Figure 3a-d. A set of 4.4x3arcmin identical sky regions taken from near the field centre of 4 UKST exposures of standard galactic plane survey field 213. The top left image (figure 2a) is from the A-grade survey R plate taken with IIIaF emulsion and the OG590 R-band filter with 63mins exposure and a visual point source detection limit of R~21.5. The top right is from the equivalent exposure taken with Tech-Pan and the new H-alpha filter with 120mins exposure (figure 2b). The bottom left (figure 2c) is the short red A-grade survey plate taken with 098-04 fast but coarser grained emulsion with a narrower 630 red filter (600A) in 10minutes exposure whilst the bottom left (figure 2d) gives the A-grade `I' survey plate taken with IV-N emulsion with a 715 filter and 90minutes exposure.
It is estimated that for point source detection the H-alpha 120 minute Tech-Pan exposures goes at least 0.5 magnitude deeper than the short red and is about 1 magnitude less deep than the R survey but with considerably superior resolution and is also about 0.5 magnitude deeper than the I survey. We are not yet sky-limited with the 2 hour H-alpha exposure.