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:
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
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;
Estimating elapsed Time (single tile, one filter Overheads)
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.
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):
Jitter pattern (Njitter):
Number of pointings (Npaw):
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.
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.
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.
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.