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SkyCalc - ESO Sky Model Calculator

SkyCalc Web Application

SkyCalc is a web application based on the Cerro Paranal Advanced Sky Model, which was developed in particular to be used in the ESO Exposure Time Calculators, by a team of astronomers at the Institute for Astro- and Particle Physics at the University of Innsbruck, as part of an Austrian in-kind contribution to ESO.

The in-kind contribution also includes two tools to correct observations for atmospheric telluric absorption and emission (Molecfit and Skycorr), which can be found here.

The citations for the Cerro Paranal Sky Model are Noll et al. (2012, A&A 543, A92) and Jones et al. (2013, A&A 560, A91).

A library of telluric transmission spectra with various resolutions and atmospheric conditions has been calculated with the Cerro Paranal Advanced Sky Model. A description of the usage of the spectra can be found in the article Moehler et al. (2014, A&A 568, A9).

ESO SkyCalc Command Line Interface (CLI)

A command-line tool skycalc_cli is available to execute skycalc

Broadband Magnitude Calculations

SkyCalc can compute the broad-band magnitudes per square arc second in the photometric bands UBVRIZYJHKLMNQ. This is done by integrating, across each band pass filter, a low resolution (R=60,000) emission spectrum model, corresponding to the radiance contributions selected in the input form. Note that only the radiance components selected in the input page are included in these magnitude calculations.

The following table provides the photometric zero points (Vega system) and the broad-band filter curves used in the magnitude calculations. These values and filter curves correspond to the ones used in the ESO ETC system.

Band Filter curve Photometric zeropoint in ergs/s/cm2/A
(flux for zero mag in Vega system)
U U filter curve U0 = 4.18023e-09
B B filter curve B0 = 6.60085e-09
V V filter curve V0 = 3.60994e-09
R R filter curve R0 = 2.28665e-09
I I filter curve I0 = 1.22603e-09
Z Z filter curve Z0 = 7.76068e-10
Y Y filter curve Y0 = 5.973e-10
J J filter curve J0 = 3.12e-10
H H filter curve H0 = 1.14e-10
K K filter curve K0 = 3.94e-11
L L filter curve L0 = 4.83e-12
M M filter curve M0 = 2.04e-12
N N filter curve N0 = 1.23e-13
Q Q filter curve Q0 = 6.8e-15

Temperature and Bolometric Radiance

Sky Model

The Sky Model is configured for the conditions at Cerro Paranal (VLT) as explained in the documentation, except for the observatory altitude above sea level.

Two additional observatory heights 3060m and 5000m were added in version 2.0.0. They correspond to the height of Cerro Armazones (future site of the ELT) and a generic 5000m site. The configuration of the Sky Model for those two sites is the same as for Cerro Paranal except the altitude which affects the pressure parameter.

The output from the SkyCalc includes plots of the transmission and emissivity spectra and optionally the individual components of these. A link to a FITS is also included, the FITS header contains a short summary of the input parameters and legends of the seven data columns:

  COMMENT column lam:     wavelength in micron
  COMMENT column flux:    sky emission radiance flux in ph/s/m2/micron/arcsec2
  COMMENT column dflux1:  sky emission -1sigma flux uncertainty
  COMMENT column dflux2:  sky emission +1sigma flux uncertainty
  COMMENT column dtrans:  sky transmission
  COMMENT column dtrans1: sky transmission -1sigma uncertainty
  COMMENT column dtrans2: sky transmission +1sigma uncertainty



Dates and Times


Sunrise, Sunset, Moonrise and Moonset, Culmination, Twilights




Version Information

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