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Standard Stars, Zero Points and Chip to Chip Offsets

Table of contents:

Zero Points Measurements Using Standards - Introduction

Standard stars are observed every photometric night with WIRCam, on average 3 times per night for each of the J,H,Ks filters. Our standards are picked from the UKIRT (FS) list. Except for engineering purposes and for the sake of characterization, standards are not observed for narrow-band filters (unless included in a PI program). Here, we use the four well modeled spectrophotometric standards (CALSPEC models) and filter transmission curves to determine the expected magnitude through narrow-band filters.

Measurements are made automatically based on the WCS in the headers of the processed images and the known star position. Note that wide band standards are generally observed with the telescope defocused by 0.5mm to prevent saturation. FLUXBEST of SExtractor is used to obtain each measurement in ADU on individual (not stacked) images.

The Zero Point vs. Date plots show the ZP after chip to chip offset corrections for different observing runs (top X axis) and as a function of time (bottom X axis - the day at the start of each observing run, since standard stars started being observed). Each color represents a different standard star. The Chip to Chip Offset histograms are made of a subsample of observations, i.e. sequences where a star was positionned on each of the detectors within a few minutes. The chip to chip offset for narrow-band filters shows a high spread due to not enough measurements having been done. Use the corresponding wide-band filter chip to chip offset.

last update: Thu Aug 16 14:04:51 HST 2007

Plots of WIRCam Zero Points and Chip to Chip Offsets

FILTER Comment Zero Point vs. Date and Camera Run ID Chip to Chip Zero Point Offset
Y  
J  
H  
Ks  
LowOH1  
LowOH2  
CH4On  
CH4Off  
H2  
Kcont  
BrG  
 

Table of WIRCam Zero Points and Chip to Chip Offsets

 # April 3 2007 - Loic Albert
 #	Created.
 # July 27 2007 - Loic Albert
 #	Added one column, ZP_WONLC (Zero point without non-linearity correction) which gives the
 #	offset to apply to the zero point for data that has not been linearized.
 #	Added a VERSION column to tell which sets are used in different IIWI versions.
 # August 16 2007 - Loic Albert
 #	Added entries for use with iiwi version 1.0 with non-linearity correction, but only valid for semesters 06B/07A. Will
 #	need to analyze 05B/06A standards to add their values to use with iiwi 1.0.
 #
 # This file tabulates the filter zero points and chip to chip zero point offsets. Here, the zero point is the Vega magnitude at which a flux of 1 ADU/sec is detected. The
 # values are based on measurements of standard stars analyzed in the wircam_analyzestandard procedure. For chip to chip offset, the reference detector is chip #60 (extension 4).
 # A jump in zero point occured when the WIRCam gain was changed from ~2.5e-/ADU to ~3.7e-/ADU (an exact factor of 1.5) in August 2006 but the reality is that, if expressed as 
 # the magnitude at which 1 e-/sec is detected, the "real" sensitivity of WIRCam has not changed. Note that the chip to chip offsets adopted for most narrow-band filters is that
 # of its corresponding wide-band filter (e.g. H2, BRG and KCONT are that of KS), simply because not enough measurements exist for narrow-band filters. No noticeable trend of
 # zero point with time has been observed so far with WIRCam. Flux measurements are made on fully processed single images using FLUXBEST of SExtractor. 
 #
 #VERSIO	STARTDATE		ENDDATE			FILTER		ZP	ZP_ERR	ZP_WONLC	ZP_EXT1	ZP_EXT1_ERR	ZP_EXT2	ZP_EXT2_ERR	ZP_EXT3	ZP_EXT3_ERR	ZP_EXT4	ZP_EXT4_ERR	COLTERM	GAIN
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	Y		24.289	0.099	-0.000		+0.021	0.011		-0.144	0.009		-0.004	0.007		+0.000	0.011		0.0	2.5	
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	J		24.120	0.040	-0.000		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	H		24.280	0.063	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	KS		23.552	0.049	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	LOWOH1		21.477	0.072	-0.000		+0.054	0.050		-0.109	0.053		-0.017	0.056		+0.000	0.033		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	LOWOH2		25.0	9.999	-0.000		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	CH4ON		23.167	0.053	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	CH4OFF		23.156	0.044	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	H2		20.999	0.078	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	KCONT		25.0	9.999	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 0.9.4	2005-11-01T00:00:01	2006-08-15T23:59:59	BRG	       	20.848	0.079	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 # Starting with semester 06B, the gain was changed x1.5 to 3.7 e-/adu
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	Y		23.820	0.099	-0.000		+0.021	0.011		-0.144	0.009		-0.004	0.007		+0.000	0.011		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	J		23.651	0.040   -0.000		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	H		23.811	0.063	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	KS		23.083	0.049	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	LOWOH1		21.008	0.072	-0.000		+0.054	0.050		-0.109	0.053		-0.017	0.056		+0.000	0.033		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	LOWOH2		25.0	9.999	-0.000		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	CH4ON		22.698	0.053	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	CH4OFF		22.687	0.044	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	H2		20.530	0.078	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	KCONT		25.0	9.999	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 0.9.4	2006-08-16T00:00:01	2007-08-31T23:59:59	BRG	       	20.379	0.079	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 #VERSIO	STARTDATE		ENDDATE			FILTER		ZP	ZP_ERR	ZP_WONLC	ZP_EXT1	ZP_EXT1_ERR	ZP_EXT2	ZP_EXT2_ERR	ZP_EXT3	ZP_EXT3_ERR	ZP_EXT4	ZP_EXT4_ERR	COLTERM	GAIN
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	Y		24.289	0.099	-0.000		+0.021	0.011		-0.144	0.009		-0.004	0.007		+0.000	0.011		0.0	2.5	
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	J		24.120	0.040	-0.010		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	H		24.280	0.063	-0.020		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	KS		23.552	0.049	-0.010		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	LOWOH1		21.477	0.072	-0.000		+0.054	0.050		-0.109	0.053		-0.017	0.056		+0.000	0.033		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	LOWOH2		25.0	9.999	-0.000		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	CH4ON		23.167	0.053	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	CH4OFF		23.156	0.044	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	H2		20.999	0.078	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	KCONT		25.0	9.999	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 0.9.5	2005-11-01T00:00:01	2006-08-15T23:59:59	BRG	       	20.848	0.079	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	2.5
 # Starting with semester 06B, the gain was changed x1.5 to 3.7 e-/adu
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	Y		23.820	0.099	-0.000		+0.021	0.011		-0.144	0.009		-0.004	0.007		+0.000	0.011		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	J		23.651	0.040   -0.010		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	H		23.811	0.063	-0.020		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	KS		23.083	0.049	-0.010		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	LOWOH1		21.008	0.072	-0.000		+0.054	0.050		-0.109	0.053		-0.017	0.056		+0.000	0.033		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	LOWOH2		25.0	9.999	-0.000		+0.027	0.021		-0.114	0.019		-0.013	0.015		-0.003	0.013		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	CH4ON		22.698	0.053	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	CH4OFF		22.687	0.044	-0.000		-0.001	0.053		-0.078	0.041		-0.039	0.052		-0.005	0.032		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	H2		20.530	0.078	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	KCONT		25.0	9.999	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 0.9.5	2006-08-16T00:00:01	2007-08-31T23:59:59	BRG	       	20.379	0.079	-0.000		+0.019	0.025		-0.060	0.023		-0.029	0.023		-0.001	0.017		0.0	3.7
 #VERSIO	STARTDATE		ENDDATE			FILTER		ZP	ZP_ERR	ZP_WONLC	ZP_EXT1	ZP_EXT1_ERR	ZP_EXT2	ZP_EXT2_ERR	ZP_EXT3	ZP_EXT3_ERR	ZP_EXT4	ZP_EXT4_ERR	COLTERM	GAIN
 # Starting with semester 06B, the gain was changed x1.5 to 3.7 e-/adu
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	Y		23.700	0.055	-0.000		+0.007	0.028		-0.142	0.022		-0.022	0.030		-0.007	0.019		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	J		23.668	0.050   -0.010		+0.016	0.031		-0.114	0.026		-0.032	0.042		-0.005	0.021		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	H		23.866	0.066	-0.020		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	KS		23.097	0.052	-0.010		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	LOWOH1		21.012	0.162	-0.000		+0.016	0.031		-0.114	0.026		-0.032	0.042		-0.005	0.021		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	LOWOH2		25.0	9.999	-0.000		+0.016	0.031		-0.114	0.026		-0.032	0.042		-0.005	0.021		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	CH4ON		22.713	0.042	-0.000		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	CH4OFF		22.680	0.042	-0.000		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	H2		20.520	0.069	-0.000		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	KCONT		20.200	0.265	-0.000		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7
 1.0	2006-08-16T00:00:01	2007-08-31T23:59:59	BRG	       	20.355	0.100	-0.000		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7