The COS optical design includes an external shutter, two science apertures, two
calibration apertures, two Optics Select Mechanisms (OSM1 and OSM2), and separate NUV and FUV detectors. COS also has an independent calibration lamp assembly containing two Pt-Ne and two deuterium lamps, which can illuminate the detectors with an emission line or a continuum spectrum, respectively. The COS optical design and elements are displayed in Figure 1.2
External light enters the aperture mechanism through either the PSA or the BOA
and illuminates OSM1, which contains the three FUV gratings and a mirror. Each grating can be set to one of several positions, to obtain different wavelength ranges. The positioning of the OSM1 mechanism is not precisely repeatable, and this can cause small, but significant, variations in how the spectrum or image is projected onto the detector. This non-repeatability can be corrected in post-observation data processing using separate or concurrent (tagflash
) calibration lamp exposures (wavecals). The FUV gratings correct for aberration in the dispersion direction only, and disperse the incoming light onto the FUV XDL detector. The COS FUV channel optical path is illustrated in Figure 1.3
If the OSM1 is set to the mirror position, the incoming light is directed to a
collimating mirror, and then to OSM2, which contains a mirror for imaging and the four NUV gratings. Each grating offers multiple positions. As is the case with OSM1, the positioning of OSM2 does not repeat exactly, and the data need to be corrected in post-observation data processing via either separate or concurrent wavecals. If a grating is in place on OSM2, the dispersed light is imaged onto the NUV detector by three separate parallel camera mirrors (NCM3a, b, c). This results in three spectra, or stripes, covering different wavelength ranges. Full wavelength coverage may be obtained through multiple observations with different grating positions. Alternatively, if the plane mirror is in place on OSM2, the undispersed light is sent to the middle camera mirror (NCM3b) and then imaged onto the NUV detector. The plane mirror on OSM2 may be used in either of two settings, designated as MIRRORA and MIRRORB. The MIRRORA setting employs a direct reflection from the plane mirror. For the MIRRORB setting, the plane mirror is slightly offset to provide primary reflection off the front surface of its coating and hence an attenuation factor of approximately 25 compared to the MIRRORA setting. The COS NUV channel optical path is illustrated in Figure 1.4
A series of beam-splitters and fold mirrors direct light from the calibration lamp
assembly (see Figure 1.2
), through either the WCA or FCA and into the optical path. The calibration lamp assembly can provide continuum illumination with its deuterium lamps and emission line illumination with its Pt-Ne lamps to both the NUV and FUV detectors. The Pt-Ne lamps may be operated during science exposures in order to produce concurrent wavelength calibrations (TAGFLASH
The FUV channel uses a large-format, windowless solar-blind cross delay line
(XDL) detector. This is a two-segment photon-counting detector with microchannel plates feeding a XDL anode. The data are digitized to a 16384 x 1024 pixel format for each segment; however the active area is only 14200 x 540 for Segment A (FUVA) and 14150 x 400 for Segment B (FUVB). Because there are no physical pixels, fiducial electronic pulses are recorded at specific times throughout an observation to permit alignment of data to a standard reference frame. These electronic pulses are referred to as “stim pulses”. Figure 1.5
schematically shows the COS FUV XDL segments with the locations of the active areas and stim pulses. When active, the stim pulses emulate counts located near the edges of the anode, beyond the illuminated portions of the detector. A zoomed-in image of one of the FUV stim pulses on segment B is shown in Figure 1.6
. An example of an FUV external science spectrum taken with Segment B is shown in Figure 1.7
, with a simultaneous wavelength calibration spectrum.
Wavelength calibration spectra for FUV segment B with G160M at 1600 obtained during
ground testing. The upper spectrum is from the internal wavelength calibration lamp obtained through the WCA. The lower spectrum is from an external lamp obtained through the PSA. The bright streak at the bottom is due to an area of enhanced background on the detector segment. Note that the size of the active area is somewhat less than the overall digitized area, and that the Y
axis has been stretched. The STIMs are also visible in the upper left and lower right corners.
With each recorded event on the XDL detector, the total charge in the associated
electron cloud incident on the anode is recorded. For FUV TIME-TAG data this pulse height amplitude (PHA) is sent to the ground along with the position of the event and can be used during data analysis to identify non-photon events. For FUV XDL ACCUM
mode data, only an integrated pulse height distribution (a histogram of the PHA data) for the entire segment is available, see Figure 1.8
A photon landing on an FUV detector segment creates an event (a cascade of
electrons) at the backside of the detector which is characterized by a pulse height amplitude (PHA) that is detected by the electronics. The detector electronics distinguishes between real and electronic noise events by the value of the PHA, with noise events having low PHAs and real events large PHAs. However, as a portion of the detector is exposed to more and more light, the PHAs that it produces become smaller, an effect called “gain sag”. Gain sag results in two effects: the mis-registration of the event positions
and localized sensitivity loss.
Mis-registration of event positions
as a function of PHA is termed “walk”. Walk has been identified in both the dispersion (X) and cross-dispersion (Y) directions. Currently, a simple Y-walk correction is made by the COS calibration pipeline (see Section 3.4.5
), and work has begun on a better correction for both X and Y.
Localized sensitivity loss
occurs when the PHAs for some pixels become too small to be distinguished from background events, causing events to be missed or filtered out. This results in a localized region of low sensitivity. Eventually, the PHAs of all of the pixels in a region become so small that photons landing on that location no longer create events with non-zero PHAs. In that case, no events are registered and the region is termed a “dead spot”. When this occurs, it is necessary to either increase the high voltage applied to the detector (which increases the PHAs of all the pixels), or to move the aperture so that the science spectra land on a different portion of the detector (which has not been exposed to as much light). The COS FUV detectors have already experienced localized gain sag on regions of the FUVB and FUVA detectors exposed to the bright Ly α airglow line when the G130M and G140L are used respectively. As a result, the detector high voltage was increased on FUVB on 10 March 2011 and on FUVA on March 26 2012. Furthermore, beginning on July 23 2012 the default location for the science spectra and target acquisitions on the detector was moved to a different position, also known as “new lifetime position”. For more information on the new lifetime position consult Appendix A:COS New Spectra Position
The NUV channel uses a 1024 x 1024 pixel Multi-Anode Micro-channel Array
(MAMA) detector. This has a semi-transparent cesium telluride photocathode on a magnesium fluoride window, which allows detection of photons with wavelengths from 1150 to 3200 Å. The NUV MAMA provides no pulse-height information, but may be used in both ACCUM
mode. The NUV channel creates three spectrum stripes on the MAMA detector, resulting in three separate stripes for the science data and three for wavelength calibration data as shown in Figure 1.9