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Standard Stars and Zero Points

Table of contents:

Zero Points Measurements Using Standards - Introduction

Standard stars are observed every photometric night with WIRCam, on average 2 times per night for each filter. Our standards are no longer picked from the UKIRT (FS) list, but from HST CALSPEC spectrophotometric standards, for which spectra have been measured both in visible and NIR, and magnitudes in all filters are computed from total throughput at CFHT, including atmosphere, optics, filters, detectors.

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 measured for each run since 2011, with additional measurements from 2007. It can be seen that the instrument has been stable for 10 years. Chip-to-chip offsets are no longer measured as ZP are calibrated on chip #4 and chip-to-chip offsets now correspond to the ratios of the medians of the flat-fields in each chip with respect to chip #4.

last update: December 11, 2017

Plots of WIRCam Zero Points

FILTER Zero Point vs. Modified Julian Date
Y J LowOH1 LowOH2
H CH4On CH4Off W
Ks H2 BrG KCont CO
H
 

Old 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.
 # February 26 2008 - Loic Albert
 #	Added the 08Aw01 data.
 # April 3 2008 - Loic Albert
 #	Make ready for version 1.5 of `I`iwi.
 # May 29 2008 - Loic Albert
 #	Modified to provide temporary ZP for the 08AW02 and 08AW03 runs.
 # June 2 2008 - Loic Albert
 #	Updated for 08Aw01 to 08Aw03 runs.
 # June 27 2008 - Loic Albert
 #	Updated temporarily to process standards of 08Aw04.
 # September 11 2008 - Loic Albert
 #	Updated temporarily to include 08Bw02.
 # October 15 2008 - Loic Albert
 #	Updated to include 08Bw03.
 #
 # 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	-0.000		+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	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
 0.9.5	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
 0.9.5	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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	2007-12-31T23:59:59	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
 #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
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	Y		23.864	0.028	-0.000		+0.007	0.028		-0.142	0.022		-0.022	0.030		-0.007	0.019		0.0	3.7 #measured on ext4 only
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	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 #no measurement
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	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 #no measurement
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	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 #no measurement
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	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 #not in
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	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 #not in
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	CH4ON		22.743	0.035	-0.000		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7 #measured on ext4 only
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	CH4OFF		22.719	0.048	-0.000		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7 #measured on ext4 only
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	H2		20.568	0.023	-0.000		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7 #measured on ext4 only
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	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 #not in
 1.0	2008-02-16T00:00:01	2008-02-26T23:59:59	BRG	       	20.403	0.015	-0.000		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7 #measured on ext4 only
 #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
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	Y		23.819	0.059	-0.000		+0.007	0.028		-0.142	0.022		-0.022	0.030		-0.007	0.019		0.0	3.7 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	J		23.726	0.040   -0.010		+0.016	0.031		-0.114	0.026		-0.032	0.042		-0.005	0.021		0.0	3.7 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	H		23.896	0.047	-0.020		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	KS		23.134	0.042	-0.010		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	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 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	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 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	CH4ON		22.775	0.041	-0.000		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	CH4OFF		22.750	0.052	-0.000		-0.02	0.04		-0.084	0.049		-0.05	0.04		-0.005	0.041		0.0	3.7 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	H2		20.600	0.047	-0.000		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	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 
 1.5	2008-02-16T00:00:01	2008-10-27T23:59:59	BRG	       	20.441	0.050	-0.000		+0.004	0.036		-0.061	0.027		-0.032	0.036		-0.001	0.022		0.0	3.7