The final CCD frame consists of two sets of spectra, one set taken at the object position and one set taken at the sky position (figure ). The object and sky spectra are separated on the CCD image by the number of rows equal to the pixel offset used for the shuffle. We obtained data using a 700 pixel shuffle (roughly 1/3 of the total size of the array). Simply shifting the frame by 700 pixels and subtracting the shifted frame from the original data results in excellent sky subtraction, as shown in figure .
Figures and show the effect of the N&S subtraction on a single row in two different wavelength regions. It can be seen that subtraction is possible to the shot-noise limit of the sky, and that the residuals of the strong emission lines do not show strong `derivatives' that generally result from standard background model subtraction techniques.
Experimentation with individual nod exposures of 15 sec, 30 sec, 60 sec, 120 sec, and 180 sec, suggests (as one might expect) that the shorter times sample sky variations better and result in better sky subtraction (figure ). In general, we found that individual nod exposures shorter than 1 min are preferable; nod exposures as long as 3 min work, but the OH removal leaves some residuals, suggesting that the lines vary on timescales 1min. Table shows sample statistics for two BRACKET mode observations: a `short' nod exposure (30 sec 30 nods) and a `long' nod exposure (180 sec 5 nods). Although the statistics are limited, it can be seen that the mean sky value is closer to zero for the `short' nod compared to the `long' nod. However, the RMS values appear to be larger for the `short' nod exposure. The reason for this is unclear at present: it could be due to noise introduced by the shuffling (the `short' nod exposure has 6 times as many nods as the `long' nod exposure) and partly due to the longer total integration time (i.e., the time between the start of the observation and the readout) for the `short' nod exposure (2759 sec for `short' versus 2094 sec for `long'). A comparison of the histogram of counts in two frames:
|storage region||inbetween obj&sky||bottom of frame|
|number of pixels||419211||209500||90400|
|a0077 (180s x 5 nods):||0.45+/-1.87||2.93+/-2.29||3.05+/-2.25|
|a0078 (30s x 30 nods):||0.49+/-1.81||4.38+/-2.56||4.09+/-2.51|
Comparison of BRACKET and ALTERNATE modes.