Calibration

Several internal calibration measurements are made during normal observations. Some are made at the beginning of the observing night or after a change of the fringe tracker parameters (DARK, BRIGHT) and others are taken once for every source (BACKGROUND, FOREGROUND, RATIO and RATIO2). DARK measurements are shuttered internally, BACKGROUND are off-source, BRIGHT are on an internal light source, FOREGROUND are on-source but off-fringe and RATIO/RATIO2 are on-source, off-fringe with either Keck I (RATIO) or Keck II (RATIO2) shuttered internally. RATIO/RATIO2) measurements are made as 5 second sequences are interleaved with 25 secs of fringe tracking data. For data reduction purposes DARK and BACKGROUND are equivalent (in the near-infrared).

The detector bias terms are measured and removed from the data on the sample time scale. The bias terms for X, Y and N (called BX, BY and BN) are calculated from the either the DARK or BACKGROUND data using equations 2-3. The X, Y, N values for each data sample are corrected by subtracting the bias terms and the corrected value of NUM is

NUM^* = (X - BX)² + (Y - BY)². (7)

Two additional bias terms come from the photon noise and the read noise. The variance of the read noise term, Brn, is calculated from the average value of NUM^* during the DARK or BACKGROUND observations. The variance of the photon term is kN, where k is the number of detected counts per photoelectron. A high-level calibration observation (BRIGHT) is used to measure k which is given by

$$\hat{{N}}$$ (8)

where the notation $\hat{{N}}$ is used to denote bias corrected values. The final corrected value for the energy measure can now be calculated as

$$\widehat{{NUM}} \hat{{N}}$$ (9)

Control of when to calibrate the data and which data are used to calculate the bias values is described in Sec. 3.1.2. Note that the value of k currently used is set by internal measurements rather than nightly measurements due to the detector characteristics.

The RATIO and RATIO2 measurements are averaged for each integration and the ratio correction calculated as

$${\frac{{(1+R)^2}}{{4R}}}$$ (10)

where R is the ratio of photon counts for each arm. The ratio correction is calculated for each spectral channel and is included on the SUM line (Table 5) for wide-band and summed spectrometer outputs and on the SPEC line for the individual spectrometer channels. Note that data taken before September 2002 only contain RATIO data and not RATIO2. In this case the FOREGROUND and RATIO data are used to calculate the ratio correction.