PFS Expected Performance

The sensitivity information below is as of Dec. 2021, and subject to change in the future.

Basic Information

Arm Wavelength range Throughput(1) Resolving Power Continuum sensitivity(2) Emission line sensitivity(3)
[AB mag] [10-17 erg/s/cm2]
[nm] mean(4) representative(5) mean(4) representative(5)
Blue 380 - 450 12% ∼2300 21.8 21.9 (@415nm) 4.7 4.5 (@415nm)
450 - 550 21% 22.3 22.3 (@505nm) 2.2 2.1 (@505nm)
550 - 650 23% 22.1 22.2 (@605nm) 2.0 1.8 (@605nm)
Red Low Res. 630 - 750 25% ∼3000 22.1 22.4 (@680nm) 1.8 1.4 (@680nm)
750 - 850 25% 21.9 22.3 (@796nm) 1.7 1.3 (@796nm)
850 - 970 24% 21.6 22.1 (@912nm) 1.8 1.3 (@912nm)
Mid. Res. 710 - 775 22% ∼5000 21.5 21.6 (@741nm) 2.0 1.7 (@741nm)
775 - 825 23% 21.4 21.6 (@796nm) 1.8 1.6 (@796nm)
825 - 885 21% 21.3 21.5 (@856nm) 1.9 1.6 (@856nm)
NIR 940 - 1050 18% ∼4300 20.9 21.5 (@993nm) 2.7 1.7 (@993nm)
1050 - 1150 19% 21.0 21.4 (@1100nm) 2.2 1.6 (@1100nm)
1150 - 1260 16% 20.9 21.2 (@1208nm) 2.2 1.7 (@1208nm)

Note: These estimates are based on the PFS exposure time calculated developed by C. Hirata arXiv:1204.5151 under the following assumptions:

  • FWHM=0.8 arcsec seeing condition (the fraction of incoming flux to fiber aperture is ∼62% at the field center and ∼54% at the edge if the fiber is perfectly aligned with a stellar object)
  • Observation at a zenith angle of 45 deg.
  • Observation at a dark night
  • Observation near the field edge (0.675 deg. from the center)
  • No Galactic dust extinction
  • Recent sky model provided by Jim Gunn
  • Systematic sky subtraction error of 1%
  • Diffused stray light on the detector originating from 2% of incoming sky flux
  • Dark current [e-/pix/s] of 0.0002 (blue and red) and 0.01 (NIR)
  • Read-out noise [e-RMS/pix] of 3.0 (blue and red) and 4.0 (NIR)

(1) The total throughput including primary mirror reflectivity, WFC transmission, and PFS instrument. See here. The fiber aperture effect is not included because it depends on seeing condition and object type. The vignetting effect, ∼94% at the field center and ∼71% at the field edge, is not included either because it depends on the field position. The continuum and emission-line sensitivity information, however, are calculated taking these factors into consideration.
(2) Continuum sensitivity in case of point source, to achieve S/N=5 for 1-hour on-source exposure (8×450 sec.), after 3 pixel binning.
(3) Emission-line sensitivity in case of point source, to achieve S/N=5 for 1-hour on-source exposure (8×450 sec.). Here, the line width is assumed to be σ=70 km/s.
(4) The average limiting magnitude and line flux in the wavelength range. This value may be affected by the sky emission line.
(5) The representative value at the wavelength where the spectrum is not affected by the sky emission line.

Continuum Sensitivity Line Sensitivity

Fig. 1. S/N expected for continuum (left) and emission line (right) in case of the total on-source integration time of 1 hour. Flat continuum of 22.5 AB mag and line flux of 1 &times 10-17 erg/s/cm2 are assumed. For the red arm, only the low resolution result is shown.

Continuum Sensitivity Line Sensitivity

Fig. 2. Limiting magnitude for continuum (left) and limiting flux of a single emission line (right) to achieve S/N=5 in case of the total on-source exposure time of 1 hour. The gray horizontal bar indicates the average value while the open circle shows the representative value in the specific wavelength range. For the red arm, only the low resolution result is shown.

Continuum Sensitivity Line Sensitivity

Fig. 3. A similar plot to Fig. 2, but for the medium resolution mode.

Moonlight Effects

Moonlight effects on the limiting magnitude are presented below. Using the PFS exposure calculator (see above), the limiting magnitude requried for S/N=5 in case of 900-sec exposure time is compared among three cases: Night skys with new moon, half moon, and full moon. The assumptions in the calculation are the same as the above, except for the systematic sky subtraction error (no error is assumed here).

Fig.4. Comparison of the limiting magnitude (S/N=5 with a 900-sec exposure) for continuum at 415 nm (left), 796 nm (middle), and 1100 nm (right), as a function of the separation angle between the object and the moon. The difference in the magnitude from the dark night case (i.e. new moon) is plotted. At a separation angle lower than 60 deg., calculations were performed for various combinations of the zenith angles of the moon and the object keeping the separation angle constant, and the variation among the individual results is indicated by the vertical stretch of the plot at a given separation angle. In the red arm, the effects in the MR mode are also plotted with a small horizontal shift.

Fig.5. The same as Fig. 4, but for a emission line with sigma=70 km/s. The ratio of the limiting flux to a dark night is plotted.