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SDSS-III APOGEE visit reduction

Introduction

The first stage of APOGEE data reduction (apred) reduces raw data, taken from multiple exposures of a given plate on a given night (a visit), to individual spectra of each of the objects on the plate. This stage of the pipeline applies basic calibration to the images, extracts one dimensional spectra from the raw exposures produced by the spectrograph, calibrates them in wavelength and flux, attempts to remove sky emisssion and correct for absorption within the Earth's atmosphere, and combined individual spectrally-dithered exposures into single spectra for each object. It also makes an initial estimate of the radial velocity of each object.

The basic steps of APOGEE data reduction are described here. More details are available in Nidever et al. (2013)

Spectroscopic Observing

To understand the steps of APOGEE visit reduction, one needs to understand a bit of detail about how the data are taken.

Plate Plugging (plug)

When the observatory is ready to observe a plate, the observatory staff plugs optical fibers into the holes drilled into the plates, and maps which fiber corresponds to which hole (and therefore which object) by shining light through each fiber. This data is incorporated into one of the Header and Data Units (HDUs) of the apPlate file described below.

Raw Data Collection

Observers mount cartridges containing the drilled, plugged plates on the telescope, and generally collect a series of 500s exposures on each plate. For most APOGEE plates, 8 exposures are obtained on any given night, although this number can vary based on available time, observing conditions, etc.

The resolution of the APOGEE spectrum, combined with the pixel size of the APOGEE detector, leads to the property that a single spectrum slightly undersamples the resolution at the short wavelength end of the recorded APOGEE spectrum. To avoid the challenges of working with undersampled data, APOGEE spectra are taken in pairs, with the detectors moved slightly (a distance of a half-pixel) between the two exposures of the pair; we refer to these as observations at different dither positions. A standard 8-exposure APOGEE observing sequence consists of exposures at the two differ dither positions (A and B), in the pattern: ABBA ABBA. In any case, the data reduction requires exposures in dither pairs; any unpaired exposure is discarded.

The infrared detectors that are used have the capability to be read "non-destructively", in which the amount of charge per pixel can be detected without affecting that charge, allowing the levels on the detectors to be measured as the exposure is proceding. This ability to read the detectors multiple times allows for a reduction in the readout noise, which can be significant for a single read of the detectors. For APOGEE, the detectors are read in an "up-the-ramp" mode where the detectors are read every 10.7 seconds. A single exposure generally consists of 47 readouts, corresponding to an exposure time of 500 seconds. Because of the mulitiple readouts, the raw APOGEE data for an exposure is actually a "data cube", with two of the dimensions representing the location on the detectors, and the third dimension representing the time sequence.

APOGEE Visit Data Reduction

The apred software consists of three sequential steps:

ap3d

For each readout of each exposure, the raw data are first corrected for bias variations in the IR detectors and electronics. This is accomplished by using a reference array of pixels that are generated by the readout electronics, as well as a set of reference pixels around the edge of each detector.

Each individual readout is then corrected for a contribution from dark current, by subtracting a calibration dark current frame made from a combination of multiple individual dark frames.

The data are then collapsed from the 3D data cubes into 2D images. This is done on a pixel-by-pixel basic. At the most basic level, a linear function is fit to the series of up-the-ramp readouts for each pixel to determine the best-fitting slope. This slope, multiplied by the total exposure time, is taken to be the flux at this pixel location for the exposure.

The 2D images are then corrected for variations in pixel-to-pixel response by dividing them by a calibration flat field, which is constructed from an average of multiple frames illuminated by an internal light source.

ap2d

ap2d takes the calibrated 2D images and extracts individual 1D spectra for each exposure. This is accomplished by modelling the distribution of the light from each fiber, separately for each individual wavelength. The flux from all 300 fibers is fit simultaneously, allowing for contributions from each fiber into the two adjacent spectra. The profiles for each fiber are derived from a calibration frame taken immediately after the exposure sequence on each plate. The contribution of light from one fiber into the adjacent fiber is estimated using calibration observations where only every sixth fiber is illuminated.

After the 1D images are extracted, a wavelength calibration is applied, as determined from observations of some arc calibration lamps. Since the APOGEE spectrograph is in a fixed location and has been kept under the same vacuum and at the same temperature since the beginning of the survey, the form of this wavelength correction is very stable, and we have been using a single wavelength calibration to determine the non-linear terms in the conversion between pixel location and wavelength. Note the the wavelength scale for each fiber is slightly different because of the different locations of the fibers in the pseudo-slit.

The SDSS/APOGEE data describing spectral line wavelengths use vacuum wavelengths. However, the wavelengths of atomic transitions are usually quoted at standard temperature and pressure (S.T.P.); this is how the CRC Handbook of Chemistry and Physics lists them for any transitions redward of 2000 Ångstroms. Thus, recognizing spectral lines associated with atomic transitions may require converting the SDSS data to the equivalent values at S.T.P.

For APOGEE data, we have used the conversion from Ciddor (Applied Optics, Vol 35, p 1566, 1996) to convert between vacuum and air wavelengths. For a vacuum wavelength (VAC) in Ångstroms, convert to air wavelength (AIR) using the equation:

AIR = VAC / (1.0 +  5.792105E-2/(238.0185E0 - (1.E4/VAC)^2) + 1.67917E-3/( 57.362E0 - (1.E4/VAC)^2)

There are small linear shifts in the wavelength scale between different exposures, which result from (1) the intentional dithering of the detectors between exposures to allow for well-sampled combined images, and (2) small flexure in the instrument that causes small shifts as the liquid nitrogen tank gets depleted (a larger shift occurs when the tank is filled, but this always happens during the daytime). The linear shifts are measured using prominent night sky emission lines that appear in every spectrum, and these shifts are applied to the wavelength solution.

ap1dvisit

The first stage in ap1dvisit is to determine to high accuracy the linear shifts between each exposure in a visit that result from the dithering of the detectors. This can be done at higher accuracy than the determination of the wavelength zeropoint from the sky lines by cross-correlating the different exposures with each other.

Each fiber of each exposure is then corrected for contribution of night sky emission. The IR portion of the spectrum includes significant numbers of very bright OH emission lines. There can also be some continuum sky contribution, especially when there is significant moonlight (and even more so when thin clouds are present). Sky subtraction is accomplished using 35 sky fibers that are distributed across the plate. Multiple fibers are used because the IR sky can be spatially variable. For each object, the sky is estimated from nearest four sky fibers. However, since the wavelength scale is not identical for each fiber, the sky spectra need to be shifted slightly before they can be subtracted. Also, since the profiles of the lines differ slightly from fiber to fiber, there are small differences that lead to imperfect sky subtraction, in particular, of the bright night sky lines. As a result, the sky subtraction of the bright night sky lines is very imperfect, and essentially, the small regions surrounding each line are rendered useless for science. This is an area for potential improvement in the pipeline, but we note that even with perfect sky modelling, the signal-to-noise under bright sky lines would be significantly degraded compared with the surrounding spectra.

The Earth's atmosphere also leads to significant absorption in the observed spectra, which arises from absorption of CO_2, H_2 O, and CH_4 bands in the APOGEE spectral window. A correction for this telluric absorption is derived from observations of 35 "telluric" standards spread across the plate, where these stars are chosen by their intrinsic color, with the goal of targetting hot stars with relatively few spectral features in the APOGEE wavelength region. Multiple telluric stars are chosen because the absorption can vary across the field of view. For each telluric star, the amplitude of the absoprtion in each of CO2, H2O, and CH4 is estimated by fitting model absorption spectra to the observed. A surface is fit to these scaling factors, and this surface is used to predict the appropriate scale factors to be used for each individual fiber. These scaling factors, along with model telluric spectra that are convolved with the fiber-specific line spread function, are used to correct each individual spectrum. This method seems to work reasonably well in many cases, but the telluric correction is still imperfect in some cases; the leading hypothesis for this is a combination of errors in the wavelength solution, inaccuracies in the telluric model, and inaccuracies in the determination of the instrumental LSF. Improvements in telluric correction are a high priority for potential pipeline improvements.

After sky correction, pairs of dithered frames are combined to produce well-sampled images. All of the different pairs are then combined to produce a single spectrum of each object for the visit.

The final visit spectra are then approximately flux calibrated. The relative flux calibration is performed using a calibration frame which tabulates the instrument spectral response, as determined from an observation of a blackbody source. The absolute level of the spectrum is then determining using a scaling with the objects catalogs H-band magnitude. We note that subsequent analysis for stellar parameters and abundances (ASPCAP) normalizes the spectra to a pseudo-continuum, so the flux calibration done here is not critical.

Finally, an initial radial velocity (RV) estimate is made by cross-correlating each visit spectrum with a grid of synthetic spectra. The best matching one serves as a template, and the derived shift between the observed spectra and the best-fitting templates provide an initial RV estimate. Note that this estimate is later refined using multiple visits to the same object, since these provide a higher signal-to-noise spectrum.

output visit spectra: apVisit files

The final dither combined spectra from a given visit are written into individual apVisit files, as described in detail in the apVisit data model.

Most stars are observed in more than one visit. Spectra from multiple visits are shifted to rest wavelength, resampled, and combined as part of the visit combination portion of the pipeline.

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