Merging: simultaneous use of the SW and LW sections

As explained in section 3.3 and in Fig. 3.1 (p. ) it is possible to use both instrument sections simultaneously. In practice this will always happen, even if the observer does not request it .
The principle of merging allows that the observer, by a proper choice of the wavelength ranges to be observed, obtains either free additional observations or a better SNR of the desired wavelength range. Knowledge of a few simple rules is sufficient to get optimum results from an observing proposal. Rules and gains are different for AOT SWS07 with respect to AOTs SWS02 and 06.

For AOTs SWS02 and SWS06 it is important to propose per aperture the same number of observations for both the LW and the SW sections. The AOT logic selects LW-SW pairs on the basis of closely similar integration times, and per pair the longest integration time is charged to the observer. Thus the largest gain is made when the two individual observations within a pair require nearly the same integration time. It is up to the proposer to obtain this goal by a proper selection of SNR and wavelength ranges. When the proposer does not (fully) use the possibilities of merging, the AOT logic will optimize the proposal by itself. This will be done in the following way:
- the requested SNR of the shortest measurement within each LW-SW pair will be increased until the integration times become closely similar.
- a measurement in either section, but within the same aperture, may be repeated in order to form a LW-SW pair with a measurement in the other section, which otherwise would not have been paired up.

For AOT SWS07 the observer should follow a different strategy. This strategy, as implemented in the AOT logic, does not lead to a pure rise in efficiency, because simultaneous use of SW-grating and LW-Fabry-Pérot will increase the overheads  and because merging is not possible through aperture 3. During the whole period in which the LW section is being used through aperture 1 or 2 to make Fabry-Pérot scans, the SW section is available to make grating scans through the same aperture. The practice of one to one pairing of LW and SW observations, as explained above for AOTs SWS02 and SWS06, does not apply to AOT SWS07. Instead, AOT SWS07 permits filling in of a Fabry-Pérot scan with several grating scans up to a maximum equal to the time needed to complete the Fabry-Pérot scan. Use of the grating during a Fabry-Pérot scan involves a loss in efficiency for the Fabry-Pérot scan. This loss amounts to an extra overhead of approximately 30 percent of the time needed to complete all SW observations.

To illustrate both merging procedures, two examples   of full range SWS observations are considered below. In both cases a constant flux density of 20 Jy and a requested SNR =  20 has been assumed. The results for AOT SWS06 are tabulated in Table 6.1, those for AOT SWS07 in Table 6.2.

 

Table 6.1: Merging of observations in AOT SWS06. The integration time in the last column shows which observation (SW or LW) determines the charged integration time

Total integration time in AOT SWS06.
Aperture	SW grating		LW grating
	band	time (s)	band	time (s)	charged times (s)
1	1A	378	--	--	378
	1B	522	3A	1024	1024
2	1E	434	--	--	434
	1D	642	--	--	642
	2B	688	3C	940	940
	2A	1058	3D	1262	1262
3	2C	2138	4	1484	2138

 

Table 6.2: Merging of observations in AOT SWS07. The integration time in the last column shows which observation determines the charged integration time

Total integration time in AOT SWS07.
Aperture	SW grating		LW F-P
	band	time (s)	band	time (s)	charged times (s)
1	1A	378	--	--
	1B	522	5B	2496	2496
2	--	--	5D	34216	34216
	1E	434	--	--
	1D	642	--	--
	2B	688	--	--
	2A	1058	5C	26520	26520
3	2C	2138	6	27800	28938

 

Table 6.3: Possible combinations of reset time , step size in scanner steps, and number of scans for each of the SWS AOT's

AOT	Band									notes
SWS01	all		1	2	2	2
			32	32	16	8				a
			2	2	2	2				1 up, 1 down
			24	24	24	24				b
		resolution	R/8	R/8	R/4	R/2				c
		duration	1172	1944	3846	6570				seconds
SWS02	1		1	2	4				..
			1	1	1				..
			2	2	2				..	up, down, etc.
			110	110	110				..	b
		duration	1.3	2.7	5.4				..	minutes
SWS02	2		1	2					..
	3		1	1					..
			2	2					..	up, down, etc.
			110	110					..	b
		duration	1.3	2.7					..	minutes
SWS02	4		1	2					..
			2	2					..
			2	2					..	up, down, etc.
			90	90					..
		duration	1.3	2.7					..	minutes
SWS07 F-P	5-6		1	2					..
										1/4
			2	2					..	unidirectional
										3 times or more

Table 6.4: Continuation of possible combinations of reset time , step size in scanner steps, and number of scans for each of the SWS AOT's

AOT	Band									notes
SWS06	1		1	2	2	4	4	4	..
+			4	4	2	2	1	1	..
SWS07			2	2	2	2	2	4	..	up, down, etc
SW section			27	27	55	55	110	220	..
SWS06	2-3		1	2	2	2	2	..
+			4	4	2	1	1	..
SWS07			2	2	2	2	4	..		up, down, etc
SW section			27	27	55	110	220	..
SWS06	4		1	2	2	2	2	2	..
			6	6	3	2	1	1	..
			2	2	2	2	2	4	..	up, down, etc
			30	30	60	90	180	360	..

Notes to Tab. 6.3 and  6.4:
a) Effectively. Actually 8 steps per sec
b) , the number of integrations per resolution element. This is the product of , the number of detectors in an array (12), and the ratio between resolution and stepsize.
c) effective spectral resolution