Introduction

PMAS, the Potsdam MultiAperture Spectrophotometer, is an integral field ("3D") spectrograph, which has been in operation at the Calar Alto 3.5 m Telescope since 2001 [1]. The instrument is optimized for the UV-visual wavelength range from 0.35 to 1 ^.m, and offers two different integral field units (IFUs): a fiber-coupled lens array (LARR) for contiguous, seeing-limited sampling with 16^16 spatial elements (spaxels), covering a field-of-view

(FOV) from 8x8 arcsec2 to 16x16 arcsec2, depending on the choice of magnification, and a bare fiber bundle (PPAK) with projected fiber diameters of 2.7 arcsec, covering a hexagonal footprint of 65x74 arcsec. While the FOV of the LARR IFU is relatively small in comparison with other existing instruments, it has demonstrated excellent sampling properties, enabling PMAS to record the point-spread-function of point sources with a centroiding accuracy on the m.a.s. level. PPAK, however, is presently the largest FOV IFU available worldwide. Combined with the large light-collecting area of its spaxels (2.7 arcsec fibers), it is an ideal instrument for low surface brightness objects, surpassing even 8 m class telescopes with conventional slit spectrographs and other integral field spectrographs [2].

The fiber spectrograph has a field of view of 60x60 mm2, which is ideally suited for a 4Kx4K 15 ^m detector, providing 4096 pixels in the spectral direction, and the same number along the spatial direction. This is rather generous for the relatively small number of 256 spectra of the lens array, and sufficient for the 331 spectra of the PPAK IFU. The spacing for the former is 14 pixels per spectrum, which allows for an extra spectrum between each pair of nominal spectra without introducing significant crosstalk. Given this arrangement of spectra for the lens array IFU, a special variant of Nod & Shuffle (N&S) spectroscopy proposed by Glazebrook & Bland-Hawthorn, which makes use of this space, was introduced at the previous detector workshop SDW2002 [3]. The PMAS variant simply shifts the entire CCD image by 7 rows up and down the parallel charge transfer direction between sky and object exposures, thus resulting in an interlaced pattern of object and sky spectra (Figure 1).

object fx16l sky

object fx16l sky

The interlaced N&S scheme used for the PMAS LARR IFU.

The interlaced N&S scheme used for the PMAS LARR IFU.

This arrangement has the advantage of 100% detector efficiency for object+sky spectra, as opposed to 66% of the conventional technique, which uses one third of the available detector space as storage area. Also, the charge transfer occurs over just 7 rows, reducing sensitivity to low charge transfer efficiency. This new interlaced N&S mode of operation was commissioned at Calar Alto in 2002 and is now available for common user programs. Here we describe the first results from two applications: (1) measuring the line strength of the Calcium triplet lines in the optical near infrared for stellar population studies in elliptical galaxies, and (2) measuring faint diagnostic lines in the haloes of planetary nebulae, to study the mass loss history of stars at the AGB.

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