![[ ESO ]](/images/eso-logo.gif){kind=logo}
ESO Optical Instrument Simulator
| HOME | INDEX | SEARCH | HELP | NEWS |
|
|||||
|---|---|---|---|---|---|
ESO Optical Instrument Simulator | |||||
|
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.
See also: Frequently Asked Questions
The Optical ETC is an exposure time calculator for the ESO optical instruments SUSI, EMMI, VLT Test Camera, FORS, and the WFI (Wide Field Imager on the 2.2m in La Silla). 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. As the ETCs are maintained on the ESO Web servers, they provide the most up-to-date information reflecting the known performance of ESO instruments.
These programs provide an HTML/Java based interface and consist of two pages. The observation parameters 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 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.
The model includes an input spectrum (e.g. a template star spectrum), atmospheric parameters , optical instrument path and observation criteria. The model generates output graphs describing the spectral illumination of the CCD, the instrument efficiency or the signal to noise as a function of the exposure time or seeing.
The target model is a spectral distribution constant with the wavelength.
The target model is a blackbody defined by its temperature and monochromatic apparent magnitude at a given wavelength. Temperature is expected in Kelvin and wavelengt in one of the band filters U, B, V, R or I.
The target model can be defined by a template spectrum . As with the blackbody it will be scaled to the provided magnitude and band filter U, B, V, R or I.
Seeing limited sources are point-like sources. The signal to noise is computed over an area of diameter twice the seeing.
Indicate the object magnitude in the broad band filter associated to the filter that you define in the Instrument Setup. The effective central wavelengths are respectively B=440nm, V=550nm, R=640nm, I=790nm. The spectrum is scaled after integration in the corresponding photometric filter using the formula log10(F) = -0.4*M-Z, with M the magnitude in the band, F the monochromatic flux in W/m2/micron, and Z the zero point. The photometric zero points and effective wavelengths for (B,V,R,I) filters are taken from the reference: Bessel, 1979, PASP, 91, 589. The values used are Z(U)=7.379, Z(B)=7.180, Z(V)=7.442, Z(R)=7.64, Z(I)=7.91.
The signal to noise for extended sources is given per pixel on the detector. The magnitude is given per square arcsecond. Extended sources are supported only in imaging mode.
Here you define the sky brightness in mag/arcsec2. The following table (Walker 1987, NOAO Newsletter) provides a reference to the observed sky brightnesses versus moonphase.
Note for MIT-LL CCD: Since the MIT-LL CCD has a response shifted toward the red, the ETC is correcting the listed general sky magnitudes in I- and z-bands:| Days from new moon |
Sky Brightness | |||||
|---|---|---|---|---|---|---|
|
U |
B |
V |
R |
I |
z |
|
|
0 |
22.0 | 22.7 | 21.8 | 20.9 | 19.9 | 18.8 |
|
3 |
21.5 | 22.4 | 21.7 | 20.8 | 19.9 | 18.8 |
|
7 |
19.9 | 21.6 | 21.4 | 20.6 | 19.7 | 18.6 |
|
10 |
18.5 | 20.7 | 20.7 | 20.3 | 19.5 | 18.3 |
|
14 |
17.0 | 19.5 | 20.0 | 19.9 | 19.2 | 18.1 |
The airmass can be computed with the Sky Calendar Tool page.
The seeing is given in arc seconds.
Here you will see the combined efficiency of the M1-M3 mirrors.
Set the corresponding FORS collimator resolution (Standard or High)
The filter is selected from a list with an option menu. Information on the filters is available on the Web page and in the instrument user manual: EMMI, SUSI2, FORS, WFI, EFOSC2
This option reduces the transmission by 50% to take the polarisation Wollaston prism into account
Select the disperser. Characteristics of the FORS grisms are described in the FORS manual . Echelle modes can be selected in the REMD EMMI mode. See EMMI manual for details. This model computes results corresponding approximately to one Echelle order centered at the wavelength selected by the user. The blaze function is not taken into account in this model. Here is a link to the instrument webpages for EFOSC2.
Select the slit width from the list of predefined values.
The detector for all instruments are listed on the detector characteristics page . The simulation takes into account different read-out noise levels in fast and slow read-out mode. For details see the User Manual.
A curve of signal-to-noise as a function of exposure time is generated. The centre value and a range to both sides of the centre value are provided.
A curve of signal-to-noise as a function of exposure time is generated. The centre value and a range to both sides of the centre value are provided (in seconds).
The result graphs are Java based applications. A static version of the graphs is also provided in GIF and ASCII format.
Source Geometry: "Point source" or "Extended" depending on the source geometry. Some results are computed differently for point-sources ort extended sources.
Probability of realisation of seeing: The probability for the seeing to be better or equal than the specified value.
Signal to Noise over PSF area: Signal to noise computed using the formula: S/N = object_signal / sqrt (object_signal + sky_signal*Npsf + Npsf*CCDnoise**2)
Number of pixels for PSF area: For point-sources this is the area over which the S/N is estimated. It is a circular area of radius given by the seeing divided by the plate scale. This value corresponds to "Npsf" in the signal-to-noise formula.Plate scale: The plate scale of the system, in arcsecs per pixel.
Electrons in the PSF area: The total flux contribution from the object, integrated over the PSF area, and expressed in electrons. This value corresponds to "object_signal" in the signal-to-noise formula.
Sky background value: The flux contribution from the sky for one pixel of the detector. This value corresponds to "sky_signal" in the signal to noise formula.
Detector read-out noise level: CCD read-out noise in electrons/pixel. This value corresponds to CCDnoise in the signal-to-noise formula.
Peak pixel value: This value is the sum of the sky background level (sky_signal) and the fraction of the object signal falling on one pixel at the center of the seeing profile (object_signal_max).
Detector saturation: The detector saturation level. A message will be displayed if the maximum intensity is greater than this limit. Please note that the actual saturation level may depend on the CCD readout-mode.
PSF extension: number of pixels over which the signal-to-noise is estimated. This value is computed as twice the seeing divided by the plate scale.
Signal to Noise at central pixel: Signal to noise on the central pixel, computed using the formula: S/N = object_signal_max / sqrt (object_signal_max + sky_signal + CCDnoise**2). Only this signal to noise is computed for extended sources.
The input flux distribution for the selected target is diplayed in units of 1e-20 ergs/cm**2/s/A
This option will display a curve showing the efficiency in terms of detected photons against wavelength.
Toggling this option will display the object spectrum as seen by the detector, in units of e-/Angstrom/sec
Toggling this option will display a curve showing the evolution of Signal to Noise Ratio against seeing in arcseconds.
Toggling this option will display a curve showing the Signal to Noise Ratio as a function of Exposure Time.
Spectroscopy results such as efficiency, signal, signal-to-noise estimates are dependent on the wavelength and given over the wavelength range in graphics form. A summary of results is provided in text form for the central pixel of the range (also corresponding to the central wavelength).
Wavelength Range: The respective wavelength associated to the first and last pixel of the detector for
the given configuration and dispersion, in nanometers.
For the Echelle modes of the spectroscopic EMMI ETC, the wavelength range corresponds approximately to one Echelle order, centered at the user-specified central wavelength. The blaze function is not taken into account in this model.
Central Wavelength: The wavelength of the central pixel, in nanometers.
Dispersion: The dispersion of the spectrum, in nanometers per pixel.
Plate scale: The plate scale of the system, in arcsecs per pixel.
Probability of realisation of seeing: The probability for the seeing to be better or equal than the specified value.
FWHM of the seeing spatial profile: The full-width at half-maximum of the slit spatial profile. This value is the seeing divided by the plate scale.
Efficiency at central wavelength: Total efficiency of the system at central wavelength, including atmospheric extinction, telescope transmission, optics and detector efficiency, in percent.
Object signal at central pixel: The total flux contribution from the object, integrated over the slit, and expressed in electrons per pixel along the dispersion direction. The value is given at the central wavelength and corresponds to "object_signal" in the signal-to-noise formula.
Sky background level at central pixel: The flux contribution from the sky for one row along the dispersion direction, in electrons per pixel along the dispersion direction. The value is given at the central wavelength and corresponds to "sky_signal" in the signal to noise formula.
Max. intensity at central pixel (object+sky): This value is the sum of the sky background level and the fraction of the object signal falling on one pixel at the center of the slit profile.
AD/Detector saturation level: The truncation level of the AD converter for the default gain mode. A message will be displayed if the maximum intensity is greater than this limit. Please note that the actual saturation level may depend on the CCD readout-mode, and that the saturation is here tested only for the central wavelength.
Detector read-out noise level: CCD read-out noise in electrons/pixel. This value corresponds to CCDnoise in the signal-to-noise formula.
Detector dark current: CCD dark current in e-/pixel/hour. This value corresponds to DarkCurrent in the signal-to-noise formula.
PSF extension: number of pixels over which the signal-to-noise is estimated. This value is computed as twice the seeing divided by the plate scale. This value corresponds to "Npsf" in the signal-to-noise formula.
Signal to Noise at central pixel: Signal to noise at central wavelength, computed using the formula: S/N = object_signal / sqrt (object_signal + sky_signal*npsf + Npsf*DarkCurrent*ExpTime + Npsf*CCDnoise**2).
The total integrated counts contribution from the object, in e-/pixel. The integration is done along the slit. The counts are expressed in electrons per pixel along the dispersion direction.
The sky contribution on each row of the detector, in e-/pixel. This value is not integrated along the slit.
The input flux distribution for the selected target is diplayed in units of 1e-20 ergs/cm**2/s/A
Toggling this option will display a curve showing the evolution of Signal to Noise Ratio against wavelength.
This option will display a curve showing the total efficiency of the system, and a second graph showing the dispersion relation.
Produces a FITS file with the 2D simulated spectrum as seen on the detector
In the optical imaging ETCs, the "no filter" option has been removed from the filter list. V is now the default filter.
Corrected list of offered grisms on FORS: Added "300I+17" and removed "grism 600I+25".
Added support for "merged" FORS ETCs "FORS(P83+)", in addition to existing FORS1 and FORS2 ETCs.
Bug fixed in case of single emission lines in the optical spectroscopy ETCs.
By mistake, the numeric results referred to the central wavelength of the spectral setting, instead of the wavelength of the emission line as they should.
The results have been null in all cases except if the emission line has happened to be close to the central wavelength of the spectral setting.
For other input types, the ETCs are completely unchanged.
EFOSC2: Moved from 3.6m to NTT telescope. Additional detector binning modes
FORS1: Transmission adjusted to match observed zero points.
Code optimized.
Corrected a bug in the validation of wavelength range.
FORS1 ETC spectroscopy: Warning note added: S/N<15 for λ>650nm. See the FORS User Manual P80 section 2.8.2 for details.
FORS1 ETCs (ima and spect.) updated for P80: E2V detector supported, list of filters and grisms changed.
EMMI Spectroscopy: Corrected efficiency profiles for REMD grating#6 and grating#7.
WFI imaging: Added filters.
FORS1/2 spectroscopy: Corrected general wavelength offsets due to off-center slit positions.
FORS1/2 spectroscopy: Corrected central wavelength for grism 150I with order sorting filter < 590nm.
FORS1 spectroscopy: Added VPH grism (Grism_1200B+97)
| |||
![[Observing Facilities and Operations]](/images/icon-observing.gif){kind=icon}
![[ESO Exposure Time Calculators]](/images/icon-up.gif){kind=icon}
![[ESO]](/images/icon-home.gif){kind=icon}
![[Index]](/images/icon-index.gif){kind=icon}
![[Search]](/images/icon-search.gif){kind=icon}
![[Help]](/images/icon-help.gif){kind=icon}
![[News]](/images/icon-whatsnew.gif){kind=icon}