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Observing Conditions Example: NIRI Spectroscopy of an Extended Object
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.
Consider a program to take an H-band (1.7µm) spectrum across the star-forming core of the galaxy M82 to measure the relative H and He recombination line strengths. Good spectral resolution is required, both to separate the Brackett-series H lines and to achieve good line-to-continuum contrast against the underlying stellar emission. NIRI in its f/14 configuration was selected as spectral resolution is more important than slit throughput for this program.
- Image quality - the target is spatially extended and thus the flux through the slit is very insensitive to observing conditions. To maximise the likelihood of the observation being executed, "any" (100%-ile) conditions was chosen.
- Sky transparency (cloud cover) - although the program is designed to measure relative line fluxes, photometric conditions are required in order to derive the dust extinction using data at other wavelengths. The 50%-ile conditions ("photometric") was chosen. Had photometry not been required, 70%-ile ("thin cirrus") would likely have been selected, even though it would increase the integration time, to improve the likelihood of observation execution.
- Sky transparency (water vapour content) - from the transmission spectra of this wavelength region, there is very little sensitivity to water vapour content, therefore "any" conditions was chosen.
- Sky background - Aside from the period soon after sunset and before sunrise, the variation in OH background is adequately small, therefore 80%-ile conditions was chosen.
Note that the statistical likelihood of execution of this observation, if all of the observing conditions are truly uncorrelated, is 100% * 50% * 100% * 80% = 40% of the time when the target is accessible in the sky. (In fact one would expect some mild correlation and so this is a slight underestimate). Note that 40% is not the probability of observations being completed successfully in the queue since a project will be allocated time when conditions are right for that project.