ELT Spectroscopic Exposure Time Calculator

This ETC is for ELT Spectroscopy. A detailed description is available in the ELT Spectroscopy ETC document.
For imaging, use the ELT Imaging ETC.

ELT Spectroscopic Mode Version 6.4.0
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Target Input Flux Distribution

Blackbody: T= K
Spectral Type: (only U-K bands)
MARCS Stellar Model
Powerlaw Index:           F(λ) ∝ λindex
Upload Spectrum

Target Brightness :
Vega mag = The input spectrum will be scaled to have this magnitude in the selected system.
AB mag
Flux mJy The input spectrum will be scaled to this flux at the observing wavelength (λref) chosen below.

The magnitude or flux is given per square arcsec for extended sources.

Spatial Distribution:
Point Source
Extended Source

Telescope Setup

Observatory Site:
Telescope Diameter:

Sky Conditions

Seeing: arcsec (FWHM in V band)
Airmass: In the current ETC model, the telescope diameter parameter affects only the light collecting area, not the resolution (PSF), see the doumentation for details.

Instrument Setup

AO Mode:  
Observing wavelength: band (λref):
λref: nm
The numeric results will refer to λref and the wavelength range of the graphs will be centered there.
Radius of circular S/N ref. area:
.i.e. over which the S/N is calculated.
mas (on the sky) DLC=1.22 λ/D is the radius of the diffraction limited PSF core for a D=42m telescope.
Number of spectra on the detector:
i.e. number of spatial elements (spaxel) in the S/N ref. area
Spectral resolution R=λ/Δλ: IMPORTANT: please consult the documentation when using R>100,000


S/N ratio: S/N = DIT = s
Exposure Time: NDIT =

The total exposure time is the product of DIT (Detector Integration Time) by NDIT (number of DITs). Instrument and telescope overheads are not taken into account.

Plots: Toggle All / No Plots

Resultant spectrum including sky
Object Spectrum only
Sky Emission Spectrum
Sky Transmission Spectrum
S/N as a function of wavelength
System Efficiency and Wavelength Range
Input spectrum in physical units