Appendix E: NIR photometry  

Near infrared JHK images of NGC 128 and of a set of standard stars were secured at the Tirgo telescope with the Arnica NICMOS3 camera in October 1995. The scale-length on the detector is $1\hbox{$.\!\!^{\prime\prime}$}0$ per pixel for a total field of view of $4' \times 4'$. The seeing was about 1.5'' FWHM and the quality of the sky was photometric.

A sequence of on-off exposures have been taken for each filter on the sources and the nearby sky with exposures ranging from 5 to 12 s. The telescope was dithered each time in different position so that the stellar images do not appear in the same place. The standard stars have been taken from Hunt et al. (1996).

The images have been reduced following the Arnica package provided by Hunt et al. (1994) for the IRAF system. Bad pixels have been corrected, after flat-fielding, by linear interpolation using a reference mask. The cosmic rays and the residual bad pixels have been eliminated using IRAF.

The Arnica zero point of the night for each filter was derived through the aperture photometry of the standard stars observed before and after the target galaxy. We used the average atmospheric extinction coefficients given by Hunt et al. (1996). The mean level of the sky background was $\mu_J = 14.95\pm 0.10$,$\mu_H = 13.24\pm 0.10$, and $\mu_K = 12.23\pm 0.10$ mag arcsec^-2. The sky counts level was measured at the corner of the frames.