2 Observations

Two set of observations are used in this survey. The first is from 4 runs at the CFHT in 1989-91, the second from one night at the TBL (Telescope Bernard Lyot) of the Pic du Midi. Tables 1 and 2 list the used frames with some relevant informations. Since the frames were intended to measure central light profiles, the exposure times were generally short, and many frames are of rather poor S/N ratio. A classification of S/N in terms of G (good), F (fair) and P (poor) is given in the table.

  

Table 1: List of Cassegrain CFHT frames used in this work. F = filter. Exposure times are in seconds. W = FWHM in arcsec. Code for S/N: G = good; F = fair P = poor. R = approximate range of measured radius in arcsec. When the file code contains an m, it was obtained by merging two frames. Individual notes: NGC 0821: nearby bright star. NGC 3115: Frames used here for technical tests

  

Table 2: List of HRCam CFHT frames used in this work. See previous table for explanations. Individual notes: NGC 4660 frame h073: a defect near the galaxian core could be corrected using another frame of lesser S/N

2.1 The CFHT observations

The CCD camera at the Cassegrain focus was used in the runs of December 1989 and April 1990. It gives a scale of 0.107 arcsec/pixel, with a good sampling even at the best seeing. For the two runs of 1991, the so called HRCam was used at the Prime focus: it compensates for telescope guiding errors using an auxiliary star in the frame field, with some improvement of the seeing. The image scale is practically the same as for the Cassegrain camera. Since the effective entrance aperture for HRCam is reduced to 120 cm, the S/N is much less good, at equal exposure time, than with the original Cassegrain camera.

The used frames are nearly all of sub-arcsec resolution. A small statistics of mesured stellar FWHM is given in Table 4. The overall means are 0.72 arcsec in R and 0.86 in B. On the other hand, the usable field is $641\times 1011$ pixels, or only $68\times 108$ arcsec, so that the determination of the sky level is rather crude. In order to get a star suitable for PSF measurement within the field, it was often necessary to position the galaxy well away from the frame centre, with further reduction of the measurable range. Due to specific technical constraints this difficulty is more acute with the HRCam.

  

Table 3: List of Pic du Midi frames used in this work. See previous table for explanations

  

Table 4: Statistics of measured stellar FWHM's at the CFHT. Night: date of night beginning in Hawai time. N: number of used observations for each filter. W_B: mean FWHM in B (arcsec). $\sigma_B$: standard deviation. W_R: mean FWHM in R. $\sigma_R$: standard deviation

2.2 The TBL observations

The observations for 6 galaxies are from a single good night (see Table 3). The camera, at the then used focal reducer, gives a scale of 0.315 arcsec/pixel, with adequate sampling for the encountered seeing. For the frames under study, the average stellar FWHM are 0.92 arcsec in R with $\sigma = 0.08$ and 1.13 in B with $\sigma = 0.20$. Their field is $107\times 174$ arcsec.