EUROPEAN SOUTHERN OBSERVATORY La Silla Science Operations: NTT
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The imaging quality of EMMI is limited by the pixel size (0.1665 arcsec in the red arm, and 0.35 in the blue) and by the optics. The latter limits the best achievable image size to about 0.5-0.6 arcsec. Under better seeing condition SUSI is recommended instead.

The count rates of the instruments are monitored and scaled to 15th mag A star at unit airmass. Most recent measurements from observations for the Red CCD taken in May 2002 during the commissioning are given in the Table below. For the evolution of the zero-points and the extinction coefficients with time, please consult this link.

Remind that an exposure time calculator for the imaging mode is available.

Approximate color equations for the BVRI filters used in RILD and for the UB filters used in BIMG have been derived from observations done in May 2002 and August 2000 respectively. The stability of the instrumental system is reasonably good, but these equations should not be relied upon for accurate photometry. To obtain photometry better than 0.1 mag, the color terms should be measured.

Imaging throughput for a 15th mag A star at airmass = 1.

Red arm		Blue arm
Filter	Count rate (e-/s)	Filter	Count rate (e-/s)
Bb	13439	U	2450
V	25712	B	19290
R	31185
I	17915

To calculate the expected S/N, the seeing and the sky background should be taken into account. Optimal seeing in EMMI corresponds to 0.6 arcseconds, limited by the optics. Measured median seeing in EMMI is about 0.8 arcseconds, and is generally within 10-20 per cent of the ambient seeing measured with the seeing monitor. If the seeing is, or in the case of service observing, should be better than 0.6 arcsec, we recommend to use SUSI , although 0.6 arcsec images have been obtained with EMMI during good nights.

The following table gives the sky brightness as measured at CTIO as function of the phase of the Moon. Especially during bright Moon, the measured values at the NTT may be higher due to scattered light inside the dome.

Sky brightness as function of the Moon phase for several filters.
The values are taken from a NOAO newsletter (Walker 1987) and measured at Tololo, near La Silla .
These values are in agreement with those measured in La Silla (see also the SUSI sky page).