Appendix C: CCD broad band photometry  

CCD images of NGC 128 and of a set of standard stars were taken in two nights in September 1994 through the B and R filters at the 1.5 m ESO Danish telescope. The frame dimensions are of $1025\times1022$ pixels, with a scale length of $0\hbox{$.\!\!^{\prime\prime}$}38$ per pixel on the sky. The exposure times were respectively of 40 and 20 minutes. The seeing was approximately 1'' FWHM.

After the standard make-up procedures the images have been calibrated using both a set of standard stars observed the same night and the photoelectric aperture photometry taken from the Longo & de Vaucouleurs (1983, 1988) catalogs. The standard stars were chosen from the set of Landolt (1992).

The calibration through the standard stars uses the following equations:

$$\begin{eqnarray} B &\!\!=\!\!& -2.5 \log(I_B) + h_B (B-R) + k_B\\ R &\!\!=\!\!& -2.5 \log(I_R) + h_R (B-R) + k_R\end{eqnarray}$$ (C1) (C2)

where I_B and I_R are the fluxes in adimensional units of the stars obtained after the normalization of the images to the standard exposure of 1 s and the correction for the extinction of the atmosphere, h and k are respectively the color term and the zero point of the CCD for each night in both filters. They give the transformation from our photometry to the B Johnson and R Kron-Cousin systems.

We also built the growth curves by integrating the flux through circular apertures in order to compute the total luminosity of the galaxy. By indicating with

$$C(\rho) = \int_{S(\rho)}^{} I(x,y) {\rm d}x{\rm d}y$$ (C3)

the growth curve of the galaxy, where $S(\rho)$ is the area of the circles of radius $\rho$ centered on the galaxy nucleus, we converted in magnitudes the growth curves through the set of equations:

$$\begin{eqnarray} m_B(\rho) &\!\!=\!\!& -2.5\,\log C_B(\rho) + h_B (m_B(\rho)-m_R... ...!\!=\!\!& -2.5\,\log C_R(\rho) + h_R (m_B(\rho)-m_R(\rho)) + k_R~ \end{eqnarray}$$ (C4) (C5)

adopting the zero point and color terms derived from the standard stars.

Following de Vaucouleurs et al. (1991) we considered the galactic extinction toward NGC 128 very small and applied no correction. The R magnitude was corrected to the Johnson system using the transformation given by Longo & de Vaucouleurs (1988).

The sky surface brightness was estimated using the standard stars and the photoelectric aperture photometry of the galaxy for both nights with an internal accuracy of 0.05 mag arcsec^-2. We got $\mu_B = 22.70$, $\mu_R = 21.20$(first night) and $\mu_B = 22.66$, $\mu_R = 21.00$ (second night). The sky counts were measured at the corners of the frames.