The interim instrument is a simple CCD imager with filters.
Detector
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
| Binning | Photoactive Pixels | Pixel Size | Pixel Size |
|---|---|---|---|
| (px) | (px) | (µm) | (arcsec) |
| 1 × 1 | 2184 × 1472 | 6.8 | 0.351 |
| 2 × 2 | 1092 × 736 | 13.6 | 0.702 |
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
| Binning | Read Mode | Read Noise | Gain | Read Time |
|---|---|---|---|---|
| (px) | (MHz) | (e) | (e/DN) | (s) |
| 1 × 1 | 1 | 9.4 | 6.3 | 4.0 |
| 2 × 2 | 1 | 10.7 | 6.2 | 3.9 |
| 1 × 1 | 6 | 19.5 | 6.4 | 1.2 |
| 2 × 2 | 6 | 25.8 | 6.4 | 1.0 |
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.
Filters
The interim instrument has an FLI CFW-1-5 filter wheel equipped with BVRI filters and a clear w filter.
The BVRI filters are Bessell filters supplied by Custom Scientific.
The w filter is an Edmund Optics fused silica window with VIS-NIR coatings (part number #84-458).
All filters are 50 mm in diameter and 5 mm thick.
Efficiency
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
| Filter | Pivot Wavelength |
|---|---|
| (nm) | |
| B | 439 |
| V | 539 |
| R | 635 |
| I | 819 |
| w | 630 |
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
| Filter | Zero-Point | Magnitude Type |
|---|---|---|
| B | 4.6 × 108 | Vega |
| V | 4.8 × 108 | Vega |
| R | 5.7 × 108 | Vega |
| I | 2.8 × 108 | Vega |
| w | 2.4 × 109 | AB |
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.