Long Exposure Experiment Results

2MASS Long Exposure Experiment Analyses

I. Introduction

Several test survey scans were made at the southern 2MASS facility at CTIO using R2 dwell times two and four times longer than the nominal R2 exposure of 1.3 sec. The R1 exposure was not changed from the nominal 50 msec for these tests. The objective of these tests was to obtain a data set that would allow empirical investigation of the 2MASS system linearity, completeness and reliability at the faint limits of the survey, flux overestimation at low signal-to-noise ratio, and response to increasing confusion noise. These tests also allow further testing of the telescope tracking performance.

Long exposure scans were made of three different survey tiles. The first tile, 315820 (l=252, b=+2.3), was selected to be near the Abell 3558 galaxy cluster. The second two tiles, 310798 (l=357, b=+18) and 322672 (l=308, b=+11), were selected to have increasing point source densities.

II. Observation Log

Tile	Obs. Date (UT)	Scan	R2 Exp	Conditions
315820	990620s	027	1x	Clear
315820	990620s	028	2x	Clear
315820	990620s	029	4x	Clear
310798	990624s	055	1x	Clear, high airglow
310798	990624s	056	2x	Clear, high airglow
310798	990624s	057	4x	Clouds, high airglow
310798	990625s	049	1x	Clear, high airglow
310798	990625s	050	2x	Clear, high airglow
310798	990625s	051	4x	Clear, high airglow
322672	990626s	029	1x	Clear, high airglow
322672	990626s	030	4x	Clear, high airglow

III. Processing Notes

The only modification to the 2MAPPS v2.1 pipeline that was necessary to run the longer exposure scans was to adjust the instrumental zero point constants for the R2-R1 photometry. The adjustment was made by adding a constant = 2.5log (exp time/1.3s). All magnitude computations within the pipeline use these instrumental zero points, which are carried in a sinlge parameter file.

Instrumental frame calibration was carried out using special dark scans obtained at the longer R2 exposure times, and with the most recent flat-field frames built from southern data.

Several caveats must apply to the processing of the longer exposure data:

Although the chi-squared distribution appears nominal, there is no guarantee that the profile-fit photometry is optimal for the longer exposures. The PSF's and variance maps may not be adequately matched to the image profiles in the longer exposures. Aperture photometry should be fine, of course.

Most of the MAPCOR artifact identification is run using parameters that are dependent on the brightness of the parent star. These parameters were tuned from empirical examination of nominal survey data. In the longer exposure scans, fainter stars will produce artifacts and they may not be properly tagged by MAPCOR.

Photometric zero point offsets for the respective nights have not been derived and applied yet. Only the uncalibrated *.bfpts source listings for each scan are currently available. The relative photometry between scans on each night should be accurate. For the cases of tile 310798, we have a normal survey scan taken under photometric conditions on 980626s.

Data processing for the test scans was run on holly. The processed scans can be found on holly:/k4/OPS/.

IV. Links to On-Going Analysis Reports

Sky Conditions

D. Kirkpatrick - 990620s Backgrounds
D. Kirkpatrick - 990624s Backgrounds

Telescope Performance

H. McCallon - RA Tracking Stability

Point Source Results

E. Kopan - 990620s Noise and Detection Statistics
W. Wheaton - Chi-Squared and Photometric Uncertainty Distributions
L. Fullmer - Scan-Scan Position and Photometric Overlap Stats
(Note the effects of flux overestimation in the delta-magnitude versus magnitude plots.)
J. Rho - 990620s Photometric Offsets, Completeness and Reliability

Extended Source Results

T. Jarrett - 990620s Galworks Results
J. Rho - 990620s Photometric Offsets, Completeness and Reliability

R. Cutri - IPAC
Last Updated - 30 June 1999