This page discusses the optical and infrared sky background on Mauna Kea and Cerro Pachon. Files of sky emission spectra for the two sites are available at the bottom of the page.

Mauna Kea

The Mauna Kea sky background is described here for the following wavelength regimes:

• Optical brightness variation and sky spectrum
• Near-infrared (1-2.5µm)
• Near-infrared (3-5µm)
• Mid-infrared (7-25µm)

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The optical sky background depends on a number of parameters including the target - moon angular separation, lunar phase, ecliptic latitude, zenith angle, and phase of the solar cycle (e.g. Krisciunas 1997, PASP, 209, 1181; Krisciunas and Schaefer 1991, PASP, 103, 1033; Benn and Ellison 1998, La Palma Technical Note 115). A model was constructed following the prescription of Krisciunas (1997) and Krisciunas and Schaefer (1991). The graph below shows the cumulative probability distributions of V-band sky brightness at an arbitrary phase in the solar cycle for three model observation scenarios. In the first model the target is always at the zenith. The second and third models are more realistic Monte Carlo realisations of likely queue and classical programs. In the second model targets were chosen with a gaussian distribution in Hour Angle with sigma=1 hour, and with a distribution in Declination between -20 and +90 degrees based on the surface area of the celestial sphere. The third model is the same as the second, but with the further constraint that the target must be at least 30 degrees from the moon. It may seem surprising that the results of these 3 models differ so little. This is due to the fact that the primary dependence of night sky brightness is on lunar phase, and secondarily on moon - target distance.

The results of these calculations indicate that the sky at Mauna Kea is fainter than 20.78 mag/arcsec² for 50% of the time and fainter than 21.37 mag/arcsec² for 20% of the time for any random target. (For an unbiased distribution of queue-mode nights the moon is below the horizon for about half the time, of course).

The values presented in the observing constraints table, and used in the integration time calculator, are valid for semesters 2000B - 2001B and have been adjusted from the graph above to those expected for a nominal solar cycle variation (currently close to the mid-point). These values will be modified slightly each year or so.

The colour of the sky changes with lunar phase. Adopted values are shown in the table below (taken from ESO, by scaling V for inferred equivalent lunar phase, and from Walker, NOAO Newsletter No. 10). The conversion between the sky background category and the number of nights from new moon indicates the constraints that are applied to schedule classical observations and do not necessarily correspond to conventional definitions of dark, grey and bright time.

Note that the V-band sky is brighter at low ecliptic latitude by ~0.4 mag (Benn and Ellison 1998. La Palma Technical Note 115).

Data from HM Nautical Almanac Office, showing sun and moon rising and setting times and lunar phases for Mauna Kea and Cerro Pachon/Tololo, are available at (just substitute the year you want in the URL to get other years): 2007 Mauna Kea and Cerro Pachon (actually La Silla)

The broad-band sky brightnesses given in the table above have been used to scaled a model optical sky spectrum . These spectra are used in the Integration Time Calculators. The sky spectrum is patched to the near-IR sky spectrum at a wavelength of 920nm. An example is shown below (for 50%, 'dark' conditions) and the data file is available.

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The near-IR (1-2.5µm) sky background is dominated by many intrinsically narrow hydroxyl (OH) emission lines. A few other species (e.g. molecular oxygen at 1.27µm) also contribute, as do H2O lines at the long wavelength end of the K window. During the night the hydroxyl lines vary in brightness on a timescale of 5-15 minutes and with an amplitude of 5-10% as atmospheric wave phenomena change the local density of species. As the emission occurs via a radiative cascade, all of the lines vary together in brightness to first order.

The strength of the OH lines also exhibits a steady decline for the first 1-2 hrs after sunset. Spectroscopic observations at low and intermediate resolutions are usually not advisable during twilight, especially in the J and H bands, as it can be difficult to achieve accurate sky subtraction. Imaging observations of bright objects are possible however, albeit with an increased and varying background.

The Integration Time Calculators employ model high resolution sky emission spectra, an example of which is shown below. In addition to the OH lines, the models incorporate zodiacal emission (approximated by a 5800K black body) (each then scaled by the atmospheric transmission), and thermal emission from the atmosphere (treated as a 273K black body for Mauna Kea and a 280 K blackbody for Cerro Pachon multiplied by 1 - transmission), at a variety of airmasses and water vapor columns. Note that moonlight, which is not included can be the dominant background source (e.g., much more so than in the model spectra), especially in the J band when the moon is bright and especially when the target is close to the moon.

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The near-IR (3-5µm) sky background is due primarily to thermal emission from the atmosphere. The transmission (and therefore the emission) varies with atmospheric water vapour content and air mass. See the descriptions of atmospheric transmission and the adopted water vapour conditions for more information. An example model background spectrum, smoothed to a resolution of 2cm^-1, is shown below.

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The mid-IR (7-25µm) sky background behaves similarly to the 3-5µm background, but is much more intense. See the descriptions of atmospheric transmission and the adopted water vapour conditions for more information.

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Cerro Pachon

The sky background has been modeled for Cerro Pachon as well and is used in its ITCs. Data files are similar to those for Mauna Kea.

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Sky Background Data Files

The raw sky emission data files 0.9-5.6 and 7-26 microns for both Mauna Kea and Cerro Pachon are available via the following tables. These files are sky background only; they do not include the emission from the telescope itself and from the instrument (both of which are included in the IR Integration Time Calculators). Any use of the data in these tables should reference Lord (1992) and acknowledge Gemini Observatory. The 1-5 micron models are in ASCII two-column format: (a) wavelength with a sampling of 0.00002µm and a resolution of 0.0004µm (0.04nm) and (b) emission in ph/sec/arcsec^2/nm/m^2 . The 7-26 micron models have a sampling of 0.0001µm and a resolution of 0.0002µm (0.2nm) for Mauna Kea and 0.002µm and 0.004µm (4nm) for Cerro Pachon.

The files were manufactured starting from the sky transmission files generated by ATRAN. These files were subtracted from unity to give an emissivity and then multiplied by a blackbody function of temperature 273 for Mauna Kea and 280 for Cerro Pachon. To these were added the OH emission spectrum (available from the European Southern Observatory's ISAAC web pages) a set of O lines near 1.3 microns with estimated strengths based on observations at Mauna Kea, and the zodiacal light, approximated as a 5800K gray body times the atmospheric transmission and scaled to produce 18.2 mag/arcsec^2 in the H band.

Note the crudity of the approximation of the thermal background, which assumes that all sources of sky opacity are at T=273K on Mauna Kea and T=280 on Cerro Pachon. In reality the sources occur at various altitudes above each site. Thus, these files should overestimate the thermal background. Note also that the OH lines can vary in intensity from night to night and vary greatly during the night as well. Finally note that background from moonlight is not included, but it can be very significant in the J, H, and K bands, especially if the target is located close to the bright moon

(In most browsers, hold down the shift key when clicking to save the data to a file). The numbers in the titles of the files are 10X the water vapor in mm and 10X the airmass.

 

 

 

 

Last update October 20, 2009; Tom Geballe
In original form December 12, 1999; Phil Puxley, Ted von Hippel and Tom Geballe