This Grating Mode AOT covers a wavelength range specified by the observer, between 43.0 and 196.7 um. The minimum range that can be covered is 1.25 um for wavelength below 90.5 and 2.5 um for wavelengths above. The spectral resolution is 0.29 um below 90.5 un and 0.6 um above 90.5 um. This translates to (2000 km/s - 950 km/s) from 43.0 to 90.5 um, and the same range again from 90.5 to 197 um. The observer gives a set of "exposures" that list minimum S/N to be achieved or time to be spent at a set of up to ten wavelengths within the range. The time per grating position satistfying all exposure requirements is then determined and used. Data is recorded from all 10 setectors while the range is being scanned. The observer has a choice, in fractions of a resolution element, of the spectral sampling interval.
LWS02 Line Mode
This Grating Mode AOT produces medium resolution spectra at one to ten spectral line wavelengths specified by the observer. Data is recorded from all 10 setectors while the lines are being scanned. The observer has a choice, in fractions of a resolution element, of the spectral sampling interval and also selects the number of additional resolution elements to scan on either side of the central resolution element.
LWS02 Photometry Mode
In the Narrow-Band Photometry mode of this AOT (used by setting the scan width to zero) an under-sampled medium resolution spectrum over the full wavelength range of the LWS (43.-196.7) is produced: ten photometric points - one in each detector channel - at a resolution of 0.29 in the five channels at wavelengths less than 90.5 um and 0.6 um in the 5 channel above 90.5 um. In this usage of LWS02, the grating is not scanned, but rather remains in a fixed position.
This AOT results in a high-resolution spectrum covering a wavelength range, specified by the observer, up to the full range of the LWS (43.0-196.7) using the Fabry-Perot Interferometer. Data will be recorded only from the detector(s) in the range, with a single detector sampled as the range is scanned. The observer has a choice, in fractions of a resolution element, of the spectral sampling interval. The observer gives a set of "exposures" that list minimum S/N to be achieved or time to be spent at a set of up to ten wavelengths within the range. The time per grating position satistfying all exposure requirements is then determined and used. Note that the Fabry-Perot's low transmission lends this mode of observing to only the brightest sources.
This AOT produces high resolution (lambda/delta lambda = 8000-9800, or 31-38 km/s; see the LWS Manual for details) spectra for one to ten lines. Data will be recorded only from the detector at the line wavelength, with the single detector being sampled as the line is scanned. The observer has a choice, in fractions of a resolution element, of the spectral sampling interval and also selects the number (three or more) of additional resolution elements to scan on either side of the central resolution element. Note that the Fabry-Perot's low transmission lends this mode of observing to only the brightest lines.
Q. What do you mean "I get 9 Detectors Free?"
A. Yes, if you are using the LWS01 or LWS02 AOT, your ISO data will typically contain a sparse sampling of wavelengths across the entire LWS band between 43.0 to 196.7 um, gathered from the "other" nine detectors not selected for the primary part of your specified measurement. This is one of the truly wonderful attributes of the LWS - you can look at a line, detect it, and get a good measurement of the FIR continuum of an object at the same time completely free!
You can calculate the noise at each grating step from the other detectors, approximately, by running the time estimator, lws-te, and seeing how much time is spent at each grating step in your measurement. You can then use the inverse of formula 6.8 of the LWS Observers Manual to compute the noise to expect in a grating step from the other detectors:
Noise = NEFD / [ (2 t ) ^ 1/2 ]
where the NEFD is obtained from table 3 or 6 in the LWS Observer's Manual and the Noise is expressed in units of 10^(-17) W/m^2. One must choose a "sample" NEFD for each detector, since it is impossible to predict beforehand the exact wavelengths which the other detectors will be seeing. Typically you will get mant steps in thes other detectors. If, for example, you have specified an LWS02 observation with a spectral step size of 4 and a scan width of 2, you will have 20 data points (wavelengths) in you line measurement. In each of 9 other detectors, you may have this many data points as well.
Caveat: You are not assured of getting data from all nine detectors, because your observation may be accomplished in a scan mode or an overscan mode that may place the incoming radiation "out of reach" of the wavelength coverage of some of the other nine detectors. Also, it is yet unclear if the same calibration routines that will be applied automatically for you for your specified AOT data will also be applied to the output of the other nine detectors; there may well be further calibration work necessary on the part of the observer to recover this addition data.
Q. When do Detectors saturate ?
A. For the detectors in grating mode the saturation limits are as follows:
|Saturating Flux (Jy)...
|...at wavelength (um)
FP limits are about a factor of 100 higher. The LWS PI has stated that these numbers should not be taken as definitive because the definition of "saturation" is somewhat arbitrary and could change in the light of experience with data-processing. He believes that only solar system observers should be concerned about them and they should use them only with expert guidance (MB & P. Clegg). When the saturation is approached, the linear ramps become non-linear and only the linear portions are considered. Thus, the result of near-saturationis not a loss of calibration but a loss of data and thus S/N.
What's the wavelength range scanned by the LWS grating for each detector
A. In the figure below it is reported the grating encoder setting as a function of wavelength. The ten dotted lines (light and bold) are the ten detectors. The numbers represent the location of well known fine strucrture lines.