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Echelle Spectroscopy ETCs |
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The model includes an input spectrum (e.g. a template star spectrum), atmospheric parameters , optical instrument path and an observation criterium. The model generates as default the spectral format in a table format reporting for each order number the wavelength of the central column, its y position in pixel units and arcseconds units, the Free Spectral Range (FSR) size, minimum and maximum wavelength, the order starting and ending wavelength and size (Template Spectra range). If required the ETC also calculates for each significative order (in the single line case only for the order where is detected the line) the total efficiency (% units), the Object, Sky and maximum expected counts (in electron units) and the Signal to Noise ratio for three points, the FSR minimum, central column and FSR maximum wavelength. For the wavelength of the central column it is reported also the wavelength value and the spectral bin size (over which the Object, Sky, Imax counts are integrated). The information contained in the spectral format table, relative to the central column wavelength value, can be displayed also in form of graph selecting the appropriate check button in the input page. In this case the ETC generate applet graphs and links to the corresponding data in ASCII format and in gif images format.
The target model can be defined by a spectral template spectrum.
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 | ||
| Parameter | Number of unique values |
Unique Values |
| model | 1 | "st" |
| [Fe/H] | 4 | -4.00,-2.00,-1.00,0.00 |
| Teff/K | 9 | 4000,4500,5000,5500,6000,6500,7000,7500,8000 |
| log(g) | 5 | -0.50,0.00,1.00,2.00,3.50 |
| geometry | 1 | "s" |
| microturbulence | 1 | 2 |
| mass | 2 | 1,5 |
| total (product) | 360 (this is the number of possible combinations, but only 87 models exist) | |
| MARCS subset: Plane Parallel Geometry | ||
| Parameter | Number of unique values |
Unique Values |
| model | 1 | "st" |
| [Fe/H] | 6 | -1.00,-2.00,-4.00,0.00,0.50,1.00 |
| Teff/K | 9 | 4000,4500,5000,5500,6000,6500,7000,7500,8000 |
| log(g) | 1 | 4.00 |
| geometry | 1 | "p" |
| microturbulence | 1 | 2 |
| total (product) | 54 (this is the number of possible combinations, but only 50 models exist) | |
Users can upload a file with the spectral flux distibution to the ETC server.
Supported formats:
In both cases the values in the first column should be the wavelength in nm units and ascending order; the second column is the the flux density in a unit proportional to erg/cm2/s/A.
The absolute flux scale is not significant since the spectrum will be scaled to the given magnitude in the given band. The maximum file size is 2 MB.
NOTE! The wavelength range of the uploaded spectrum must cover the spectral range of the selected instrument mode as well as the wavelength range of the photometric band in which the magnitude is given.
The target model is a Planck blackbody spectrum defined by the temperature T
The target model is a power law spectrum of the type F(λ) ∝ λp, where p is the spectral index.
In all of the above cases, the object spectrum is scaled to the given broad-band magnitude after integration over the given photometric band, using photometric zeropoints and bandpass profiles as listed here. When a redshift is applied to the spectrum, it is redshifted before it is scaled. Magnitudes are given per arcsec2 for extended sources
In this case the source is a single emission line of characteristic wavelength, FWHM, and Flux in 10-16 ergs/s/cm2 for point sources or 10-16 ergs/s/cm2/arcsec2 for extended sources.
Seeing limited sources are point-like sources.
The signal to noise for extended sources is given per wavelength bin on the detector (as indicated in the output table). The magnitude is given per square arcsecond. The detected counts reported on the output table integrates over the solid angle omega determined by the product of PSF (in arcsec) and the slit width (in arcsec).
With the advent of instruments using new adaptive optics (AO) modes, new turbulence parameters need to be taken into account in order to properly schedule observations and ensure that their science goals are achieved. These parameters include the coherence time and the fraction of turbulence taking place in the atmospheric ground layer, in addition to the seeing. Starting from Period 105, the turbulence constraints are standardised to the turbulence conditions required by all instruments and modes, whether they are seeing-limited or AO-assisted.
The handling of atmospheric constraints thus changes for both Phase 1 (proposal preparation) and Phase 2 (OB preparation). In Phase 1, the seven current seeing categories are replaced by seven turbulence categories for all instruments. Each category can be defined by other parameters than a pure seeing threshold, depending on the instrument. For all instruments, all categories share the same statistical probability of realisation, which is key for an accurate time allocation process. In Phase 2, the image quality will still be the only applicable constraint for seeing-limited modes, whereas the same turbulence category as for Phase 1 will be used for diffraction-limited modes.
Users are encouraged to read the general description of these changes for Phase 1 and Phase 2 on the Observing Conditions webpage, as well as instrument User Manuals for specifics per instrument.
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The definitions of seeing and image quality used in the ETC follow the ones given in Martinez, Kolb, Sarazin, Tokovinin
(2010, The Messenger 141, 5)
originally provided by Tokovinin (2002, PASP 114, 1156) but corrected by Kolb (ESO Technical Report #12):
Seeing is an inherent property of the atmospheric turbulence, which is independent of the telescope that is observing through the atmosphere; Image Quality (IQ), defined as the full width at half maximum (FWHM) of long-exposure stellar images, is a property of the images obtained in the focal plane of an instrument mounted on a telescope observing through the atmosphere. The IQ defines the S/N reference area for non-AO point sources in the ETC. With the seeing consistently defined as the atmospheric PSF FWHM outside the telescope at zenith at
500 nm, the ETC models the IQ PSF as a gaussian, considering the gauss-approximated transfer functions of the atmosphere, telescope and
instrument, with s=seeing, λ=wavelength, x=airmass and D=telescope diameter:
For fibre-fed instruments, the instrument transfer function is not applied. The diffraction limited PSF FWHM for the telescope with diameter D at observing wavelength λ is modeled as:
\(F_{\text{Kolb}} \) is the Kolb factor (ESO Technical Report #12):
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The sky background model is based on the Cerro Paranal Advanced Sky Model, also for instruments at la Silla, except for the different altitude above sea level. The observatory coordinates are automatically assigned for a given instrument.
By default, the airmass and moon phase parameters are entered manually. The sky model will use fixed typical values for all remaining relevant parameters (which can be seen in the output page by enabling the check box "show skymodel details").
Alternatively, a dynamic almanac widget can be enabled to facilitate assignment of accurate sky model parameters for a given target position and time of observation. The sky radiation model includes the following components: scattered moonlight, scattered starlight, zodiacal light, thermal emission by telescope and instrument, molecular emission of the lower atmosphere, emission lines of the upper atmosphere and airglow continuum.
The almanac is updated dynamically by a service on the ETC web server, without the need to manually update the web application.
Notes about the algorithms, resources and references for the almanac are available here. A more advanced version of the almanac is included in our SkyCalc web application, which provides more input and output options.
Hovering the mouse over an input element in the almanac normally displays a pop-up "tooltip" with a short description.
The upper left part of the almanac box refers to the date and time of observation.
This can be done with a UT time or a MJD. A date/time picker widget will appear when
the UT input field is clicked, but the UT can also be assigned manually. In any case, the
UT and MJD fields are dynamically coupled to be mutually consistent.
The two +/- buttons can be used to step forward or backward in time by the indicated step and unit per click. The buttons can be held down to step continuously until released.
The third of night corresponding to the currently selected time is indicated. This is an input parameter to the airglow component in the sky model. Twilight levels (civil, nautical and astronomical) referring to the sun altitude ranges are also indicated in the dynamic text. These levels refer to the sun altitude:
The target equatorial coordinates RA and dec can be assigned manually in the two input
fields or automatically using the SIMBAD resolver to retrieve the coordinates.
If the lookup is successful, an "info" link will open a window in which the raw SIMBAD response can be inspected.
The units can be toggled between decimal degrees and hh:mm:ss [00:00:00 - 23:59:59.999] for RA and dd:mm:ss (or dd mm ss) for dec. A whitespace can be used as separator instead of a colon.
The table dynamically displays the output from the server back-end service, including temporal and spatial coordinates for the target, Moon and Sun. The bold-faced numbers indicate the parameters normally relevant in the phase 1 proposal for optical instruments. The numbers appear in red color if they are out of the range supported by the sky model.
The chart dynamically shows the altitude and equivalent airmass as function of time for the moon and target,
centered on midnight for the currently selected date.
The green line, which refers to the currently selected time,
can be dragged left and right to change the time, dynamically coupled with the sections in the Time section.
For the X-SHOOTER VIS arm, the sky background is reduced to correspond to the continuum between emission lines.
The offsets applied to the normal table of night sky brightness (mag/arcsec2) are: U:0 , B:0, V:0.242, R:0.139, I: 0.633, Z: 1.237.
The ETC allows the user to set the following:
| IS | IS width (arcsec) | IS heigh (arcsec) | slit width (arcsec) | slit heigh (arcsec) |
| #1 | 2.0 | 2.6 | 0.68 | 8 |
| #2 | 1.8 | 1.9 | 0.44 | 8 |
| #3 | 1.5 | 1.5 | 0.30 | 10 |
Slit.
The ETC allow the user to select the slit width. The slit heigh is kept fixed at 10 arcsec. See below to know how the Obj, Sky, Imax and S/N is calculated.
FLAMES Fiber Feed
Instead of a slit, the FLAMES Medusa fibers can be used to feed light to UVES. The fiber diameter is 1 arcsec, which then serves as the slit. The total sky aperture is 0.785398 arcsec2. ( = pi*(0.5 arcsec)2)
HARPS
The echelle spectrograph HARPS (La Silla 3.6m) is basically very similar to UVES, but has a simpler set of configurations. The efficiency of Fiber A is factor ~ 1.6 higher than fiber B. For details see this web page. Note that the polarimeters are splitting the light in 2 channels (equally if the star is unpolarized), i.e. for a "lossless" polarimeter circ_A/no_pol_A = lin_A/no_pol_A = 0.5. You can also see page 18 of the user manual. The ETC applies measured polarimeter efficiency factors.
Observation Mode.
The instrument works in 4 instrument modes: Red Arm, Blue Arm, Dichroic1, and Dichroic2 where the dichroic modes allow the simultaneous exposure of the Red and the
Blue Arm. The definition of an 'Observing Mode' requires the selection of the instrument mode, the crossdisperser to be used (Blue Arm: CD1 or CD2, Red Arm: CD3 or CD4), and
the central wavelength.
If 'Standard Template' is selected, the predefined central wavelength as given in parentheses (lam_c) in the pull-down menu is used.
This wavelength setting corresponds to the wavelength in the Standard Template as provided by the 'Phase 2 Proposal Preparation (P2PP)' tool.
If 'Free Template' is selected, the central wavelength can be set in the wavelength range as given in brackets [lam_0 < lam_c > lam_1]
in the pull-down menu for the given instrument mode and crossdisperser. Note, that the suggested instrument mode, crossdisperser, and central wavelength combinations are
predefined to allow senseful instrument setups only.
The user can insert in the optical path no filter (option None) or set one of the the filters listed in the following table:
| Filter | Cross Disperser |
|---|---|
| BBS6-HER5 | CD#1 or CD#2 |
| BBS2-BG24 | CD#1 |
| RBS1-BG40 | CD#3 |
| RBS2-SHP700 | CD#3 |
| RBS9-BK7_5 | CD#3 or CD#4 |
| RBS3-OG590 | CD#4 |
| RBS12_HALPHA | CD#3 or CD#4 |
| RBS13_HBETA | CD#3 |
| RBS14_OIII5007 | CD#3 or CD#4 |
| RBS15_OIII4363 | CD#3 |
| RBS16_NII5755 | CD#3 |
| RBS17_OI6300 | CD#3 or CD#4 |
| RBS18_SII6724 | CD#3 or CD#4 |
| RBS19_HeII4686 | CD#3 |
Recommended combinations are colored yellow.
Detailed information on filters, optical components and detectors is available in the relevant instrument user manuals.
The ETC calculates as default the predicted spectral format. This is presented in a table format. The table reports for each order number the wavelength of the central column, its y position in pix units and arcseconds units, the Free Spectral Range (FSR) size, minimum and maximum wavelength, the order starting and ending wavelength and size: Template Spectra range (TS range).
The FSR is the wavelength range over which two adjacent orders are not overlapping, correspondent to the distance between wavelengths at which the Blaze function is 0.5.
The central wavelength of the FSR is
wc=2*sin(alpha_blaze)/Kech/m
where alpha_blaze is the echelle incidence angle at blaze wavelength, Kech is the echelle constant (grooves/mm) and m is the order number.
The size of the FSR it is approximatively given by
FSR_size=lambda_blaze/m
where lambda_blaze is the blaze wavelength and m the order number.
If required the ETC also calculates for each significative order (in the single line case only for the order where is detected the line) the total efficiency (% units), the Object, Sky and maximum expected counts (in electron units) and the Signal to Noise ratio for three points, the FSR minimum, and maximum and the central column wavelength. For the FSR central wavelength it is reported also the wavelength value and the spectral bin size (over which the Object, Sky, Imax counts are integrated).
To evaluate the total number of counts expected, the ETC use the following "zero order" formula:
For point sources:
N_point=F*D*T*E*S/P For extended sources:
N_extended=F*D*T*E*S*Omega/P
Where
F=Incident Flux (in ergs/s/cm2/A for point sources and ergs/s/cm2/arcsec2/A for extended sources).
D=wavelength bin
T=Exposure time
E=Total efficiency (atmosphere, telescope, optical components, filters, detector, slit losses in case of point sources)
S=Telescope Surface
P=Energy of one photon
Omega=Solid Angle subtended by a rectangle of size equal to the product of the slit width (in arcsecs) and the pre-slit PSF FWHM projected on the sky.
To evaluate the signal to noise ratio the ETC use the following expression:
S/N=N_Obj/sqrt(N_Obj+ S_Sky+ nPixY*n_dark*T/3600+ nBinY*n_RON2)
Where N_Obj and N_Sky are the number of predicted detected counts predicted for the object (using the appropriate expression if point source or extended one) and the sky (extended source).
nPixY is the number of pixels along the Y detector direction equivalent to twice the PSF FWHM (or to the appropriate size if an Image Slicer is inserted).
n_dark is the dark current (1e/pix/h).
T the exposure time in seconds.
nBinY is the number of bins equivalent to nPixY.
n_RON is the read out noise (for the particular chip used in the arm red or blue at the specified read out speed and gain).
The information contained in the spectral format table, relative to the central column wavelength value, can be displayed also in form of graphs selecting the appropriate check button in the input page. In this case the ETC generate applet graphs and links to the corresponding data in ASCII format and in gif images format.
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