[ ESO ]  

VIRCAM Exposure Time Calculator

HOME INDEX HELP

General description

The HTML/Java based interface allows to set the simulation parameters and examine interactively the model generated graphs. The ETC programs allow easy comparison of the different options relevant to an observing program, including target information, instrument configuration, variable atmospheric conditions and observing parameters. Being maintained on the ESO Web servers, the ETCs are regularly updated to reflect the known performance of ESO instruments.

The exposure time calculator consists of two pages.
Input page: The observation parameters page presents the entry fields and widgets for the target 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.
Output page: The results page presents the computed results, including number of counts for the object and the sky, signal-to-noise ratios, instrument efficiencies, etc. The optional graphs are displayed in several formats. In addition a summary of the input parameters is appended to the result page.

Note: These tools are only provided for technical assessment of observation feasibility. Variations of the atmospheric conditions can strongly affect the required observation time. Calculated exposure time do not take into account instrument and telescope overheads. Users are advised to exert caution in the interpretation of the results and to report any result which may be suspected to be inconsistent.

The exposure time calculator models the observation chain which includes the target spectral distribution, atmosphere parameters, instrument configuration, and detector setup. An instrument description for VISTA is available on the instrument page.

Input Spectrum

The following options are available to describe the input spectrum of the target.


Atmosphere

Instrument Setup


Results

The DIT (Detector on-chip integration time per single shot in seconds) is needed as input for all of the following cases:

Specify an exposure time per pixel,
OR
Specify a signal-to-noise ratio to be achieved,
OR
Specify an observing strategy.

The observing strategy constitutes of:

In all cases, the exposure time for each exposure is DIT*Ndit, with the specified DIT. However, the VIRCAM focal plane is not fully covered with detectors and therefore some larger offsets (pawprint offsets) are used to obtain a tile, an image with more uniform coverage of the focal plane. The total exposure time per pixel after taking NxM microstep exposures + Njitter dithered exposures + Npaw pawprint offsets + Nexp exposure loops depends on the Npaw parameter value. In case one chooses Npaw=1, then each pixel will be exposed once for each pawprint offset and the total exposure time per pixel will be given by Ndit*Nexp*NxM*Njitter. The Npaw=3 (Tile3 patterns) result in vertical stripes, where the pixels along these vertical stripes are exposed at least twice (except for the edges of the tile). In case of Npaw=6 each pixel in a final tile will be exposed at least twice (except for the edges of the tile). Therefore most of the pixels in a tile covered with Npaw=3 and Npaw=6 will have total exposure time per pixel defined by Ndit*Nexp*NxM*Njitter*2. For more details please see the VISTA User manual.

The output form will give you estimates for SNR or Exposure Time, together with output graphs you selected.

Do not confuse exposure time (which is the total exposure time per pixel used for calculation of S/N) and total observation time, the latter being a sum of exposure time, including additional offset exposures and overheads in the telescope and instrument. Please consult the user manuals for guidance on the choice of the integration parameters.

Possible graphical Outputs


Version Information


[Observing Facilities and Operations] [ESO] [Index] [Help]