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What is it?
This repository contains idl code to calculate the sensitivity of a ground-based mid-infrared instrument for astronomy. The code was written during 2008-2009 for the Phase A study of the instrument METIS (, the Mid-Infrared E-ELT Imager and Spectrograph, for the 42-m European Extremely Large Telescope. The code and all input parameters are highly tailored for the particular design parameters of the E-ELT and METIS, however, the programme is structured in such a way that the parameters can easily be adjusted for a different system, or alternative input files used.
Basic features of the E-ELT and METIS are:
- a 42-m aperture
- wavelength coverage: 3-14 micron (L, M and N bands)
- a ground-based telescope, i.e. contributions from atmospheric radiance and a strong thermal background from the telescope
- adaptive optics for atmospheric turbulence correction using either single conjugate natural guide star AO (aomode='scao') or laser guide star tomographic AO (aomode='ltao')
- instrument performance is assumed to be thermal background-limited at all wavelengths, in all modes. The METIS instrument is cryogenically cooled to achieve this
- two sites are considered according to the info provided by ESO at
What is it not?
- the code does not generate synthetic images or spectra
- no coronagraphic or polarimetric observing modes
- no specific detector readout schemes or subarrays
The package contains 3 key files for sensitivity calculations in METIS' 3 main modes of observation:
1. -> Imager mode
- Imager specific parameter file:
2. -> Low-resolution long-slit spectroscopy mode (R~1000-3000)
- low-res spectroscopy specific parameter file: (same as for imager)
3. -> High-resolution integral field spectroscopy (R~100,000 in LM, 50,000 in N band; image slicer design)
- IF spectrosocpy specific parameter file:
Zip files
There are three zip files which each contain all the files you need for a particular mode of observation, as above.
Parameter files:
- general input parameters related to telescope and atmosphere. Contains parameters for setting file locations to be adjusted for your own system
- input parameters related to the pre-optics of METIS
Atmospheric profiles:
- atm/paranal_*.dat: atmospheric profile files for a Paranal-like site, containing both transmission and radiance info. Profiles have been pre-convolved to resolutions appropriate for the different modes of observation: 100 for imaging, 3000 for slit spectroscopy, 50000 for N band IFU spectroscopy and 100000 for LM-band IFU spectroscopy
- atm/high_and_dry_*.dat: dito Paranal for a high & dry site at 5000 m
Encircled energy lookup tables:
- ee_files/: this folder contains 2 lookup tables for AO-corrected encircled energies. (i) EnsquaredEnergy_AO_LMN_V10.dat is for single conjugate AO and is calculated with AO simulation tool PAOLA. Please refer to the tech note by Kendrew for detailed input details; (ii) atlas_eeav.dat is the same for the laser tomography AO mode, provided by the team designing the LTAO module for E-ELT at ONERA, France. These values were converted from encircled to ensquared energies, and this introduces a marginal error. See tech note for more info.
The profiles/ folder contains a number of input files loosely grouped for being "profiles" of sorts.
-> bandic_prof.dat: input for calculation of the transmission profile of the dichroic that splits LM and N band radiation
-> filters.dat: filter transmission details for imager mode
-> grisms.dat: grisms info for low-resolution spectroscopy
-> metis_ifures_lm.dat: resolving power info for the LM band IFU spectrograph based on the METIS optical design
-> metis_ifures_n.dat: dito N band
-> znse_abs.dat: absorption profile for a ZnSe window - required for calculating the instrument throughput
-> znse_trans.dat: dito transmission profile
Auxiliary functions:
- calculates the transmission profile of the band-splitting dichroic
- calculates the detector quantum efficiency. This is pretty much hard-coded in, not much actual calculation, but based on realistic manufacturers' values
- calculates throughput of the instrument for any of the modes
- calculates the transmission profile of the dichroic that splits the incoming beam between the wavefront sensor unit (~< 1. micron) and the instrument.
- calculates the thermal background from the ZnSe entrance window.
This code was written using IDL 7.0 under Windows XP SP3 and should be compatible with most reasonably current versions of IDL (> 6.x) although I haven't tested it extensively. I've used it regularly under linux, which requires some minor modifications to the directory structure. The input directories are specified in the file.
The IDL Astronomy Library is referenced throughout - you can download it here:
Who wrote it?
This code was written by Sarah Kendrew, Postdoc in Astronomy Instrumentation at Leiden Observatory, PO Box 9513, 2300 RA Leiden, during 2008-2009. For more info, you can contact me on sarahaskendrew AT
If you use this code, either for METIS-specific calculations or for your own project, please cite:
Kendrew et al., Mid-infrared astronomy with the E-ELT: Performance of METIS, Proc. SPIE vol. 7735, in press (2010).
If you use the code extensively, I'd like to hear about it and would appreciate being added as a co-author to any resulting publications.
Further reading
A detailed description of the calculation method, inputs and results for METIS are in the tech note in the repository by Kendrew, 2009. The calculation method is based on that of the E-ELT exposure time calculator provided by ESO, albeit with quite different inputs, freely available via the ESO E-ELT Design Reference Mission:
The atmospheric profiles are also available on these pages, at R~100,000.
For more information about the METIS instrument and project, see
S. Kendrew was funded by NOVA, the Dutch Research School for Astronomy, when this work was carried out.
METIS is an international collaboration of NOVA (Leiden/Dwingeloo, NL), the Max Planck Institute for Astronomy (Heidelberg, D), CEA Saclay (Paris, F), the UK Astronomy Technology Centre (Edinburgh, GB) and the KU Leuven (Leuven, B).
--- S. Kendrew, June 2010