Subsections

• 4.1.1 ICS

4.1 AOT Introduction

Astronomical observations with ISO are carried out via observing modes known as AOTs, Astronomical Observation Templates. These are modes in which the basic operating parameters for the instruments are predefined (e.g. which sub-instrument to use at what wavelength resolution), leaving the user to select wavelengths to scan between, enter flux levels etc.

There are two main observing modes with the SWS.

1.

Grating-Only Observations (Medium spectral resolution). The spectral region of interest is directed to an array of 12 detectors. The spectral range instantaneously covered by the array is about 8 grating resolution elements, with gaps between the individual detectors (except for Band 3). For an observation, the grating is scanned in small steps to fully sample the desired wavelength range and provide a certain redundancy in case of detector failures or detector memory effects. Different scan schemes are used to obtain high fidelity line profiles, scans of wavelength ranges, and quick low-resolution full scans of the SWS wavelength range.

2.

Fabry-Pérot / Grating Combination Observations (High spectral resolution). The wavelength is selected by setting the Fabry-Pérot unit to a gap where one of the transmitting orders matches the correct wavelength. The F-P unit is then scanned in steps of about 1/4 F-P resolution element. The grating is used as an order sorter, with the maximum of the grating transmission tuned to the desired wavelength so that unwanted F-P orders are suppressed. Because the two detectors of a F-P detector pair look at slightly different grating resolution elements, this tuning can normally be done only for one detector. The other detector will not simultaneously deliver a useful signal as use of this redundant detector would require commanding of a slightly different grating position.

Figure 4.1 gives an overview of the wavelength ranges that could be simultaneously covered by the SW and LW or FP detector systems.

Figure 4.1: Overview of possibilities for simultaneous use of LW grating and SW grating (top) and FP and SW grating (bottom). Only combinations within the hatched area were allowed, and simultaneous observations through aperture 3 when observing with the FP were not allowed. The corresponding AOT band designations and aperture numbers are indicated.

During an AOT observation, SWS scans the source spectrum by rotating the scanning mirror of the grating or varying the gap of the Fabry-Pérot. Each AOT may perform one or more exposures with an internal calibrator for a photometric check and a number of dark current measurements with the aperture closed. Hence, the data produced by any single AOT is generally of several different types.

Four AOT's, listed below, are defined for SWS. Operating modes have changed since the start of PV, and any such changes are noted in this document.

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AOT 1: a single up-down scan for each aperture. It has four possible scan speeds and is primarily intended to quickly scan the entire SWS wavelength range at degraded resolution.

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AOT 2: slow short up-down scans, designed to measure line profiles.

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AOT 6: medium length grating scans with various options. Programmed as long up-down scan, possibly with reference scan. Used to observe long wavelength regions

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AOT 7: LW section devoted to FP observations. The SW section does almost the same as in AOT 6. In this AOT priority is given to the FP.

Tables 4.1 and 4.2 summarise their properties. In all of these AOT's the measurements of photometric check, dark current and science data are interleaved in the ERD and SPD, while the AAR only contains science detector data for which an unique wavelength could be assigned to the light falling on that detector.

Table 4.2: Possible combinations of reset interval t_r, step size L_s in scanner steps, and number of scans n_scan for each of the SWS AOT's

AOT	Band									notes
SWS01	all	tr	1	2	2	2
		Ls	32	32	16	8				a
		nscan	2	2	2	2				1 up, 1 down
		nint	24	24	24	24				b
		resolution	R/8	R/8	R/4	R/2				c
		duration	1172	1944	3846	6570				seconds
SWS02	1	tr	1	2	4	4			..
		Ls	1	1	1	1			..
		nscan	2	2	2	4			..	up, down, etc.
		nint	110	110	110	110			..	b
		duration	1.3	2.7	5.4	10.8			..	minutes
SWS02	2	tr	1	2	2				..
	3	Ls	1	1	1				..
		nscan	2	2	4				..	up, down, etc.
		nint	110	110	110				..	b
		duration	1.3	2.7	5.4				..	minutes
SWS02	4	tr	1	2	2				..
		Ls	2	2	2				..
		nscan	2	2	4				..	up, down, etc.
		nint	90	90	90				..
		duration	1.3	2.7	5.4				..	minutes
SWS07 F-P	5-6	tr	1	2					..
		Ls								1/4
		nscan	2	2					..	unidirectional
										3 times or more
SWS06	1	tr	1	2	2	4	4	4	..
+		Ls	4	4	2	2	1	1	..
SWS07		nscan	2	2	2	2	2	4	..	up, down, etc
SW section		nint	27	27	55	55	110	220	..
SWS06	2-3	tr	1	2	2	2	2	..
+		Ls	4	4	2	1	1	..
SWS07		nscan	2	2	2	2	4	..		up, down, etc
SW section		nint	27	27	55	110	220	..
SWS06	4	tr	1	2	2	2	2	2	..
		Ls	6	6	3	2	1	1	..
		nscan	2	2	2	2	2	4	..	up, down, etc
		nint	30	30	60	90	180	360	..

a.

Effectively. Actually 8 steps per sec.

b.

n_int, the number of integrations per resolution element. This is the product of n_scan, the number of detectors in an array (12), and the ratio between resolution and stepsize.

c.

Approximate effective spectral resolution cf AOT 2 - see section 4.2.4.

Example output from all AOTs are given in sections 4.2 to 4.5.

4.1.1 ICS

In all cases AOTs were actually commanded as sets of ICSs, Instantiated Command Sequences. These are more basic instrument commands to open the shutter, start the grating moving etc. The eight ICSs defined for SWS are listed in table 4.3. The use of these ICSs is described in the following sections.