CFH12K - Optics and optical performance


Filters transmission curve, quantum efficiency and sky spectrum
The following diagram shows the quantum efficiency for the two types of CCDs populating the CFH12K: the EPI and the HiRho parts. The transmission curves for the broad-band filters are plotted while the narrow-band filters central wavelength is simply indicated on the axis.

A typical dark sky specturm collected at CFHT is also given. This spectrum will change (continuum and emission lines) depending of the solar activity over the years (average value given here).

Notice the very sharp edges of these interference filters (made by Barr). This raises an issue in the OH lines as discussed in the data section.

Figure 1: Filters transmission curve, CCD quantum efficiency and Mauna Kea sky spectrum


Filters characteristics
The following table provides precise characteristics of all the filters currently available for CFH12K.

Figure 2: CFH12K filters characteristics


Photometric performance
The total efficiency of the primary mirror plus the three lenses from the WFC is 73%. With a transmission of 99% for the cryostat front window, the overall efficiency of CFH12K at prime focus at 650 nm is 57% when one includes all the telescope optics plus filter and CCD.

The parameters for the photometric equation are:

magnitude = -2.5*log(counts[e-/sec]) - a*X + b*color + Co

with ``a'' the airmass term, ``X'' the airmass, ``b'' the color term and ``Co'' the photometric zero point at zero airmass in electrons per second. The following table gives the average ``a'', and ``b'' parameters over the twelve CCDs (for the four main photometric bands (parameters vary by a few percent from chip to chip). The photometric zero point ``Co'' is given for CCD09. The zero point for the other CCDs can simply be derived from the table of quantum efficiency numbers provided in the "CFH12K focal plane" section.

For example, the zero point in the R band for CCD03 is obtained by adding:

2.5*log(QE_R_CCD03 / QE_R_CCD09) = 0.2

to CCD09 zero point, hence C0(CCD03) = 26.77 (in e-/sec). From that point, to obtain the zero point in ADU/sec, 2.5*log(gain_CCD03) simply needs to be subtracted to that value.

Figure 3: CFH12K photometric performance (data 2000)

Sky brightness
The sky brightness was derived from real data taken on the sky. The sky brightness will vary over the course of the years depending on the solar activity (the following values are based on data taken in 2000, hence up by 0.2 magnitude compared to the values recorded at first light in January 1999).

The first table gives the magnitude per square arcsecond while the second table gives the actual numbers in electrons per second per pixel measured on CCD09. Sky brightness is given for two airmasses: 1.0 and 1.5 (sky brightness is constant from 1.0 to 1.2 and then increases for higher airmasses).

The last line gives the offset to apply to the given magnitudes or flux for a gery sky. The moon is supposed to be 40% up and far away (more than 40 degrees) from the astronomical target.

All these tables are based on real data obtained on the sky.

Figure 4: Sky brightness and offset to apply for a grey sky (50% moon)

Comments on the CFH12K pages to J.-C. Cuillandre: