Echelle Spectrographs ETC
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 is a blackbody defined by its temperature and monochromatic apparent magnitude at a given wavelength. Temperature is expected in Kelvin and wavelength in one of the band filters U, B, V, R or I.
The target model is a power law spectrum defined, at the wavelength w, from a continuum flux level Fo, a reference wavelength wc and the power low index p, a floating number, as:
Fo=10-(0.4*M + Z)
where M is the Object Magnitude and Z is the zero order point in the selected observing band (see table below).
The target model is a spectral distribution constant with the wavelength. This case is a subcase of the Power Law one selecting index p=0.
The target model can be defined by a spectral type. As with the blackbody it will be scaled to the provided magnitude and band filter U, B, V, R or I.
Indicates the object magnitude in the broad band filter associated to the filter defined in the Instrument Setup. The effective central wavelengths are respectively B=440 nm, V=550 nm, R=640 nm, I=790 nm. 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 references: for bands UBVRI Bessel, 1979, PASP, 91, 589; for bands JHK Wilson ApJ 177, 533 (1972).
In this case the source is a single line of characteristic wavelength l, FWHM, and emitting a selectable Flux (in 10-16 ergs/s/cm2 units).
Seeing limited sources are point-like sources. The signal to noise is computed over an area of diameter twice the seeing.
The signal to noise for extended sources is given per wavelength resolution 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).
Here you define the sky brightness in mag/arcsec2. The following table provides a reference to the observed sky brightnesses associated to the days from full Moon. Note: Values for the Z band correspond to an extrapolation and will be updated when measurements are available.
For X-SHOOTER, the sky background is reduced to refer to the continuum between emission lines.
The offsets applied to the normal table of night sky brightness are: U:0 mag, B:0 mag, V:0.242 mag, R:0.139 mag, I: 0.633 mag, Z: 1.237 mag.
The airmass can be computed with the Skycalc page.
The seeing is given in arc seconds.
The ETC allows the user to set the following:
|IS||IS width (arcsec)||IS heigh (arcsec)||slit width (arcsec)||slit heigh (arcsec)|
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)
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.
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:
|BBS6-HER5||CD#1 or CD#2|
|RBS9-BK7_5||CD#3 or CD#4|
|RBS12_HALPHA||CD#3 or CD#4|
|RBS14_OIII5007||CD#3 or CD#4|
|RBS17_OI6300||CD#3 or CD#4|
|RBS18_SII6724||CD#3 or CD#4|
Yellow color highlighted combinations are recommended.
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
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
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:
F=Incident Flux (in ergs/s/cm2/A for point sources and ergs/s/cm2/arcsec2/A for extended sources).
D=wavelength resolution bin
E=Total efficiency (atmosphere, telescope, optical components, filters, detector, slit losses in case of point sources)
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 twice the seeing (in arcsecs) 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 seeing (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.
Defined "fudge" factors files, one for each CD, to match
actual observed counts as from UVES commissioning 2. At
the moment they are constant with wavelength
(within each CD):
CD1: 1.0 (no corrections)
Refined physical model parameters for Spectral Format
determinations with results from commissioning 2.
N.B. The Spectral Format displayed from the ETC, reports orders which are at least for half on the active detector area
(0 <= y(central column) <= 2048).
Slightly corrected the cetral wavelength for a few Standard
Arm Blue, Cd1: from 342 nm to 346 nm
Dic2, Arm Blue, Cd1: from 395 nm to 390 nm
Dic2, Arm Red, Cd3: from 556 nm to 364 nm
Added the 1x2 CCD binning entry and included the
combinations which are actually implemented in the
instrument only as reported in the following table.
|Read Mode||Gain||CCD Binning|
Modified table reporting RON and CVFCT for the arm selected and the CCD detector as a function of detector Gain and Read Out Speed as measured in December 1999.
Assigned Dicroic 1 to CD2 (390 nm) and CD3 (564 nm) as appropriate.
Eliminated ND1-5 filters from Pre Slit Optics menu.
Inserted Depolarizer and Iodine Cell entry in Pre Slit Optics menu.
Added a small check to not to display results for single line source when its central column wavelength falls outside the active detector area.
Added on the summary report of the Simulator Setup a missing entry about the Input Flux Spatial Distribution.
Eliminated an empty list item appearing when selecting as Input Flux Distribution a Template Spectra.
Clarified or corrected some notation in the Spectral Format Table.
Rearranged items in Dicroic-Arm-CD-(wav) and in CCD Binning-Read Out Sped-Gain selectors to improve clarity.
Limitated the use of the last 4 Dicroic-Arm-CD menu choices in the list to free templates.
Renamed Pre Slit Filter in Pre Slit Optics.
Changed background color of BBS2-BG24 filter to white to emphasize that now it is not anymore recommended to use with CD1.
Inverted the order of binning notetion to uniformize it to the UVES manual and the P2PP notation.
Cleaned input output html forms.
Fixed a bug affetting the setting of M1 efficiency curve setting. Before the M1 efficiency was rescaled to have at 670 nm the value 75%.
Corrected the Al_12month curve before used for M1, M2 and M3, applying a 0.945 cedrrection factor to M1, and a 0.973 factor to M3. The M2 efficiency curve is of Al_12month revised from Hans Dekker 3 Dec 99.
Corrected the value of Telescope Effective Surface:
M1 optical diameter 8 m (before was 8.18) => area = 50.26548 m^2
M2 area including baffle diameter (1.2 m) => area = 1.13097 m^2
spiders area: 4 X (3.7 m X 0.04 m) => area = 0.59200 m^2
Result: Total Area = 48.54251 m^2
Fixed a problem in inserting camera efficiency curves.
Explicitly reported what Dicroic-Arm-CD the counts table and the graphs are referring to.
Fixed a bug affecting PowerLaw input distributions, corrected QSO and elliptical galaxies template spectra
First public release