Twilight flats of each quadrant are visually checked for star removal, vignetting and structure and compared with a standard master. The following parameters are monitored: structure and noise.These values are accessible via the trending page.

Products.

Photometric Zeropoints
relevant FITS keys are:

TPL ID = VIMOS_img_cal_Photom,
DPR CATG = CALIB,
DPR TYPE = STD,
DPR TECH = IMAGE

Reduced (i.e. bias-subtracted, flat-fileded) standard stars image in the R filter and quadrant 1. The overscans have been removed and the vignetting is apparent on three sides. The photometric field shown is PG0231 (RA 02:33:23.2, DEC 05:13:34.1 J2000) and the Stetson photometric standard stars used to compute zeropoins are indicated with circles.

Purpose. Photometric standard stars are observed almost every night VIMOS is operational in some of the five filters U, B,V,R,I and they are used to compute the magnitude zeropoints. This parameter provides information on the total throughput of the telescope and the instrument. On Paranal the zeropoints are computed on a frame-by-frame basis at the beginning of the night to check the total throughput. QC Garching computes the night zeropoints per filter and per quadrant using all the night standard stars observations. The photometric night zeropoint are then used to calibrate science imaging observations and to monitor the total throughput.

Recipe. Two recipes are used by QC garching to compute the night magnitude zeropoints:

vmImObs does the bias-subtraction, overscans removal and flat fielding of each single photometric field image, then runs S-Extractor ( Bertin & Arnouts 1996 (A&A 117, 393) ) to find all the objects in the field and to compute their instrumental magnitude (i.e. the SExtractor MAG_BEST). The photometric standard stars are identified using a photometric catalog, and written in a table together with their instrumental and catalog magnitudes. The recipe also outputs the reduced standard star frame.

vmImCalOpt computes the airmass and color corrected magnitude zeropoints of the night. The input is the set of all available tables per filter and per quadrant produced by vmImObsStare , the number of table is equal to the number of frames per filter and per quadrant observed during the night. The output is a zeropoint table containing the magnitude zeropoint value (in ADU), the color term, the color index and the extinction coefficient used. This zeropoint is then used in ADU to calibrate science images and it is converted into electrons for trending.

QC checks.

Night zeropoints in electrons corrected by extinction and color are monitored for each quadrant and each filter by QC Garching (see trending page). Color Terms and extinction coefficients are computed in general every 6 months

Products.

Note The standard star fields are in general observed with the standard setting for imaging: 1x1 binning and low gain.

MOS flat

TPL ID = VIMOS_mos_cal_FLAT, VIMOS_mos_cal_NightCalib
DPR CATG = CALIB,
DPR TYPE = FLAT,LAMP
DPR TECH = MOS

Raw and reduced MOS flat frame of quadrant 1, here for the LowRes Red grism. Fringing is visible in the redder regions (i.e. toward the right) of the reduced frame

Purpose.

MOS flats are in general taken the morning following the science observations. When required that are also taken during the night together with an arc-lamp exposure, just after the science acquisition. Spectroscopic flats in principle contain the lamp function the slit function, the fixed pattern noise. The red flats show also fringes, that are different from the science exposure. For this reason, the flat-fielding introduces artificial features in the science frames, therefore science data in the red grisms are not flat-fielded. The flat fields are instead used to refine the spectral curvature model.

Recipe. The vmSpFlat recipe expects a stack of frames that can also be obtained with different mask shutter positions to avoid spectra contamination. The product is a master flat normalized.

QC checks.

Each product is visually checked and the trended parameter is the lamp flux at reference wavelength for the slit closest to the center (see trending page).

Products.

MOS arc lamp,

TPL ID = VIMOS_mos_cal_arc, VIMOS_mos_cal_NightCalib
DPR CATG = CALIB,
DPR TYPE = WAVE,LAMP,
DPR TECH = MOS

Raw MOS arc frame of quadrant 1 obtained with the LR_red grism. Cleary visible are the spectra of the two reference slits (at the top-right, and the low-middle regions)

Purpose. Arc lamp frames are in general taken the morning after the science observations. When required, they are taken also during the night together with 3 flat exposures, just after the science acquisition. The position of the arc lines is used for wavelength calibration.

Recipe. The vmSpCalDisp recipe expects one arc lamp exposure that has written in the header the first guesses of the inverse dispersion solution (ESO PRO IDS MAT_i_j_k), of the spectral curvature (ESO PRO CRV MOD_i_j_k) and optical distorsions models (ESO PRO OPT DIS X/Y_i_j). A master flat field can be also given in case the refined curvature and distorsion models are to be computed. The inverse dispersion solution, and the spectral curvature are computed for each line spectrum of each slit and written in a table (VI_PWDM). The RMS of the inverse dispersion solution residuals is also evaluated.

QC checks.

Products.

The inverse dispersion solution table (product name:VI_PWDM, category name: EXTRAC_TABLE) contains the following columns used for MOS science spectra calibration and for spectra extraction:

Y is the dispersion direction coordinate. The image position X₀,Y₀ of the center of the slit x,y on the mask, for a given wavelength lambda₀ assumed as the reference wavelength and written in the grism table, is derived using the optical distorsion model written in the header coefficients "ESO PRO OPT DIS X_i_j" = O_ij, "ESO PRO OPT DIS Y_i_j" = Q_ij

Then the spectral curvature model (i.e. the curvature on the CCD of spectrum corresponding to the point x,y of the mask) is refined for each row using the slit borders of the flat-field and starting from the first guess to find the coefficients CRV_POL_i = C_i:

X-X_s=C₁(Y-Y₀)+C₂(Y-Y₀)²+...

Y-Y₀=D₀+D₁(lambda-lambda₀)+D₂(lambda-lambda₀)²+...

MOS spectrophotometric standards

DPR CATG = CALIB, DPR TYPE = STD, DPR TECH = MOS

Raw MOS spectrum with grism LR_red of a photometric standard star (visible in slit 7).

Purpose.Specrophotometric calibration of science frames (not yet pipeline supported). The spectrophotometric standards are acquired with an 8-slits mask, and the telescope is shifted in order to have the same standard in each of the four quadrants.

Recipe. The recipe vmSpCalPhot is under testing.

Products.

IFU flat and IFU arc lamp

TPL ID = VIMOS_ifu_cal_NightCalib, VIMOS_ifu_cal_DayCalib
DPR CATG = CALIB,
DPR TYPE = FLAT,LAMP (for flats)
DPR TYPE = WAVE,LAMP (for arc-lamps)
DPR TECH = IFU

Raw (left) and extracted (right) IFU flat frame of quadrant 1, taken with the LowRes Red grism. In Low Resolution all the 4 IFU pseudo-slits with 1600 fibers are exposed (multiplexing 4). The 1600 extracted fiber spectra are wavelength calibrated and are stored in the image successively, pseud-slit by pseudo-slit counting fiber from left to right. In the raw frame, since the data are of quadrant 1, the first pseudo-slit is the one at the bottom (the first pseudo-slit is at the top in quadrant 3 and 4). Then, the first row of the extracted image corresponds to the first fiber to the left of the sequence at the bottom of the raw image (i.e. pseudo-slit1), and the last row of the extracted image corresponds to the last fiber to the right of the uppermost fiber sequence (i.e. pseudo-slit 4). The zero order contamination is clearly visible in pseudo-slits 1 and 2 (i.e. the first two pseudo-slits starting from the bottom of the frame). Fringing is visible in the redder regions (toward the right side) of the extracted frame

Raw (left) and extracted (bottom) IFU flat frame of quadrant 1, here for the HighRes Red grism. In high resolution only the central IFU pseudo-slit, with 400 fibers, is exposed. The extracted fiber spectra are wavelength calibrated and are stored in the image successively starting from the first fiber to the left of the raw frame. Fringing is visible in the redder regions (toward the top on the raw and toward the right in the extracted frame).

Raw IFU arc lamp frames of quadrant 1. On the left the exposure is taken with the LowRes Red grism and all the 4 pseudo-slits (1600 fibers) are used. On the right the exposure is taken with the HighRes Red grism and only the central pseudo-slit is used (400 fibers). Redder wavelengths are toward the top of the frame.

Purpose. IFU arc lamp and flat frames are taken during the night (template VIMOS_ifu_cal_NightCalib), immediately after the science acquisition template, with the same instrument rotation angle as the science. They are also taken during the day (template VIMOS_ifu_cal_DayCalib). Due to the instability of the instrument, the night calibration are required for the science reduction. The flat-fields are used for identifying and tracing the fibers and for computing their relative transmission, the arc-lamps are used for the wavelength calibration.

Recipe. The vmifucalib recipe expects one arc lamp and at least one flat field exposure. The recipe uses also a fiber identification file containing information on the fiber positions. The spectral distorsion are computed by tracing the flat field spectra. The wavelength calibration is done after extracting the arc lamp spectra along the flat-field traces. The RMS of the inverse dispersion solution residuals is also evaluated and written in the header keyword "HIERARC ESO QC IFU IDS RMS"

Products.

The inverse dispersion solution file (product tag PWDF, category name: IFU_IDS) contains one table extension for each active pseudo-slit. Each table contains the coefficients of the polynomial fits for each of the 400 fibers, starting from the first fiber to the left.

The extraction file (product tag:PTCF, category name: IFU_TRACE) contains two table extension for each active pseudo-slit. Each table contains the coefficients of the polynomial fits for each of the 400 fibers, starting from the first fiber to the left. The first table extension coefficients are referred to the whole spectral range, the second extension coefficients are obtained by linear fitting of the traces on a shorter spectral range.

The transmission file (product tag: PTNF, category name: IFU_TRANSMISSION) contains the fiber to fiber relative transmission correction factors for each of the 400 (High or Medium resolution) or 1600 fibers (Low resolution), starting from the first fiber spectrum of the first pseudo-slit.

IFU spectrophotometric standards

TPL ID = VIMOS_ifu_cal_Specphot
DPR CATG = CALIB,
DPR TYPE = STD
DPR TECH = IFU

Raw (left) and reduced (right) IFU spectrophotometric standard star spectrum of quadrant 1 obtained with grism HighRes Orange. . The standard star is LTT-7379. The reduced spectrum is the sum of all the fiber extracted spectra, it is wavelength calibrated and the signal is given in ADU per wavelength interval. The wavelength interval used is listed in the header keyword CDELT1. For the HighResOrange grism CDELT1is 0.62 A/pix. To have the signal per Angstrom one should then divide for 0.62.

Purpose.Specrophotometric calibration of science frames is not yet pipeline supported. The standard star frames are reduced but no spectral response curve is computed. The acquisition of spectrophotometric standards consists in four exposures with the telescope shifted in order to have the same standard star in each of the four quadrants.

Recipe. The recipe vmifustandard requires in addition to the raw standard star frame, the three files produced with the vmifucalib recipe (product tags: PWDF, PTCF and PTNF) that contain information on the inverse dispersion solution, fiber extraction and transmission respectively. The standard star spectra are extracted and resampled to a constant wavelength step written under the header keyword CDELT1 (CDELT1 is 0.62 A/pix for the grism HighResOrange). The extracted spectra are transmission corrected, subtracted and stored in the product with product tag STRF. The extracted spectra are stored in the output image in the usual order: successively from pseudo-slit1 to pseudo-slit4 counting fibers from left to right. The blue wavelengths are on the left. The sum of all the fiber extracted spectra is stored in the product with tag STXF, and the median sky spectrum that have been subtracted in the product with tag STSF.

Products.

Send comments to <qc_vimos@eso.org>  Last update: Jul 5, 2004