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Quality Control and
Data Processing
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FORS2: Calibration recipes DPR.CATG = CALIB, DPR.TYPE = BIAS, DPR.TECH = IMAGE
Creates a master bias from a set of bias frames.
The master bias is created from a set of raw bias frames, for which the highest and the lowest value for each pixel are rejected and the average of the remaining pixels is computed. The recipe should therefore not be used on fewer than 5 raw bias frames. The overscan regions, if present, are removed from the result. The master frame has 2 extensions, with the actual product being in the first extension and its errors being in the second extension.
The pipeline measures median values, read noise and the standard deviation of the input frames. The new master frame is compared with the corresponding certified master frame from the calibration database.
DARK frames: fors_dark
DPR.CATG = CALIB, DPR.TYPE = DARK, DPR.TECH = IMAGE
Creates a master dark from a set of dark frames.
After bias subtraction, the median of the corrected input frames is calculated. The combined frame is finally normalised to the unit time. The overscan regions, if present, are removed from the result. The master frame has 2 extensions, with the actual product being in the first extension and its errors being in the second extension.
Dark master calibration frames are generally not used in further data reduction steps: dark current in the FORS detectors is negligible, and trying to subtract it would just add noise to the data. Dark frames are just produced for monitoring the instrument.
IMG_SCREEN_FLAT frames: fors_img_screen_flat
DPR.CATG = CALIB, DPR.TYPE = FLAT,LAMP, DPR.TECH = IMAGE
Creates a master screen flat from a set of screenflat images.
After bias subtraction, the input frames are averaged. The overscan regions, if present, are removed from the result. The combined frame is then normalised dividing it by its large scale illumination trend. The large scale trend is obtained by applying a median filter with a large kernel. The master frame has 2 extensions, with the actual product being in the first extension and its errors being in the second extension.
The pipeline measures median values, conad and fixed pattern noise from the input frames. The new master frame is compared with the corresponding certified master frame from the calibration database.
IMG_SKY_FLAT frames: fors_img_sky_flat
DPR.CATG = CALIB, DPR.TYPE = FLAT,SKY, DPR.TECH = IMAGE
Creates a master twilight flat from a set of twilightflat images. Warning:The twilight images of FORS2, which are used to create master sky flats, show evidence for a feature, whose position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information please look at the FORS Secondary Standard and Absolute Photometry Project. In order to eliminate the contributions of field stars on the jittered sequence of flat fields, the frames are combined using a median rather than on a simple average. Before combining the individual frames, each is normalized with the median of its flux. The overscan regions, if present, are removed from the result. The master frame has 2 extensions, with the actual product being in the first extension and its errors being in the second extension.
The new master frame is compared with the corresponding certified master frame from the calibration database.
Zeropoint frames: fors_zeropoint
DPR.CATG = CALIB, DPR.TYPE = STD, DPR.TECH = IMAGE
Do a standard reduction of the input frame (debias, flatfielding), search for objects, determine their luminosities and align the derived photometry table with a standard star reference catalog. Warning:The twilight images of FORS2, which are used to create master sky flats, show evidence for a feature, whose position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information please look at the FORS Secondary Standard a nd Absolute Photometry Project.
The fors_zeropoint recipe is used to estimate the magnitude zeropoint or the atmospheric extinction from one imaging exposure on a photometric standard stars field. The bias master calibration is subtracted from the raw exposure. The debiased signal is then divided by the normalised sky flat field, and the overscan regions, if present, are removed from the result. The frame is not normalized 1 second exposure time. The calibrated image is then sent to a source detection and extraction application (SExtractor 2.5.0). The detected sources are compared to a catalogue of standard stars for identification. The comparison is made, whenever possible, applying point-pattern-matching techniques. If pattern matching either fails or is not applicable (e.g., too few standard stars are present in the field-of-view), then stars identification will be entirely based on the sky-to-CCD transformation specified in the input image FITS header. Finally, the difference between the catalog magnitude (corrected for the transmission curve difference between the used filter and the catalog filter, i.e. the colour term) and the instrumental magnitude (based on electron counts and corrected to airmass zero), is optimally averaged on all the identified standard stars. The derived quantity is conventionally referred to as the frame zeropoint. The corresponding extinction coefficient is computed as E = Eph + (Zph - Z)/A where Z is the computed frame zeropoint, Zph the expected zeropoint on a photometric night, Eph the atmospheric extinction coefficient of a photometric night (that was used in the computation of Z), and A is the airmass. The assumptions on extinction coefficients Eph and instrument zeropoints Zph for photometric nights are listed here for each FORS filter. They are also contained in the photometric tables ued by the pipeline and delivered to the PIs. The zeropoint and the atmospheric extinction coefficients computed by this recipe have the sole purpose of monitoring the instrument+telescope system and the quality of the atmosphere. With only one exposure it is impossible to obtain actual determinations of either the atmospheric extinction coefficient (mag/airmass) or the instrument zeropoint without making assumptions on the other unknown. In order to evaluate them both we would need at least two different exposures of standard star fields (not necessarily of the same field), obtained at (very) different airmasses.
Mis-identified standard stars In May 2011 a problem was noticed with some standard star field results, where the pipeline mis-identified standard stars. Usually such mis-identifications are caught during the QC certification process, but a few such cases (≤5%) have slipped through. In such cases typically only few of the identified stars carry high weight. The new master frame is compared with the corresponding certified master frame from the calibration database.
FORS_CALIB: fors_calib
DPR.CATG = CALIB, DPR.TYPE = FLAT,LAMP/WAVE,LAMP, DPR.TECH = SPECTRUM DPR.CATG = CALIB, DPR.TYPE = FLAT,LAMP/WAVE,LAMP, DPR.TECH = MOS DPR.CATG = CALIB, DPR.TYPE = FLAT,LAMP/WAVE,LAMP, DPR.TECH = MXU DPR.CATG = CALIB, DPR.TYPE = FLAT,LAMP/WAVE,LAMP, DPR.TECH = POLARIMETRY
Creates all necessary calibration products to process SCIENCE data.
The recipe identifies reference lines on LSS and MOS/MXU arc lamp exposures, and traces the spectral edges on the associated flat field exposures. With this information the spectral extraction mask to be applied in the scientific data reduction is determined. From the input flat field exposures a normalised flat field frame is also derived. The input arc lamp and flat field exposures are assumed to be obtained quasi-simultaneously, so that they would be described by exactly the same optical distortions.
Missing rows in wavelength calibration for MOS/MXU: Sometimes the pipeline does not find a wavelength solution for all CCD rows in MOS/MXU data. Interpolation across such missing rows sometimes does not work, resulting in missing rows (= flux identical to zero) in mapped science data. These are the narrow black lines seen in the lower slit in the screen shot below.
Wavelength Calibration for 150I: For some offsets both in LSS and MOS the pipeline cannot correctly align the redmost part of the 150I arc spectrum. This can be done manually by changing the parameters of the pipeline slightly. Cosmic correction/spectrum extraction: The correction for cosmic rays done in the new pipeline since 2006-10-01 can distort line profiles, esp. of narrow lines. Users are therefore urged to check their extracted spectra carefully. Sky background in MOS/MXU observations: The assumption of the new pipeline (since 2006-10-01) that at least 50% of the pixels contain only sky background is violated in many cases, so that the sky will be overestimated. Comparing the mapped science frame and the mapped sky frame allows to check this effect. Sky background in LSS observations: The use of a median to estimate the sky background (since 2006-10-01) does not correct for large spatial gradients. Flat-fielding in MOS mode: Due to the problems induced by the stray light contamination, in some cases the pipeline may introduce small-scale, artificial patterns in the flatfielding. These patterns are mostly located close to the slit edges and are below the 3% level. Artifical patterns can also be introduced close to bad pixels regions. The presence of these patterns does not hamper noticeably the accuracy of the wavelength calibration and the quality of the science data. The new master frame is compared with the corresponding certified master frame from the calibration database.
FORS_SCIENCE: fors_science
DPR.CATG = CALIB, DPR.TYPE = STD, DPR.TECH = LSS/MOS/MXU DPR.CATG = SCIENCE, DPR.TYPE = SKY, DPR.TECH = SPECTRUM
Processes on-sky (STD and SCIENCE) spectroscopic data This recipe is used for reducing FORS2 LSS, MOS, and MXU scientific spectra applying the extraction mask and the normalised flat field created by the recipe fors_calib. The slit spectra are bias subtracted, flat fielded if requested, and remapped eliminating the optical distortions. The input wavelength calibration can optionally be adjusted to a number of reference sky lines. Finally, objects are searched and extracted from all the slit spectra. This recipe is also used to reduced spectroscopic standard star observations. In case of standard star observations the SCI acronym in the products names should also be read STD. In these cases the recipe also determines the efficiency of the system from associated flux standard star tables as well as response curves. These response curves are then combined into master response curves after some time, which will be used to flux-calibrate the science data.
FORS_PMOS_CALIB: fors_pmos_calib
DPR.CATG = CALIB, DPR.TYPE = FLAT,LAMP/WAVE,LAMP, DPR.TECH = POLARIMETRY
Creates all necessary calibration products to process PMOS SCIENCE data
This recipe identifies reference lines on PMOS arc lamp exposures, and traces the spectral edges on the associated flat field exposures. With this information the spectral extraction mask to be applied in the scientific data reduction is determined. The recipe takes into account the fact that every slit in the mask produces two slits in the observed frames. From the input flat field exposures a normalised flat field frame is also derived. The input arc lamp and flat field exposures are assumed to be obtained quasi-simultaneously, so that they would be described by exactly the same optical distortions.
Association of PMOS spectra for different numbers of objects in different slits: If there is more than one active slit, and at least one slit has a number of objects different from the other slits, then the association between spectra belonging to the same object (in the two corresponding beams) was ALWAYS wrong for pipeline versions up to 4.7.3. As long as the number of objects per slit is the same for all slits (including the case of only 1 active slit) the association was correct.
FORS_PMOS_SCIENCE: fors_pmos_science
DPR.CATG = CALIB, DPR.TYPE = STD, DPR.TECH = POLARIMETRY DPR.CATG = SCIENCE, DPR.TYPE = SKY, DPR.TECH = POLARIMETRY
Processes on-sky (STD and SCIENCE) PMOS data This recipe is used for reducing FORS2 PMOS, scientific spectra applying the extraction mask and the normalised flat field created by the recipe fors_pmos_calib. The slit spectra are bias subtracted, flat fielded if requested, and remapped eliminating the optical distortions. The input wavelength calibration can optionally be adjusted to a number of reference sky lines. Finally, objects are searched and extracted from all the slit spectra. Then the appropriate polarimetric parameters (for linear and circular polarization) are derived by combining the spectra from the various exposures. This recipe is also used to reduced spectroscopic standard star observations. In case of standard star observations the SCI acronym in the products names should also be read STD. The pipeline reduces sets of 2 or 4 frames for circular polarimetry and sets of 4 or 8 frames for linear polarimetry for versions up to 4.7.3. Later versions have no such limit. Pipeline versions up to 4.7.3 also had the following bug: If there was more than one active slit, and at least one slit had a number of objects different from the other slits, then the association between spectra belonging to the same object (in the two corresponding beams) was ALWAYS wrong. As long as the number of objects per slit is the same for all slits (including the case of only 1 active slit) the association was correct. In the case of standard stars the recipe also determines for polarized standard the polarization angle and the offset in polarization. For unpolarized standard stars only the absolute polarization is determined.
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