NACO Exposure Time Calculator

General description

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.

Laser Guide Star (LGS) Status

This is now supported by the NACO ETCs (imaging and spectroscopy).

Target and Reference Source Spectrum

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.

Spectral Type

In imaging a stellar spectral type is used to make the color correction.

In spectroscopy, a template spectrum is scaled to the provided magnitude.

Target / Reference Source Magnitude

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.

Separation of Target and Reference Source

Target Spatial Distribution

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:

• Point Source

• 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.

• Extended Source

• 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.

Atmosphere

• Seeing

• The seeing at zenith at wavelength 500 nm.

Seeing statistics:


The Paranal seeing statistics is based on the so-called UT seeing measurements obtained from the UT1 Cassegrain Shack-Hartmann wavefront sensor used for active optics. The measurements are deconvolved in order to represent the seeing outside the dome (i.e. they are corrected for the instrument+telescope resolution).

These data come from http://www.eso.org/gen-fac/pubs/astclim/paranal/seeing/singcumul.html

• Airmass

• The airmass of the observed target. The airmass must be ≥ 1.

• Sky

• The sky flux is based on a model with absolute flux calibration that takes into account thermal background and OH lines.

Instrument Setup

• Neutral Density Filter

• 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).

• Wollaston prism system

• 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.

• NAOS WFS/Dichroic

• 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).

• CONICA Filter

• 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.

• CONICA Objective

• Here you can choose the objective for the CONICA camera. Five objectives are available, S13 , S27/L27 and S54/L54 , 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.

• Spectroscopy

• In spectroscopy mode, the Wollaston prism and the neutral density filter are not available, only cameras S54/L54 and S27/L27 can be used, and you must select the grism/filter combination and the slit width. For more information, please refer to sect. 4.4.2 of the NACO user's manual.

Results

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.

• Total exposure time

  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.

• DIT

  Detector on-chip integration time for one exposure (in seconds). Limits for saturation and non-linearity depend on DIT for small DITs.

• BLIP integration time

  The so-called BLIP integration time (Background LImited Performance) is the DIT for which the photon noise from the background equals the readout noise contribution of the detector (RON²). Definition: BlipTime = RON² / BackgroundPerPixelPerSecond (with the RON and background flux in electrons).
  
  
  • In general, the S/N is readnoise-limited, when DIT < BlipTime. (The ETC will issue a warning).
  • To be truly background-limited, set DIT < BlipTime.
  
  Note that in the ETC, the unit of the listed value "Total bkg. value" is e- per pixel per DIT (not per second).

• S/N Ratio

• 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.

• Exposure Time

• The Exposure Time is the product of DIT and NDIT .

  • DIT is the detector integration time (in seconds)
  • NDIT is the number of exposures of duration DIT for a saved image. The NDIT is rounded up to the nearest integer value and this value is used when calculating the S/N. If the quoted NDIT value is zero, the target star is extremely bright and will be overexposed.

  The total exposure time is therefore DIT x NDIT .

• Plotting Range

• The Range box sets up the computation interval for the output curve of S/N versus exposure time. The percentage is the interval (in % of the total exposure time) shown beyond the end of the exposure time interval.

Possible Outputs in Imaging

For the purpose of presenting the results of the calculations of the NACO Imaging ETC, you can choose from the following output options:

Show 2-D PSF image (jpg and FITS)

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.

Encircled Energy as a function of Aperture Radius

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).

S/N as a function of Exposure Time

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.

Text Summary Results

All magnitudes are in the Vega system unless explicitly indicated otherwise

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.

• FowlerNsamp(HighSensitivity) FowlerNsamp and HighSensitivity (For 1 to 2.5 micron observations)
• Double_RdRstRd(HighDynamic) Double_RdRstRd and HighDynamic (For 1 to 2.5 micron observations)
• Uncorr(HighDynamic) Uncorr and HighDynamic (For observations in NB_3.74 and NB_4.05)
• Uncorr(HighWellDepth) Uncorr and High HighWellDepth (For observations in L')
• Uncorr(HighBackground) Uncorr and HighBackground (For observations in M')

NACO Transmission Model:

Version Information

• Version 6.0.0 (February 26, 2015) A unified convention for seeing and image quality applied in all ETCs offered from CfP P96.
  Spectroscopy ETC: A new advanced sky model enabled. The Imaging ETC uses a simpler sky background model.
• Version 5.0.0 (July 1, 2013)
  All ETC version numbers have been aligned at 5.0.0 as the ETC system infrastructure was re-factored and installed on a new web server. Due to implementation changes, some numerical results may differ slightly from previous versions. Please report significant discrepancies to the ESO user support group usd-help@eso.org
  
  

Send comments and questions to usd-help@eso.org