The interim instrument is a simple CCD imager with filters.


The detector is FLI ML3200 with a Kodak KAF-3200ME CCD.

The field is 12.8 × 8.7 arcmin with the long axis roughly N-S. The detector can be binned and windowed. Table 1 gives the format and pixel scale for binning 1 × 1 and 2 × 2.

Table 1: Detector Format and Scale

BinningPhotoactive PixelsPixel SizePixel Size
1 × 12184 × 14726.80.351
2 × 21092 × 73613.60.702

The detector has two read modes (1 and 6 MHz). Table 2 shows the read-out parameters as functions of binning and read mode.

Table 2: Detector Read-Out Parameters

BinningRead ModeRead NoiseGainRead Time
1 ×
2 × 2110.76.23.9
1 × 1619.56.41.2
2 × 2625.86.41.0

The ADC saturates at 8k DN.

For science applications, we currently typically read the detector at 1 MHz (for lower noise), binned 2 × 2 (since the telescope currently has image quality problems), and unwindowed. The 6 MHz mode and windowing might be useful for applications which require less overhead and are not limited by read noise. Binning 1 × 1 might be useful for applications which are limited by the bright limit.

The detector has good peak quantum efficiency in 550-700 nm but is poor to the blue of 400 nm and to the red of 800 nm.

The detector is typically cooled to -30 C. The median dark current at this temperature is about 0.005 e/s/px, but there is a tail of hot pixels. About 6700 pixels (0.21%) have dark currents of 0.1 e/s/px or more, about 237 pixels (0.0072%) have dark currents of 1 e/s/px or more, and about 27 pixels (0.00084%) have dark currents of 10 e/s/px or more.


The interim instrument has an FLI CFW-1-5 filter wheel equipped with BVRI filters and a clear w filter.

All filters are 50 mm in diameter and 5 mm thick.


Firgure 1 shows the estimated system efficiencies — including the atmosphere at an airmass of 1.5, telescope, and instrument.

Figure 1. The estimated system efficiency.

The efficiency in w below 380 nm is uncertain. The AR coating on the filter is optimized for 400 nm and above, and we have no data on its performance below 380 nm.

Table 3 gives the pivot wavelengths of the filters, defined by equation (A16) of Bessell & Murphy (2012).

Table 3: Filter Pivot Wavelengths

FilterPivot Wavelength

Zero-Points and Transformations

Table 4 gives the zero-points for the filters, the count rates in electron/second for a magnitude zero star. The magnitude system is Vega-based for BVRI and AB for w.

Table 4: Filter Zero-Points

FilterZero-PointMagnitude Type
B4.6 × 108Vega
V4.8 × 108Vega
R5.7 × 108Vega
I2.8 × 108Vega
w2.4 × 109AB

For normal stars, the AB w magnitude is given in terms of the SDSS g and r magnitudes by

w - r ≈ 0.12 (g - r).

That is, w is effectively slightly bluer than SDSS r.