A case for Schmidt photography anno 1997

To date the photographic plate has become a rarely used detector in astronomy. Imaging is now commonly done with CCDs which have a much higher quantum efficiency and a linear response. In addition, the general tendency is to make larger and larger telescopes with a decreasing detector field-of-view, which is often less than a few arcmin. This makes them unsuitable for studying degree sized astronomical objects. In contrast, IIIa-type and Technical Pan (Tech Pan) detectors (Schmidt plates/films) have a high contrast emulsion and a large photon detection area (e.g. UKST square degrees). They are therefore excellently suited to study extended objects with a low surface brightness, e.g. low surface brightness galaxies (Phillips & Parker, 1993; Schwarzenberg et al. 1995, 1996) and galactic clouds (Stark 1995). A disadvantage of photographic emulsion is its non-linearity. But, for low surface brightness objects a small increase in photographic density is linearly correlated with a small step in surface brightness. This means that for high quality uniform plates the relative surface brightness can directly be derived, and calibrated if the skybrightness is measured during the exposure. In this way faint objects with mag/ can readily be studied (de Vries & Le Poole 1985; Stark 1995) with IIIa-J plates. The sensitivity of the IIIa-F emulsions used for R-band imaging is about the same as IIIa-J, but the night sky is about a magnitude brighter in the R-band, so the detection of faint objects is correspondingly poorer. With the advent of Tech Pan films which have finer grains than IIIa-emulsions and a much higher detection quantum efficiency, it is possible to study faint structures in R and H down to a brightness limit which is comparable to IIIa-J emulsions (Phillips & Parker 1993). The best way for a detailed quantitative analysis is to digitise Schmidt films/plates at a resolution which properly samples the point spread function of the system (seeing, telescope and emulsion), i.e. with 10 m pixels 0.67 arcsec (see MacGillivray 1998).

In this paper we will discuss surface brightness measurements of galactic clouds in comparison with far-infrared/submillimetre CO, [C ], and [C ] emission. We distinguish between diffuse ( mag), translucent ( mag) and dark ( mag) clouds.

© Copyright Astronomical Society of Australia 1997