Observational Astronomy Primer Exercises: 1.) Viewing FITS Images

To help you become familiar with astronomical data files – .FITS format – I would like to give you a small task: please experiment with some supernova images in DS9.   Here is what I’m thinking for some small goals:

  • Download data for one astronomical target from two different telescopes and display them together, side-by-side.
  • Stretch the image scaling to appropriate levels and experiment with color maps.
  • Match the coordinates.
  • Make labels.
  • Create regions.
  • Output/print an image (png, jpg, etc.) of your results & save ‘backup’ file.


I discuss photometry in another post & how to do simple flux extraction with DS9 and HIPE later.

Try Kepler’s SNR (G4.5+6.8) in IR  http://chandra.harvard.edu/photo/openFITS/multiwavelength_data.html   and try to find one more image from another band that is not listed on that page.  I will discuss some good online databases tomorrow, but one good option would be a radio image from the VLA – you can download VLA data from the NRAO image archive retrieval tool  https://archive.nrao.edu/archive/archiveimage.html




Notes on image headers

A good .fits file will include necessary WCS (pointing) and brightness units.  And beam size information if needed.


The main header ‘card’ to look for is ‘BUNIT’, which should be something like Jy/beam, MJy/sr, W/m2/pixel, etc.  Sometimes images you download (such as HST, GALEX…) will still be in units of counts/sec or other system that will need to be converted to physical units before you can integrate fluxes.  If you need to perform these conversions, try looking at the instrument webpages or papers in the literature to get the conversion factors, otherwise you can try to contact the PI who owns the data.

To convert the beam area – that is, to calculate the beam size as pixels/beam or sr/beam etc – our job is easy if the PSF can be approximated by a 2D Gaussian.  Properly-formed radio, sub-mm and IR headers should have the beam parameters listed as BMAJ, BMIN, and BPA – the beam major axis FWHM, minor axis FWHM, and position angle, respectively.  All 3 are normally in units of degrees.  The PA in the FITS standard is defined as counter-clockwise from ‘image north’ or the upper y-axis, as opposed to the mathematical standard right x-axis assumed by python and other programming languages.

WCS (World Coordinate System)

The information about which pixel corresponds to what RA/DEC on the sky is given in the header as well.  Here is the NASA dictionary of standard header cards:  http://heasarc.gsfc.nasa.gov/docs/fcg/standard_dict.html
Basically, all the header cards beginning with C (CTYPE, CDELT, …) have to do with the the axis and pointing information.  Here is a brief description of some:
CTYPE gives the projection type – SIN, TAN, …
CRPIX gives the reference pixel number (starting from 1)
CRVAL gives the pointing coordinate of the reference pixel
CUNIT gives the units of the reference coordinate values (default is degrees)
CDELT gives the width of each pixel in sky coordinates
CROTA gives the image rotation angle, if applicable.

The number after the main keyword denotes which image axis – 1 is for the x-axis (RA), 2 is for the y-axis (DEC), 3 is for the z-axis (velocity/frequency/wavelength) in a 3D data cube, 4 is for the Stokes axis.  So, CRPIX1 is the reference pixel for the x-axis, CDELT2 is the pixel width in the y-axis, CUNIT3=’Hz’ means that the spectral axis channels are stepped in frequency, etc.

Instead of CRDELT/CROTA etc. you may find your image using an alternate grid reference system based on the cards CD1_1, CD2_1, CD1_2 and CD2_2.  These four parameters define a matrix grid of the pixel scale and rotation.

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