NACO Exposure Time Calculator Help Page

Important notes and bug reports

Please note the version information at the bottom of this page.

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

General description

The NACO ETC is an exposure time calculator for the ESO high-resolution near-infrared camera, CONICA, which uses the Nasmyth Adaptive Optics System, NAOS. 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. The ETCs are maintained on the ESO web servers to always provide up-to-date information reflecting the known performance of ESO instruments.

These programs provide an HTML based interface and consist of two pages. The observation parameters page presents the entry fields and widgets for the target and reference source 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. 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 within Java applets allowing interactive manipulation. The results are also provided in ASCII and GIF formats for further analysis and printing. Finally, a summary of the input parameters is appended to the result page.

Instrument descriptions are available for NACO

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

The target model can be defined by the target's spectral type. It uses a template spectrum, which is scaled to the provided magnitude and filter. The spectral type is used to make the color correction. Currently, template spectra are only covering JHK bands (wavelength < 2.5 microns).

• 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.

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:

• 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.

• Seeing

The seeing FWHM in V band, at Zenith(airmass=1). NACO is the only instrument that requests users to enter the seeing at Zenith in their phase I proposals.

• Airmass

The airmass at which the observation is performed.

• 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.

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.

• Chopping / Non-chopping modes

  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.

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.

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.

• 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
  Source magnitude: The magnitude of the target in the chosen filter, in magnitudes for point sources, and magnitudes per square arc second for extended sources.
  Source geometry: Geometry of target (point source or extended).
  Reference Source separation: Separation of reference source and target in arc seconds.
  Reference Source spectral type: The spectral type of the reference source.
  Reference Source magnitude: The magnitude of the reference source.
  Reference Source geometry: Geometry of the reference source (always point source)
  Seeing: The selected seeing (FWHM).
  Sky Magnitude: The sky magnitude in the selected filter, in magnitudes per arc second, at Paranal.
  Sky Temperature: The sky temperature in Kelvin, on an average night at Paranal.
  Sky Variability: Variability of the sky brightness (in magnitudes).
  NAOS Wavefront sensor: The selected wavefront sensor (visible / IR)
  NAOS Transmission: Name of wavefront sensor and transmission in %.
  CONICA Filter: The used CONICA filter.
  CONICA Grism: The used CONICA grism (spectroscopy only).
  CONICA Slit: The used CONICA slit (spectroscopy only).
  CONICA Objective: The used CONICA objective.
  Detector Read-out mode: The used detector read-out mode. We offer the five combinations of read-out mode and detector mode:
  • 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')
  User Requested: The user requested exposure time or S/N setting.
  DIT: Detector integration time for one exposure.
  S/N: Selected signal-to-noise ratio
  NDIT: Number of detector integrations.
  NACO Transmission Model:
  Signal-to-noise: The signal to noise is given for a target of the selected magnitude, in the filter selected and for the selected exposure time or S/N settings.
  Total exposure time: NDIT*DIT, in seconds and without overhead, is the time required to reach the signal-to-noise quoted above.
  Number of detector integrations: NDIT, is the number of detector integrations needed to reach the signal-to-noise quoted above.
  Radius of the S/N reference area: Imaging: The radius in arc seconds of the ref. area for the quoted S/N value.
  Spectroscopy: The spatial extent of the rectangular S/N reference area. The slitwidth is the extent in the perpendicular direction.
  Number of pixels...: Same as above, but quoted in pixels.
  Total number of e-...: The number of photo-electrons on the detector array within the PSF area, counted in one DIT
  Detector Saturation and Linearity Limits: Saturation refers to the maximum number of photo-electrons that can be counted in one pixel. Additional photo-electrons are either lost, or bleed out into neighboring pixels. The Linearity Limit refers to the max. counts for which the detector response is proportional to the received photo-electrons.
  In some cases, the linearity- and saturation levels is observed to depend on DIT (for small DITs, when the ratio DIT/DIT_min is less than a few). In practice, this is implemented the following way, currently only for the readmode FowlerNsamp(HighSensitivity), although it may also be relevant in other cases.
  
  • if ( DIT/DIT_min < 2) then Satlim and LinLim are both halved
  • if ( 2 < DIT/DIT_min < 4) then Satlim and LinLim are linearly interpolated, to be between 1/2 and 1 times Satlim and LinLim, respectively
  • if ( DIT/DIT_min > 4) then Satlim and LinLim are not changed
  Encircled Energy: This is the fraction of light within the considered PSF area. This number must be between 0 and 1, and quantifies the efficiency of the AO system. The higher the number, the better is the correction. In spectroscopy, this is effectively the slit transmission factor.
  Strehl Ratio: Ratio of the peak intensity of the observed PSF to that of a perfect diffraction limited PSF.
  Pixel Scale...: The sky-projected angular size of a pixel element (pixel side).
  Overall Transmission: The total transmission coefficient of the NACO optical system. In spectroscopy, this number does not include the slit transmission factor.
  Sky Variability Table: This table contains some of the information given above, but for three different sky brightness cases (imaging only):
  Average: Average sky brightness.
  Bright Sky: Numbers in this column correspond to a sky brighter than an average night sky, by an amount equal to the sky variability.
  Dark Sky: Numbers in this column correspond to a sky darker than an average night sky, by an amount equal to the sky variability.

  In the last row of the table, the so-called BLIP integration time for each sky brightness is displayed.

Version Information

• Version 3.2.6 (June 16, 2008)

  The NACO ETCs (imaging and spectroscopy) now obtain the AO-information by calling the NAOS-PS server over the internet. Using this approach more realistic AO-performances are obtained, in particular because the dependency on airmass is considered.
  
  The LGS AO-mode is now supported.
  
  A new dichroic choice "FREE" has been introduced; with that option the NAOS-PS will try to find an optimum dichroic for the configuration.
  

• Version 3.2.1 (Mar 19, 2008)

  On Wednesday, 19 March it was discovered that a bug in the NACO ETC caused off-axis S/N values to be systematically overestimated (that is, cases where the target and AO reference objects differed). This bug only affected the imaging ETC. This bug has now been fixed.

• Version 3.2.1 (Mar 7, 2008)

  Spectroscopy and Extended sources only: Irrelevant AO information removed from result page.

• Version 3.2.1 (Aug 31, 2007)

  Graphs improved.

• Version 3.2.0 Imaging (July 30, 2007)

  Bug fix: A wrong error message prevented use of the NB_4.05 filter.

• Version 3.2.0 (June 21, 2007)

  Prepared for variable aperture in NAOS-PS interfaced version (not yet supported by NAOS-PS). Backward compatible.

  Spectroscopy: Optional sky transmission spectrum added.

• Version 3.1.3 (March 20, 2007)

  Imaging (via NAOS-PS):Bug fix: When the NACO imaging ETC is invoked via the NAOS-PS software, the peak pixel value was wrong in the previous version.
  

• Version 3.1.3 (March 7, 2007)

  Imaging:Implemented variable aperture radius.
  

• Version 3.1.2 (August 31, 2006)

  Imaging:AO-database updated.
  

  Interface with the NAOS-PS software updated.
  

• Version 3.1.1 (August 31, 2006)

  Imaging: The option to display the 2D Point-Spread Function (PSF) (FITS file and jpg image) is now re-enabled and updated (showing an additional jpg image with logarithmic color table).
  

• Version 3.1.1 (August 18, 2006)

  Spectroscopy: Corrected a bug in the integration of PSFs over the slitwidth and central pixel.
  The effect of the bug in earlier versions was that the fraction of object flux entering the slit (and consequently the estimate of object counts) has been too high, typically 10-15%. The resulting S/N ratio was thus slightly over-estimated. The peak counts (the maximum detected counts in the central pixel) can have been over-estimated by up to a factor 2.

• Version 3.1.0 (July 25, 2006)

  The parameter space of the (NGS) AO-database has been extended and finer sampled, i.e. now offering more choices of the AO-input parameters GS/target separation and GS spectral type.

  Added note about LGS status.

• Version 3.0.7 (January 2006)

  The ETC has been renamed from "CONICA" to "NACO".

• Version 3.0.6 (September 2005)

  Limits for saturation and non-linearity depend on DIT, for small DITs. Currently only in read-mode FowlerNsamp(HighSensitivity)).
  
  Imaging ETC:
  BLIP integration time info added.
  
  Spectroscopy ETC:
  Added spectroscopic modes and removed deprecated modes.
  Atmosphere model updated.
  Bug fixed in the scaling of the graph of object counts.
  

• Version 3.0.5 (March 3, 2005)

  Spectroscopic ETC:
  S/N formula corrected. In most cases the S/N values have only changed marginally.

• Version 3.0.4 (January 7, 2005)

  Spectroscopic ETC:
  Corrected a bug: the transmissions of dichroics N20C80 and N90C10 were not considered correctly in previous versions.
  Improved model for background; now considering thermal emission of dichroics.
  New file for telescope efficiency.
  New file for atmosphere transmission.
  Bug fixed: The band in which the input magnitude is given was not considered.
  
  Imaging ETC: Code updated, but no changes expected.

• Version 3.0.3 (September 13, 2004) Spectroscopic ETC

  Corrected a bug for input spectrum of type Blackbody: The user-provided temperature value was ignored. The bug is fixed in this version.
  For an input spectrum of type Single Line, the default value of the FWHM line width changed from 1nm to 2nm.

• Version 3.0.2 (August 2, 2004) Spectroscopic ETC

  Enabled singleline input option. Corrected a general bug (index of reference in spectra was offset by 1 pixel).
  Since the ETC displays the values of signal from object and sky at central wavelenth of the chosen spectral setting, a consequence of the correction is that these values are altered with respect to the previous version. Peak value and S/N are also different compared to version 3.0.1.

• Version 3.0.1 (July 28, 2004) Spectroscopic ETC

  Now validating the chosen combination of parameters.
  Corrected a problem with template input spectra.
  

• Version 3.0.0 (July, 2004)

  Spectroscopic ETC:

  New model.
  

  Imaging ETC:

  New Aladdin3 detector.
  

• Version 2.9.3 (March 8, 2004)

  Spectroscopic ETC:

  11 new grism/filter modes.
  Added option to show plot of object spectrum.
  Computation of max. intensity temporarily disabled.
  

• Version 2.9.2 (December 19, 2003)

  Improved implementation of linearity level checking.
  PSF database performance improved.
  Removed irrelevant PSF information in case of extended sources.
  Exposure times for chopping templates updated.

• Version 2.9.1 (August 26, 2003)

  Spectroscopy mode only: Corrected formula for the max intensity at central pixel.
  

• Version 2.9.0 (August 4, 2003)

  Sky backgrounds in bands J,H adjusted. Values of typical variablity adjusted in all bands.
  Spectroscopy mode released.
  Imaging: Blackbody source emission distribution added, more restrictive criteria for detector linearity/saturation, dark current value adjusted.

• Version 2.8.5 (February 21, 2003)

  Updated version for P71

• Version 2.8.1 (March 01, 2002)

  Calibrated version for Phase I, P70

• Version 0.2 (June 27, 2001)

  PS supported & dichroic information added

• Version 0.1 (Feb. 12, 2001)

  Delta Version

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