fringez 1.3.5

Removing atmospheric fringes from ZTF i-band images

fringez

Getting Started

ZTF i-band images are contaminated with atmospheric fringes that significantly effect the photometric solution of sources within the image. fringez uses principal component analysis to generate a fringe model from these i-band images. Each ZTF readout channel generates similar atmospheric fringe patterns and therefore requires its own fringe model. That fringe model can then be used to generate a fringe bias image for each contaminated i-band image which, when subtraction from the i-band image, removes the atmospheric fringes. This results in a clean i-band image.

Diagram of fringez Scripts

fringez installs three executables.

• fringez-download: Downloads pre-generated fringe models from the NERSC web portal
• fringez-clean: Cleans contaminated i-band images using fringe models
• fringez-generate: Generates new fringe models using contaminated i-band images

Installation

Preferred method is through pip:

pip install fringez

Latest version can also be installed from github:

git clone https://github.com/MichaelMedford/fringez.git
cd fringez
python setup.py install

Downloading Fringe Models

Users must have fringe models on disk in order to clean i-band images with atmospheric fringes. These fringe models are what generate a fringe bias image for each contaminated i-band image which, when subtraction from the i-band image, removes the atmospheric fringes. This results in a clean i-band image.

Pre-generated fringe models can be downloaded from the NERSC web portal with the fringez-download executable. Select a -fringe-model-date from the below list to download a version of the pre-generated fringe models. The script will also download the model lists, containing the list of images used to create the fringe model.

Current model versions:

• 20200723 (32 GB)

Fringe models can either be downloaded for all 64 readout channels with the -all argument, or only a single fringe model can be downloaded with the -single argument. If the -single argument is selected, then only the model with the -fringe-model-id argument is downloaded. The -fringe-model-id is the cid and the qid of the fringe model combined in the following syntax: c01_q1

By default the fringe models and fringe model lists will be downloaded to the current directoy, but can also be sent a specific directory with the -fringe-model-folder argument.

Generating Clean Images

Contaminated images can be cleaned with the fringez-clean executable.

Contaminated images can either be cleaned one at a time with the -single-image argument, or all images in a folder can be cleaned with the -all-images-in-folder argument. All clean images will have the same filename as the science images, but with the extension sciimg.clean.fits. To save the fringe bias images to disk for debugging, select the -debug argument.

Cleaning all contaminated images in a folder

Cleaning a folder of contaminated images requires specifying the folder where fringe models are located. fringez-clean will automatically pair the correct fringe model with each contaminated image.

From within the directory where all of the science images are located, execute fringez-clean -all-images-in-folder -fringe-model-folder={FRINGE_MODEL_FOLDER}. The -fringe-model-folder argument must be set when -all-images-in-folder is selected.

When cleaning all images, fringez-clean can either clean one image at a time with the -serial argument, or clean images in parallel with the -parallel argument. Images are cleaned in -serial by default. Parallelization is executed with the mpi4py package and simply splits the list of images in the folder across the processes used to launch the fringez-clean executable.

Cleaning a single contaminated image

Cleaning a single contaminated image requires specifying a fringe model. This fringe model should match the readout channel ID of the contaminated image. The image and the model will both be labelled with the same quadrant ID and ccd ID. For example, if ztf_20190703184838_000481_zi_c02_o_q1_sciimg.fits is the contaminated image, then fringe_PCArandom_comp06.c02_q1.20190618.model would be the correct model because of the matching c02 and q1 parameters.

From within the directory where the contaminated science image is located, execute fringez-clean -single-image -image-name={IMAGE_NAME} -fringe-model-name={FRINGE_MODEL_NAME}. The -image-name and -fringe-model-name arguments must be set when -single-image is selected.

Measuring the Uniform Background Indicator (UBI)

The presence of correlated background noise can be determined by measuring the Uniform Background Indicator, or UBI, or an image. The measurement is made by performing aperture photometry on random background locations and measuring how well the scatter in background flux measurements is captured by locally determined estimates of the flux error.

An ideal UBI = 1 indicates no correlated background noise, with larger values indicating more correlated noise. Here are the results of calculating UBI on an image of random noise for various aperture sizes, as well as three example images to qualitatively show how UBI scales with the presence of atmospheric fringes.

Ideal UBI and Example Images

To measure the UBI on an image:

from fringez.metric import calculate_UBI
UBI, UBI_err = calculate_UBI(sciimg_fname, mskimg_fname, updateHeader=True)

The science image header will be updated with the value of the image's UBI.

Generating Fringe Models

Note: Downloading pre-generated models using the fringez-download executable is recommended. Most users will not need to generate new fringe models. New models must be generated from images that have not already been cleaned.

Fringe models are generated with the fringez-generate executable.

The models which will be generated are listed in $PATH_TO_FRINGEZ_DIR/model.py:return_estimators. The number of components in each model fit are set with the -n-components argument. Plots of the eigenimages can be generated for debugging with the -plots argument.

To generate fringe models:

1. Place images containing fringes into a directory. All images within the directory must have the same RCID. Models are made separately for each ZTF RCID. Image names are expected to begin with ztf and end with sciimg.fits.
2. From within this folder, execute fringez-generate. By default the script will choose six components and will not generate debugging plots.
3. The current directory will now contain a model or models of the fringes, named fringe_{MODEL_NAME}_comp{N_COMPONENTS}.c{CID}_q{QID}.{DATE}.model. By default the script will only generate PCArandom models, but more models can be tested by editing the $PATH_TO_FRINGEZ_DIR/model.py:return_estimators function. The current directory will also contain a *.model_list file for each model listing the images that went into the creation of the model.

Requirements

• Python 3.6

Authors

• Michael Medford [e-mail]
• Peter Nugent [e-mail]

Citation