The CASCADe pipelines¶
The CASCADe package combines basically two distinct pipelines: The first can be considered a “classical” spectroscopic pipeline to extract spectra from spectral images or cubes. The focus of the second pipeline is on the calibration of spectral timeseries and lightcurve fitting to extract a transit or eclipse spectrum. In the following we will explain both pipelines in more detail.
Using CASCADe to extract a spectral timeseries¶
In this section of the manual we will discuss all the pipeline steps needed to extract a spectral timeseries. As a first step, to be able to run the CASCADe code, first import the cascade package:
This module will provide all needed functionality. To start with the spectral extraction, one first creates a transit spectroscopy object (TSO) which will hold all data and intermediate pipeline data products:
tso = cascade.TSO.TSOSuite()
Next we need to initialize the TSO object using the initialization files which define the data, model parameters and behavior of the causal pixel model implemented in CASCADe. For details on these files, please consult the CASCADe Initialization section of the documentation. To initialize the TSO instance, execute the following command:
path = cascade.initialize.default_initialization_path tso.execute("initialize", "cascade_WASP19b_extract_timeseries.ini", "cascade_WASP19b_object.ini", path=path)
The files used in this example are for spectral data of the transit of WASP 19 b observed with the WFC3 instrument of HST and are part of the example use cases part of the CASCADe distribution. For further details see the CASCADe examples sections of the documentation. Before running the code, make sure that the paths specified in the initialization files are correctly set in addition to the path settings of the CASCADe environment variables (see the CASCADe Environment section of the documentation for further details). If the initialization files are not in the standard directory where CASCADe expect these file to be, an additional path keyword can be set. In the example above we set the path the the standard initialization path. If the path is the standard initialization path or that specified by the CASCADE_INITIALIZATION_FILE_PATH environment variable, the path keyword can be omitted. Also note that a relative path can be specified. In this case the path will be relative to the the standard initialization path or that specified by the CASCADE_INITIALIZATION_FILE_PATH environment variable. The loaded paramters can be found under tso.cascade_parameters
In case the user whiches to re-initialize the TSO object, it is highly advised to first reset the object with the following command:
After which all previously defined control and data parameters will be reset and a cleaned re-initialization can be made.
After initialization of the TSO object we can load the observational dataset by executing the following pipeline step:
This will load all spectroscopic data specified in the initialization files, including any background data or, like with the WFC3 data, will fit a background model to the data. The loaded data can be found tso.observations
The next pipeline step is the subtraction the background from the spectral images:
This will subtract the observed or fitted background from the data under tso.observations.dataset and set the self.observation.dataset.isBackgroundSubtracted flag to True.
The next step is to identify and flag bad pixels, and to create a cleaned dataset and a spectral extraction profile. For this we use a directional filter. To execute this step use the following command:
This will update the mask of the background subtracted dataset found in tso.observations.dataset and create two new data product: the cleaded dataset tso.cpm.cleaned_dataset and a smoothed, filtered dataproduct tso.cpm.filtered_dataset on which the extraction profile for the optimal spectral extraction will be based.
Continuing, next we need to determine the pointing movement of the telescope as this has a direct impact on the wavelength registration. To determine the relative position, rotation and scale change of the source spectrum from the input spectroscopic data set, run the following pipeline step:
We use a cross-correlation in phase space in combination with a polar transform to determine the translational and rotational movements of the telescope. The movements are relative to a reference observation. To minimize the risk of using a reference spectral image which a problem, we compare to a number of reference images and than take the median values. The results of this step are stored in tso.cpm.spectral_movement.
The movement of the telescope can now be used to correct the initial wavelength associated with the detector pixels for telescope movements:
This will correct the wavelengths for each time step of the data and the cleaned and filtered data product.
The last step before the spectral extraction is setting the extraction area on the detector within which the spectrum will be extracted. This is done with the following command:
The extraction aperture is centered around the spectral trace with a certain width which is defined in the initialization files, and takes into account the movements of the telescope determined in the determine_source_movement step. This ensures that also for larger movements, like nodding of spatial scanning like with the HST/WFC3 observations, the extraction aperture is always properly centered. The extraction mask in time is stored under tso.cpm.extraction_mask.
The final step is then the spectral extraction of the spectral timeseries of the star & planet:
This step will extract the spectral timeseries using both optimal as well as aperture extraction, and rebin the resulting spectra to a uniform wavelength grid, for which the rebin factor is specified in the initialization files. The results are stored in tso.observation.dataset_optimal_extracted, respectively tso.observation.dataset_aperture_extracted. The extraction profile and its associated mask are stored under tso.cpm.extraction_profile and tso.cpm.extraction_profile_mask. Apart from storing the results in the TSO object, the spectra are also saved as fits spectral tables at the location specified by the CASCADE_DATA_PATH environment variable and the ‘observations_path’ variable defined in the initialization files. The fits files of the optimal extracted spectra are labeled with ‘COE’ (Cascade Optimal Extracted) and the aperture extracted spectra are labeled with CAE (Cascade Aperture Extracted).
Using CASCADe to calibrate the spectral timeseries and determine the planetary spectrum¶
After extracting the spectral timeseries, we can now proceed to determine the systematics on the timeseries and extract the transit or eclipse spectrum of the planet. The first few pipeline steps are identical to those of the previous section. First we import the CASCADe package:
than we create an instance of the timeseries object:
tso = cascade.TSO.TSOSuite()
next we initialize the TSO object:
path = cascade.initialize.default_initialization_path tso.execute("initialize", "cascade_WASP19b_calibrate_planet_spectrum.ini", "cascade_WASP19b_object.ini")
note that again we use the WASP-19 b example provided in the examples coming with the CASCADe package.
We then load the spectra into the tso object:
For completeness we also execute the subtract_background step:
as there is the possibility that the user provides not background subtracted spectra. If the spectra are background subtracted, this can be indicated in the initialization files by switching the observations_has_background variable to False. In that case the subtract_background step will be skipped.
As with the spectral extraction pipeline, we also execute the filter_dataset pipeline step:
to flag any spurious spectral data points and create a cleaned dataset. Note that in this case we use a simple median filtering in contrast to the directional filtering used for the spectral data cubes and images.
In case of HST/WFC3 spectra, a check of the wavelength solution for an overall wavelength shift is made with the following pipeline step:
For this, a sample model of the observed spectrum is created using the stellar parameters defined in the initialization files together with the sensitivity curve of the spectrograph. Using a cross correlation between the model and the observed time averaged spectrum, an overall wavelength shift is determined and corrected for. Note that this step is already done during spectral extraction with the CASCADe pipeline. For other instruments this step can be ignored by switching the processing_determine_initial_wavelength_shift parameter to False
After all the previous steps we now can run the main pipeline task for calibrating the spectral timeseries and fitting the transit or eclipse:
The derived transit spectra from the regression fit and bootstrap analysis are stored under tso.exoplanet_spectrum The used transit model under tso.model and the results from the regression analysis under tso.calibration_results’.
The final pipeline step will plot the resulting spectra and save the transit or eclipse spectrum as a fits file:
All results from this pipeline will be stored in the location defined by the CASCADE_SAVE_PATH environment variable and the cascade_save_path parameter from the initialization files.