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Observing Conditions Example: NIRI Imaging of an Extended Object

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[weather icon] This is an example of aspects to consider when choosing observing condition constraints. The science drivers for each program are distinct, of course, and thus the appropriate set of conditions for your own observations may be quite different. The example is taken from the NIRI System Verification plan.

The observation proposed would allow distance determination to a distant galaxy using the surface brightness fluctuation (SBF) technique. The SBF procedure requires deep, high spatial-resolution images of early-type galaxies. The SBF signal increases linearly with resolution (image quality). Photometric weather is required to put the measured fluctuation magnitudes on an absolute distance scale. Thus, the observing conditions were requested as follows:


  • Image quality - Requested 70% seeing, expecting to get good images at K with tip-tilt correction. There is a trade-off, of course, between integration time required to achieve a particular S/N and the seeing. The best 20% seeing would have been requested had the galaxy been more distant. However, this image quality would have been overkill for nearer galaxies and the likelihood of getting the best conditions is lower. The balanced approach of 70% seeing was selected; in a real proposal, 20% conditions would have been requested for the most-distant subset of the sample. 

  • Sky transparency (cloud cover) - Photometric weather is required to make sure a distance can be computed from an apparent fluctuation magnitude. The 50% or better sky transparency will ensure that the photometric conditions required are met.

  • Sky transparency (water vapour content) - in the near-IR K-band my observations are insensitive to atmospheric water vapor, therefore "any" conditions was selected.

  • Sky background - for broad K-band imaging, there was no real concern about the sky background (OH airglow), and the 80% conditions was chosen. Of course, with making frequent sky exposures and subtracting them from science exposures, and as long as this is done on time scales of a few minutes, the OH airglow and thermal components of the sky background can be removed adequately.


caution Note that the statistical likelihood of execution of this observation, if all of the observing conditions are truly uncorrelated, is 70% * 50% * 100% * 80% = 28% of the time when the target is accessible in the sky. (In fact one expects some mild correlation and so this is a slight underestimate). The chances of conditions being favorable for the observations at any particular time are thus quite small even with the relaxed image quality constraint. Therefore if these observations had been planned to be carried out in classically-scheduled time, statistically it would have required an allocation of four nights to be 'assured' of one night with the conditions required. Note that 28% is not the probability of the observations being completed successfully in the queue since a project will be allocated time when conditions are right for that project.

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