CCF computations#
The present tutorial relates to the cross-correlation of 2D echelle spectra from the star or from the planetary atmosphere with weighted masks. We illustrate the tutorial with 2D ESPRESSO spectra of TOI-421c acquired on 2023-11-06. After being converted with the relevant module, all subsequent processing of the datasets by the workflow will be performed under CCF form.
We assume that you have already set up your system and dataset and reduced your spectral time-series.
Disk-integrated CCFs#
For this conversion we assume that you have processed your spectral transit time-series.
First, you need to define the radial velocity (rv) grid upon which CCFs will be computed, by choosing its boundaries and step size (in km/s):
gen_dic['start_RV'] = -100.
gen_dic['end_RV'] = 100.
gen_dic['dRV'] = None
Boundaries are defined in the solar barycentric rest frame. By default the step size is left undefined, so that CCFs are automatically computed using the spectrograph pixel size and the introduction of correlated noise is limited.
One the major interests of ANTARESS
, however, is a careful propagation of covariances in the processed datataset and its accounting in fits. This allows you to oversample CCFs compared to the instrumental pixel size to potentially better resolve the stellar line profiles.
Then, you need to provide a weighted mask as:
gen_dic['CCF_mask'] = {'ESPRESSO':mask_path}
The mask is specific to a given spectrograph because the spectral coverage and resolution may change the contrast and wavelength distribution of the measured stellar lines.
The configuration file provide more details about the format and content of the mask file.
We note that masks customized to your target star can be computed from the processed datasets with ANTARESS
.
By default the full spectrograph range is considered for CCF computation. You can however limit the computation to specific orders using:
gen_dic['orders4ccf']={'ESPRESSO':order_idx}
Where order_idx is a list of order indexes, counted from 0.
Tip
Computing CCFs over a selection of orders can be useful to search for chromatic effects associated with the spectrograph or the Earth atmosphere. It is also a way to check whether an apparent Rossiter-McLaughlin signature is truly arising from a transiting planet, as we do not expect a strong chromatic dependence of the RM signal.
Activate the CCF conversion for disk-integrated spectra
module (gen_dic[‘DI_CCF’] = True) and set it to calculation mode (gen_dic[‘calc_DI_CCF’] = True).
The workflow converts 2D echelle spectra into CCFs after they have been reduced but before they have been aligned in the star rest frame. This allows using disk-integrated CCFs to search for systematic trends in the stellar line shape and position over the processed visit, and to measure the systemic rv of the system, as described in the detrending tutorial.
Tip
You can recompute the CCFs after activating the detrending module, to check the quality of your correction.
Once all datasets have been converted you can set the present module to retrieval mode (gen_dic[‘calc_DI_CCF’] = False) if you want to further process the disk-integrated CCFs. Otherwise they can be retrieved for external use in the /Working_dir/Star/Planet_Saved_data/DI_data/CCFfromSpec/ directory.
Intrinsic CCFs#
For this conversion we assume that you have processed your spectral transit time-series and extracted the intrinsic stellar profiles.
The rv grid, mask, and order selection are set up in the same way as for the disk-integrated CCFs (section). The rv grid is automatically shifted by the pipeline from the solar barycentric rest frame to the star rest frame, based on your input system properties and systemic rv
Activate the CCF conversion for intrinsic spectra
module (gen_dic[‘Intr_CCF’] = True) and set it to calculation mode (gen_dic[‘calc_Intr_CCF’] = True).
The module will also convert out-of-transit differential spectra into CCFs, so that you can evaluate the noise quality of the dataset and better interpert intrinsic CCFs.
Once all datasets have been converted you can set the present module to retrieval mode (gen_dic[‘calc_Intr_CCF’] = False) if you want to further process the intrinsic CCFs. Otherwise they can be retrieved for external use in the /Working_dir/Star/Planet_Saved_data/Intr_data/CCFfromSpec/ directory (/Diff_data/CCFfromSpec/ for out-of-transit differential CCFs).
Atmospheric CCFs#
WIP