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MOS Performance


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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