Marco C Lam
Time-domain Astrophysics is entering its golden age with a number of new telescopes coming online, generating large volume of high cadence quality data. Rapid follow-up of the transient astronomical events discovered by them are essential to enable science.
IRAF has been the "industrial standard" in astronomical data reduction since the 1980s, from undergraduate class to data reduction for the state-of-the-art observatory facilities at a volume rate of hundreds of GB per night. The deprecation of its support by Space Telescope Science Institute and the incompatibility with the 64-bit system will slowly paralyse the data reduction process. A replacement is essential in the future of all branches of Observational Astronomy.
ASPIRED is a new spectral reduction package written in PYTHON 3, the most popular programming language among the current generation of Astrophysicists. It facilitates simple and rapid orchestration of tailor-made reduction pipelines fine-tuned for the users' specific requirements, making it a candidate to replace IRAF. It is a concurrent development with RASCAL, a wavelength calibrator.
An electromagnetic spectrum is the entire range of wavelengths of electromagnetic radiation. Each source has its characteristic emission or absorption features. Spectrographs are used to disperse the incident light into a spectrum and record the data with a detector (e.g. CCD). The “rainbow spectrum” is the visible range that can be seen by the naked eye, from 390 to 700 nm. (A figure is included to illustrate the basic design of a spectrograph works as described above.)
Doing Science with a Spectrum
The spectra below show four strong and broad absorption features. The physical processes behind them always produce them at those specific wavelengths in the source. Measuring the strength, shape, shift and broadening of the features allows us to derive the intrinsic properties of the source that created and modified the appearance of the spectrum as observed on Earth. (A plot is used to show different responses recorded by the camera of the same source (ASASSN14gh) using red and blue optimised dispersion gratings on the SPRAT spectrograph on the Liverpool Telescope. The two settings deliver different quality in the signal at different wavelengths: red has the better quality for the longer wavelength, while blue is better at the shorter wavelengths.)
Data Processing and Extraction
Spectral data extraction follows 4 steps:
1 Image Flattening - This step corrects for the varying optical and detector behaviour across the image. The processed image reproduce the signal that a uniform detector should produce.
2 Spectral Tracing & Extraction - The spatial positions of the 2D spectrum are identified along the dispersion direction. The signals are then summed to give the response as a function of the dispersion.
3 Wavelength Calibration - The dispersion-to-wavelength relation has to be applied to the spectrum before it can enable science. It works by comparing against the spectrum from an arc lamp with the known position-to-wavelength relation.
4 Flux Calibration - Detector sensitivity varies as a function of wavelength, so the signal requires a scaling. This is done by applying the sensitivity of the instrument computed from a standard star with well-known flux.
(Four annotated figures are provided side-by-side to those steps to illustrate the steps.)
Software Stack
The ASPIRED uses a number of popular and well-maintained packages including ASTROPY, NUMPY, SCIPY, RASCAL, SPECTRES, and their associated dependencies. They allow simple maintenance and housekeeping. Software Versioning, Continuous Integration and Automated Documentation are enabled with GitHub, Travis CI and Readthedocs (Rtd). With every new commit is made to GitHub, Travis CI will be triggered automatically to test the compilation as well as any other test cases provided, while Rtd generates new documents to update the differences made in the most recent commit. The installation guide and user menu including examples are available at the Rtd link (see below).
GitHub: https://github.com/cylammarco/ASPIRED
Readthedocs: https://aspired.readthedocs.io/en/latest/
arXiv: https://arxiv.org/abs/1912.05885
Usage
ASPIRED is still undergoing development, but three data pipelines are already building on top of it: an upgrade of the LT/SPRAT pipeline, a new instrument SAAO/Mookodi, and a an observation broker BlackholeTOM at the University of Warsaw. The continuous development will carry on for at least 2 more years, funded by the Tel-Aviv University from October 2020.