Echelle Spectrographs ETC
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Important notes and bug reports

Input Parameters

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.

• Input Flux Distribution

  • Blackbody

    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.

  • Power Law

    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:

    F(w)=Fo*(w/wc)^p

    where

    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).

  • Continuum

    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.

  • Template spectrum

    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.

  • Object Magnitude

    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).

    	Photometric Band
    	U	B	V	R	I	J	H	K
    wc	360	440	550	640	790	1250	1650	2160
    Z	7.3788	7.1804	7.4425	7.6408	7.9115	8.5058	8.9431	9.4045

  • Single Line

    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).

• Spatial Distribution

  • Seeing Limited

    Seeing limited sources are point-like sources. The signal to noise is computed over an area of diameter twice the seeing.

  • Extended Source

    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).

• Sky Conditions

  • Sky Brightness

    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 the NIR arm in X-SHOOTER, the sky background is computed by a physical atmosphere model like in the NIR ETCs e.g. SINFONI, CRIRES, NACO.

    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

  • Airmass

    The airmass can be computed with the Skycalc page.

  • Seeing

    The seeing is given in arc seconds.

• Instrument Setup

  • Mirrors (M1 to M2/M3 Depending on location of Instrument on Telescope). UVES (M1,M2,M3).

    The ETC allows the user to set the following:

  • Pre Slit Optics: The pre slit filter is selected from a list with an option menu. The ETC allow the following settings: None (no pre-slit filter inserted), ADC (Atmospheric Dispersion Compensator), Depolarizer, Iodine cell.

  • Image Slicers:

    The UVES instrument can observe with or without inserting image slicers along the optical path. The use of image slicers is important for bad seeing conditions or the highest spectral resolution. The Image Slicers mode is selected from a list with an option menu. The ETC allow the following settings: None (no image slicer inserted), IS#1, IS#2, IS#3. The following table contains the characteristic image slicers data.

    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 following slit width values: 0.3-1.2 arcsec with a step of 0.1 arcsec and 1.2-3.0 arcsec with a step of 0.3 arcsec. 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. A help page dedicated to the HARPS mode of the ETC is available here.

  • 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.

  • Below Slit Filters.

    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

    Yellow color highlighted combinations are recommended.

    Information on UVES filters, optical components and detectors is available in the instrument user manual.

  • Detector

    The ETC allows the user to set the read out mode (slow/fast) and the gain (high/low) of the detector.

    The following table reports for each detector of the two arms the corresponding Read Out Noise for the two read out modes and the corresponding conversion factor to ADUs.

    ARM	CCD	BINNING	GAIN	SLOW		FAST
    				RON	CVFCT	RON	CVFCT
    BLUE	EEV	1x1	LOW			4.1	2.06
    		2x2	HIGH	2.1	0.57
    		1x2	LOW			3.9	1.95
    		2x3	HIGH	2.1	0.57
    RED	MIT-LL	1x1	LOW			3.8	1.5
    		2x2	HIGH	3.4	0.57
    		1x2	LOW			3.3	1.53
    		2x3	HIGH	3.0	0.53
    	EEV	1x1	LOW			3.4	1.58
    		2x2	HIGH	1.97	0.57
    		1x2	LOW			3.3	1.53
    		2x3	HIGH	2.0	0.53

    Yellow color highlighted combinations are the modes offered to public.

    Saturation. The following table shows the approximate saturation levels for the modes of each detector.

    ARM	CCD	GAIN	BINNING	SATURATION e-	SATURATION ADU
    BLUE	EEV
    		LOW	1x1, 1x2	120000	60000
    		HIGH	2x2, 2x3	34000	60000
    RED
    	EEV
    		LOW	1x1, 1x2	100000	65000
    		HIGH	2x2, 2x3	36000	65000
    	MIT-LL
    		LOW	1x1	65000	60000
    		LOW	1x2	65000	60000
    		HIGH	2x2, 2x3	23000	60000

    In case the ETC predict a maximum intensity greater or equal of the correspondent number this number is displayed on a red background. As the ETC rounds off the number of significant digits, it may happen that a number slightly lower than the limit is displayed as equal to the limit, but in this case not on a red background.

  • Mask Binning

    The ETC allow the user to set three mask binning (spatial X spectral):(1 X 1) or (2 X 2) or(2 X 3) or (1 X 2).

  • Exposure Time

    The ETC allow the user to set the exposure time in seconds.

Results

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 resolution 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 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.

Version Information

Version 3.2.6 (June 20, 2008)

Code updated, fixed a problem for X-shooter in emission line mode.

Version 3.2.2 (Nov 21, 2007)

Bug fixed in the scaling of the power-law input spectrum.

Version 3.2.1 (Aug 31, 2007)

New layout of results in output page.
Prepared support for X-shooter.
Graphs changed. Added saturation warning in Imax graph

Version 3.2.0 (July 30, 2007)

Bug fixed: A bug prevented the use of UVES "1x1 ultrafast" red detector readout mode.


Version 3.2.0 (June 21, 2007)

UVES detetor parameters and efficiency curves updated.

Bug fixed: The atmospheric extinction was erroneously applied to the sky background itself.

Code optimized.


Version 3.1.1 (February 9, 2007)

UVES: Added new "1x1 ultrafast" detector readout mode (Red Arm only).


FEROS only: Version 3.1.0 (July 25, 2006)

Added support for ADC to FEROS ETC.


Version 3.0.7 (February 20, 2006)

Corrected the value of the height of ImageSlicer#3 from 1.5 to 2.0 arcsec.


Version 3.0.7 (February 17, 2006)

In ImageSlicer-mode: The formula for computing the maximal intensity (Imax) has been corrected.


Version 3.0.6 (September, 2005)

UVES: Eight narrow "below slit filters" added.
A threshold is applied to values in S/N tables; values<1.0e-9 are shown as zeros.

Version 3.0.5 (March 3, 2005)

Support for FEROS added.

Version 3.0.4 (January 7, 2005)

Code updated. In general, the results may differ slightly with respect to previous versions.
FLAMES-UVES: The model now assumes a 0."3 decentering of the science object on the fiber.

Version 2.9.3 (Aug, 2004)

Added a setting: DIC2, red arm, CD4, range=[570-940] wave_c=760.
New blue CCD.
Parameters updated for all CCDs, in particular linearity/saturation limits.

Version 2.8.6 (Feb. 27, 2003)

RON and gain conversion factors for HARPS CCDs corrected/updated.

Version 2.8.5 (Feb. 20, 2003)

Added support for the La Silla HARPS spectrograph

Version 2.8.4 (December 20, 2002)

The computation of the maximal intensity (Imax) has been corrected. The correction does not affect anything else.
Gain conversion factor is displayed.

• Version 2.8.1 (January 9, 2002)

  Two new Below Slit Filters added: HER_5 and BK7_5. Filter CuSO4 disabled. Efficiency of CD1 changed to that of final version. Total transmission of instrument adjusted to fit recent calibration data. Code changed to automatically select the default Below Slit Filter for the Standard Templates: HER_5 for CD1 & CD2, SHP700 for CD3 and OG590 for CD4.

• Version 2.8 (September 1, 2001)

  Three new Standard Templates enabled: 'Red Arm, CD3, range wave_c[450-750] (600)nm', 'Dic2, Blue Arm, CD1, range wave_c[330-400] (346) nm' and 'Dic2, Blue Arm, CD2, range wave_c[360-480] (390) nm'.

• Version 2.5.2 (December 20, 2000)

  Updated transmission curve for CD#4 to reflect the november 2000 upgrade of the instrument. The maximum wavelength range for CD#3 was extended to 750nm.

  Version 2.5.1 (June 14, 2000)

  Version 2.5 (February 3, 2000)

  Main changes:

  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)
  CD2: 0.9
  CD3: 0.9
  CD4: 0.8

  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 Template cases:
  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
  Fast	Low	1x1
  Slow	High	2x2
  Fast	Low	1x2
  Slow	High	2x3

  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.

  Minor changes

  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.

  Version 2.4.2 (November 18, 1999)

  Main changes:

  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
  subtract:
  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.

  Minor changes:

  Explicitly reported what Dicroic-Arm-CD the counts table and the graphs are referring to.

• Version 2.4.1 (Sectember 3, 1999)

  Fixed a bug affecting PowerLaw input distributions, corrected QSO and elliptical galaxies template spectra

• Version 2.4 (August 5, 1999)

  First public release

Send comments to <usd-help@eso.org> Last modified: January 10, 2007