COS NUV Extraction Box Heights
for Faint Targets
The COS team studied the effect of varying the HEIGHT parameter in the XTRACTAB reference file (COS Data Handbook Section 3.7.11) for COS/NUV data. An extraction height of 8 pixels is used when estimating the signal-to-noise ratio in the COS exposure time calculator (ETC), while CalCOS uses a height of 57 pixels to create the final spectra. This is because the flux calibration for bright sources is performed using a spectrum extracted with a box large enough to encompass all the flux, while for faint sources, the maximum possible S/N is obtained by minimizing the dark rate and thus reducing the extraction height.
We studied the effect of varying the HEIGHT parameter in the XTRACTAB for targets of different brightness levels to examine whether there are benefits to lowering the value of 57 pixels used by CalCOS. We found that in order to avoid having to derive flux calibrations for each possible extraction height value, the extraction height could only be reduced to ~37 - 43 pixels. This did not improve the S/N of faint sources significantly, while possibly having a negative effect on the flux accuracy of brighter targets. We recommend PIs use a smaller box height for faint sources (i.e., when the dark rate is similar to the source count rate), but do not reduce the height below 13 pixels to ensure no significant signal is lost. To do this, PIs will need to customize an XTRACTAB, edit the HEIGHT column, and recalibrate as outlined in the COS Data Handbook Section 3.6.
Lastly, we point out that the centering of the spectral stripes may cause some flux to be left out of the extraction region when the extraction height is reduced. Due to target centering uncertainties (sometimes up to 13 pixels in the cross-dispersion direction), the B_SPEC value listed in the XTRACTAB file may not be perfectly centered on the middle of the stripe. While this will not have a significant impact on the flux accuracy in spectra with a 57-pixel height, this effect may result in significant vignetting and subsequent loss of signal for small height values. Therefore, customizing the B_SPEC values may be necessary as well.
More information about this work will be published in ISR 2017-03, “Analysis of COS NUV Extraction Box Heights” in the near future.