FMOS Spectrum Simulator


This calculator is still under development and results are subject to change as the performance and characteristics of the instrument are better determined.

This is a simulator of a fully reduced and calibrated spectrum when your target is observed with FMOS. This is expected to allow observers to investigate feasibilities of line strength and continuum brightness of their targets, with the effect of the OH suppression masks.

By using the items listed below, you can assume an intrinsic SED type, apparent magnitude, type and amount of reddening, and redshift of your target with some observing conditions. Detailed information for some of the items are described at the bottom of this page.

Please enjoy a virtual observation with FMOS !

Notes --

  1. It takes about a minute to simulate one spectrum and show the result.

  2. This simulator assumes that noise comes solely from photon shot noise, instrument background, and detector properties. Additional sources of noise, such as poor flat-fielding, incorrect sky subtraction, and so on, will result in worse sensitivity than this tool suggests.

  3. Some of the template spectra come from observational data and thus already include some noise. Also, some of them do not have very high spectral resolution. Therefore, this simulator would not be ideal to precisely evaluate feasibilities of observational quantities such as velocity dispersion and spectral index.

  4. This simulator is still in process of development -- If you found something strange in the calculations, could you let us know (the contact address is at the bottom of this page). Any comments and suggestions are also welcome.

Defining your target

Spectral type : galaxy :
star :
others :

Redshift = * See below (1).
Extinction : E(B-V) = * See below (2).
Magnitude : = * See below (3).

Additional lines - See below (4).

Give additional lines to the selected spectrum if necessary. Provide the information as follows in the box on the right for one line per row.

Format: (X1) (X2) (X3) (X4)

  • X1 - "e" (for emission) or "a" (for absorption)
  • X2 - Observed wavelength of line center in unit of micron
  • X3 - Line flux in unit of erg cm-2 s-1
  • X4 - FWHM of the line in km/s.
Example: e 1.2 1.0e-16 300.

Note: The width of an OH suppression mask is Δ&lambda = 7.5 A or wider.

Instrument setup

Spectrograph : * See below (5).
Observing mode :
Additional flux loss at the fiber entrance : % * See below (6).
Airmass (sec z) =
Total on-source integration time = sec
Sampling method CDS (Number of readouts = ) * See the 2nd item of "Notes for operation" in "Observation procedure"
for some explanations about this sampling method.
RAMP (Max. # of readouts is assumed.)
Binning size : pix in the λ direction * See below (7)(8).

For presentation of your result

Wavelength range of interest Autoscale
Manual Min: μm * See below (9).
Max: μm
Flux density range of interest Autoscale
Manual Min: cgs/A
Max: cgs/A


  • (1) When you select one of the stellar spectra as a template, this parameter is automatically set to zero.
  • (2) For "Milky Way", "LMC" & "SMC" extinction curves, the analytic formulas by Pei (1992, ApJ, 395, 130) are used. If you select "Starburst", the analytic formula by Calzetti et al. (2000, ApJ, 533, 682) is applied. RV (= AV/E(B-V)) is assumed 3.1 for the "Milky Way" and "LMC", 2.72 for "SMC", and 4.05 for "Starburst".
  • (3) This defines the total magnitude of your target.
  • (4) In "Additional lines", you can add artificial emission/absorption lines on the selected continuum spectrum. The line profile is assumed to be a Gaussian. (No saturation effect is taken into account while it is likely to be important for deep absorption lines. If the intensities are calculated to be negative in a part of the line profile, they are replaced with a very small positive value).
  • (5)The differences between IRS1 and IRS2 on this simulator are only the readout noise of the detector and conversion factor (e-/ADU). The throughputs are assumed to be the same.
  • (6) The throughput data used inside this simulator include the "best" estimate of the flux loss at the fiber entrance for point sources. This mainly comes from the spectra of relatively bright stars obtained in the engineering observations. The data of faint galaxies tend to indicate more loss, although they also show a large scatter due perhaps to the intrinsic variety in their size & morphology as well as the measurement error in the estimation). If you want to assume an additional loss of flux from the fiber aperture, you can set a non-zero value in percent (%) here. For example, input "50" if you try to estimate feasibility when only half of the flux falls onto the fiber (i.e. twice more is lost) compared with the "best" case.
  • (7) The resolving power and dispersion of Low Resolution Mode are ∼ 500 and 5 A / pixel, respectively. For High Resolution Mode, they are ∼ 2200 and 1 A / pixel.
  • (8) The size of a re-imaged fibre core is ∼ 5 pixels in FWHM on the detector. Therefore 5 pixels in dispersion direction approximately corresponds to a spectral resolution element (∼ 25 A in Low Resolution Mode and ∼ 5 A in High Resolution Mode). An 8 pixels aperture is assumed in all calculations to extract a fiber spectrum from an image on the detector.
  • (9) The spectral coverage of FMOS is 0.9 -- 1.8 μm, and this is entirely covered by a single exposure in LR. In HR, this is divided into 4 bands: J-short, J-long, H-short, and H-long. If you choose Auto, the entire region of each band is presented as a result of simulation. Note that there will be no useful data at 1.35 - 1.40 microns because the fiber slit blocks the part of the primary spectra formed on the mask mirror.

Questions and comments regarding this page should be directed to Kentaro Aoki ( ).

Last updated: Jan 31, 2012

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