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Observing Conditions Example: NIRI Imaging of an Extended Object

[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 I have requested are as follows:

  • Image quality - I have 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. I would have requested the best 20% seeing 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. I opted for the balanced approach of 70% seeing; in a real proposal I would request 20% conditions for the most-distant subset of my 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 I get the photometric conditions I require.

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

  • Sky background - for broad K-band imaging, I am not very concerned about the sky background (OH airglow), and choose the 80% conditions. Of course, I will be making frequent sky exposures and subtracting them from my science exposures, and as long as I do this on time scales of a few minutes, I can remove the OH airglow and thermal components of the sky background 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 we expect some mild correlation and so this is a slight underestimate). The chances of conditions being favorable for my observations at any particular time are thus quite small even with my relaxed image quality constraint. Therefore if I had planned these observations to be carried out in classically-scheduled time, statistically I would have required an allocation of four nights to be 'assured' of one night with the conditions I require. Note that 28% is not the probability of my observations being completed successfully in the queue since a project will be allocated time when conditions are right for that project.