Purpose

Spectrophotometric standard stars are observed for all grisms on a regular basis, dependent on the usage of the settings for science. Their purpose is to monitor the overall instrument efficiency (telescope+instrument+detector) and to provide a measure of the spectral response. They are measured for MOS and IFU separately.

Although a single standard star observation may be well suited for providing a response curve for relative flux calibration, usage of individual measurements is often limited due to two reasons: standard stars are not always observed under optimum weather conditions or under conditions similar to the science observation and instrumental instabilities or scheduling requirements may prevent observation of standards in time.

Master Response Curves

In order to handle the above mentioned shortcomings, standardized response curves which are averaged from individual measurements over a dedicated time period can be very helpful. Such curves are called master response curves in the following.

The VIMOS pipeline extracts standard star spectra for MOS in the same way as science observations. In case of IFU, the fibre spectra are, in addition to the procedure for science, also sky subtracted and summed. The response curve is calculated as:

R = F_std / f_std

with

F_std: tabulated standard star flux in erg/cm**2/s/A
f_std: observed flux corrected for airmass and extinction
f_std = f_raw 10**(0.4*airmass*extinction), f_raw in electrons/s/A.

Master response curves have been created from pipeline-reduced standard star observations by applying the following steps:

1. Displaying all response curves measured during a dedicated period for one instrumental setting
2. Deselecting curves that clearly deviate
3. Averaging the remaining curves.

Verification

For verification of the master response curves, several standard star observations for all settings have been treated as science and the extracted spectra have been flux-calibrated using the master curves. The results have been compared to the tabulated flux values.

Example for MOS: Observation of CD -32 9927 from 2006-03-31 with HR_blue grism (no filter). The left image shows the tabulated flux and the extracted spectra (not flux-calibrated) from quadrants 1 and 3. The spectra differ significantly at longer wavelengths. In the middle image, the flux-calibrated spectra from all four quadrants are compared to the tabulated flux. The spectra from all quadrants have nearly the same level now. The right image shows the flux-calibrated spectra divided by the tabulated flux. These residuals are better than 10% over most of the spectral range.

Example for IFU: Observation of EG 21 from 2006-08-23 with LR_red grism, OS-red filter, and 2:1 IFU magnification. The spectra from all fibres have been summed and an estimated background subtracted (black curves). The tabulated flux is shown in red. (Please note that the binning of the tabulated flux is much lower than the resolution of the extracted spectra.) The observations show artefacts from filter transmission (not corrected during reduction) but the overall level agrees well.

Accuracy and limitations: Comparison of flux-calibrated spectra with tabulated flux values show that, in general, an accuracy of 10% can be achieved for relative calibration within each spectrum. The master response curves represent, however, a snap shot in time and changes in the overall efficiency of the telescope+instrument cannot be accounted.

Availability and Usage

Master response curves have been created for most MOS and IFU settings. Details can be found in the tables below. The response curves for the HR_red grism apply only to the new holographic grisms which are operational since November 2005. Please note that the VIMOS detectors have been exchanged in June 2010. Two different sets of response curves are, therefore, available which should be used for data obtained until May 2010 or after August 2010, respectively.

Table 1: available MOS settings. The time range from which standard star observations have been taken is indicated. The number of actually used observations for averaging can vary between quadrants.

Grism	Filter	Detectors	Time range of STD observations	Number of observations
LR_blue	OS-blue	OLD	2005-09-08 to 2006-07-22	24..29
LR_blue	free	OLD	2005-09-25 to 2006-07-29	3..5
LR_red	OS-red	OLD	2006-08-21 to 2006-11-26	9..10
MR	GG475	OLD	2006-09-13 to 2006-11-26	15..16
MR	OS-red	OLD	2006-01-01 to 2006-05-30	16..19
HR_blue	free	OLD	2006-01-04 to 2006-07-01	10..11
HR_orange	GG435	OLD	2006-08-16 to 2006-11-26	4
HR_red	GG475	OLD	2005-11-30 to 2006-04-25	2..3
LR_blue	OS-blue	NEW	Oct 2010 to Mar 2011	12..15
LR_blue	free	NEW	not available
LR_red	OS-red	NEW	Oct 2010 to Mar 2011	11..12
MR	GG475	NEW	Nov 2010 to Apr 2011	5..8
MR	OS-red	NEW	Apr 2011	2..3
HR_blue	free	NEW	Oct 2010 to Feb 2011	2..3
HR_orange	GG435	NEW	Nov 2010 to Apr 2011	7..10
HR_red	GG475	NEW	Dec 2010 to Mar 2011	3..4

Table 2: available IFU settings. The time range from which standard star observations have been taken is indicated. The number of actually used observations for averaging can vary between quadrants.

Grism	Filter	IFU magnification	IFU shutter	Detectors	Time range of STD observations	Number of observations
LR_blue	OS-blue	1:1	off	OLD	2006-07-18 to 2006-10-27	8..11
LR_blue	OS-blue	2:1	off	OLD	2005-11-10 to 2006-04-30	4..6
LR_red	OS-red	1:1	off	OLD	2005-09-06 to 2006-05-25	9..14
LR_red	OS-red	2:1	off	OLD	2005-09-23 to 2006-08-22	5..8
MR	GG475	1:1	on	OLD	2006-11-13 to 2006-11-26	7..8
MR	GG475	2:1	on	OLD	2005-11-10 to 2006-02-01	4..5
HR_blue	free	1:1	on	OLD	2006-09-20 to 2006-11-14	7..9
HR_blue	free	2:1	on	OLD	2006-09-18 to 2006-11-19	8..9
HR_orange	GG435	1:1	on	OLD	2006-10-06 to 2006-11-16	8..10
HR_orange	GG435	2:1	on	OLD	2006-10-11 to 2006-11-13	4..5
HR_red	GG475	1:1	on	OLD	2006-02-23 to 2006-11-25	3..6
HR_red	GG475	2:1	on	OLD	2006-02-22 to 2006-08-19	2
LR_blue	OS-blue			NEW	not available
LR_red	OS-red			NEW	not available
MR	GG475	1:1	on	NEW	Oct 2010 to Feb 2011	6
MR	GG475	2:1	on	NEW	not available
HR_blue	free	1:1	on	NEW	Oct 2010 to Mar 2011	10..11
HR_blue	free	2:1	on	NEW	Nov 2010 to Jan 2011	4..6
HR_orange	GG435	1:1	on	NEW	Oct 2010 to Mar 2011	14..15
HR_orange	GG435	2:1	on	NEW	Oct 2010 to Nov 2010	3
HR_red	GG475	1:1	on	NEW	Oct 2010 to Dec 2010	2..3
HR_red	GG475	2:1	on	NEW	not available

Usage: Master response curves R_mst can be applied by hand to extracted spectra with the following procedure:

F_obs = R_mst * f_raw * 10**(0.4*airmass*extinction)

with f_raw in electrons/s/A. The VIMOS pipeline flux-calibrates spectra when a response curve is provided with the tags MOS_SPECPHOT_TABLE or IFU_SPECPHOT_TABLE, respectively, in the set of frames and if the appropriate recipe options are set (--photometry=true in case of vmmosscience and --CalibrateFlux=true for vmifuscience).

Download

Sets of master response curves for MOS and IFU are available as tar files:

	MOS	IFU
pre-upgrade	VI_GRSM_030101.tar	VI_GRSF_030101.tar
post-upgrade	VI_GRSM_101001.tar	VI_GRSF_101001.tar
new HR_blue grism, after March 2012	VI_GRSM_HRb_120315.tar	VI_GRSF_HRb_120315.tar

The file names have the following formats.

For MOS:

VI_<TYPE>_<DATETAG>_<ORDFLT>_<GRISM>_Q<QUAD>.fits

For IFU:

VI_<TYPE>_<DATETAG>_<ORDFLT>_<GRISM>_<IFUMAG><IFUSHUT>_Q<QUAD>.fits

With:

<TYPE>: either GRSM for MOS or GRSF for IFU master response curves
<ORDFLT>: order separation filter (OS-blue, OS-red, GG435, GG475, or "_" for free)
<GRISM>: grism (LR_blue, LR_red, MR, HR_blue, HR_orange, HR_red)
<QUAD>: quadrant (1, 2, 3, or 4)
<IFUMAG>: IFU magnification 1 (for 1:1) or 2 (for 2:1)
<IFUSHUT>: IFU shutter ON or OF (OFF)

The response files are fits tables with 7 columns for compatibility with the VIMOS pipeline but only columns 1 (WAVE) and 7 (RESPONSE) are filled.