Scattered and Excess Light

Dark Current

The shutter for bHROS is located just after the image slicers (slit) and controls light entering the entire instrument enclosure, not just to the detector. Therefore, any excess light within the enclosure will mimic an enhanced dark current on the CCD. The traditional dark current for the bHROS detector is approximately 1 e^- per hour. Excess light within bHROS is an additional 5 e^- per hour, as measured from bias corrected long dark exposures. Thus, the total dark current for the instrument is 6 e^- per hour.

This enhanced dark current is well behaved and simply adds into the scattered light that can be removed using standard processing techniques. Most programs will not require dark exposures. If dark exposures are required, they must be specifically requested. See the Calibrations page for more information.

Please note: This dark current can be a significant source of noise for faint targets.

Scattered Light

In most instances, the scattered light within bHROS measures less than 5% (typically 1-2%) of the continuum and is well-behaved (i.e. smoothly varying) and sampled well between orders. Thus, the scattered light component can be easily removed using standard image processing techniques. However, in the case of emission line sources, the scattered light can be problematic. The scattered light pattern from a single (off-CCD) emission line can contain structure that is not easily modeled nor removed using standard processing techniques. For emission line sources, it may be advisable to use the available filters to block unwanted light outside of the wavelength region of interest.

In order to assess the level of scattered light in the spectral direction along an echelle order, the saturated core of interstellar Na I D2 in the spectrum of HD 152249 was observed during commissioning. The amount of scattered light in the spectral direction (not including the above sources of scattered light) was measured to be 0.31%.

Last update March 10, 2006; Steven Margheim