Using the parameters that were typed into the PGA software each observation is translated into a timed sequence of instrument commands. For every observation the exact sequence of executed commands is slightly different. However, there is a general structure for the command sequences that is different for the grating and the Fabry Perot and for range and line scans. Knowledge of the sequence of instrument commands is important for a better understanding of the instrument operations and the order in which the data are written in the data products. In this section we describe the sequences of operations (or commands) that are issued for each of the four LWS AOTs.
The commanded LWS sample list defines which analogue channels are sampled and
the length of the list determines the constant rate at which they are read out
during each format period (one format period is 2 seconds).
The interval between two detector resets, the
integration period, is also a commandable parameter which is determined
when the AOT is expanded into its ICS's (Instrument Command Sequences)
and which can lie between 0.25 and 0.5 seconds
(a quarter of a format period) duration. Note that the shorter reset
interval is only used for very bright sources (typically brighter
than 10000 Jy at 100 
m).
For slow scanning (previously described in the LWS Observers Manual as normal scanning) these integration periods will be repeated a number of times (at least 4) -- determined from the instantiated AOT -- at each position of the LWS mechanism (that is, at constant grating and, if applicable, Fabry Perot positions). The LWS mechanism position (s) are then varied to carry out a scan which will in general also be repeated a number of times (at least 3). In fast scanning mode the integration period is not repeated in the same scan, instead the mechanism is moved to carry out the scan, and then the whole scan is repeated a number of times (at least 3). Therefore each individual scan is carried out as fast as possible. In Figure 3.1 the concept of samples, resets and reset periods is explained in a graphical way.
Figure 3.1: The concept of samples, resets and reset intervals.
Each LWS spectral scan is divided into a given number of mechanism steps (either Fabry-Perot or Grating). For every step a number of integrations is performed, each integration corresponding to one detector ramp. Each ramp consists of a number of samples of the detector read out. These sampled data are the basic raw data product of the LWS called the Edited Raw Data or ERD. The total integration time for any observation is achieved by repeated scans of the mechanism, each with the same measurement structure. The bullets here give a concise overview of the measurement structure and define the terms used:
The values of NSAMPLE, NRESETS and NSTEPS can be found in the housekeeping data - see section 8.2.3.
Transparent data are AOT specific data that are not processed bt the satellite, but are passed directly from the uplink side of the ground station to the down link side ('transparent' in this case thus means bypassing the satellite). The Transparent Data (TDATA) contains information generated during the processing of the observer's input which may be required when processing the data for an observation, but is not required by the instrument to execute the observation on the satellite. The TDATA messages either relate to a complete observation (or AOT) or to the execution of a particular Instrument Command Sequence (ICS) and are written to the EOHA and EOHI files respectively. The main TDATA information appears as the fields EOHAAOTV and EOHIMSG1 in the EOHA and EOHI files. Their contents are shown in Tables 3.1 and 3.2.
| Offsets | Length | Type | Description |
| (bytes) | (bytes) | ||
| 0-3 | 4 | 2 I*2 | Raster Dimensions |
| 4-8 | 5 | Observation duration (seconds) not including slew | |
| 9-19 | 11 | Date of AOT to OCT logic processing as YYDDDHHMMSS | |
| 20-29 | 10 | unused | |
| 30 | 1 | FPS used flag (0: no, 1: yes), blank for grating AOTs | |
| 31 | 1 | FPL used flag (0: no, 1: yes), blank for grating AOTs | |
| 32-33 | 2 | spare | |
| 34-35 | 2 | I*2 | Total number of spectra (number of lines for line |
| scan AOTs, number of spectra to build up the range | |||
| for wavelength range AOTs | |||
| 36-37 | 2 | I*2 | Detector used for the start wavelength (only AOT L01) |
| 38-39 | 2 | I*2 | Detector used for the end wavelength (only AOT L01) |
| OR | |||
| 36-39 | 4 | I*4 | Start zone of the series of zones (only AOT L03) |
| 40-43 | 4 | I*4 | End zone of the series of zones (only AOT L03) |
| 44-47 | 4 | I*4 | First zone number corresponding to FPL (breakzone) |
| (only AOT L03) | |||
| 48-55 | 8 | F8.4 | Start wavelength of requested range (AOT L01 and L03) |
| 56-63 | 8 | F8.4 | End wavelength of requested range (AOT L01 and L03) |
| Offsets | Length | Type | Description |
| (bytes) | (bytes) | ||
| 0-3 | 4 | I*4 | Requested S/N for this line or range |
| 4-15 | unused | ||
| 16-17 | 2 | I*2 | Spectrum number (line number or part of range) |
| 18-23 | 6 | I*6 | Current scan number |
| 24-27 | 4 | I*4 | Current zone number (only AOTs L03 and L04, |
| see below | |||
| 28-35 | 8 | F8.4 | Wavelength (current line for line spectra or |
| reference line that determined integration | |||
| time for range spectra) | |||
| 36-43 | 8 | E8.3 | Incident power (for line in line spectra or |
| for reference line for range spectra) | |||
| 44-47 | 4 | I*4 | Maximum scan half width (only for line spectra) |
| 48-49 | 2 | I*2 | Active detector (line spectra: detector for |
| current line; Range spectra: detector for | |||
| reference line) | |||
| 50-55 | 6 | I*6 | Total number of scans to be completed |
| 56-59 | 4 | I*4 | Total number of measurements |
| 60-63 | 4 | I*4 | Number of scans between illuminator flashes |
As the TDATA is not sent to the satellite it is not synchronised accurately with the execution of ICSs by the instrument, or to the telemetry stream from the instrument. A counter has therefore been implemented in the LWS housekeeping which increments when each relevant ICS is executed allowing the data produced to be associated with the correct TDATA information by the ISO data processing software.
In table 3.3 a list of all LWS Instrument Command Sequences is given. These ICSs are uplinked to the instrument in order to command it to perform operations for the measurements. They are used in the next sections where the AOTs are described in detail.
| name | purpose |  |  |
| LS8100 | Perform grating scan | 4.0 | 6.0+(LENGTH*2.0) |
| LS8110 | Perform fixed grating observation | 4.0 | 10.0+(LENGTH*2.0) |
| LS8300 | Execute FPS scan measurement | 4.0 | 6.0+(LENGTH*2.0) |
| LS8301 | Execute FPS scan measurement | 3.0 | 4.0+(LENGTH*2.0) |
| LS8310 | Setup FPS mode | 8.0 | 30.0 |
| LS8320 | Switch off FPS subsystem | 4.0 | 8.0 |
| LS8400 | Execute FPL scan measurement | 4.0 | 6.0+(LENGTH*2.0) |
| LS8401 | Execute FPL scan measurement | 3.0 | 4.0+(LENGTH*2.0) |
| LS8410 | Setup FPL mode | 8.0 | 30.0 |
| LS8420 | Switch off FPL subsystem | 4.0 | 8.0 |
| LS8600 | Setup for opaque grating mode illuminator flash | 8.0 | 32.0 |
| LS8601 | Setup for grating mode illuminator flash | 4.0 | 10.0 |
| LS8603 | Setup for opaque FPS mode illuminator flash | 6.0 | 16.0 |
| LS8604 | Setup for opaque FPL mode illuminator flash | 6.0 | 16.0 |
| LS8605 | Reset from opaque grating mode illuminator flash | 8.0 | 38.0 |
| LS8606 | Reset from grating mode illuminator flash | 5.0 | 18.0 |
| LS8608 | Terminate opaque FPS mode illuminator flash | 6.0 | 26.0 |
| LS8609 | Terminate opaque FPL mode illuminator flash | 6.0 | 26.0 |
| LS8610 | Part 1 of grating mode illuminator flash | 7.0 | 42.0 |
| LS8611 | Part 2 of grating mode illuminator flash | 7.0 | 42.0 |
| LS8901 | Initialize instrument for grating mode operations | 7.0 | 18.0 |
| LS8906 | Reset instrument to HOLD mode after grating mode operations | 7.0 | 20 |
The internal Illuminators of the LWS instrument are used to correct the data for changes in the responsivity of the detectors (on a timescale of typically 30 minutes). There are two different kinds of illuminator flashes that are regularly performed.
The first one is an opaque illuminator flash. In this case one of the Fabry Perot is rotated in the beam and the etalons are put in a non-parallel position. This way the illuminators are observed relative to the dark signal. This kind of illuminator flash is performed at the start and end of every grating AOT and is used as the standard illuminator flash for FP observations. These flashes can also be used to determine the dark signal on the detector. The LWS Auto Analysis uses the data of these flashes to correct for the dark signal and to correct for responsivity drifts.
The second one is a normal grating mode illuminator flash. In this case the grating is put in the rest position and the illuminators are observed on top of the signal of the astronomical source. These illuminator flashes are only used to check for the responsivity changes over the AOT.
The illuminators should be flashed at least every 20 minutes to be able to correct for short term detector responsivity changes. The AOT to OCT logic therefore decides when the illuminator flashes should be done. This could be only at the start and end of the AOT or at the start of every scan (i.e. every line for line AOTS, every zone for FP AOTs).
The operations for raster scans is very similar to the operations for single pointing. For every point in the raster a observation is performed similar to the normal single pointing observations. The main difference are the illuminator flashes. In the case of raster scans these can occur at the following times depending on the length of the observation:
In figure 3.2 a schematic flow diagram is given of the operations of an LWS raster scan. The central dashed box represents the operations for the grating or Fabry-Perots as described below in sections 3.2.6 and 3.2.7.
Figure 3.2: Schematic overview of
the operations performed during a raster.
The general approach to grating spectra is to make multiple scans of the spectrum for the same observation where every scan contains three or more integrations per spectral point for slow scanning and one integration per spectral point for fast scanning. The minimum number of scans is three. A user will therefore end up with at least nine integrations for a single spectral point in the case of slow scanning and at least three integrations for a single spectral point for fast scanning. These integrations give, when added together, the requested S/N. In figures 3.3 and 3.4 a schematic diagram is given of the operations for an L01 and L02 AOT. These flow diagrams are represented in figure 3.2 by the central dashed box. They should help the observer to understand the order in which the data is written in the data products (especially the ERD and SPD).
Figure 3.3: A schematic overview of the
operations performed during an grating range scan AOT (L01).
Figure 3.4: A schematic overview of the
operations performed during an grating line scan AOT (L02).
The general approach to Fabry-Perot spectra (like for grating spectra) is to make multiple scans of the spectrum for the same observation where every scan contains three or more integrations per spectral point for slow scanning and one integration per spectral point for fast scanning. The minimum number of scans is three. A user will therefore end up with at least nine integrations for a single spectral point in the case of slow scanning and at least three integrations for a single spectral point for fast scanning. These integrations give, when added together, the requested S/N. For the Fabry-Perot scan itself, there is a difference between the range scan AOT and the line AOT. For the range scan AOT (L03) the grating is set to certain calibrated positions, and the FP is scanned over a range that could be the full range, to give a sub-spectrum over one grating element. The grating is then moved to the next position etc. For the line scan AOT the wavelength of the line is used to obtain the optimum position of the grating to get maximum transmission for the FP. The FP is then scanned over the requested scan width. The grating in this case is not set to a pre-calibrated position (although the position will of course be calibrated in the processing). When the Long Wavelength Fabry Perot (FPL) is scanned the offsets that are required to keep the etalons parallel are continuously changed in order to keep the etalons parallel. This way, the spectral resolution and the transmission of the FPL will be constant during a scan. The following description of the logic does not include this changing of the offsets. It is implicitly assumed to be done during an FPL scan.
In figures 3.5 and 3.6 a schematic diagram is given of the operations for an L03 and L04 AOT. These flow diagrams are represented in figure 3.2 by the central dashed box. They should help the observer to understand the order in which the data is written in the data products (especially the ERD and SPD).
Figure 3.5: A schematic overview of the
operations performed during an Fabry-Perot range scan AOT (L03)
Figure 3.6: A schematic overview of the
operations performed during an Fabry-Perot line scan AOT (L04).