The sensitivity of MOS in imaging, in terms of optical throughput, was
measured on M92 and NGC7006 standard star fields (Christian et al.,
1985, P.A.S.P., 97) observed under photometric
conditions with the Lick2 CCD. We found the following transformation
equations ( is the airmass):
The same relations are valid for the Loral3 CCD; with the new thinned
CCDs, a gain of approximately 2 in sensitivity is expected in
V and R (and even more in blue), i.e. the zero
points in magnitudes should be increased by roughly 0.75.
The photometric coefficients in the V band, compared to
expected star fluxes outside the Earth's atmosphere, indicate that the
total efficiency atmosphere+telescope+MOS+ CCD(LICK2) ~10%
(compared to 11.4% for FOCAM in Nov. 91). Without the CCD the
efficiency atmosphere+telescope+MOS = 31%.
One can then estimate the efficiency of the MOS optics in the
V band, with the following assumptions on
transmissions/efficiencies: atmosphere = 0.85; 2 telescope mirrors =
0.822; MOS mirror = 0.97; filter = 0.82; CCD dewar window = 0.952; CCD
QE = 0.32 (5500Å):
Within the uncertainties of the above calculation this is close to the
predicted 80% in imaging mode.
The efficiency of MOS in spectroscopic mode has been measured on
standard spectrophotometric stars (Oke J., 1974, Ap.J. Suppl.,
27, 21; Stone R., 1977, Ap.J., 218,
767). The combined efficiency atmosphere + telescope + MOS + grism
+ CCD for several grisms is shown in Figure 28.
FIGURE 28; Total Efficiency: Atmosphere + Telescope + MOS + Grism + CCD
Figure 29 shows a typical sky spectrum obtained with the R300 grism
and a slit 2" wide. The accuracy of the sky correction is on the
order 0.2% r.m.s. of the sky intensity. The residual of the sky
correction was derived with the task background in IRAF, with
a third order Chebyshev polynomial, The r.m.s of the residuals are
0.2% in the interval 6000-9000Å, and higher at the edges because
of lower grism efficiency.
FIGURE 29; Sky spectrum with R300 (top) and residual of the sky correction (as a percentage of the sky intensity; bottom).
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