Exposure Time Calculator (ETC) Description

• all of this page,
• the recommended values to use for DIT,
• the ETC Help,
• the ETC Release Notes
• the VISTA Overview,

The actual system peformance may ultimately differ from that assumed in the ETC.

Ultimately it is expected that the ESO VISTA ETC should be used, however at present there is a discrepancy between them which is in the porcess of being resolved. Users are advised the VDFS ETC (v1.4) gives plausible nos. for the broadband filters and that apparently the largest differences with the ESO ETC are due to the way the expected sky background level is computed in the ESO version.

To ensure you are informed of any significant errors that may be found in the Exposure Time Calculator please joining the mailing list

Estimating on-Sky Observing Time

The Exposure Time Calculator can be used in three modes (see Observing Setup section of ETC) to determine

First set the parameter

Detector on-chip integration (DIT): seconds.

to your chosen value - a separate page gives recommended values to use for DIT - for modes 1 and 2 below you may wish to start with the maximum recommended values as these will minimize the effect of read noise (a total object exposure of Y secs with DIT=X secs will produce Y/X lots of read noise) and give you a lower limit on the time needed.

Mode 1: click

Object exposure time: seconds

where the exposure time is given (60 sec in this example) and the signal to noise reached in this time on source of a given magnitude is calculated - using a different value for DIT will give a (slightly) different result due to the amount of readnoise;

Mode 2: click

S/N Ratio:

where the signal to noise is given and the exposure time needed on source of a given magnitude is calculated - using a different value for DIT will give a (slightly) different result due to the amount of readnoise;

Mode 3:

Estimating elapsed Time (single tile, one filter Overheads)

click

Observing Strategy:

where the adopted total exposure time can be split down into an observing strategy for a single tile in a single filter, and the ETC then returns the total on sky time and the elapsed time (on sky time + overheads) so that the observing strategy can be adjusted to maximise the survey efficiency.

The strategy involves breaking down the total on source time (as found with either the "Object exposure time" or "S/N Ratio" options above) into a number Ndit of individual integrations, each DIT seconds long, with the Ndit*DIT long exposures repeated Nexp times with a number of jitters, Njitter, and if required a NxM microstep pattern. Npaw will usually be left at the default value of 6.

A separate page gives recommended values to use for DIT which can be input to the ETC as

Detector on-chip integration (DIT): seconds.

The other parameters which must be set for the overheads mode are shown (in the ETC) as

Exposure coadds (Ndit):
Exposure loops (Nexp):
Microstepping pattern (NxM): 1x1 2x2
Jitter pattern (Njitter):
Number of pointings (Npaw):

Bad pixels

A typical VISTA detector has ~2% 'bad' pixels. Those of you that did not plan to jitter should carefully consider if this is indeed the correct strategy. ESO may well want to see a justification (agreed with those who pipeline the data) of how bad pixels will be handled in any proposal that decides not to jitter, and any resulting effect on achieving the science (& legacy) goals.

Seeing

The ETC "seeing" input should be the FWHM of the PSF seen at the detector in the filter you are using.

• the ETC does NOT convolve the atmospheric seeing with the FWHM of the instrument PSF (instrument PSF expected ~0.45" - but spec is 0.5").
• the ETC does NOT scale seeing the (wavelength dependent) atmospheric seeing from 0.5mu (at which the ESO DIMM seeing monitor reports seeing - see www.eso.org/paranal/site/paranal-figs.html) to any other wavelength.

QE

There is a variation of Quantum Efficency between different detectors

Aperture Correction (loss)

When examining the ETC output note that the "aperture correction (loss)" reflects the amount of the magnitude of the object that was taken not to lie within the specified aperture assuming a Moffatt profile (Moffatt 1969, A&A 3, 455) with beta=2.

e.g. In 0.8" seeing, in a 2" aperture and a point source the ETC tells you that the aperture correction is 0.35mag.

Suppose then you input a magnitude 16 object and 0.8" seeing and 2" aperture - the ETC will calculate the signal to noise of a 16+0.35=16.35 mag object all of whose flux falls within the aperture.

If you are familiar with the terms being used

Run Exposure Time Calculator