The LVM Data Reduction Pipeline (DRP) takes raw exposures from our LVM-I system of 3 spectrographs, with 3 spectral channels each, fed by fibers from 4 telescopes (1 science, 2 sky, and 1 spectrophotometric) to produce a flux calibrated and sky subtracted row-stacked spectra (RSS) file. The current tagged version of this pipeline is available on github. We provide an overview of the reduction steps below.
Preprocessing
Due to the fixed fiber bundle and overall stability of the instrument, master calibration frames (e.g. biases, pixel masks, pixel flat fields, fiber models, fiber centroids, wavelength models, line spread function models) are constructed every few months or when there was a possible change to the instrument configuration. Preprocessing of any individual science exposure includes overscan and bias subtraction, application of the pixel flat frame, and conversion to electrons using the gain values. Any quasi-static CCD artifacts (e.g., hot pixels, bad columns, etc.) are added to a pixel mask extension. Given our 900s exposure times and that the majority of observations are not dithered, we use L. A. Cosmic, the Laplacian CR rejection algorithm, to detect and flag cosmic rays. We also model and subtract stray-light.
Extraction of 1D spectra
Our 1D spectra are then extracted, assuming a Gaussian profile for each spectrum on the CCD image. Due to the design of the spectrograph system, this is carried out across each of the 3 observed spectral channels (b, r, z) to produce 3 RSS files. The pipeline also accounts for sub-pixel thermal shifts by cross-matching a subset of columns against the fiber model.
Wavelength calibration
In addition to the wavelength calibration obtained from our arc lamp images, associated with our master calibration frames, the LVM pipeline accounts for potential thermal shifts in the wavelength model. For this, we measure the Gaussian centroids of strong and isolated sky lines within the science data. This is done separately for each arm of the spectrograph, using 1-4 lines per spectrograph. Typical wavelength corrections are ~0.01 Angstroms within a single night. Finally, all spectra are sampled to a common wavelength grid, with a wavelength sampling of 0.5 Angstroms.
Heliocentric velocity corrections are calculated based on the observation time and sky position. These are added to the metadata associated with the file in the FITS header, but are not applied to the spectra.
Astrometric solution
Images from the acquisition and guide cameras, that bracket the science fiber bundle, are used to derive the final astrometric solution associated with the science field. Our astrometric accuracy is typically XX”. This is currently not used to populate any of the primary header keywords, but will be in the future DRP release. However, this astrometric solution is used to determine the location of each individual fiber associated with the science IFU. A similar astrometric solution is also determined for the 2 sky telescope positions. All individual astrometric positions for each fiber are stored in the position table, available in the ‘SLITMAP’ extension of the RSS files.
Flux calibration
The LVM Observing Strategy is designed to provide co-temporal observations of 12 sequentially observed F-type stars during the course of one science exposure. By observing a large number of stars, we aim to mitigate any effects due to errors in the spectral typing, following the strategy previously pursued by MaNGA. These 12 stars are acquired using the spectrophotometric telescope, but fed through the same spectrograph system, with 4 stars exposed on to each spectrograph. These spectra are therefore calibrated and reduced identically to the science spectra, following the steps described above.
These stars were selected based on their magnitude, isolation (to dominate our 35.3″ spaxels) and Apsis parameters from Gaia DR3. Our preliminary method of determining the spectral sensitivity of the instrument is based on a comparison of our observed spectra with the Gaia XP spectra, however the Gaia spectra have significantly lower spectral resolution and cannot be used for telluric corrections. To overcome these limitations, ongoing pipeline development work focuses on updating this procedure to instead carry out detailed stellar template fitting.
Once the spectral response function has been determined and applied, data from the three spectrograph channels is combined to produce a single spectrum covering our full 3600-9800 Angstrom wavelength range.
When there were not enough spectrophotometric stars observed for a given science observation, or during the science commissioning when the calibrations were still being modified, another method for the flux calibration was used. Instead of using the stars from the spectrophotometric (STAN) telescope, stars within the Science IFU were used and compared with Gaia spectra. In the header, there is a keyword `FLUXCAL’ which is either `STAN’ when the spectrophotometic stars observed with the STAN telescope were used or `SCI’ when stars within the Science IFU were used for flux calibration.
Sky subtraction
The LVM Observing Strategy is also designed to provide co-temporal observations of two sky fields. Since many of the science cases involve very spatially extended sources (e.g., Milky Way emission), we use a novel strategy to observe and model the sky in order to not subtract possible science signal along with the sky background. This is done by monitoring in parallel in two separate locations on the sky. One location is selected from a grid of 768 positions to be relatively dark region within 10 degrees of the science field, and is intended to provide constraints on the sky continuum level. The second location is selected to be very dark, particularly in Milky Way foreground emission, in order to provide constraints on the time-variable sky line features, including geocoronal emission. These two fields are observed with small fiber bundles of ~60 fibers each, which are fed through the same spectrograph system, distributed approximately uniformly across each of the spectrographs. These spectra are therefore calibrated and reduced identically to the science spectra, following the steps described above. Calibrated sky spectra are then analyzed separately for their constraints on the sky continuum and sky lines, and extrapolated to the location of science field.
Final data product
Our sky subtracted, flux calibrated RSS files resulting from the LVM DRP are designated with the `SFrame’ naming convention. The typical filenames follow the format `lvmSFrame_{expnum}.fits’, where {expnum} is the exposure number for a given observation. The intermediate DRP data products will not be part of this data release, but for completion, these are the `lvmFrame’ after flat fielding, `lvmFFrame’ after flux calibration, and `lvmCFrame’ after the spectral channels are combined.
Within this single RSS file, we include different extensions. For each spectrum, we provide the flux measurements `FLUX’, the associated errors (reported as inverse variances `IVAR’), masks to identify data issues `MASK’, line spread function constraints `LSF’, and the subtracted sky spectra and error. All spectra are on a fixed wavelength grid that we provide (`WAVE’), along with a position table associating fiber locations to right ascension and declination coordinates, `SLITMAP’. A full data model for our SFrame files is provided here: LVM SFrame data model.
A description of the current LVM DRP caveats can be found at LVM caveats.