Surface Brightness Measurements of Extended Galactic Nebulae

We discuss the importance of digitised sky-limited Schmidt Band R-band, and H images of extended translucent (A B ' 1 ? 5 mag) and dark (A B > 5 mag) clouds. In translucent clouds (e.g. high-latitude cirrus) photons can penetrate through the whole cloud and control the physical and chemical processes. Comparison of the B images with far-infrared/submillimetre CO, C I], and C II] emission provides important information on the non-homogeneous cloud structure, carbon chemistry, and energy balance. In dark clouds the interior is completely shielded from the interstellar radiation eld. L1204 is a dark cloud with at its edge the well-known H II rim S140. The R-band image shows S140 as well as the diiuse extended H II region around the exciting B-type star. However, the sensitivity for H in the R-band is rather low and there is contamination with diiusely scattered red light from dust. Deep H and R-band imaging of an 20 0 20 0 eld has recently been done with the 1m Hoher List telescope of Bonn University. A wealth of small-scale structures can been seen on continuum subtracted images which provide new insight on the geometry of this cloud. 1 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 eeciency and a linear response. In addition, the general tendency is to make larger and larger telescopes with a decreasing detector eld-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//lms) have a high contrast emulsion and a large photon detection area (e.g. UKST 40 square degrees). They are therefore excellently suited to study extended objects with a low surface brightness, e.g. 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 1