I. Introduction

Source lists from the SPICES fields were supplied by Peter Eisenhardt and Daniel Stern for the purpose of comparison with objects in the 2MASS Point Source Working Database. These lists were limited to objects with measured SPICES photometry having J<18.5 mag or K<17.0 mag. The SPICES sources have been positionally associated with objects in the 2MASS Point Source Working Database and the relative photometric agreement of the two is discussed below.

Important Caveat: The comparison has been made using the results of the preliminary-processed 2MASS data. While we believe that these data meet the basic quality minimums of the 2MASS survey, they have not undergone the stringent quality assurance and validation afforded released catalog data. In addition, the 2MASS photometric calibration applied to these data is preliminary, and will change slightly during 2MASS final data processing which will commence in the late Spring of 2001.

II. Matching Procedure

A search of the 2MASS Working DB was made for all sources within 3 arcseconds of the position of each SPICES source. All of the SPICES fields cover boundaries between at least two 2MASS "Tiles", so there there can be multiple sightings of sources in the Tile overlap regions. In addition, some of the Tiles covering the SPICES fields were observed more than once during the survey, so there can be multiple sightings in these cases. Positional associations from all 2MASS epochs were extracted and included in the analysis below. The associated 2MASS sources were manually filtered to remove any objects flagged to be artifacts or otherwise contaminated measurements.

The second column of Table 2 contains links to figures showing the difference between SPICES and 2MASS J (top panels) and K (bottom panels) magnitudes plotted as a function of the SPICES magnitude for each matched source in the four fields. Vertical lines in each plot indicate the nominal 2MASS completeness level and magnitude at which the 2MASS photometry has SNR>10:1. The vertical error bars represent the 2MASS photometric measurement uncertainties only. No uncertainties were provided for the SPICES photometry.

Field Name	Mag Offset Plot	Average Delta J (mag)	Average Delta K (mag)
Cetus		-0.015 + 0.024	-0.026 + 0.027
Pisces		0.065 + 0.011	0.009 + 0.027
Lynx		-0.182 + 0.044	-0.005 + 0.039
SA57		-0.078 + 0.025	0.043 + 0.039

The following features can be seen in the plots:

• Down to ~1 mag fainter than the 2MASS SNR>10 limits, there are only small, constant offsets between SPICES and 2MASS photometry. These presumably represent differences between the relative calibrations and photometric systems of the two data sets. The average and standard deviation (sigma mean) of the J and K offsets over the ranges 13 < J < 16 and K < 15 are listed for each field in the last two columns of table 2.



• In the Pisces and Lynx fields, SPICES reports photometry fainter than 2MASS for sources with J_SPICES<12. We suspect this is due to saturation in the SPICES data.



• In all the fields and in both J and K, the 2MASS photometry becomes systemmatically brighter than the SPICES photometry with decreasing magnitude beginning at approximately 1 mag fainter than the 2MASS SNR>10 limits. This is the well-known flux-overestimation bias. When measurements of sources very near the detection limit are made, objects will be preferentially detected when positive noise excursions drive their apparent brightness up. Similarly, when negative noise excursions drive their apparent brightness down, they are not detected. Thus, on average the measurements of sources near the detection limits will be systemmatically brighter than their true flux level.



• The plot for the SA57 field shows points marked with crosses that corresp to measurements of three sources that fall systemmatically below the general trend. These objects are all extended sources, resolved by 2MASS. This analysis includes only 2MASS point source photometry which is derived from psf-fitting. Psf-fitting will underestimate the total flux of a source that has an extent significantly broader than the psf, as these three objects are. The extended sources are not included in the derivation of the mean J and K photometric offsets for SA57.