The Cosmic Origins Spectrograph
(COS) is an HST
fourth generation spectrometer, designed to enhance the spectroscopic capabilities of HST
at ultraviolet (UV) wavelengths. COS was built by Ball Aerospace Corporation to the specifications of Dr. James Green, the Principal Investigator (PI), at the University of Colorado at Boulder in conjunction with the COS Instrument Definition Team (IDT). Designed to primarily observe faint point sources, COS is optimized for maximum throughput, and provides moderate and low resolution spectroscopy in the UV and limited imaging in the NUV.
The FUV channel employs a large format cross delay line (XDL) detector
consisting of two 16384 x 1024 pixel segments, referred to as FUV segments A and B. The segments are separated by a physical gap of 9 mm, which makes it impossible to obtain a continuous spectrum across the two segments with a single setting. The supported central wavelength positions were selected to enable full wavelength coverage of the gap. Table 1.2
shows the wavelength ranges of both segments for all possible FUV grating and central wavelength combinations.
To retain efficiency utilizing the square format of the NUV detector, three mirrors
simultaneously image three, fully aberration-corrected, spectra onto a single 1024 x 1024 Multi-Anode Micro-channel Array (MAMA) detector. Consequently, three separate regions of the spectrum are imaged onto the detector. These spectral regions, referred to as stripes A, B, and C, each span the physical length of the detector in the dispersion direction - but are not contiguous in wavelength space. The allowable grating positions were defined with two objectives: the capability of obtaining full spectral coverage over the NUV bandpass and maximizing scientific return with a minimum number of grating positions. As a result, several of the supported central wavelength positions were selected to maximize the number of diagnostic lines on the detector in a single exposure. Table 1.3
shows the wavelength ranges of the three stripes for all possible NUV grating and central wavelength combinations
For each NUV and FUV central wavelength setting there are four grating offset
=1-4) available to move the spectrum slightly in the dispersion direction. This allows the spectrum to fall on different areas of the detector to minimize the effects of small scale fixed pattern noise in the detector. Figure 1.1
shows the shifts in uncalibrated x pixel coordinates of the stripe B spectra for all four FPPOS
This figure shows spectra obtained at all four FPPOS
positions using the G185M grating with a central wavelength setting of 1850. The individual plots show the collapsed counts from the stripe B spectra versus the uncalibrated x pixel coordinates. Note that the three features marked 1, 2, and 3, shift slightly for each FPPOS
COS imaging may only be done with the NUV channel and the spectral coverage
includes the entire NUV bandpass from ~1650-3200 Å. This mode utilizes a flat mirror with two available mirror settings, MIRRORA and MIRRORB. The first setting uses a primary reflection off the mirror surface, and the second setting provides an attenuated reflection. MIRRORB and/or the BOA may be used to obtain images of brighter objects, but MIRRORB produces a secondary image and the BOA produces an image with coma that degrades the spatial resolution (Figure 5.2
and Figure 5.3
). While the spatial resolution of COS NUV MIRRORA (Section 1.2
) images can be quite good, the field of view is very small. Furthermore, because the optics image the sky onto the detector – not the aperture – the image includes some light from sources out to a radius of about 2 arcsec. However, only point sources within about 0.5 arcsec of the aperture center have essentially all their light imaged, and so the photometric interpretation of a COS image can be inherently complex.
COS has two modes of data collection, TIME-TAG and ACCUM, and only one
mode can be used for a given observation. In TIME-TAG mode the position, time, and for FUV, pulse height of each detected photon are tabulated into an events list, while in ACCUM mode the photon events are integrated onboard into an image. TIME-TAG data have a time resolution of 32 ms, and can be screened as a function of time during the post-observation pipeline processing to modify temporal sampling and exclude poor quality data. COS is optimized to perform in TIME-TAG mode, although ACCUM mode is fully supported in the pipeline processing. ACCUM mode should be used primarily for UV bright targets that can not be observed in TIME-TAG mode due to high count rates. Users should note that FUV data taken in ACCUM mode use sub-arrays since the 18MB of onboard memory cannot hold a complete FUV image (containing both detector segments). ACCUM mode omits only the wavecal region and unused detector space, therefore the FUV ACCUM subarrays contain all of any external spectrum. The FUV ACCUM subarrays, whose sizes are 16384 x 128, are shown in Figure 2.2