of the focal planes. This is averaged down slightly by the in-scan sampling, but is still visible in the column-averaged residuals as a 1% middle-to-edge bias.

The amplitude and shape of the residuals observed in the new H-band array are consistent with those observed in the J and Ks arrays. However, the uncertaintisy in the residuals evaluated for H-band are systemmatically larger than the other two bands, which is not unexpected given the QE variations across the array.

The amplitude and shape of the photometric residuals measured in these tests are very similar to those measured in earlier tests of the northern system. Therefore, we conclude that the flat-fielding performance of the new H-band array is as good as before.

R. Cutri - IPAC
Last Update - 19 September 1999