APOGEE Visit Combination

TODO: Update for changes for DR19

Most APOGEE fields are observed multiple times to build up the signal-to-noise ratio for the faintest stars and to detect stellar binaries from their radial velocity variation. For analysis to be performed at maximum S/N, these multiple “visit” spectra (as output by ap1dvisit) are combined into one spectrum for each star. This process is called “visit combination” and involves several steps.

The output of this process is a single apStar file for each star. This file contains the combined spectrum as well as all of the resampled visit spectra on the same wavelength scale.

Visit Combination Steps

  1. Re-determination of Doppler shift (i.e., radial velocity) for each visit spectrum.
  2. Resampling of each visit spectrum onto the same rest wavelength scale (i.e., the Doppler shift is removed).
  3. Continuum normalization of each visit spectrum using a median filter.
  4. Weighted combination of all the resampled visit spectra.
  5. Re-application of mean continuum shape.

Resampling

To combine the visit spectra, their individual Doppler shifts must be removed and they then must be resampled onto the same wavelength scale. With the radial velocity determined in step (1) above, the wavelengths are corrected to the rest wavelength values (λrest=λobs/(1+RV/c), where c is the speed of light). Using the tabulated rest wavelengths, the spectrum is resampled onto the final logarithmically-spaced wavelength scale using sinc-interpolation.

Continuum Normalization

Because the visit spectra are taken under different conditions (e.g., airmass) and the flux calibration is not perfect, the relative fluxes as a function of wavelength can vary from visit to visit. Therefore, each visit spectrum is roughly continuum normalized before the ensemble may be combined. A 500-pixel median filter that excludes bad pixels is used to calculate the continuum and normalize each spectrum. This continuum is saved for a final re-normalizing step at the end.

Weighted Combination

The final step is to combine the rest-frame shifted, resampled and normalized visit spectra. The combination is done in two different ways: (1) global weighting, where each visit spectrum is weighted by its (S/N)2, and (2) pixel-by-pixel weighting, where each pixel is weighted by its (S/N)2. In both cases, bad pixels in individual visit spectra are discarded before the combination. Also, the uncertainties of pixels affected by persistence are inflated, resulting in reduced weight of these pixels to the combined spectrum. The two schemes give similar results in most cases, but both are saved in the apStar FITS file. For visit spectra with 1 < S/N < 10, the spectra’s weights are set to zero if its S/N is less than three standard deviations below the median S/N of all of the visit spectra for the star. For visit spectra with S/N < 1, the spectra’s weights are always set to zero.

Finally, the combined spectra are multiplied by the average (over the multiple visit spectra) of the continuum shape to return the continuum to the combined spectrum.

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Output Star Spectra: apStar files

As described in the data access description, the combined spectra are provided in apStar files in FITS format. The primary HDU of each file contains an image which gives the two versions of the combined spectrum mentioned above for each object, plus the individual visit spectra that went into these combinations.

For these files, all of the individual spectra have been resampled to a common logarithmically-spaced wavelength scale, with the radial velocity of each individual visit spectrum removed. Note that the APOGEE wavelength scale is based on vacuum wavelengths. The logarithmic wavelength grid spacing is the same for all objects (log10 λi+1 – log10 λi = 6E-6) with a common starting wavelength of 15100.802 Angstroms.

These spectra are roughly flux-calibrated. Additional HDUs contain the estimated uncertainties in each pixel, masks, and other information. HDU2 stores the uncertainty per pixel. The pixel mask information is stored in HDU3. These images yield a bitmask for each pixel, in particular the APOGEEPIXMASK bitmask. As the final spectrum is a combination of three or more individual exposures, it may be that some bits were flagged in some exposures but not in others.

The headers also include a global STARFLAG bitmask that flags global conditions about the spectra. The global STARFLAG bitmask starts as a bitwise OR of the individual visit STARFLAG bitmasks, while the ANDFLAG bitmask is a logical AND of the individual visit bitmasks; however, in addition, to the the visit-level bits, there are bits related to the radial velocities used for the combination that can be set in the global STARFLAG.

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