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Observing Conditions - how sky conditions affect near-IR observing

Introduction

In the near-infrared (1-5 microns) a number of factors (telescope elevation / airmass, clouds, OH emission, water vapor column density, and moonlight) determine the transparency of the sky and the amplitude of fluctuations in the background radiation, both of which affect the performance of the instruments used in this wavelength region. Some of these factors are strongly wavelength dependent. The Integration Time Calculators attempt to simulate the effects of airmass, cloud cover, and water vapor, as well as image quality, and one can use the ITCs to explore the effects. The following is a rough summary of how all of these factors affect photometry and spectroscopy.

Photometry

The near-infrared JHKL'M photometric filters are designed to transmit in the clearest portions of the 1.0-1.35, 1.45-1.8, 2.0-2.4, 2.9-4.1, and 4.5-5.2 micron "windows" and in some cases cover less than half of the aforementioned wavelength ranges.

In the J, H, and K photometric filters the atmospheric transmission is high and absorption by water vapor is not a significant contributor and thus there are essentially no water vapor constraints to obtaining optimal sensitivity.

Noise due to fluctuations in OH sky background emission (which occurs in various OH ro-vibrational bands in the 1.0-2.3 micron interval) is the most important source of noise in the J, H, and K photometric filters; it is most dominant in J and H. The ITCs use the nominal values for the background; but occasionally there are nights or portions of nights when these values can be exceeded by a factor of 2-3 (or can be less by that factor). OH emission varies diurnally; it is strongest in the daytime and at twilight is still typically 2-3 times higher than an hour or two later, and it doesn't increase again until just before sunrise. Thus measurements at J and H will be less sensitive shortly after the end of twilight than an hour or two later. Note also that OH emission roughly scales with airmass so the noise due to backgound fluctuation is higher at higher airmass.

In the L' and M filters absorption by water vapor is more significant, but still generally less important than other atmospheric constituents such as CH4, CO2, CO, and O3. Overall the atmospheric transmission is lower than in the JHK filters. It is recommended that photometric measurements in the L' and M filters avoid the highest airmasses and water vapor columns (i.e., do not specify WV "any").

Moonlight (which is not included in the ITC) can be a non-negligible source of sky background in the J band; it is much less of a problem in the H and K bands, although if the moon is very close to a faint astronomical target the performance in those bands can also be affected.

Spectroscopy

Most of the factors discussed above also affect spectroscopic performance, but the effect of each varies from band to band, and indeed from wavelength to wavelength (e.g., a sought-after emission line may or may not correspond in wavelength to a strong telluric absorption line). There also are some instances where one can characterize one part of a window as affected in a different ways than another part. For example, the 2.85-3.25μm region contains numerous strong lines of H2O, but beyond 3.25μm the both density of H2O lines and their strengths decrease markedly. Thus an investigator whose science resides in the 2.85-3.25μm region should request drier conditions than one who is interested only in 3.25-4.15μm.

The above issues mean that the investigator should use the ITCs to check the transmission and background at specific wavelengths or wavelength intervals of interest, and should vary conditions such as airmass and water vapor to see their effects.

Accurate removal of telluric lines by ratioing the spectrum of the science target by that of a calibration star is generally achievable if the airmass match between the two objects is good. However it in regions where lines of telluric H2O are important, it can fail, because unlike other important absorbing species, the water column varies temporally as well as well as with airmass. Investigators whose science lies at wavelength affected by H2O lines may wish to specify, possibly by using some science time, that calibration stars are observed more frequently than normal, in order to mitigate this problem.