Spectrometer

Fig.1Credit:MPE	Fig.2.-focal plane onto spectrometer arrays Credit:MPE/MPIA

Integral Field Spectrometer

- Simultaneous two bands 57-105 & 105-210 µm spectroscopy. Field of view, 47''x 47'' ( 5x5pixels ) FOV rearranged via an image slicer on two 16x25 Ge:Ga detector arrays.

- Resolution ~ 1500 km/s

- Sensitivity: ~ 2-8 x10^(-18) W/m^2 (5 sigma, 1h)


Spectrometer description


The spectrometer covers the wavelength range from 57 to 210 microns. It provides a resolving power of 1000-4000 (i.e. a spectral resolution of ~75-300km/s) with instantaneous coverage of ~1500km/s and simultaneous imaging of a 47"x47" field of view, resolved in 5x5 pixels. An image slicer employing reflective optics is used to re-arrange the 2 dimensional field of view along a 1x25 pixels entrance slit for a grating spectrometer, as shematically shown in Figure 2. The Littrow-mounted grating with a length of ~30cm is operated in first, second or third order, respectively, to cover the full wavelength range. The first order covers the range 105-210 micron, the second order 72-105 micron, and the third order 57-72 micron.
The light from the first diffraction order is separated from the light of the two other orders by a dichroic beamsplitter and passed into two optical trains feeding the respective detector arrays (stressed/unstressed) for the wavelength ranges 105-210 micron and 57-105 micron. Anamorphic re-imaging optics is employed to independently match the spatial and spectral resolution of the system to the square pixels of the detector arrays. the filter wheel in the short-wavelength path selects the second or third grating order.

(a) Fig.3- Image Slicer  (b) Fig.4- Qualification model of the grating unit. A torquer motor is used to actuate the grating angle which is measured with sub-arcsecond precision by an Inductosyn angular resolver (credits:MPE/MPIA)

Grating

The grating blank has a length of 320mm with a groove period of 8.5 ±0.05 grooves/mm, with a total of approximatively 2720 grooves. The reflection grating is operated in the first (105-210 micron), the second (72-105 microns) and the third diffraction order (55-72 micron). Grating deflections from 28 degrees to 68 degrees are possible to cover the full wavelength range of each order.

An effective resolution of l/dl ~ 940-5500, (cdl/l ~ 55-320 km/s) can be obtained. The instantaneous 16 pixel spectral coverage is ~100-500 (600-2900 km/s), corresponding to ~0.15-1.00 micron wavelength coverage. The table 2 shows summarises the grating characterisation in terms of velocity resolution, spectral coverage and typical grating step sizes for a given order/wavelength.

order	#	grating angle angle	FWHM	16 pix coverage	pixel per FWHM
	[micron]	[deg]	[km/s] [micron]	[km/s] [micron]
1	105	26.5	318 / 0.111	2856 / 1.000	1.78
1	158	42.18	239 / 0.126	1572 / 0.828	2.43
1	175	48.05	212 / 0.124	1280 / 0.747	2.65
1	210	63.19	140 / 0.098	720 / 0.504	3.11
2	72	37.73	164 / 0.039	1840 / 0.442	1.42
2	105	63.19	80 / 0.028	720 / 0.252	1.78
3	55	44.53	114 / 0.021	1448 / 0.266	1.26
3	72	66.63	55 / 0.013	615 / 0.148	1.42

Table 2- Relation between grating angle and wavelength

Filters

The PACS filters, in combination with the detectors, define the wavelength bands of the instrument. There are in total 3 bands in the PACS spectrometer: 55-72 microns, 72-105 microns and 105-210 microns. The graph (Fig.5) represent the overall transmission of the combined filters in each band of the spectrometer.

Fig.5- Filter transmissions of the spectrometer QM filters	Fig.6- High stress module close-up (credits:MPE/MPIA)

Photoconductor arrays

The spectrometer employs two Ge:Ga photoconductors arrays (stressed and unstressed) with 16x25 pixels each to perform integral field spectroscopy in a field of 47"x47", resolved in 5x5 pixels, with a spectral resolution of ~ 75-300 km/s. The Ge:Ga photoconductor arrays are a completely modular design : 25 linear modules of 16 pixels each are stacked together to form a contiguous, 2-dimensional array. For the long-wavelength band a sophisticated stressing mechanism ensures a homogeneous stress within each pixel along the entire stack. The second "unstressed" array with improved short-wavelength responsivity is almost identical to the long--wavelength array, except for the mechanical stress on the pixels which is reduced to about 10% of the level needed for the long-wavelength response.The figure 6 shows 25 stressed modules integrated into their housing.

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Credits

The contents of this page are based on the PACS observer's Manual produced by Bruno Altieri, Roland Vavrek and the PACS Instrument Control Centre

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For more information, see Observer Manual

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