2 Previous DIB surveys

In recent years the number of known DIBs has experienced considerable growth. This rapid increase is due to the improvement of CCDs and higher resolution instruments in the optical range. Herbig (1975) announced a total of 39 identified diffuse interstellar bands between 4400 and 6850 Å. In high signal to noise scans of reddened stars Herbig (1988) reported the discovery of 29 new lines between 6767 and 6862 Å. In a later publication Herbig & Leka (1991) added 22 more DIBs to an ever-growing list. Most of these surveys concentrated on the star HD 183143. It was clear at this point that the main barriers in revealing more DIBs was instrumental. To date, the largest DIB survey was reported by Jenniskens & Désert (1994). This survey included all the previously identified DIBs, and yet higher resolution than before resulted in the identification of more DIBs. The survey, from 3800 Å to 8680 Å, revealed a total of 229 DIBs. This list included 153 certain DIBs as well as 76 probable and questionable DIBs. The survey covered a wider wavelength range than the present one of BD+63$^{\circ }$ 1964, but some regions of the spectrum were omitted in their data. A subsequent smaller survey, but using a resolution of 60000 concentrated on several different wavelength regions (Krelowski et al. 1995). These were mostly $\sim$80 Å windows centred at 5790 Å, 5915 Å, 6010 Å, 6250 Å, 6400 Å and 6665 Å. This survey resulted in the tentative identification of 42 weak DIBs. These DIBs were labeled either as certain or as doubtful DIBs.

The total number of diffuse interstellar bands identified in the wavelength range 3800 Å to 6815 Å until now is 184, 103 of these being classified as "certain'' by Jenniskens & Désert (1994). What makes this present survey of these three reference targets both unique and necessary is: (i) the enhancement of narrow DIBs in BD+63$^{\circ }$ 1964, mentioned below, enabling us to confirm many of the weaker DIBs already reported, as well as to discover many new DIBs. (ii) Never before has there been a survey of diffuse interstellar bands from 3980 Å to 6820 Å in one echelle spectrum. To cover a wide wavelength range it was necessary for previous surveys to take a number of different exposures of the same stars over a period of several months. This created numerous complications such as different seeing and telluric conditions, and calibration drifts, which do not arise in our data. We have analysed a consistent, deep exposure over the desired wavelength range of the star BD+63$^{\circ }$ 1964 which shows, to date, the strongest DIBs ever measured. (iii) We measured a number of unreddened targets to optimize instrumental, telluric and stellar line corrections. (iv) The detected DIB candidates have been checked for confirmation in deep spectra of two very reddened targets, HD 183143 and BD+40$^{\circ }$ 4220, which have very different spectral types and rotation.