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Дата изменения: Tue Nov 24 21:32:27 1998
Дата индексирования: Sat Sep 11 22:12:37 2010
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Поисковые слова: crab nebula

CCD photometry

The R,I photometric observations of SBS 0335-052W were obtained occasionally during the study of SBS 0335-052 with the 3.5m telescope in Calar Alto on 1991 October 6. The CCD camera was installed at prime focus of the 3.5m telescope. At f/3.5, with the GEC CCD 1152$\times$770 pixels, (22.5 pixel^-1), the field of view is 73$\times$49. Two exposures of 600 s and 900 s were taken in R and I, respectively. Two clusters, M92 and NGC 7790 were observed as photometric standards. The photometric calibration was made using the data of Christian et al. (1985). The observations were done under photometric conditions with a seeing of FWHM = 144 $\pm$ 011 and 122 $\pm$ 002 in R and I respectively. Airmasses were 1.45 for R and 1.46 for I.

All data reduction was done with MIDAS⁷. The frames were corrected for bias, dark, flat field and, additionally, in the I passband, for fringe-pattern. Unfortunately, the fringe-pattern correction was not very good and there was some residual structure in both the horizontal and vertical directions as result of additional noise in the CCD readout. This structure was corrected by applying median filters separately in X- and Y-directions.

Aperture photometry was performed on the standard star observations using MAGNITUDE/CIRCLE task with the same aperture for all stars. The standard star measurements were used for the determination of the magnitude zero points and the transformation of the instrumental CCD magnitudes into the international RI system. For the standards stars, the comparison of the CCD magnitudes with the catalogue magnitudes shows a scatter of $\sigma$ = 0.08 mag for R and $\sigma$ = 0.12 mag for I. No color term has been included in the transformation, because tests showed that no significant color term exist.

For photometry of extended objects, we used the package SURFPHOT as well as dedicated software for adaptive filtering developed at the Astrophysical Institute of Potsdam (Lorenz et al. 1993). The adaptive filter allows one to reduce pixel noise by 3-4 times without loss of spatial resolution of bright cores of stars and galaxies and to retain the object's total flux as well. The smoothing scale of the adaptive filter was 11$\times$11 pixels. Before applying an adaptive filter a special mask was built, where all bright stars and galaxies were masked out. Such a mask is necessary for proper determination of noise statistics used by the filter. The code of fitting sky background from the package for adaptive filtration was used. It constructs background within masked regions with no use of any polinomial approximation. Instead the so called algorithm of 'stretched skin' is used, which iterativly fill background inside the mask by interpolation of that from outside the mask. Total magnitudes are R = 19.03 $\pm$ 0.09 mag and I = 19.08 $\pm$ 0.14 mag for SBS 0335-052W and R = 16.57 $\pm$ 0.08 mag and I = 16.92 $\pm$ 0.12 mag for SBS 0335-052. The total magnitude I for SBS 0335-052 is in a good agreement with total I = 16.88 mag from TIL97.

Elliptical fitting was performed with FIT/ELL3 in SURFPHOT package where isophotes of galaxies were fitted by ellipses and the deviations were analysed by means of Fourier techniques as is described by Bender & Moellenhoff (1987). To construct a surface brightness profile we used the effective radius as the geometrical average, R_eq=$\sqrt{ab}$. The brightness profile was decomposed into two components: with gaussian distribution in the central part and exponential disk. For this FIT package was used. Weights in fitting procedure were accepted as proportional to 1/$\sigma$, where $\sigma$ is the instrumental accuracy of surface brightness profile.

The R images, isophotes and profiles with their decomposition for SBS 0335-052 and SBS 0335-052W are shown in Fig.4. The error bars for surface brightness profiles in both R and I are less than symbol size. Therefore they are shown only on the (R-I) profiles. The error bar for the first point shows the accuracy of the transformation to the standard system. The error bars for the rest points show the instrumental accuracy of the color and take into account the detector gain and photon statistics for each frame. The best linear fits to the outer parts of the R surface brightness distributions are given by:

$$0335-052~: \mu_R = (21.25 \pm 0.05) + (2.03 \pm 0.02)r,$$ (1)

$$0335-052W: \mu_R = (22.22 \pm 0.12) + (1.60 \pm 0.07)r,$$ (2)

The corresponding exponential scale lengths in pc are $\alpha_R$ = 535$\pm$5 pc and $\alpha_R$ = 422$\pm$18 pc for SBS 0335-052 and SBS 0335-052W respectively.