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Astron. Astrophys. 361, 877-887 (2000)

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2. Observations and data reduction

2.1. Imaging and deconvolution

N11C was observed on 20 November 1997 using the ESO New Technology Telescope (NTT) equipped with the active optics SUperb Seeing Imager (SUSI). The detector was a Tektronix CCD (#42) with [FORMULA] pixels of 24 µm (0".13 on the sky), and the seeing varied between 0".56 and 1".14 (FWHM ).

The observations were performed in the uvby Strömgren photometric system using the ESO filters # 715, 716, 713, and 714 respectively. We were particularly careful to keep most of the brightest stars in the field under the detector's saturation level to have at our disposal high quality Point Spread Function (PSF) stars. This led us to adopt exposure times of 180, 130, 150 and 90 seconds in u, v, b and y respectively. We also used ditherings of 5" -10" for bad pixel rejection and in order to be able to use the full oversampling capabilities of the MCS deconvolution algorithm. Indeed when performing simultaneous deconvolution of several frames, the algorithm uses the different frame centerings as a constraint while decreasing the pixel size (Magain et al. 1998). We took a grid of 7 dithering positions for each filter. Luckily, the objects of interest within N11 C are close enough to hold in a single SUSI field of view. Unfortunately, the u images could not be used for the photometry due to their insufficient quality.

Photometry was derived in the Strömgren v, b and y filters according to the following procedure: after bias subtraction and flat-fielding, the seven frames were co-added in each of the filters. The photometry of the stars situated outside of the compact clusters was performed on the resulting frames through the PSF fitting algorithms ALLSTAR and DAOPHOT (Stetson 1987) implemented in the ESO MIDAS reduction package. Multiple object subtraction was performed to clean the images, but only those objects found during the first activation of the FIND subroutine were retained for subsequent photometry. This yielded the photometry of 344 stars lying outside the subfields of the compact clusters Sk-66o41 and HNT.

These clusters were obviously too crowded for DAOPHOT to work properly. They were instead processed with the MCS deconvolution algorithm proposed and implemented by Magain et al. (1998). The deconvolution was performed on 128 [FORMULA] 128 pixel (16".64 [FORMULA] 16".64) sub-frames of the same co-added frames. Nevertheless, one of the original frames had to be removed from the sum in the y filter for cluster HNT because of a badly placed cosmic ray impact, so that only 6 frames were co-added for that filter before deconvolution of the HNT cluster. The original pixel size was reduced by a factor of two for the Sk-66o41 deconvolution but it was conserved for the restoration of the slightly less crowded HNT cluster, thus leading to final PSFs of 0".13 and 0".26 (FWHM) for the Sk-66o41 and HNT clusters respectively.

The MCS code results from a new approach to deconvolution taking care not to violate the Shannon (1949) sampling theorem: the images are deliberately not deconvolved with the observed PSF, but with a narrower function, chosen so that the final deconvolved image can be properly sampled, whatever sampling step is adopted to represent the final data. For this purpose, one chooses the final, well-sampled PSF of the deconvolved image and computes the PSF which should be used to perform the deconvolution. The observed PSF is constructed from several stars close enough to the clusters in order to avoid any possible PSF variation across the field.

The deconvolved frames unveil 63 and 70 objects in and around Sk-66o41 and in HNT respectively. Three of the stars in the Sk-66o41 subfield did not appear in all filters and were not included in the subsequent photometric treatment. Moreover, seven stars in HNT (#83, 86, 97, 99,103, 113, 128) and three stars in Sk-66o41 (#18, 38, 54) were excluded from further treatment because of suspicious photometry. The omitted stars in both cases are exclusively faint components, most of which appear in or close to the densest and brightest parts of the clusters, which strongly decreases their already low intrinsic S/N. The final sample is thus made of 57 stars in the Sk-66o41 subfield and 63 in HNT.

A technical problem prevented us from using the standard star observations to calibrate the photometry. Instead we first deconvolved three bright and isolated stars in the field independently in order to fix the zero point between deconvolved and DAOPHOT photometry. Then, we fixed the y magnitudes using the V magnitudes published in Paper I: out of the 22 stars of the region around Sk-66o41 for which these authors published photometry, 14 fall into our field. The zero point in y was established by matching our instrumental y magnitudes to the published V magnitudes for 13 of them, since the fourteenth revealed a strong discrepancy with respect to the others. The result has an rms uncertainty of [FORMULA] mag. Finally, we calibrated the v and b magnitudes by making use of the three stars for which we possess both photometry and spectral type (Wo597, Wo622 and star #204), by matching their ([FORMULA]) and ([FORMULA]) colors with those calibrated by Balona (1994) for stars with equivalent spectral types. The standard deviations of this operation are 0.03 in v and 0.01 in b.

The final photometric results for the two clusters are presented in Table 1 and Table 2, while the resulting color-magnitude diagrams for the individual clusters and the field stars are shown in Fig. 4.


Table 1. Photometry of the Sk-66o41 components. To allow reference to our earlier work, the last column provides a cross-identification with the numbering adopted in Paper II.


Table 2. Photometry of the HNT cluster

2.2. Spectroscopy with NTT/EMMI

The EMMI spectrograph attached to the ESO NTT telescope was used on 21 November 1997 (BLMD mode) to obtain several long slit spectra. The grating was # 12 centered on [FORMULA] 4350 Å and the detector was a Tektronix CCD (# 31) with 10242 pixels of size 24 µm. The range was [FORMULA][FORMULA] 3810-4740 Å and the dispersion 38 Å mm-1, giving FWHM resolutions of [FORMULA] pixels or [FORMULA] Å for a 1".0 slit. At each position we first took a short 5 min exposure followed by one or two longer 15 min exposures. The instrument response was derived thanks to observation of the calibration stars LTT1020, LTT1788, EG21.

The seeing varied from 0".7 to 1".4. These atmospheric conditions allowed us to obtain relatively un-contaminated spectra of some of the components of the HNT cluster, but we were unable to resolve the more compact Sk-66o41 cluster spectroscopically.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001