INSTRUMENT DESCRIPTION Counter-Dispersed Imaging Demonstration Why is this such a Good Idea Project Status


  • Polarization Options
  • Simulated Data and Algorithms
  • H-alpha extension
  • Image Gallery
  • Current instrument parameters


    To use PNe to study galaxies one needs to detect them and then to measure their line-of-sight velocity. Normally these two steps are distinct: one does narrow-band imaging (in the [O III] line) to find the PNe, and subsequently takes spectra of the PNe using multi-slit or multi-fibre spectrographs. We have discovered an elegant alternative in which the detection phase can be skipped entirely: counter-dispersed imaging (CDI). The galaxy field of interest is simply imaged through a slitless spectrograph tuned to the [O III] line as shown in the lower half of the figure below. When this image is studied, the background light of the galaxy and the images of foreground stars will be found to be blurred, but the PNe are instantly recognisable as bright point-like images. This is because they, and perhaps an occasional giant cloud of ionised gas, are the only objects with a powerful emission line at this wavelength (5007Å). Each PN will be slightly displaced from its true position on the sky by an amount determined by its exact emission wavelength, and hence by its velocity.

    A second image is taken with the spectrograph rotated by 180o (upper half of figure). This can be done by taking two consecutive, reversed exposures (as has been implemented on our first CDI programmes). Or the two images can be taken simultaneously using duplicate spectrograph arms (the approach used in the PN.S). The PNe seen in the first image will be readily identified in this second image, but with the sense of their displacement reversed. Obviously, these two images taken together yield (1) position and (2) velocity, while the (3) brightness can be obtained from either image. So in one night we can do as well as, or better than, the traditional procedure does in two or three.

    DEMONSTRATION: The following pair of images illustrate the idea (these data were taken at the WHT in April 1997). Click on them to `blink' them. It will be obvious that the displacement of various PNe differs (notice especially how the shape of groups of objects changes). These differences arise because the PNe have different velocities.

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    What are the merits of the PNS idea over other techniques?


  • PN.S Version 3 (for TNG and WHT) is being built under various contracts.
  • ASTRON has delivered its part of the mechanical work, MSO is currently evaluating these components. Telescope flanges still to be made.
  • The optics have also been delivered and are being tested at MSO - some corrective work is required as the tight tolerances have resulted in some problems fitting the optics to the mechanical units
  • The gratings have arrived at MSO

    last modified by NGD, 25 Aug, 2009