Nov. 99 Long Exposure Experiment Results

2MASS Long Exposure Experiment - November 1999

I. Introduction

A second epoch of long exposure test survey scans were made at the southern 2MASS facility at CTIO in Novmber of 1999. These scans were made using R2 dwell times six 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 long expsure scans were targetted on Abell galaxy clusters and the LMC, so science investigations were also an objective.

The 6x exposure scans were accompanied by normal survey exposure scans on all nights except 991113s. All of the tiles observed during these tests have also been observed already as part of normal survey operations. The date and scan numbers of the archived observations for each tile are provided in the table below.

In June of this year, long exposure experiments were made at CTIO using scans with 2x and 4x the normal survey exposure time. The results from ongoing analyses of those data can be found here.

II. Observation Log

The table below provides a log of the long exposure experiments. Listed are the observation date, the scan number on that date, the survey tile number, the exposure time relative to nominal survey exposure, the J, H and K_s PFRAC and average frame background values, the location of the processed data on disk, and finally the date and scan number of the original survey scan of each tile.

November 1999 2MASS-South Long Exposure Test Scan Summary

Dateh   scn   Tile  ex Field     Jpfrc Hpfrc Kpfrc   Jbg  Hbg   Kbg  Location                    Prv_obs scn  Q
-----------------------------------------------------------------------------------------------------------------
991106s	042  303074 6x Abell_85  0.772 0.733 0.725  3300 14000 15000 holly:/k4/OPS/991106s/s042  981002s 067 10
        043  303074 1x Abell_85  0.766 0.780 0.732   604  2720  2730 elvis:/k4/OPS/991106s/s043  981002s 067 10     

991108s 084  328151 1x LMC       0.740 0.765 0.725   570  2980  2350 wilma:/b0/OPS/991108s/s084  981202s 042 10
        085  328151 6x LMC       0.700 0.728 0.693  3200 16000 13400 holly:/k4/OPS/991108s/s085  981202s 042 10
        092  327035 1x LMC       0.751 0.774 0.742   403  1850  1935 wilma:/b0/OPS/991108s/s092  981026s 086 10
	093  327035 6x LMC       0.737 0.762 0.731  2150  9400 10900 holly:/k4/OPS/991108s/s093  981026s 086 10

991109s	025  300116 1x Abell_119 0.677 0.716 0.672   538  2420  2140 wilma:/b0/OPS/991109s/s025  980914s 068 10
        026* 300116 6x Abell_119 0.748 0.771 0.741  3150 13500 12000                             980914s 068 10 
	033  303226 1x Abell_277 0.734 0.780 0.736   500  2255  1999 wilma:/b0/OPS/991109s/s033  981023s 070 10
	034  303226 6x Abell_277 0.732 0.771 0.722  2900 12400 11500 holly:/k4/OPS/991109s/s034  981023s 070 10

991110s 040  300116 6x Abell_119 0.736 0.746 0.711  3400 14000 12400 holly:/k4/OPS/991110s/s040  980914s 068 10

991113s 060  328153 6x LMC       0.765 0.772 0.739  3550 15700 15000 holly:/k4/OPS/991113s/s060  981202s 044 10
	067  328155 6x LMC       0.764 0.780 0.745  3350 14000 14700 holly:/k4/OPS/991113s/s067  981202s 046 10
	074  327023 6x LMC       0.754 0.771 0.737  3250 14100 13400 holly:/k4/OPS/991113s/s074 981025s 086 10
	075  327025 6x LMC       0.741 0.758 0.726  3350 14200 14200 holly:/k4/OPS/991113s/s075 981025s 094 10

* Ks data bad in this scan - failed in pipeline

III. Processing Notes

Initially, the only modification to the 2MAPPS v2.1 pipeline that was made 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 single parameter file. The effect of the corrected instrumental zero point is to bring the instrumental magnitudes in the 6x scans perhaps to within 10-20% of the calibrated levels.

After processing the 6x exposure scans, it was discovered that all of the H and K_s profile-fit photometry failed, because of a threshold in PROPHOT that stops operation on a source if the background exceeds 9999.99 DN. This threshold exists to minimize the time PROPHOT will spend trying to fit large numbers of spurious sources in the wings around bright stars. As can be seen from the observing log above, the H and K_s frame backgrounds exceeded 9999 DN in nearly all of the 6x exposure scans, so the profile fitting failed.

A specially modified version of PROPHOT was be constructed that increased the BG threshold to 39999.99 DN for the long exposure scans only. All of the 6x scans were rerun with the modified PROPHOT, producing nominally valid profile-fit photometry. The disk locations listed in the above table refer to the rerun versions of the 6x scans.

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 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, although confusion will result in aberrant aperture photometry in dense fields.

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 long exposure scans have not been applied. Only the uncalibrated *.bfpts source listings for the 6x scans are currently available. The relative photometry between scans on each night should be accurate. Preliminary photometric calibrations have been derived and applied to the 1x scans of each of the test fields. The photometry in these scans should be used to bootstrap calibrations in the 6x exposure scans.

IV. Links to Default Pipeline Source Lists and Images

The links below point to gzip'd tar files containing the source lists produced by default pipeline processing for the normal survey scans and 6x exposure scans for each of the test fields. If you gunzip and untar each file, they will fill a subdirectory structure of the form: YYMMDDh/sNNN/ , where YYMMDDh is the date/hemisphere, and NNN is the scan number. The 1x and 6x point source lists have the form sNNN.cal and sNNN.bfpts, respectively. The 1x and 6x extended source lists have the form gNNN.cal and gNNN.gals, respectively.

1x Point Source Lists

6x Point Source Lists

1x Extended Source Lists

6x Extended Source Lists

The Atlas Images for each of the new scans can be obtained via anonymous ftp from:

anon-ftp.ipac.caltech.edu:/pub/2mass/minutes/nov99/

A listing of that directory follows:


ftp> cd pub/2mass/minutes/nov99
250 CWD command successful.
ftp> dir
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 2686416
-r--r--r--   1 ipac     99336858 Nov 24 17:14 s025.images.tar.gz
-r--r--r--   1 ipac     102190586 Nov 24 17:15 s033.images.tar.gz
-r--r--r--   1 ipac     94681058 Nov 24 17:17 s034.images.tar.gz
-r--r--r--   1 ipac     95144948 Nov 24 17:21 s040.images.tar.gz
-r--r--r--   1 ipac     95819754 Nov 24 15:54 s042.images.tar.gz
-r--r--r--   1 ipac     99632766 Nov 24 15:57 s043.images.tar.gz
-r--r--r--   1 ipac     96143108 Nov 24 17:23 s060.images.tar.gz
-r--r--r--   1 ipac     95836571 Nov 24 17:27 s067.images.tar.gz
-r--r--r--   1 ipac     98576430 Nov 24 17:31 s074.images.tar.gz
-r--r--r--   1 ipac     99439967 Nov 24 17:36 s075.images.tar.gz
-r--r--r--   1 ipac     99704498 Nov 24 16:56 s084.images.tar.gz
-r--r--r--   1 ipac     93885215 Nov 24 17:11 s085.images.tar.gz
-r--r--r--   1 ipac     104541405 Nov 24 16:19 s092.images.tar.gz
-r--r--r--   1 ipac     99631780 Nov 24 16:24 s093.images.tar.gz

Each entry in the ftp directory is a gzip'd tar file containing all of the J, H and K_s Atlas Images (aka coadds) from each respective scan, having the normal pipeline naming convention. Use the table in section II. to match scan number with tile. Note that only the images for the scans taken in November 1999 are currently available.

V. Links to On-Going Analysis Reports

Point Source Results

Comparison of WSDB and 6X over-scanned LMC fields

Source Count Statistics

Extended Source Results

Source Count Statistics

R. Cutri - IPAC
Last Updated - 3 December 1999