NACO Exposure Time Calculator
The input 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 output 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 as images and interactive Java applets as well as ASCII and PDF formats for further analysis and printing.
This is now supported by the NACO ETCs (imaging and spectroscopy).
In the Target Input Flux Distribution field, you can select a spectral type and filter magnitude for the target. Alternatively, you can choose to specify the target with a blackbody temperature (and a filter magnitude). In spectroscopic mode, you can choose to specify a single emission line instead. In the Reference Source Parameters field, information about spectral type and filter magnitude is entered for the reference source.
In imaging a stellar spectral type is used to make the color correction.
In spectroscopy, a template spectrum is scaled to the provided magnitude.The Pickles template spectra covers only the JHK bands. The target spectrum can also be selected from a subset of MARCS stellar model spectra, kindly provided by Bengt Edvardsson at the Uppsala Astronomical Observatory. The parameter space of the MARCS subsets are listed the following tables. Note that not all models (referring to all possible combinations of parameters) actually exist.
|MARCS subset: Spherical Geometry|
|total (product)||360 (this is the number of possible combinations, but only 87 models exist)|
|MARCS subset: Plane Parallel Geometry|
|total (product)||54 (this is the number of possible combinations, but only 50 models exist)|
You must select the filter and filter magnitude for proper scaling of the template spectrum. Available filters are V, J, H, K, L and M. For extended sources, the magnitude must be given per square arc second.
The target is slightly off the optical axis, because another star is used as reference source. This is particularly useful if the target is very faint, since the quality of the AO correction deteriorates rapidly for reference source magnitudes greater than V=13. The reference source must be within 30 arc seconds of the primary target.
The geometry of the target will affect the signal to noise, since extended sources will cover a wider area of the detector. You can either select:
If point source is chosen, the target object 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, see the NACO instrument page or the user's manual. For imaging, the S/N is given in an area of radius equal to 1.22*lambda/D, which is 70 mas for an 8 m diameter telescope observing in the center of the K band (2.2 microns). In spectroscopy, the S/N reference area is a rectangle with dimensions*1.22 lambda/D the spatial direction and the slitwidth in the perpendicular direction.
If extended source is chosen, the target object is assumed to be an emitter with a uniform intensity distribution. In this case, the S/N on the result page is given per pixel element of the detector.
The seeing at zenith at wavelength 500 nm.
The airmass of the observed target. The airmass must be ≥ 1.
The sky flux is based on a model with absolute flux calibration that takes into account thermal background and OH lines.
The effect of the neutral density filter is to reduce the flux (count rate). Currently only applicable to some filter configurations: those with central wavelengths in range [1.0 - 2.5] microns (1.25% transmission) or in range [2.5 - 5.0] microns (2.00% transmission).
A Wollaston prism system can be placed in the beam to examine the polarization of the target. When used, two images will be produced. Since the Wollaston prism has an efficiency of 90%, each image will have 45% of the light in case the light is NOT intrinsically polarized. The numbers on the result page correspond to one such image. The Wollaston prism cannot be applied with the J filter.
The NAOS dichroic determines the waveband which is diverted from the beam to the wavefront sensor. In combination with the filter, this determines the wavefront sensor to be used. Since the light not transmitted through the dichroic is used by the AO system, it is optimal to use a dichroic that has LOW transmission in the waveband where the reference source has maximum flux, and HIGH transmission in the waveband where the target has maximum flux.
From version 3.2.6, the NACO ETCs make a HTTP call to the NAOS-PS server which returns the AO-preformance estimate to the NACO ETC. This approach makes it possible to have a new dichroic option "FREE" in the ETC; when this option is chosen, the NAOS-PS will always try to optimize image quality / the flux for NAOS, which, in some cases, may not be optimal for the science goals (and then the dichroic should be selected by the user).
Choosing the instrument filter determines in which band the observation will be performed. The magnitude parameters for object and sky are transformed (again, using the corrections given in Effective Temperatures and Intrinsic Colors for Main Sequence(V), Giant (III), and Supergiant (I) Stars by A.T.Tokunaga, and narrow filter corrections) to the observation band selected by choosing an instrument filter.
Here you can choose the objective for the CONICA camera. Five objectives are available,
, with focal ratio of 13, 27 and 54 respectively. The Nyquist sampling depends on the wavelength, thus different focal ratios offer adequate sampling of the PSF by the detector at different wavelength regions. S13 samples the PSF in the J filter, S27/L27 samples PSF in the K filter, and S54/L54 samples the PSF in the M filter.
The ETC does not support the SDI objective, but some information can be found in the NACO user's manual to estimate S/N for the SDI objective.
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.
Since P71, NACO imaging can be performed in two exposure modes, either chopping or non-chopping. The Chopping mode can only be used for observations in M' (mandatory) and L', NB_3.74 and NB_4.05 (optional).
The S/N ratio in chopping mode is approximately reduced by a factor 1/sqrt(2). Please find more details about chopping in the NACO user's manual.
For the chopping mode of NACO, the user selects the total exposure time, in minutes. The DIT (Detector Integration Time) depends on the instrument configuration. It will be set to the relevant minimal DIT. The NDIT (Number of DIT's) is then found from the total exposure time, by dividing with the DIT.
Detector on-chip integration time for one exposure (in seconds). Limits for saturation and non-linearity depend on DIT for small DITs.
Indicate here a S/N 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 .
The total exposure time is therefore DIT x NDIT .
For the purpose of presenting the results of the calculations of the NACO Imaging ETC, you can choose from the following output options:
Toggling this option will display two images (jpg format) of the Point-Spread Function (PSF), the first one with a linear color scale and the second one with a logarithmic color scale. In addition, a 2D FITS file with the PSF will be available for download. Note that these are monochromatic PSFs computed by the NAOS-PS software for the central wavelength of the filter used in the simulation.
The pixel scale in the fits image corresponds to lambda / 2D, with D being the diameter of the telescope. More information on the simulation parameters is available in the FITS header.
Toggling this option will display a plot of the Encicled Energy (EE) as a function of the aperture radius (arcsecs). A cross of two lines will indicate the selected aperture radius r and the value EE(r). The default aperture radius is 1.22*lambda/D (D=8.2m).
Toggling this option will display the S/N as a function of exposure time. For point sources the S/N is calculated in the area of the diffraction limited core, radius equal to 1.22 lambda/D, which is 70 mas for an 8 m diameter telescope observing in the central K band (2.2 microns). For extended sources, the S/N is calculated per pixel element, using the surface brightness in magnitudes per square arcsec.On the results page, the input parameters are repeated, the NACO transmission model parameters are given, sky data is given, and the S/N is plotted. See Text Summary Results for a description of the text output.Toggling both options will display a page with the input parameters, the NACO transmission model parameters, sky data and the graphs described above. Toggling no options will give you only the Text Summary Results.
The text summary results are given on all results pages. Here follows a short description of the items in the text summary results. For spectroscopy, the numbers quoted are for the central wavelength.Source spectral type: The spectral type of the target.
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