IRAF as a Workflow

8/13/2019 IRAF as a Workflow

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A common task to observational astronomers is to use a telescope equipped

with a CCD (charge-coupled device) camera to gather spectroscopic data, i.e.,

data about the light flux distribution over a region of the sky. In this task, several

factors, such as instrument imperfections and the discrete nature of light itself,

introduce errors in the measured data. Some of these errors can be random in

nature, while some of them can be deterministic. The goal of the CCD reduction

process is to minimize errors due to deterministic factors. Such a task is

commonly known as removing the instrument signaturefrom the spectroscopic

data.Although there is no global standard for the format used to store thegathered data, FITS (Flexible Image Transport System) format is commonly

used.

IRAF (Image Reduction and Analysis Facility) is a suite of programs used by

observational astronomers for the reduction and analysis of astronomical data.

IRAF installation files are readily available for UNIX systems. Once installed, theuser can interact with a myriad of IRAF utilities through the command language,

or CL. The CL is used to run the applications programs, which are grouped into

two classes, the system utilities and the scientific applications programs. Both

the CL and all standard IRAF applications programs depend upon the facilities

of the IRAF virtual operating system (VOS) for their functioning.

By using IRAF, an astronomer can execute the CCD reduction process on a

bunch of gathered data. At first, the several steps corresponding to the image

reduction process can be done interactively by an end-user through the CL: the

user can sequentially provide each FITS file to be processed by issuing theappropriate commands. However, for a large volume of data, this procedure is

very time-consuming and error prone. To automate the image reduction

process, script files can be constructed which would contain the instructions to

process the input data, for both reduction and analysis. However, this

alternative solution would require some knowledge of the CL scripting syntax,

which would take the astronomer further away from her/his final goal which is

actually doing science based on the gathered data.

We propose a user-friendly, WEB portal as an alternative to CL. Through this

WEB portal, the astronomer can interact with IRAF image reduction

functionalities. In particular, astronomers can upload files in the FITS format as

input to the image reduction process to be executed by IRAF utilities. After that,

the astronomer of this WEB portal can graphically define some input

parameters to direct for the task at hand. This WEB portal is intended to be a

frontend to a scientific workflow engine which, in a way that is transparent to the

astronomer, invokes the appropriate IRAF subprograms needed to perform all

phases of the reduction process.

The WEB portal is designed to be based on Chiron, a parallel workflow engine.Chiron is designed and implemented to execute scientific workflows in parallel


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on High Performance Computing (HPC) environments, such as clusters and

grids. Chiron was initially designed and developed motivated by the good

results obtained in solving real engineering problems by executing applications

that involve parameter explorations in clusters of computer nodes. The same

idea can be applied to the image reduction process executed using IRAF. Each

phase of the process can be implemented as an activity in Chiron. This way,

Chiron executes image reduction process as a scientific workflow thus allowing

for astronomers to benefit from HPC and provenance (i.e. lineage data)

capabilities. Chiron is based on message passage interface (i.e.MPI) so that

the engine is executed along the computing nodes of the environment. Each

computing node in the HPC environment executes an instance of Chiron, which

also gathers provenance data (start time, end time, errors) from the parallel

execution of each activity.

This way, after the definition of a reduction process by the astronomer, the WEBportal executes Chiron and each one of the workflow activities is enacted in the

appropriate order (according to the data dependency between them). Finally,

the produced results (i.e.the reduced images) are presented to the astronomer,

which can either analyze them or refine the input parameters and resubmit the

task to the subjacent workflow.

A positive aspect of our proposed WEB portal to execute IRAF is that it is

designed to facilitate the execution of exploratory studies (i.e. when the

astronomer have to execute the same process to process independent sets of

data an parameters). In this context, the astronomer can provide a range ofinput parameters the system can use to execute several data reduction

scenarios.

An example of the routines to be implemented in the proposed workflow is the

CCD reduction process for astronomical long slit spectroscopic data. This is a

process of large applicability since it can be applied to astronomical data in

different spectral regions from different telescopes. The tasks to be performed in

each step will depend on the choices made by the user.

The figure below presents a schematic view of the proposed workflow. In thefollowing, we briefly describe each step to be performed by this workflow.

Once the data is obtained by the telescope and stored in the FITS format,

calibration images of the telescopes are built (step 1). In these images is the

information necessary to correct the scientific data from preliminary instrument

conditions, as bias CCD reading fluctuations and flat field illumination patterns.

Then these initial calibration images are applied to the scientific data (step 2).

The target spectra are identified and extracted from the scientific data. The

target can be either point-source like (i.e., stars) or extended sky objects. In anycase, the correspondence of the two spatial dimensions of the CCD to one


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dimension of the spectral is applied. These spectra must be calibrated in

wavelength (of equivalent unit, as energy) ending the step 3 of the reduction.

The observed spectrum is not yet identical to the one emitted by the source, as

it may suffer a deviation due to Earth's movement around the Sun (Galacticsource), from the movement of the Sun in the Galaxy (Local Group source), or

from the universe expansion (extragalactic source). This shift correction, among

others - as relative flux calibration of the target based on observed standard

photometric stars - is applied in step 4.

Finally, some post-processing can be done in the resulting spectra, such as

identification of lines and bands (emission or absorption), adjust the intensity of

lines, effective temperature stellar spectrums, comparison with modeled spectra

etc. These tasks are incorporated in step 5.

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IRAF as a Workflow