3. Demos

Demos for instruments can be found on the demos page. 

• SPIRou

• NIRPS APERO

• NIRPS ESO

• HARPS

• ESPRESSO

4. Input/Output data format

When completed there should be the following directories inside the DATA_DIR given.

calib              // contains input calibration files required by lbl 

                        (i.e. blaze and wave files depending on instrument
lblreftable        // LBL reference tables are added here

log                // Log files are stored here

models             // Any models downloaded are placed here

plots              // Any plots saved to disk are put here

science            // input spectra are put here (each in `{OBJECT_SCIENCE}` sub-directories)

lblrdb             // LBL RDB files are added here

lblrv              // LBL RV files (in `{OBJECT_SCIENCE}_{OBJECT_TEMPLATE}` sub-directories) 

                        are stored here

masks              // mask files for each object are placed here

other              // Other downloaded files are added here

templates          // templates are put here

Radial velocity outputs:

There are two main outputs a .fits file and a .rdb file.

The rdb file is compatible with dace (https://dace.unige.ch/radialVelocities/?) 

The fits file contains several extensions:

• Wave, DV, SDV, D2V, SD2V, D3V, SD3V images (each 2D with shape number of lines by the number of files)

• RDB0   is the reference table (fits bin table format)

• RDB is the fits bin table equivalent of the .rdb file

• PTABLE is the parameters used in this LBL run (fits bin table format)

Note the .rdb file and RDB extension in the fits file consist of the following columns:

• rjd     / reduced Julian date

• vrad   / the velocity (first derivative, dv) in m/s

• svrad   / the uncertainty on velocity

• d2v, sd2v, d3v, sd3v   are the second and third derivatives, and their respective uncertainties

• fwhm, sig_fwhm, dW and sdW   (FWHM and width (defined by CARMENES), related to d2v) and their respective uncertainties

• vrad_{key} and svrad_{key}    where 'key' relates to a specific measure of the radial velocity

  • • achromatic: 

    • chromatic_slope: 

    • photometric_band: 

    • photometric_band_{xlow-xhigh}   - dividing the detector into chunks

    • {Lambda}nm    - divided into chunks (where lambda is the center of the bin)

Notes:

_tc denotes a file has been cleaned of tellurics within LBL

6.  More details on LBL

As the LBL name implies, ‘lines’ are key in the LBL algorithm. These lines are propagated through the code under different names. Here are the main steps within the LBL code in handling lines.

After creating (or inputting) a template, we find the local maxima and minima (see Figure 2 in Artigau et al. 2022). The location of these maxima and minima are reported in the _pos and _neg (positive and negative) masks in the masks/ folder. There is also a _full file that reports both positive and negative features, it is simply the combination of the two other tables.

Masks are used primarily to cut the spectral domain in individual regions over which velocities are computed. There is also a secondary use, which is to construct a CCF (cross-correlation function) on the first iteration of the code to get a rough estimate of the systemic velocity of the star. The LBL algorithm only converges if one knows the velocity of the target within ~1 full-width at half maximum of the line profile.

Within the LBL code, if one passes the _full mask, the computation of the LBL velocities will be done between lines corresponding to the local maxima (equivalent to using the _pos mask), but the CCF will be computed using all spectroscopic features (both positive and negative), which works better in very low signal-to-noise datasets. For a bright target (SNR>20), using the _full or _pos mask will not change the significance of the CCF, but for targets with SNR=3-5, this could lead to a better convergence of the velocity on the first iteration.

The _pos, _neg and _full masks are reported at a zero systemic velocity and are independent of the instrument used. One could use a SPIRou _full mask for a NIRPS dataset without issue.

When analyzing a sequence, it is important that the line list always remains the same such that the behavior of an individual line can be traced through epochs without ambiguity. When analyzing a target for the first time with a given mask, one creates a file within the lblreftable folder, which is basically the _pos line limits (all positive features, even if a _full mask was passed along) projected onto the wavelength grid of the dataset. Many lines will therefore be seen twice in consecutive orders and the LBL code will handle them as different lines afterward. A line may fall on a cluster of bad pixels within one order and one a clean area in the following one, so they are better handled individually even if they trace the same wavelength in the stellar spectrum. 

When looping through individual files for a given target, we start with the corresponding lblreftable and add a number of values derived from the spectrum into the tables lblrv/ folder. Some lines at the edges of orders can come and go because of BERV changes between spectra. We always keep the same line lists and lines that are missing are simply flagged as NaN. Furthermore, in most instruments and over parts of the domain, the blaze throughput is very low and parts of orders are useless, even though the wavelength grid is defined. The lines falling there are also seen in other orders (close to blaze peak) and are kept in the lblrv and lblreftable, but are set to NaN. One therefore expects to see a good fraction of lines set to NaN for bluer orders. The lblrv tables preserve the input header for future reference and give a number of values that can be used for statistical analysis (see Ould-Elkhim 2023, in press). By construction, all files for a given combination of target+template will have the same number of lines in their lblrv table. The wavelengths reported in these tables are those at the beginning of the ‘line’ (maxima preceding feature), and are expressed at a systemic velocity of zero and without the BERV.

The number of lines used for an individual spectrum is the number of non-NaN lines in the corresponding lblrv file, with the understanding that the ‘lines’ are lines-per-order, with some lines being counted twice. The relative weight of each line is 1/svrad2.