Photographic Enhancement of H Films

Photographic Amplification

Photographic amplification (Malin 1978) is a photographic enhancement process based on a simple, non-destructive contact copying procedure. It is intended to emphasise extended features that are much fainter than the night sky background. It has been especially useful in research on galaxies (e.g. Malin and Carter 1983), where features 5.5 magnitudes fainter than the sky are routinely detectable on sky-limited plates. The process works as well on Tech Pan film as on plate material. Its practical advantage is its simplicity and utility. Finished prints revealing faint features of large areas of plate can be available in less than an hour after receipt of the original processed plate (the process does not work well on copies). The process reveals features that are as faint or fainter than those derived digitally (Couch et al. 1984, Bothun et al. 1991).

The value of a narrow-band filter, well matched pixel size and photographic amplification are well shown in Fig. 1. Three of these four images compare photographically amplified derivatives from original UKST exposures of identical parts of the Vela supernova remnant. These enlarged images are made from a series of plates taken over more than 20 years.

 
Figure 1: Enlargements of sections of UKST images of part of the Vela supernova remnant, exposures made on 098-04, IIIa-F and Tech Pan emulsions. Figs 1(a-c) are after photographic enhancement. Fig. 1(d) is as Fig. 1(c), but without photographic enhancement. Each image is about 20 arcmin high.

Fig. 1(a) shows an early unhypersensitised exposure on an Eastman Kodak Type 098-04 plate. This fast but coarse grained emulsion was exposed behind the Meaburn (1976) filter for 90 minutes. Stars and structured nebulosity are evident. In Fig. 1(b) a sky limited exposure was made on the successor to 098-04, type 127-04, later to be known as IIIa-F. This exposure was made on a hypersensitised plates with the broad-band RG 630 filter and an exposure time of 90 minutes. In Fig. 1(c) and (d) we see the same part of the sky photographed with the new, narrow-band UKST H filter and Tech Pan, with Fig. 1(c) being photographically enhanced, Fig. 1(d) before enhancement.

Fig 1(c) shows the effect of the filter in eliminating much continuum from stars, while the imaging properties of the film ensure that the star images are small. This, combined with the good response of the film to H allows the complex structure of the faint H emission to be seen much more clearly, revealing an evident `billowing' appearance for the first time. It is also noted that the faint, diffuse emission in this part of the Gum nebula is recorded in the lower right (SW) corner of Fig. 1(c).

The importance of minimising the star image size through both filtration and small point spread properties is shown in the series of images in Fig. 2, which follow the same sequence as Fig. 1, but at an increased enlargement, and without the un-enhanced plate. Again, the decreased stellar background and finer grain allows much more fine structure in the emission nebula to become apparent.

 
Figure 2: Further enlargement of the same plates used for Fig. 1. Each image is 7.5 arcmin high.

© Copyright Astronomical Society of Australia 1997