The supported central wavelength positions were selected to enable full wavelength coverage of the gap. Recorded wavelengths 2. Therefore, only segment A is available for these settings.
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. The NUV detector's sensitivity at these wavelengths is extremely low. Their dispersion is twice that of the first-order spectrum.
At these wavelengths, the flux calibration applied by calcos is unreliable. 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. The first setting uses a primary reflection off the mirror surface, and the second setting provides an attenuated reflection. 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.
Home Instrument Handbooks Instrument Handbooks. Data Handbooks Data Handbooks. Close Menu On this Page. Menu Navigation. FUV Channel. NUV Channel. But as the average pulse height in a particular region approaches and then drops below this threshold, real photon pulses are increasingly misidentified as background, causing a decreasing effective throughput. Since the amount of gain sag increases with the total amount of the previous illumination, the effects first appear in regions of the detector that are illuminated by the bright Lyman Alpha airglow line, but eventually, the entire spectrum is affected.
A two-zone spectrum extraction algorithm is used by the COS pipeline to calibrate data obtained at LP3 and subsequent lifetime positions. This extraction method is optimized only for observations of point sources using the PSA aperture. See the COS webpage for updated information on the calibration of new lifetime positions. Fixed-pattern noise in the COS detectors limits the signal-to-noise that can be achieved in a single exposure see Section 5. A modest reduction in observational overheads will not normally be considered sufficient justification for not using all four FP-POS settings.
COS exposures may be obtained in either a time-tagged photon address TIME-TAG mode, in which the position, arrival time, and pulse height for FUV observations of each detected photon are saved in an event stream, or in accumulation ACCUM mode, in which only the positions of the photon events are recorded.
ACCUM mode is designed for bright targets with high count rates that would otherwise overwhelm the detector electronics. Because the lower information content of ACCUM data reduces their utility for archival researchers, its use must be justified for each target. For details, see Section 5. It is possible to suppress the taking of wavelength calibration spectra, but since it significantly lessens the archival quality of COS data, it must be justified.
Non-optimal observing techniques should not normally be adopted solely for the purpose of producing a modest reduction of the observational overheads; in such cases, the observer should normally just request adequate time to use the recommended optimal strategy see HST Preparation of the PDF Attachment.
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