2 Observations and data reduction

Integral-field spectra of SuWt 2 were obtained during two observing runs in 2009 May and 2012 August with the Wide Field Spectrograph (WiFeS; Dopita et al., 2007). WiFeS is an image-slicing Integral Field Unit (IFU) developed and built for the ANU 2.3-m telescope at the Siding Spring Observatory, feeding a double-beam spectrograph. WiFeS samples 0.5 arcsec along each of twenty five $38$arcsec $\times$ $1$arcsec slits, which provides a field-of-view of $25$arcsec$\times$$38$arcsec and a spatial resolution element of $1.0$arcsec$\times$$0.5$arcsec (or $1\hbox{$^{\prime\prime}$}\times 1\hbox{$^{\prime\prime}$}$ for y-binning=2). The spectrograph uses volume phase holographic gratings to provide a spectral resolution of $R=3000$ (100 kms${}^{-1}$ full width at half-maximum, FWHM), and $R=7000$ (45 kms${}^{-1}$ FWHM) for the red and blue arms, respectively. Each grating has a different wavelength coverage. It can operate two data accumulation modes: classical and nod-and-shuffle (N&S). The N&S accumulates both object and nearby sky-background data in either equal exposures or unequal exposures. The complete performance of the WiFeS has been fully described by Dopita et al. (2007); Dopita et al. (2010).

Figure: Top panel: narrow-band filter image of the PN SuWt 2 on a logarithmic scale in H$\alpha$ and [NII]6584Å taken with the European Southern Observatory (ESO) 3.6-m telescope (programme ID 055.D-0550). The rectangles correspond to the WiFeS fields of view used for our study: 1 (red) and 2 (blue); see Table 1 for more details. Bottom panels: Spatial distribution maps of flux intensity and continuum of $[$O III$]$ $\lambda$5007 for field 2 and locations of apertures ($10$arcsec$\times$$20$arcsec) used to integrate fluxes, namely `A' the ring and `B' the interior structure. The white contour lines show the distribution of the above narrow-band H$\alpha$ emission in arbitrary unit. North is up and east is towards the left-hand side.

Our observations were carried out with the B3000/R3000 grating combination and the RT560 dichroic using N&S mode in 2012 August; and the B7000/R7000 grating combination and the RT560 dichroic using the classical mode in 2009 May. This covers $\lambda \lambda$3300-5900 Å in the blue channel and $\lambda \lambda$5500-9300 Å in the red channel. As summarized in Table 1, we took two different WiFeS exposures from different positions of SuWt 2; see Fig. 1 (top). The sky field was collected about 1 arcmin away from the object. To reduce and calibrate the data, it is necessary to take the usual bias frames, dome flat-field frames, twilight sky flats, `wire' frames and arc calibration lamp frames. Although wire, arc, bias and dome flat-field frames were collected during the afternoon prior to observing, arc and bias frames were also taken through the night. Twilight sky flats were taken in the evening. For flux calibration, we also observed some spectrophotometric standard stars.

Table 1: Journal of SuWt 2 Observations at the ANU 2.3-m Telescope.

Field	1	2
Instrument	WiFeS	WiFeS
Wavelength Resolution	$\sim 7000$	$\sim 3000$
Wavelength Range (Å)	4415-5589,	3292-5906,
Wavelength Range (Å)	5222-7070	5468-9329
Mode	Classical	N&S
Y-Binning	1	2
Object Exposure (s)	$900$	$1200$
Sky Exposure (s)	-	$600$
Standard Star	LTT3218	LTT9491,
		HD26169
$v_{\rm LSR}$ correction	$-5.51$	$-25.77$
Airmass	$1.16$	$1.45$
Position (see Fig.1)	13:55:46.2	13:55:45.5
Position (see Fig.1)	$-59$:22:57.9	$-59$:22:50.3
Date (UTC)	16/05/09	20/08/12