KMOS Exposure Time Calculator
Important notes and bug reports
Note: These tools are only provided for the technical assessment of feasibility of the observations. Variations
of the atmospheric conditions can strongly affect the required observation time. Calculated exposure times do not
take into account instrument and telescope overheads. Users are advised to exert caution in the interpretation
of the results and kindly requested to report any result which may appear inconsistent.
These programs provide an HTML/Java based interface and consist of two
pages. The observation parameters page presents the entry fields and
widgets for the target and reference source information, expected atmospheric conditions,
instrument configuration, observation parameters such as exposure time
or signal-to-noise, and results selection. An "Apply"
button submits the parameters to the model executed on the ESO Web server.
The results page presents the computed results, including number of counts
for the object and the sky, signal-to-noise ratios, instrument efficiencies,
PSF size etc.. The optional graphs are displayed within Java applets allowing
interactive manipulation. The results are also provided in ASCII and GIF
formats for further analysis and printing. Finally, a summary of the input
parameters is appended to the result page.
The target model can be defined by the target's spectral type.
It uses a template spectrum, which is scaled to the provided magnitude and
filter. The spectral type is used to make the color correction.
You must select the filter and filter magnitude for proper scaling of the template spectrum. Available
filters are V, J, H and K. For extended sources, the magnitude must be given per square arc second.
Target Spatial Distribution
A point source is assumed to be an emitter with negligible angular size.
This can be selected for objects with an angular radius of much less than the sky-projected pixel size.
The reference area for the S/N is circular with a radius proportional to the seeing disk at the wavelength of observation.
This effective seeing is computed from the given seeing value (given as FWHM in the V band), using Roddier's formula:
Normally the radius of the circular S/N reference area is FWHM(λ), but due to a bug,
the KMOS ETC currently uses radius=sqrt(2)*FWHM(λ). This can lead to an underestimation of the
actual obtainable S/N by a factor which depends on the on the background noise level and target brightness.
The target object is assumed to have a uniform intensity and the S/N on the result page is given per spatial pixel on the detector.
Note that the magnitude (or the flux for an emission line) is always given per arcsec2 for extended sources.
Extended Source (with a given area)
The source is assumed to have a uniform intensity over the given area (Ω) on the sky, the number of pixels in the S/N area is
Ω / pixelScale2. To obtain the S/N per arcsec2, enter Ω=1 here. Note that the magnitude (or the flux for an emission line) is always given per arcsec2 for extended sources.
The sky background radiance and transmission are based on the Cerro Paranal advanced sky model,
developed by a team at the University of Innsbruck as part of the Austria in-kind contribution to ESO.
Seeing conditions. The value refers to the FWHM of the seeing disk in V band, at observed airmass.
The seeing FWHM in V-band.
Seeing(airmass=X) is the seeing at the airmass at which the observation is performed. Use this seeing in the phase I proposal and the OB constraints.
The values in the two seeing fields are mutually updated when one of the values changes. The field "Seeing(at Zenith)" also
depends dynamically on the airmass value. The seeing is assumed to scale with the airmass X like this: Seeing(X) = Seeing(Zenith) * X3/5.
The precipitable water vapor PWV is the vertically integrated total mass of water vapor per unit area for a column of atmosphere. PWV=2.5 mm is close to the median value for Paranal.
KMOS has a fixed camera with a single spatial scale.
The spatial scale along the slice is 0.2arcsecs and the slice width is 0.2arcsecs. This ensures the same spatial sampling of 0.2 arcsecs on the sky.
At the detector, the slice width is sampled by two pixels in the spectral direction.
This refers to the combination of filter and grating that
determine the (fixed) wavelength range of observations. The
available gratings are IZ, YJ, H, K or HK, the the latter with
spectral resolving power around half of that of the other
You must supply information about the total observation time. This can be done
in terms of DIT (Detector Integration Time), which is the duration of individual exposures,
and NDIT (Number of DIT's), which is the number of exposures. The total exposure time
is the product of DIT times NDIT. This exposure time does not take into account
instrument and telescope overheads.
Alternatively, you can specify a signal to noise ratio, in which case the ETC will compute the minimal number of
individual exposures (each of duration DIT) required to reach the requested S/N ratio.
The Signal to Noise Ratio (SNR or S/R) is defined for a point-like source
at the central observation wavelength. Indicate here a value
and choose a DIT, to get an estimate on how many exposures (NDIT) will be needed to achieve it.
The Exposure Time is the product of DIT and NDIT.
- DIT is the detector integration time (in seconds)
- NDIT is the number of exposures of duration DIT.
- INT is the total exposure time (excluding overheads). INT = DIT x NDIT
- Encircled Energy on Target:
This is the fraction of the object's total flux contained in those pixels over which the S/N is calculated, i.e. "Number of Pixels in PSF spatial profile" on the results page.
- Strehl Ratio: This is the peak intensity of the observed PSF to that of a perfect diffraction limited PSF.
- Version 5.0.1 (December 19, 2013)
Included the Moon phase as parameter in the input form.
- Version 5.0.0 (July 1, 2013)
All ETC version numbers have been aligned at 5.0.0 as the ETC system infrastructure was re-factored and
installed on a new web server. Please report significant discrepancies to the ESO user support group firstname.lastname@example.org