velocities for APOGEE stars

At stars group meeting, run by Lauren Anderson (Flatiron), new graduate student Jason Cao (NYU) showed us his work on measuring radial velocities for individual-visit APOGEE spectra. He has a method for determining the radial velocity that does not involve interpolating either the data or the model. During his presentation, Jo Bovy (Toronto) pointed out that, actually, the APOGEE team appears to do an interpolation of the data after the one-d spectral extraction. That's unfortunate! But anyway, we have a method that doesn't involve any interpolation which could be used on a survey that doesn't ever do interpolation before or after extraction. And yes, you can extract a spectrum on any wavelength grid you like, from any two-d data you like, without doing interpolation! The group-meeting attendees had many constructive comments for Cao.


  1. Please explain more the "no interpolation" part

    1. You can see the spectral extraction from 2-d to 1-d as an interpolation, so, in that light, you can't extract spectroscopy without interpolation. However, once the extraction has happened, if you interpolate again in 1-d to a new grid, without fully propagating / using the non-trivial covariance obtained by the 2-d-to-1-d extraction, you will get a spectral result whose noise properties and resolution vary with velocity shift, at least in any interpolation scheme I know.

      I think it is possible to mitigate these effects by fully propagating uncertainties, provided that they are Gaussian in the 2-d image, but I don't know of any spectroscopic pipeline that does this.

    2. Even in 2d there is an implicit interpolation when you do wavelength calibration. And that is not the only place (fringing, multiple orders, etc)

      From a practical aspect, this 2d to 1d extraction is the less noisy part to my knowledge. But I agree that something can be done.

      That said, I still don't know how you avoid interpolation for radial velocity "measurement".