0. Introduction

The Cosmic Origins Spectrograph (COS) is an ultraviolet spectrograph on-board the Hubble Space Telescope (HST) with capabilities in the near ultraviolet (NUV) and far ultraviolet (FUV).

CalCOS is the data processing pipeline which converts the raw data produced by COS's detectors onboard HST into usable spectral data. It transforms the data from a list of many individual recorded photon interactions into tables of wavelength and flux at that wavelength.

This tutorial aims to prepare you run the CalCOS pipeline to reduce spectral data taken with the COS instrument. It focuses on COS data taken in TIME-TAG mode. Note that there is another, less commonly used mode: ACCUM, which should generally be used only for UV bright targets.

Notes for those new to Python/Jupyter/Coding:

  • You will frequently see exclamation points (!) or dollar signs (\$) at the beginning of a line of code. These are not part of the actual commands. The exclamation points tell a jupyter Notebook to pass the following line to the command line, and the dollar sign merely indicates the start of a terminal prompt.

1. Setting up the environment to run CalCOS

The first step to processing your data is setting up an environment from which to run CalCOS.

1.1. Prerequisites

This tutorial assumes some basic knowledge of the command line and was built using a unix style shell. Those using a Windows computer will likely have the best results if working within the Windows Subsystem for Linux.

If you do not already have any distribution of the conda tool, see this page for instructions, and install either anaconda (more beginner friendly, ~ 3 GB, lots of extras you likely won't use) or miniconda (~ 400 MB, only what you need to make environments).

1.2. Create your conda environment

Once you have conda installed, you can create an environment.

Open up your terminal app, likely Terminal or iTerm on a Mac or Windows Terminal or Powershell on Windows.

First, add the "conda-forge" channel to your computer's conda channel list. This enables conda to look in the right place to find all the packages we want to install.

$ conda config --add channels conda-forge

Now we can create a new environment for running CalCOS; let's call it calcos_env, and initialize it with python version 3.10 and several packages we'll need.

$ conda create -n calcos_env python=3.10 notebook jupyterlab numpy astropy matplotlib astroquery

After allowing conda to proceed to installing the packages (type y then hit enter/return), you can see all of your environments with:

$ conda env list

and then switch over to your new environment with

$ conda activate calcos_env

Finally you must install the CalCOS and CRDS packages using pip:

$ pip install calcos crds

At this point, typing calcos --version into the command line and hitting enter should no longer yield the error

command not found: calcos

but rather respond with a version number, i.e. 3.4.0.

At this point, if you started this Jupyter Notebook in another Python environment, you should now quit that instance, run $ conda activate calcos_env, and reopen this Jupyter Notebook. If you're unsure whether you're already using the calcos_env environment, you can see the active environment with the following cell.

In [1]:
# Displays name of current conda environment
from os import environ
print("You are using:", environ["CONDA_DEFAULT_ENV"])
You are using: fs2

1.3. Imports and basic directories

We will import the following packages:

  • calcos to run the COS data pipeline
  • astroquery.mast Mast and Observations for finding and downloading data from the MAST archive
  • numpy to handle array functions (version $\ge$ 1.17)
  • astropy.io fits for accessing FITS files
  • astropy.table Table for creating/reading organized tables of the data
  • matplotlib.pyplot for plotting data
  • glob, shutil, and os for searching and working with system files and variables
  • pathlib Path for managing system paths
In [2]:
# Import for: The COS Data Reduction Pipeline
import calcos

# Import for: Manipulating arrays
import numpy as np

# Import for: Reading in data
from astropy.io import fits
from astropy.table import Table

# Import for: Plotting
import matplotlib.pyplot as plt
# This line makes matplotlib plots appear in the Notebook instead of possibly showing up in separate windows
%matplotlib inline

# Import for: Downloading data from archive
from astroquery.mast import Observations

# Import for: Searching for files on our system
import glob

# Import for: Making environment variables
import os, shutil

# Import for: Working with system paths
from pathlib import Path

We will also define a few basic directories in which to place our inputs and outputs.

In [3]:
# These will be important directories for the Notebook

datadir = Path('./data/')
outputdir = Path('./output/')

# Make the directories if they don't exist
datadir.mkdir(exist_ok=True), outputdir.mkdir(exist_ok=True)
(None, None)

1.4. Set up a reference file directory

CalCOS needs to be able to find all your reference files, (flat field image, bad pixel table, etc.), and the best way to enable that is to create a central directory of all the calibration files you'll need. We refer to this directory as "lref" by convention, and set a system variable lref to the location of the directory. In this section, we will create the lref environment variable; however, we need to populate the lref folder with the actual reference files. We do this in Section 2.2. If you have already downloaded the set of COS reference files you need to use into an existing lref directory, you should instead set lref to the path to this directory.

We can assign a system variable in three different ways, depending on whether we are working from:

  1. The command line
  2. A python environment
  3. A Jupyter Notebook
Unix-style Command Line Python Jupyter Notebook
export lref='./data/reference/...' os.environ["lref"] = "./data/reference/..." %env lref ./data/reference/...

Note that this system variable must be set again with every new instance of a terminal - if you frequently need to use the same lref directory, consider adding an export statement to your .bash_profile or equivalent file.

Because this is a jupyter Notebook, we set our reference directory with the cell magic below:

In [4]:
%env lref ./data/reference/references/hst/cos/
env: lref=./data/reference/references/hst/cos/

We can note the value of the system variable using the echo command:

In [5]:
!echo $lref

2. Gathering the data to run CalCOS

The CalCOS pipeline can be run either from a python environment, or directly from a Unix-style command line. The two use the same underlying machinery but can differ in syntax. For specifics on the keywords to run CalCOS with specific behaviors and arguments, see Table 3.2: Arguments for Running CalCOS in Python and Table 3.3: Command-line Options for Running CalCOS in Unix/Linux/Mac.

2.1. Downloading the raw data

First, we need to make sure we have all of our data ready and in the right spot. If you are unfamiliar with searching the archive for data, we recommend that you view our tutorial on downloading COS data. This Notebook will largely gloss over downloading the data.

To run CalCOS, we will need the following files:

  1. All the raw data from separate exposures we wish to combine as _rawtag fits files
  2. The association file telling CalCOS which files to combine as a _asn fits file.

Note that we do not generally run the CalCOS pipeline directly on the data files, but instead on an association _asn file. This allows for the calibration of related exposures into combined _x1dsum files.

If you instead use _rawtag or _corrtag exposure files files as your inputs, you will only receive the exposure-specific _x1d files as your outputs.

For this example, we're choosing the dataset LCXV13040 of COS/FUV observing the quasar 3C48. In the cell below we download the data from the archive.

In [6]:
# Guery the MAST archive for data with observation id starting with lcxv1304
q1 = Observations.query_criteria(obs_id='lcxv1304*')

# Make a list of all products we could download associates with this file
pl = Observations.get_product_list(q1)

# Filter to a list of only the products which are association files
asn_file_list = pl[pl["productSubGroupDescription"] == 'ASN']

# Filter to a list of only the products which are rawtag files
rawtag_list = pl[
    (pl["productSubGroupDescription"] == 'RAWTAG_A') | 
    (pl["productSubGroupDescription"] == 'RAWTAG_B')

# Download the two file lists to the data directory
rawtag_locs = Observations.download_products(
    rawtag_list, download_dir=str(datadir))
asn_locs = Observations.download_products(
    asn_file_list, download_dir=str(datadir))
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13ezq_rawtag_a.fits to data/mastDownload/HST/lcxv13ezq/lcxv13ezq_rawtag_a.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13ezq_rawtag_b.fits to data/mastDownload/HST/lcxv13ezq/lcxv13ezq_rawtag_b.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13f4q_rawtag_a.fits to data/mastDownload/HST/lcxv13f4q/lcxv13f4q_rawtag_a.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13f4q_rawtag_b.fits to data/mastDownload/HST/lcxv13f4q/lcxv13f4q_rawtag_b.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13faq_rawtag_a.fits to data/mastDownload/HST/lcxv13faq/lcxv13faq_rawtag_a.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13faq_rawtag_b.fits to data/mastDownload/HST/lcxv13faq/lcxv13faq_rawtag_b.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13ffq_rawtag_a.fits to data/mastDownload/HST/lcxv13ffq/lcxv13ffq_rawtag_a.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13ffq_rawtag_b.fits to data/mastDownload/HST/lcxv13ffq/lcxv13ffq_rawtag_b.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13040_asn.fits to data/mastDownload/HST/lcxv13040/lcxv13040_asn.fits ... [Done]

By default, each exposure's files are downloaded to separate directories, as is the association file.

We need to move around these files to all be in the same directory, which we do below.

In [7]:
# move the files to the base data directory
for lpath in rawtag_locs['Local Path']:
for lpath in asn_locs['Local Path']:
    asn_name = os.path.basename(lpath)

# Delete the now-empty nested subdirectories

2.2. Gathering reference files

The following process of gathering reference files is given a detailed explanation in Section 3 of our Notebook on Setting up an environment to work with COS data. Your process will be somewhat simpler and quicker if you have already downloaded the reference files.

Each data file has an associated set of calibration files which are needed to run the associated correction with (i.e. you need the FLATFILE to flat field correct the data.) These reference files must be located in the $lref directory to run the pipeline.

The Space Telescope Science Institute (STScI) team is regularly producing new calibration files in an effort to keep improving data reduction. Periodically the pipeline is re-run on all COS data. To determine which reference files were used most recently by STScI to calibrate your data, you can refer to your data file's "CRDS_CTX" keyword in its fits header (see next cell).

In [8]:
# Find all of the raw files:
rawfiles = glob.glob(str(datadir/'*raw*.fits'))
# Get the header of the 0th raw file, look for its CRDS context keyword:
crds_ctx = fits.getheader(rawfiles[0])['CRDS_CTX']
# Print it:
print(f"The CRDS Context last run with {rawfiles[0]} was:\t{crds_ctx}")
The CRDS Context last run with data/lcxv13ezq_rawtag_a.fits was:	hst_0987.pmap

The value of this keyword in the header is a .pmap or observatory context file which tells the CRDS calibration data distribution program which files to distribute. You can also find the newest "operational" context on the HST CRDS website.

To download the reference files specified by a context file, we use the crds tool we installed earlier.

If you haven't already downloaded up-to-date reference files, (as in Section 3 of "Setup.ipynb",) click to skip this cell and begin downloading the files!

If you have recently downloaded COS reference files, (i.e. if you ran Section 3 of the "Setup" Notebook,) you likely do not have to download more reference files. Instead, follow the instructions in this cell and then skip to Section 3.

If you already downloaded the files, you can simply point the pipeline to the ones you downloaded, using the crds bestrefs command, as shown in the following three steps. Run these steps from your command line if you already have the reference files in a local cache. Note also that there may be newer reference files available. To make sure you are always using the most up-to-date reference files, we advise you familiarize yourself with the newest files and documentation at the CRDS homepage.

  1. The following sets the environment variable for crds to look for the reference data online:

$ export CRDS_SERVER_URL=https://hst-crds.stsci.edu

  1. The following tells crds where to save the files it downloads - set this to the directory where you saved the crds_cache, i.e. in Section 3 of our Notebook on "Setup":

$ export CRDS_PATH=${HOME}/crds_cache

  1. The following will update the data files you downloaded so that they will be processed with the reference files you previously downloaded:

$ crds bestrefs --files data/*raw*.fits --update-bestrefs --new-context '<the imap or pmap file you used to download the reference files>'

Assuming everything ran successfully, you can now skip to Section 3.

If you have not yet downloaded the reference files, you will need to do so, as shown below:


Note that as of the time of this Notebook's last update, the pipeline context used below was hst_0989.pmap, but this changes over time. You are running this in the future, and there is certainly a newer context you would be better off working with. Take a minute to consider this, and check the HST Calibration Reference Data System webpage to determine what the current operational pmap file is.

Unless we are connected to the STScI network, or already have the reference files on our machine, we will need to download the reference files and tell the pipeline where to look for the flat files, bad-pixel files, etc.

The process in the following two cells can take a long time and strain network resources. If you have already downloaded up-to-date COS reference files, we recommend that you avoid doing so again. Instead, keep these crds files in an accessible location, and point an environment variable lref to this directory. For instance, if your lref files are on your username's home directory in a subdirectory called crds_cache, give Jupyter the following command then skip to Section 2.3:

%env lref /Users/<your username>/crds_cache/references/hst/cos/

If you have an older cache of reference files, you may also simply update your cached reference files. Please see the CRDS Guide for more information.

Assuming you have not yet downloaded these files, in the next two cells, we will setup an environment of reference files, download the files, and save the output of the crds download process in a log file:

In [9]:
%%capture cap --no-stderr 
# The above ^ allows us to redirect the cell's output into a txt file created in the next cell
#  This avoids a very long printed output
%env CRDS_SERVER_URL https://hst-crds.stsci.edu
# The above ^ sets an environment variable for crds to look for the reference data online
%env CRDS_PATH ./data/reference/ 
#The above ^ tells crds where to save the files it downloads

# The next command depends on your context and pmap file  - it looks up the specified pmap "context" file,
    # which tells it what reference files to download. It then downloads these to the CRDS_PATH directory
# You may wish to update this pmap if there is a newer pmap file - check https://hst-crds.stsci.edu
!crds bestrefs --files data/*raw*.fits  --sync-references=2 --update-bestrefs --new-context 'hst_0989.pmap' 
In [10]:
# This file will contain the output of the last cell
with open(str(outputdir/'crds_output_1.txt'), 'w') as f:

We'll print the beginning and end of that file just to take a look:

In [11]:
crds_output_dict = {}  # pair each line with its line number, start at 0
with open(str(outputdir/'crds_output_1.txt'), 'r') as cell_outputs:  # open the file
    for linenum, line in enumerate(cell_outputs):  # loop through lines
        crds_output_dict[linenum] = line[:-1]  # save each line to dict
# Get the length of the dictionary - how many lines of output
total_lines = len(crds_output_dict)

    f"Printing the first and last 5 lines of {total_lines} lines output by the previous cell:\n")
for i in np.append(range(5), np.subtract(total_lines - 1, range(5)[::-1])):
    print(f"Line {i}:   \t", crds_output_dict[i])

# Delete the contents of the dict to avoid 'garbage' filling memory
Printing the first and last 5 lines of 175 lines output by the previous cell:

Line 0:   	 env: CRDS_SERVER_URL=https://hst-crds.stsci.edu
Line 1:   	 env: CRDS_PATH=./data/reference/
Line 2:   	 CRDS - INFO -  Fetching  ./data/reference/mappings/hst/hst_wfpc2_wf4tfile_0250.rmap      678 bytes  (1 / 140 files) (0 / 1.7 M bytes)
Line 3:   	 CRDS - INFO -  Fetching  ./data/reference/mappings/hst/hst_wfpc2_shadfile_0250.rmap      977 bytes  (2 / 140 files) (678 / 1.7 M bytes)
Line 4:   	 CRDS - INFO -  Fetching  ./data/reference/mappings/hst/hst_wfpc2_offtab_0250.rmap      642 bytes  (3 / 140 files) (1.7 K / 1.7 M bytes)
Line 170:   	 CRDS - INFO -  Fetching  ./data/reference/references/hst/cos/z6n16118l_flat.fits  134.2 M bytes  (20 / 20 files) (796.1 M / 930.3 M bytes)
Line 171:   	 CRDS - INFO -  0 errors
Line 172:   	 CRDS - INFO -  0 warnings
Line 173:   	 CRDS - INFO -  169 infos
Line 174:   	 [0

Line 158 of the output should show 0 errors.

If you receive errors, you may need to attempt to run the crds bestrefs line again. These errors can arise from imperfect network connections.

It is recommended that you use this new $lref folder of reference files for subsequent CalCOS use, rather than re-downloading the reference files each time. To do this (after completing this Notebook):

  • Save this folder somewhere accessibile, i.e. ~/crds_cache
  • Add a line to your .bashrc or similar: export lref=<Path to your reference file directory>
    • If you wish to avoid adding this to your .bashrc, simply type the line above into any terminal you wish to run CalCOS from
    • If running CalCOS from a jupyter Notebook, instead add a cell with: %env lref /Users/<Your Username>/crds_cache/references/hst/cos

3. Processing raw COS data using CalCOS

Now we have all the reference files we need to run the pipeline on our data.

This following cells which run the pipeline can take a while, sometimes more than 10 minutes, so you may choose to not run the remaining cells of this Notebook on the example data. You may, instead, wish to simply look at the rendered output in the html version of this Notebook.

By default, the pipeline also outputs hundreds of lines of text - we will suppress the printing of this text and instead save it to a text file.

3.1. Running CalCOS: From a python environment

Now, we can run the pipeline program:

Note that generally, CalCOS should be run on an association (_asn) file (in this case: ./data/lcxv13040_asn.fits). You may run CalCOS directly on _rawtag or _corrtag exposure files, but this will not produce an _x1dsum file and can result in errors for data taken at certain lifetime positions. No matter what type of files you run CalCOS on, you should only specify the FUVA segment's file, i.e. the _rawtag_a file. If a rawtag_b file is in the same directory, CalCOS will find both segments' files.

In this example, we also specify that verbosity = 2, resulting in a very verbose output, and we specify a directory to put all the output files in: output/calcos_processed_1. To avoid polluting this Notebook with more than a thousand lines of the output, we again capture the output of the next cell and save it to output/output_calcos_1.txt in the cell below.

In [12]:
%%capture cap --no-stderr 
# Above ^ again, capture the output and save it in the next cell

calcos.calcos(str(datadir/asn_name), # 1st param specifies which asn file to run the pipeline on
              verbosity=2, # verbosity param: [0 = don't print much at all to the console or text file, 1 = print some, 2 = print everything]
              outdir=str(outputdir/"calcos_processed_1")) # save all resulting files in this subdirectory in our output directory
/Users/nkerman/miniconda3/envs/fs2/lib/python3.9/site-packages/calcos/fpavg.py:1203: RuntimeWarning: invalid value encountered in true_divide
  conversion = data.field("flux") / data.field("net")
In [13]:
# This file now contains the output of the last cell
with open(str(outputdir/'output_calcos_1.txt'), 'w') as f:

Again, we'll print the beginning and end of that file just to take a look and make sure CalCOS ran successfully.

In [14]:
calcos_output_dict = {}  # pair each line with its line number, start at 0
with open(str(outputdir/'output_calcos_1.txt'), 'r') as cell_outputs:  # open the file
    for linenum, line in enumerate(cell_outputs):  # loop through lines
        calcos_output_dict[linenum] = line[:-1]  # save each line to dict
# Get the length of the dictionary - how many lines of output
total_lines = len(calcos_output_dict)

    f"Printing the first and last 5 lines of {total_lines} lines output by the previous cell:\n")
for i in np.append(range(5), np.subtract(total_lines - 1, range(5)[::-1])):
    print(f"Line {i}:   \t", calcos_output_dict[i])

calcos_output_dict.clear()  # Delete the contents of the dict
Printing the first and last 5 lines of 2373 lines output by the previous cell:

Line 0:   	 Warning:  Creating output directory 'output/calcos_processed_1'.
Line 1:   	 CALCOS version 3.4.0
Line 2:   	 numpy version 1.22.3
Line 3:   	 astropy version 4.2.1
Line 4:   	 Begin 26-Apr-2022 18:11:49 EDT
Line 2368:   	 updateMempresent
Line 2369:   	 copySptFile
Line 2370:   	 Warning:  spt file not found, so not copied to product
Line 2371:   	 End   26-Apr-2022 18:13:56 EDT
Line 2372:   	 elapsed time = 127.2 sec. = 2.12 min.

3.2. Running CalCOS: From the command line

The syntax for running CalCOS from the command line is very similar. Assuming your data files, lref directory, and reference files are all where you've told CalCOS to look, you can simply run:

calcos --outdir directory_to_save_outputs_in filename_asn.fits

or, if you want to save a very verbose output to a log file log.txt:

calcos -v --outdir directory_to_save_outputs_in filename_asn.fits > log.txt

To see the full list of commands, Table 3.2:Command-line Options for Running CalCOS in Unix/Linux/Mac, or run the following cell with no arguments.

In [15]:
The command-line options are:
  --version (print the version number and exit)
  -r (print the full version string and exit)
  -q (quiet)
  -v (very verbose)
  -s (save temporary files)
  -o outdir (output directory name)
  --find yes (find Y location of spectrum)
  --find no (use Y location of spectrum from 1dx file and wavecal)
  --find cutoff (find Y location if sigma <= cutoff)
  --csum (create 'calcos sum' image)
  --only_csum (do little else but create csum)
  --raw (use raw coordinates for csum image)
  --compress parameters (compress csum image)
  --binx X_bin_factor (csum bin factor in X)
  --biny Y_bin_factor (csum bin factor in Y)
  --shift filename (file to specify shift values)
  --stim filename (append stim locations to filename)
  --live filename (append livetime factors to filename)
  --burst filename (append burst info to filename)

Following the options, list one or more association
files (rootname_asn) or raw files (rootname_raw).
ERROR:  An association file name or observation rootname must be specified.

4. Re-processing COS data with altered parameters

4.1. Altering the calibration switches

The way to alter how CalCOS runs - i.e. which calibrations it performs - is with the calibration switches contained in the fits headers.

The switches (with the exception of "XTRACTALG"), can be set to the values in the following table:

Value: "PERFORM" "OMIT" "N/A"
Meaning: Performs the calibration step Does not perform the calibration step This step would not make sense for this file

XTRACTALG instead can be set to either "BOXCAR" or "TWOZONE", to specify the spectral extraction algorithm to be used. For more information, see Section 3.2.1: "Overview of TWOZONE extraction" of the Data Handbook.

In the cell below, we get a full list of the switches by name. If you want to learn more about the calibration steps and switches, see Chapters 2 and 3 of the COS Data Handbook.

In [16]:
# Reads the header of the 0th rawfile
header = fits.getheader(rawfiles[0])
# The calib switches are found in lines 82 - 109 of the header
calibswitches = header[82:109]
TAGFLASH= 'AUTO            '   / Type of flashed exposures in time-tag          
              / CALIBRATION SWITCHES: PERFORM, OMIT, COMPLETE                   
FLATCORR= 'PERFORM '           / apply flat-field correction                    
DEADCORR= 'PERFORM '           / correct for deadtime                           
DQICORR = 'PERFORM '           / data quality initialization                    
STATFLAG=                    T / Calculate statistics?                          
TEMPCORR= 'PERFORM '           / correct for thermal distortion                 
GEOCORR = 'PERFORM '           / correct FUV for geometic distortion            
DGEOCORR= 'OMIT    '           / Delta Corrections to FUV Geometric Distortion  
IGEOCORR= 'PERFORM '           / interpolate geometric distortion in INL file   
RANDCORR= 'PERFORM '           / add pseudo-random numbers to raw x and y       
RANDSEED=                   -1 / seed for pseudo-random number generator        
XWLKCORR= 'PERFORM '           / Correct FUV for Walk Distortion in X           
YWLKCORR= 'PERFORM '           / Correct FUV for Walk Distortion in Y           
PHACORR = 'PERFORM '           / filter by pulse-height                         
TRCECORR= 'PERFORM '           / trace correction                               
ALGNCORR= 'PERFORM '           / align data to profile                          
XTRCTALG= 'TWOZONE '           / BOXCAR or TWOZONE                              
BADTCORR= 'OMIT    '           / filter by time (excluding bad time intervals)  
DOPPCORR= 'PERFORM '           / orbital Doppler correction                     
HELCORR = 'PERFORM '           / heliocentric Doppler correction                
X1DCORR = 'PERFORM '           / 1-D spectral extraction                        
BACKCORR= 'PERFORM '           / subtract background (when doing 1-D extraction)
WAVECORR= 'PERFORM '           / use wavecal to adjust wavelength zeropoint     
FLUXCORR= 'PERFORM '           / convert count-rate to absolute flux units      

Let's begin by switching off all the switches currently set to "PERFORM" to a new value of "OMIT", in every rawfile:

In [17]:
verbose = False  # Set to True to see a bit more about what is going on here

# Find each rawfile, i is just a counter variable for the files you loop through
for i, rawfile in enumerate(rawfiles):
    if verbose:
    header = fits.getheader(rawfiles[i])  # Read that rawfiles header
    # Find all calib switches
    corrections = [key for key in list(header.keys()) if "CORR" in key]

    for correction in corrections:
        if header[correction] == 'PERFORM':
            if verbose:
                print("switching\t", header[correction],
                      "\t", correction, "\tto OMIT")
            # Turn off all the calib switches
            fits.setval(rawfile, correction, value='OMIT', ext=0)

In this case, CalCOS realizes that all the switches are set to "OMIT", and exits without doing anything.

In [18]:
calcos.calcos(str(datadir/asn_name), verbosity=0,
# Run CalCOS with all calib switches OFF; allow text output this time
Warning:  Creating output directory 'output/calcos_processed_2'.
CALCOS version 3.4.0
numpy version 1.22.3
astropy version 4.2.1
Begin 26-Apr-2022 18:13:59 EDT
Nothing to do; all calibration switches are OMIT.

4.2. Running CalCOS with a specific set of switches

Now, let's set a single switch to "PERFORM", and just run a flat-field correction ("FLATCORR") and a pulse-height filter correction ("PHACORR"). Set verbosity = 1 or 2 to learn more about how CalCOS is working.

In [19]:
verbose = False

# Find each rawfile, i is just a counter variable for the files you loop through
for i, rawfile in enumerate(rawfiles):
    if verbose:
    # Change the header's keyword FLATCORR to the value PERFORM
    fits.setval(rawfile, "FLATCORR", value='PERFORM', ext=0)
    # Change the header's keyword PHACORR to the value PERFORM
    fits.setval(rawfile, "PHACORR", value='PERFORM', ext=0)
In [20]:
%%capture cap --no-stderr
In [21]:
# This file now contains the output of the last cell
with open(str(outputdir/'output_calcos_3.txt'), 'w') as f:

4.3. Running CalCOS with a different reference file

You may wish to run CalCOS with a specific flat file, bad pixel table, or any other reference file. CalCOS offers the ability to do just this on a file-by-file basis, by changing the CALIBRATION REFERENCE FILES values in the header of your data.

As an example, we check which calibration files are selected for one of our rawtag files.

In [22]:
header = fits.getheader(rawfiles[0])  # Read 0th rawfile's header
# The 110th to 138th lines of the header are filled with these reference files
refFiles = header[110:138]
# Get just the keywords i.e. "FLATFILE" and "DEADTAB"
refFile_keys = list(refFiles[2:].keys())
TDSCORR = 'OMIT    '           / switch for time-dependent sensitivity correctio
              / CALIBRATION REFERENCE FILES                                     
FLATFILE= 'lref$z6n16118l_flat.fits' / Pixel to Pixel Flat Field Reference File 
DEADTAB = 'lref$s7g1700gl_dead.fits' / Deadtime Reference Table                 
BPIXTAB = 'lref$5b91920bl_bpix.fits' / bad pixel table                          
SPOTTAB = 'lref$5b91820fl_spot.fits' / Transient Bad Pixel Table                
GSAGTAB = 'lref$5b919208l_gsag.fits' / Gain Sagged Region Reference File        
HVTAB   = 'N/A                    ' / High voltage command level Reference Table
BRFTAB  = 'lref$x1u1459il_brf.fits' / Baseline Reference Frame Reference Table  
GEOFILE = 'lref$x1u1459gl_geo.fits' / Geometric Correction Reference File       
DGEOFILE= 'N/A                    ' / Delta Geometric Correction Reference Image
TRACETAB= 'lref$5b91919sl_trace.fits' / 1D spectral trace table                 
PROFTAB = 'lref$5b91920fl_profile.fits' / 2D spectrum profile table             
TWOZXTAB= 'lref$5b919203l_2zx.fits' / Two-zone spectral extraction parameters   
XWLKFILE= 'lref$14o2013ql_xwalk.fits' / X Walk Correction Lookup Reference Image
YWLKFILE= 'lref$14o2013rl_ywalk.fits' / Y Walk Correction Lookup Reference Image
PHATAB  = 'lref$wc318317l_pha.fits' / Pulse Height Discrimination Reference Tabl
PHAFILE = 'N/A                    ' / Pulse Height Threshold Reference File     
BADTTAB = 'N/A                    ' / Bad Time Interval Reference Table         
XTRACTAB= 'lref$5b919206l_1dx.fits' / 1-D Spectral Extraction Information Table 
LAMPTAB = 'lref$5b91919tl_lamp.fits' / template calibration lamp spectra table  
DISPTAB = 'lref$5b919205l_disp.fits' / Dispersion Coefficient Reference Table   
IMPHTTAB= 'N/A                    ' / Imaging photometric table                 
FLUXTAB = 'lref$62m19539l_phot.fits' / Spectroscopic flux calibration table     
WCPTAB  = 'lref$u1t1616ql_wcp.fits' / wavecal parameters table                  
BRSTTAB = 'N/A                    ' / burst parameters table                    

For this section, let's download another Pulse Height Amplitude (_pha) table file using the crds tool (I arbitrarily choose this one):

In [23]:
!crds sync --files u1t1616ll_pha.fits --output-dir $lref
CRDS - INFO -  Symbolic context 'hst-operational' resolves to 'hst_1006.pmap'
CRDS - INFO -  Reorganizing 0 references from 'instrument' to 'flat'
CRDS - INFO -  Reorganizing from 'instrument' to 'flat' cache,  removing instrument directories.
CRDS - INFO -  Syncing explicitly listed files.
CRDS - INFO -  Fetching  ./data/reference/references/hst/cos/u1t1616ll_pha.fits    11.5 K bytes  (1 / 1 files) (0 / 11.5 K bytes)
CRDS - INFO -  0 errors
CRDS - INFO -  0 warnings
CRDS - INFO -  5 infos

Now we can use the fits headers to set this new file as the _pha file. As a demonstration, let's do this for only the raw data from segment FUVA of the FUV detector:

Note that we are still only performing two corrections, as all calibration switches aside from FLATCORR and PHACORR are set to OMIT.

In [24]:
# Find just the FUVA raw files
rawfiles_segA = glob.glob(str(datadir/'*rawtag_a*.fits'))
for rawfileA in rawfiles_segA:
    with fits.open(rawfileA, mode='update') as hdulist:
        hdr0 = hdulist[0].header  # Update the 0th header of that FUVA file
        # NOTE that you need the $lref in there if you put it with your other ref files
        hdr0["PHATAB"] = 'lref$u1t1616ll_pha.fits'

Finally, let's run CalCOS with the new _pha file for only the FUVA data:

In [25]:
%%capture cap --no-stderr
calcos.calcos(str(datadir/asn_name), verbosity=2, outdir=str(outputdir/"calcos_processed_4"))
In [26]:
# This file now contains the output of the last cell
with open(str(outputdir/'output_calcos_4.txt'), 'w') as f:

Before we go, let's have a look at the spectra we calibrated and extracted in Section 2.3

We'll make a very quick plot to show the two spectra calibrated by STScI's pipeline and by us right now. The two should agree very well. Small differences may be expected, given that the RANDSEED values may be different between the two versions.

Much more information on reading in and plotting COS spectra can be found in our other tutorial: Viewing COS Data.

(You can ignore the UnitsWarning below)

In [27]:
# Get the STScI calibrated x1dsum spectrum from the archive
                               mrp_only=True,  download_dir='data/compare/')
# Read in this STScI spectrum
output_spectrum = Table.read(
wvln_orig, flux_orig, fluxErr_orig, dqwgt_orig = output_spectrum[1][
# Convert the data quality (DQ) weight into a boolean we can use to mask the data
dqwgt_orig = np.asarray(dqwgt_orig, dtype=bool)

# Also read in the spectrum we recently calibrated in Section 2.3
output_spectrum = Table.read(
new_wvln, new_flux, new_fluxErr, new_dqwgt = output_spectrum[1][
# Convert the data quality (DQ) weight into a boolean we can use to mask the data
new_dqwgt = np.asarray(new_dqwgt, dtype=bool)

fig, (ax0, ax1, ax2) = plt.subplots(
    3, 1, figsize=(15, 10))  # Build a 3 row x 1 column figure
ax0.plot(wvln_orig[dqwgt_orig], flux_orig[dqwgt_orig], linewidth=0.5, c='C0',
         label="Processed by the archive")  # Plot the archive's spectrum in top section
ax1.plot(new_wvln[new_dqwgt], new_flux[new_dqwgt], linewidth=0.5, c='C1',
         label="Just now processed by you")  # Plot your calibrated spectrum in middle section

ax2.plot(wvln_orig[dqwgt_orig], flux_orig[dqwgt_orig], linewidth=0.5, c='C0',
         label="Processed by the archive")  # Plot both spectra in bottom section
ax2.plot(new_wvln[new_dqwgt], new_flux[new_dqwgt],
         linewidth=0.5, c='C1', label="Just now processed by you")

ax0.legend(loc='upper center', fontsize=14)
ax1.legend(loc='upper center', fontsize=14)
ax2.legend(loc='upper center', fontsize=14)
ax0.set_title("Fig 3.1\nComparison of processed spectra", size=28)
plt.savefig(str(outputdir/"fig3.1_compare_plot.png"), dpi=300)
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13040_asn.fits to data/compare/mastDownload/HST/lcxv13040/lcxv13040_asn.fits ... [Done]
Downloading URL https://mast.stsci.edu/api/v0.1/Download/file?uri=mast:HST/product/lcxv13040_x1dsum.fits to data/compare/mastDownload/HST/lcxv13040/lcxv13040_x1dsum.fits ... [Done]
WARNING: UnitsWarning: 'erg /s /cm**2 /angstrom' contains multiple slashes, which is discouraged by the FITS standard [astropy.units.format.generic]

Congratulations! You finished this Notebook!

There are more COS data walkthrough Notebooks on different topics. You can find them here.

About this Notebook

Author: Nat Kerman - nkerman@stsci.edu

Updated On: 2022-03-24

This tutorial was generated to be in compliance with the STScI style guides and would like to cite the Jupyter guide in particular.


If you use astropy, matplotlib, astroquery, or numpy for published research, please cite the authors. Follow these links for more information about citations:

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