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Observational Astrophysics in the visible light. Shai Kaspi. Technion – April 2019. Flexible Image Transport System (FITS).
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Observational Astrophysics in the visible light Shai Kaspi Technion – April 2019
Flexible Image Transport System (FITS) • Digital file format used to store, transmit, and manipulate scientific and other images. Most commonly used in astronomy. Designed specifically for scientific data and hence includes many provisions for describing photometric and spatial calibration information, together with image original metadata. • First part of the file has an ASCII header that carry keyword/value pairs which provide information such as size, origin, coordinates, binary data format, free-form comments, history of the data, etc. • Second part of the file has the actual data in a binary format. • FITS is also often used to store non-image data, such as spectra, photon lists, data cubes, or even structured data such as multi-table databases. A FITS file may contain several extensions, and each of these may contain a data object. For example, it is possible to store x-ray and infrared exposures in the same file.
Bias image • A signal that initially imposed on the CCD to prevent the signal from going negative. • Depends on temperature, readout process, electronics, etc. • Created by reading the CCD with zero exposure time.
Overscan area • Virtual pixels created after the image is read and indicate how the electronics responds to a real zero signal. Shows the variation of bias between images and is used to correct it.
Dark image • Dark current created due to thermal motion of electrons in the CCD. • Minimized by cooling the CCD. • Obtained by taking exposures with the shutter closed. • CCD are cooled.
Flat Field image • Pixel sensitivity to light is different due to pixel response, dust, vignetting • Taken with uniformly illuminating the telescope’s aperture (twilight sky).
Object image • Exposure of the CCD to the night sky through the telescope.
Fringing • Caused by interference between rays from multiple reflections within the CCD or filters. Correction is done by multiplying and subtracting.
Bad pixels • Pixels or rows which do not respond as all the other pixels.
Cosmic rays 3600 sec dark exposure.
Photometry • Measurement of the flux or intensity of an astronomical object's electromagnetic radiation. • Add up all the light from the object and subtract the light due to the sky.
Aperture Photometry • Adding up the pixel counts within a circle centered on the object and subtracting of an average sky count
Point Spread Function (PSF) Photometry • When doing photometry in a very crowded field, such as a globular cluster, where the profiles of stars overlap significantly, one must use de-convolution techniques, such as point spread function fitting, to determine the individual fluxes of the overlapping sources.
Filters • Observations are commonly done through filters to obtain light in a certain waveband.
Magnitude • Logarithmic measure of the brightness of an object, measured in a specific wavelength or bandpass, usually in UV, optical, or near-infrared wavelengths. • Two objects whose fluxes measured from Earth are I1 and I2 and will have apparent magnitudes m1 and m2 related by: • Absolute magnitude - the object's apparent magnitude as seen from 10 parsecs: • One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. A parsec is equal to about 3.26 light-year. • The star Vega has been defined as having an apparent magnitude of almost zero.
Magnitude • To compute magnitude using flux in a certain filter: m= -2.5 log10 (F) + m0 Where F measure in erg/cm2/sec Each filter has its own m0 E.g., for U filter m0U = -13.87 for B filter m0B = -12.97 for V filter m0V = -13.74 etc.
Photometry calibration Each image is observed under different conditions (different: airmass, atmospheric transparency, instrument, etc.), thus need to take calibration actions in order to compare results. Extinction - the absorption and scattering of light by matter (dust and gas) between an emitting astronomical object and the observer. Atmospheric extinction. Interstellar extinction. Galactic extinction.
Differential photometry is the measurement of changes in the brightness of an object over time. This is the light curve of the object. Can be done at the highest precision.
Relative photometry is the measurement of the apparent brightnesses of multiple objects relative to each other.
Absolute photometry is the measurement of the apparent brightness of an object on a standard photometric system, these measurements can be compared with other absolute photometric measurements obtained with different telescopes/instruments. • Catalogs for standard magnitudes. • E.g., Landolt UBVRI Standards Catalog • Each filter or instrument has its own catalogs.
IRAFImage Reduction and Analysis Facility • General purpose software system for the reduction and analysis of astronomical data. • Optical and IR as well as external packages for Hubble Space Telescope (optical), EUVE (extreme ultra-violet), or ROSAT and AXAF or Chandra (X-ray). • http://iraf.noao.edu/ • Working environment with layered packages.
DS9 • An astronomical imaging and data visualization application. DS9 supports FITS images and binary tables, multiple frame buffers, region manipulation, and many scale algorithms and colormaps. It provides for easy communication with external analysis tasks and is highly configurable. • http://hea-www.harvard.edu/RD/ds9/
