FCU ETC USER MANUAL

Welcome to the WSO-UV FCU ETC User Manual. In here you will find a detailed explanation on how to use the ETC tool.

The Exposure Time Calculator (ETC) is a tool developed by the WSO-UV SOC Team to allow the astronomers to estimate the observing time for their scientific programs, in view of the mission Call for Proposals.

With the FCU ETC the user can choose to calculate the exposure time of an observation to achieve a desired signal-to-noise ratio (S/N), or viceversa.

MainPage
Fig 1. Main page





Summary of contents



0. Basics

The Exposure Time Calculator (ETC) allows the user to estimate the exposure time corresponding to a desired S/N ratio, or, conversely, to estimate the S/N ratio obtained with a given exposure time.
  • When the exposure time is entered by the user, the ETC takes the filter transmission, throughputs of the optics and the efficiency of the detector, and calculates the expected count rate for the specified instrument configuration. The ETC will finally find the S/N ratio and the integrated counts from the source.
  • When the S/N ratio is entered as input, the ETC takes the filter transmission, throughputs of the optics and the efficiency of the detector, and calculates the expected count rate for the specified instrument configuration. It will finally find the required exposure time so that the observation will have the specified S/N. This exposure time is then used to calculate the integrated counts from the source.

The different files implementing the different FCU elements behavior (Mirrors, Filters, Dispersing elements and Detector) can be seen by using the buttons found at the top of the ETC web interface.

TopButtons
Fig 2. Top buttons allow inspection of the FCU elements behavior

Some known issues found in the current version of the ETC:

  • It only deals with point sources. All the calculations in this and other sections should be reviewed and adapted in order to consider the possibility of diffuse extended sources.
  • It does not take into account any extinction affecting the flux reaching the telescope.
  • (When dealing with spectroscopic mode) It calculates the counts per second at the given wavelength falling on a given pixel, and, later on, it multiplies them by the dispersion d (i.e., by all the photons falling into that pixel) for getting the total number of counts per second in that resolution element (pixel). The dispersion d depends on the system itself (the resolution R and the pixel size).


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1. Mode

The FCU can work in two main operational modes:
  • Imaging: The beam is focused on the selected channel (FUV or UVO).
  • Spectroscopy: The beam is directed to the prisms.

Depending on the mode selection, different filters and dispersive elements are available.


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2. Channel

The FCU has two independent channels
  • The far UV (FUV) channel has capabilities for high resolution imaging. Also, some low dispersion spectroscopic capabilities around Lyman - alpha (121.5 nm) and the C IV resonance transition at 155.0 nm are available .
  • The UV - optical (UVO) channel is designed for wide field imaging from 200 to 600nm.

Different filter options are available depending on the channel selection.

  • FUV Filter wheel 1 provides a grey filter with constant transmittance 10%, and the dispersive elements P122 and P155, which set the FCU in spectroscopic mode.
  • FUV Filter wheel 2 provides the long-pass filters with null or very low transmittance at short wavelengths and high transmittance for wavelengths above the value indicated by its label: F125LP, F140LP, F150LP and F165LP.
  • UVO Filter wheel 1 provides the broad band filters GAIA BP, F255W, F336W and F555W.
  • UVO Filter wheel 2 provides the narrow band filters F232N, F280N, F308N F656N and F673N.

Further information on the FCU can be found in:


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3. Inputs

The input data field allows the observer to specify the spectral distribution D(lambda) in erg cm-2 s-1 mstrong-1 for which the S/N or exposure time is required.
  • For the spectral line the user provides the flux in proper units.
  • For the flat continuum the user provides the flux in proper units.
  • For the black body with a given temperature, the user provides a spectral shape and its apparent magnitude which will be used for computing the normalization constant.
  • For the Kurucz model with a given temperature and surface gravity, the user provides a spectral shape and its apparent magnitude which will be used for computing the normalization constant. The ETC offers a range of stellar atmospheres models from de Castelli and Kurucz1 (2004) Atlas9 Stellar Atmosphere Models. .
  • For the uploaded spectrum, the user provides a pre-defined spectrum in ASCII format. ETC assumes an input format consisting of two columns separated by space: the first column corresponding to the wavelength (Amstrongs) and the second one to the flux density in erg s-1 cm-2 A-1. The ETC expects steps of 1 Amstrong in wavelength, and it assumes a null flux if there is not any flux value for a given wavelength.
When using the spectral line, the flat continuum or an uploaded spectrum, the flux density D (in erg s-1 cm-2 A-1) as it reaches the telescope is given. However, when using a black body or a spectrum of Kurucz's library an apparent magnitude has to be indicated in order to estimate a proper internal normalisation factor that contains both the distance factor and any other additional factor due to unit differences. We use as reference the magnitude in the Johnson V band.

All entered values must use the dot as decimal separator.


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4. Background Levels

Currently, only the option none can be used. All relevant components such as,
  • Sun light reflected on the Earth surface (Earthshine).
  • Geocoronal emission originated mainly from the hydrogen and oxygen atoms in the outer part of the Earth's atmosphere (airglow).
  • Sunlight scattered by interplanetary dust grains (zodiacal light).
  • All the other cosmic component (outside the Solar System) usually known just as diffuse radiation background.
are currently ignored by the ETC.


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5. Output

Within this field the user selects the expected return.
  • By selecting ''S/N'' ratio (signal-to-noise), the user retrieves the exposure time (in seconds) to reach the target S/N.
  • By selecting ''Exposure Time'', the user retrieves the S/N ratio for a given exposure time (in seconds).

When the spectroscopy mode is set, the user have also to give the specific wavelength for which the S/N or exposure time should be calculated.


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6. Results

A summary of the input data entered by the user appear in the top part of the page. The numerical results (S/N, exposure time and total signal) appear after that summary.

Three plots will be shown below.

  • The first plot shows the input spectral distribution.
  • The second plot shows the system throughput (telescope+optical system).
  • The third plot shows the output distribution.

The user is also allowed to retrieve these results in ASCII or in html table format.

mainOutput3
Fig 3. Results page

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