3.1. Observational settings
Data were taken on the 2 metre telescope TBL, during a 2 months period in 1994 (September 28 to November 24), and during 93 nights scattered over 6 months (July to December) in 1995. Observations were only carried out when M 31 was higher than above the horizon.
Optical device We observe at the f/25 Cassegrain focus behind a focal reducer "ISARD" that brings the aperture to f/8, in a field where the image quality is compatible with the sampling of the CCD camera.
Filters To be able to test achromaticity, we use two well separated filters: Johnson B and Gunn r.
CCD Camera The camera is a 1024 1024 Tektronix CCD camera. Pixels are 24 µm wide, which corresponds to an angular size of . The effective field covered by ISARD is only 900 780 pixels. The chip is thin and its quantum efficiency remains above 70% in the two bands we use. The array is very clean with very few bad pixels. The readout noise is and the gain, or conversion factor is .
Exposure time 20 minutes in red 2 and 30 minutes in blue.
Runs For various reasons, and in particular because the telescope we use is not dedicated, the focal reducer ISARD must often be dismounted and remounted. After such an operation, the positions of the mirrors and the camera are never exactly the same as before. We call a session between two dismounting-mounting of ISARD a "run". Our exposures were taken over a total of 10 runs in our two autumns of observation, each of which is identified by a letter a,b,c...
As the field of ISARD is small, we were led to cover the M 31 bulge with 6 fields (fields A, B, C, D, E and F of Fig. 4). An additional field, Z, centred on the nucleus of M 31 was taken at the beginning of each night, as a reference to help in the pointing of the telescope. It turned out that it was impossible to monitor all the fields in both colours each night. We decided to put a priority on the first four fields, with an emphasis on red exposures. Blue images, which require longer exposure times, were less regularly taken. Fields E and F were poorly sampled. We had altogether 76 nights of good weather over the two periods of observation. The number of images taken in each field during the whole survey is summarised in Table 1.
Raw data were processed at Pic du Midi during the observation sessions using MIDAS. Mean bias images have been constructed for each night, from a median combination of typically 10 frames, and show a good stability. Mean flat-fields have been made for each run and they correct most of the differences between runs. We come back on this point later.
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998