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Observing Constraints - Sky Transparency

The sky transparency is presented separately for Mauna Kea and Cerro Pachon.

Mauna Kea

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

The optical extinctions used in the Integration Time Calulator are model spectra that include scattering and absorption by water vapour and other atmospheric constituents.

For reference purposes only, median extinction coefficients for Mauna Kea are shown in the following table (Boulade 1988, CFHT Bulletin, 19, 16, for l<400nm; CFHT Observers Manual for l>400nm). They are insensitive to water vapour content.

Wavelength (nm) Extinction
(mag / air mass)
310 1.37
320 0.82
340 0.51
360 0.37
380 0.30
400 0.25
450 0.17
500 0.13
550 0.12
600 0.11
650 0.11
700 0.10
800 0.07
900 0.05


The near-infrared J, H and K wavebands are separated by regions of deep absorption predominantly due to water vapour. Transmission spectra were generated using the ATRAN model (Lord, S.D. 1992, NASA Technical Memor. 103957) for the adopted water vapour conditions (1.0, 1.6 and 3.0mm) at 1.0, 1.5 and 2.0 air masses. These spectra are used in the integration time calculators.

For reference purposes only, median extinctions through broad-band filters for Mauna Kea are shown in the table below (Krisciunas et al. 1987. PASP, 99, 887):

Wavelength (µm) Extinction
(mag / air mass)
1.25 (J) 0.10
1.65 (H) 0.06
2.2 (K) 0.09


The near-infrared 3-5µm atmospheric transmission is dominated by various species including water and CO2. Transmission spectra were generated using the ATRAN model (Lord, S.D. 1992, NASA Technical Memor. 103957) for the adopted water vapour conditions (1.0, 1.6 and 3.0mm) at 1.0, 1.5 and 2.0 air masses. These spectra are used in the integration time calculators.

For reference purposes only, median extinctions through broad-band filters for Mauna Kea are shown in the table below (Krisciunas et al. 1987. PASP, 99, 887):

Wavelength (µm) Extinction
(mag / air mass)
3.4 (L) 0.16
3.8 (L') 0.09
4.8 (M) 0.22


The thermal-infrared 8-25µm atmospheric transmission is dominated by various species including methane, CO2 and water. Transmission spectra were generated using the ATRAN model (Lord, S.D. 1992, NASA Technical Memor. 103957) for the adopted water vapour conditions (1.0, 1.6 and 3.0mm) at 1.0, 1.5 and 2.0 air masses. These spectra are used in the integration time calculators.

For reference purposes only, median extinctions through broad-band filters for Mauna Kea are shown in the table below (Krisciunas et al. 1987. PASP, 99, 887):

Wavelength (µm) Filter Bandwidth Extinction
(mag / air mass)
7.8 narrow 0.46
8.7 narrow 0.12
9.8 narrow 0.15
10.0 (N) broad 0.15
10.3 narrow 0.07
11.6 narrow 0.08
12.5 narrow 0.13
20.0 (Q) broad 0.42


Cerro Pachon

At present the conditions for Cerro Pachon are assumed to be the same as for Mauna Kea.


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Last update March 2, 2001; Phil Puxley