Further information Questions regarding this page should be
directed to Kentaro Aoki ( ).
Measured system throughput
There are a few restrictions applid in the operation of HR.
Applicants/observers should go to this
page and keep the notes there in mind about the use of HR.
Fig. 1 shows the measured system throughput as a function of
wavelength in the High Resolution (HR) mode
(red line) and Low Resolution (LR)
more (blue line) (Kimura, M., et al. 2010, PASJ, 62, 1135).
Note that due to the effects of the OH masks, there are a number of
wavelengths with low throughput looking like absorption lines. In
fact, there is a fiber-to-fiber variation in the throughput (about ~3
%), and the median values among the fibers are plotted over the
spectral coverage in this figure.
This throughput was measured by illuminating the Echidna fibers on the
prime focal plane through the wide-field corrector lenses with a
calibrated black-body radiation source and taking spectra with
IRS1. The result was then multiplied by 0.85 to consider the
reflectivity of the telescope main mirror. (The throughput of IRS2 has
also been recently measured by observing stars and galaxies with J & H
magnitudes known, suggesting that it is consistent with IRS1.)
For the features numbered in the plot, brief explanations are given
- (1): The peak of diffraction
efficiency of the primary grating in the spectrographs (see
details here for the optics layout) is around
1.35um. Hence the throughput decreases rather rapidly at shorter
wavelengths, instead of a higher throughput kept over the H band.
- (2): This decrease corresponds to
the wing of the strong absorption feature of fused silica at 1.4um
which are used for the correctors in the spectrographs.
- (3): The edge of the passband of
the thermal blocking filter in the camera dewar is visible here.
- (4): When the high resolution
spectra are formed on the mask mirror, the 1.35um-1.40um part has to
be blocked by the fiber slit and is therefore not available on the
- (5)It is emphasized that the
throughput is lower in LR than in HR: A VPH grating is added in the LR
mode to anti-disperse the OH-suppressed high resolution spectra and
allow the entire spectral coverage (0.9 μm to 1.8 μm) to be
covered by the detector at once. Therefore the LR system throughput is
approximately (HR throughput) x (VPH throughput).
Fig. 1 - Measured system throughput in the HR
mode (red line) and LR mode (blue
line). From Kimura, M., et al. (2010).
ATTENTION: Fig. 1 was updated in Aug
2010 and the throughput value plotted there is lower in both LR and HR
than in the same plot having been indicated until then. This is purely
because there was a bug-fix in the calculation of the throughput (a
wrong temperature, 1000 K instead of 1090 K, was used before as that
of the black body radiation source), NOT because
the actual instrument throughput became lower than the previous
semester. Consequently, there has been no change in the guideline of
Guideline of sensitivity
Guidelines of continuum level and emission line flux to achieve
S/N=5 detection with 1 hour on-source integration time are
presented below. Please pay attention to the following notes that
are valid both to LR and HR in applying this information to your
We have still been in the process of collecting systematic on-sky data
for better performance verification (especially in HR). We should
therefore mention that the sensitivity information below are still
preliminary and is subject to change in future.
- For continuum, S/N was measured after
applying four pixel binning to spectra (cf. the FWHM of
a single emission line (i.e. ~one spectral resolution element) is 4-5
pixel). Most of the data used to derive the sensitivity are actually
from faint galaxies, and the magnitudes referred here are their
total magnitudes (mostly MAG_AUTO from SExtractor).
- For emission line, S/N was measured per
emission line: It was calculated by integrating the line
profile and comparing it with the noise level expected in the
corresponding spectral element. This sensitivity estimation was
performed using the spectra of galaxies, not AGN/QSO. Observers
therefore need to be careful when applying this number to such
objects having broad lines.
- The emission line flux in the tables below were
derived assuming that an object is a point source,
i.e., most (~80%) of the energy falls on the an Echidna fiber on the
- The numbers in the following tables are
the averages in the corresponding bands. Observers need to be
careful when applying them to features near the edge of each band,
especially 0.9-1.0 μm, 1.40-1.45 μm, and 1.75-1.80 μm, where
the instrument throughput is very low (see Fig. 1).
- These sensitivity information is based on the data
that were taken by employing the cross-beam switching (CBS)
mode. We will investigate how much difference is given to the final
data quality between normal beam switching (NBS) and CBS with the
same on-source integration time.
For observers to investigate more detailed feasibilities of their
science targets as a function of wevelength,
FMOS spectrum simulator is also
- Low Resolution mode (LR)
The sensitivity varies as a function of wavelength, mainly according
to the variation of instrument throughput. Hence we divide the
spectral coverage (0.9-1.8 μm) into four bands like in the High
Resolution mode and list the limiting magnitude (flux) for continuum
(emission line) within each band, respectively.
||Emission line flux
||[erg cm-2 s-1]
||6.2 x 10-16
||1.0 x 10-16
||1.0 x 10-16
||1.9 x 10-16
Tab. 1 - Resolving power and sensivity to contiuum and emission
line in the Low Resolution (LR) mode.
- High Resolution mode (HR)
While the higher spectral resolution in HR tends to allow a smaller
number of photons to arrive at each pixel than LR, this is
compensated to some extent by its higher throughput (see Fig. 1). If
the high resolution is essential e.g. to see a line profile, only
~4-5 pixels should be binned at most and the sensitivity (per pixel
and per spectral resolution element) should be lower in HR.
Meanwhile, if the main goal is e.g. the detection of emission
line(s) and measurement of total line flux(es), a number of pixels
could be binned and more benefit would be expected from the higher
throughput (at the expense of spectral resolution instead), thanks
mainly to the negligible contribution of readout noise when the ramp
sampling method is applied.
Below we list the sensitivity information to achieve S/N=5 with 1
hour on-source integration in HR, after 4 pixel binning for
continuum and per emission line for emission line. As of Jan
2011, we have been focusing more on investigating the sensitivity of
emission lines, so we are still short of real data to estimate the
sensitivity to continuum. Therefore, we estimated it using (1) the
noise level measured on the actual HR spectra and (2) an assumption
that the sensitivity of the instrument in HR is twice as high as in
LR based on the comparison of real data for emission lines (see the
||Emission line flux
||[erg cm-2 s-1]
||1.7 x 10-16
||0.4 x 10-16
||0.5 x 10-16
||0.5 x 10-16
||0.5 x 10-16
Tab. 2 - Spectral coverage, resolving power, and sensivity to
contiuum and emission line in the High Resolution (HR) mode.
- Correlation between emission line flux and S/N
We detected a number of emission lines of faint galaxies at various
wavelengths during the engineering observations and GTOs. We
estimated the S/Ns (per emission line) of those detections and
normalized them to the values expected for 1 hr on-source
integration. In Fig. 2, these normalized S/Ns per emission line are
plotted against emission line fluxes (after the apergure effect at
the fiber entrance for a point source is applied): The data from LR
mode, J-long in HR, and H-long in HR are indicated by green, blue and
red squares, respectively. This suggests, although there is
significant scatter especially in the LR data, S/N=5 (S/N=10) can be
achieved by 1 hour on-source integration for an emission line with a
flux of 1.0 x 10-16 erg cm-2 s-1 in
LR (HR), respectively. For the LR data, a number of objects are
located below the correlation but they tend to be emission lines that
were observed at 0.9-1.1 μm or 1.7-1.8 μm where the instrument
throughput is lower, or on OH masks.
Fig. 2 - For emission lines of faint galaxies on the data from
engineering observations and GTOs, S/Ns (for 1 hr integration time,
per emission line) are plotted against emission line fluxes (after the
apergure effect at the fiber entrance for a point source is applied).
The data from LR mode,
J-long in HR, and H-long in
HR are indicated by green,
red squares, respectively.
Spectra from engineering observations
Below the spectra of a few astronomical objects are presented. The
data were taken during the engineering observation in Dec 2009, with
IRS1 in the LR mode. These spectra have been flux-calibrated taking
the energy loss at a fiber aperture into consideration.
||A low resolution spectrum of a galaxy with J(AB)~20.1 mag and
H(AB)~19.7 mag. The on-source integration time is 1.5 hours in the CBS
mode. The red line shows the spectrum after data reduction,
calibration, and 4 pix binning. The S/N of the continuum emission
(dashed line is shown for reference) is estimated to be about 5 from
1.1 μm to 1.7 μm (i.e. in J band and H band). A 2-D image of the
reduced and calibrated spectrum is indicated on top of the plot.
||An emission line galaxy at z=1.5. The on-source integration time
is 1.5 hours in the NBS mode. 3-pixel binning is applied. The emission
lines clearly detected are [OIII]5007 and Hα. A 2-D image at the
top of this panel shows the spectrum of this object, where the
positions of the emission lines are indicated by circles.
||An AGN at z=1.35. The on-source exposure time is 1.5 hours in the
CBS mode. 3-pixel binning is applied. In addition to the broad
Hα emission, the narrow [OIII]5007 emission is detected with
S/N~5, of which flux is estimated to be ~8 x 10-17 erg
cm-2 s-1. A 2-D image at the top of this panel
shows the positive and negative spectrum of this object from the CBS
observation (so the net integaration is 0.5 x 1.5 hours for each) and
the positions of the emission lines are indicated by circles.
FMOS spectrum simulator
Last updated: July 23, 2012
Copyrightę 2000-2011 Subaru Telescope, NAOJ. All rights reserved.