SARG E.T.C. Short Notes

You are going to use the SARG Exposure Time Calculator, please read this short note with attention.

The SARG ETC is a Java applet with both editable fields and curtain menus where you can define all the main parameters you want in order to obtain the S/N for a given exposure time or vice versa.

You have to fill all the fields in order to obtain a correct evaluation of the wanted result, successively you have to choice between S/N or Texp as input value. The unselected voice of the input form will be evaluated by the ETC. To start the program push the compute button.

For a quick start with the ETC select here.

Notes and bug Reports

Due to a bug in this release of the ETC, please pay attention to fill the wanted input field (S/N or Texp) with the guess value before to select it.

NOTE 1: The SARG ETC is a simple computational model of the Galileo High Resolution Spectrograph. It could be exploited in order to obtain a quasi-realistic forecast of the spectrograph behaviour. Calculated Exposure Time and S/N ratio do not account for the following items:

• variations of the atmospheric conditions
• variations of telescope mirrors reflectivity
• telescope and system overheads

The user is kindly requested to report any inconsistent result.

ETC Input Section

The input section of the SARG ETC is subdivided in four major sections:

• INPUT FLUX DISTRIBUTION: 
  • Object Magnitude: This field indicates the object magnitude in the broad band filter defined in the following field. The effective central wavelengths are: B=440 nm; V=550 nm; R=640 nm; I=790 nm. The effective wavelengths and the photometric zero points are taken from Bessel 1979, PASP, 91,589
  • Band: This field defines the photometric band for the Object magnitude and for the corresponding Sky magnitude
  • Wavelength: it is possible to select between 30 wavelength values (from 380 nm to 950 nm).
• SKY CONDITIONS:
  • Days from new moon: it defines the sky brightness in mag/sec² . The reference values are given in the following table as described in the OIG (Optical Imager Galileo) description page.
   

  Days from new moon	U	B	V	R	I
  0	22.4	23	21.4	20.6	19.1
  3	21.7	22.5	21.3	20.5	19.1
  7	20.4	21.3	21	20.4	18.9
  10	19.1	20.4	20.2	19.4	18.7
  14	17.5	19.2	19.2	19	18.3

  • Airmass: you have to compute the airmass (exploit Skycalc)
  • Seeing: input the forecast seeing value in arcsec (point source)

• INSTRUMENT SET UP:
  • Slits: select the slit you need. Refer to the slits description page for details
  • Filters: SARG has a selection of four order sorting and stray light rejection filter. Each grism is coupled with each filter (FW2 with blue grism; FW3 with the green grism; FW4 with the yellow grism and FW5 with the red grism). Beside these filter a selection of neutral filters is also given. In order to get more information please refer to the filter page.
  • Grisms: it is possible to select between four grisms. In order to have information about ranges and spectral format, please see the grism description page.
  • Bin: a large selection of  CCD mosaic reading modes are available. The first number correspond to the column (spatial direction) while the second number to the line (resolution direction).
• OPTION: you have to select the operative mode of the ETC. In order to get the Exposure Time (S/N) you have to select the S/N (Exposure time) button after you filled the correspondent input field.
  • Exposure Time: the exposure time in seconds
  • S/N: the signal to noise ratio

Once you have choice and filled all input parameter fields, start computation by pushing the COMPUTE button.

ETC Result Section

The SARG ETC computes either the necessary exposure time in obtaining a given Signal to noise ratio or, viceversa, the signal to noise ratio that it is possible to obtain with a given exposure time. Both these value are shown in the FINAL RESULTS section. Beside, in a separate section, a set of intermediate results are also given. 

• INTERMEDIATE RESULTS:
  • FLUX: the source incident flux in W/(m²*µm)
  • MSKY: the sky brightness in mag/arcsec²
  • FSKY: the sky incident flux in W/(m²*µm*arcsec²)
  • NBIN:The number of bin in the dispersion direction
  • NPIX: the number of pixels for the bin in the dispersion direction
  • NPIXT: the total number of pixels involved in the computation for the dark current contribute
  • NSKY: the number of sky photons detected. It is expressed in arcsec²/bin
  • S: the signal detected from the CCD. It is evaluated in e^-
  • ATM. EFFICIENCY: the trasparency of the atmosphere is evaluated from measured La Palma extinction coefficents:K_U= 0.49;K_B= 0.25; K_V=0.15 ; K_R=0.11 ; K_I=0.07 . 
  • TNG EFFICIENCY: the measured total reflectivity of the three (M1, M2, M3) GALILEO telescope mirrors
  • SARG EFFICIENCY: the measured efficiency of the whole spectrograph
  • TOTAL EFFICIENCY: as resulting from the product of the previously described efficencies
• FINAL RESULTS:
  • S/N:Signal to Noise for pixel as results from computation (or from setting)
  • EXPOSURE TIME:the exposure time as results from computation (or from setting)