clear trend of deterioration in the figure of the platen. Normally, such 
deterioriation might be attributed to the gradual release metallurgical 
strains, but not in the case at hand, for the DBA platen was made of an 
alloy called Precedent 71 which has a stability approaching that of fused 
quartz. Some other physical process had to be involved. At this point, 
the decision was made to measure the platen with film in place and with the 
application of the film-flattening vacuum (about 0.1 atmosphere). This 
vacuum was produced by the pump of the camera itself, for in this way there 
would be no question concerning the correctness of the degree of the applied 
vacuum. The results were startling. Relative to one corner of the format, 
the center of the platen dropped over 20 um when the vacuum was turned on. 
A full set of 169 measurements of spot elevations confirmed that a pro- 
nounced, systematic change in the figure of the platen had indeed occurred 
with the application of the film-flattening vacuum. Moreover, the actual 
figure of the platen under the film-flattening vacuum could fully account 
for the magnitude of the apparent radial distortion uncovered in the 
Vermont and Atlanta Projects. Here, then, was the physical explanation 
that had been sought. The deformation induced by the vacuum also explained 
the slowly changing static figure of the platen — with continuing use the 
platen had acquired an increasingly concave 'set' as result of repeated 
flexing of the surface in response to each application of the vacuum. 
Contour maps of the surface of the DBA Reseau Platen resulting 
from the succession of measurements referred to above are shown in Figure 
10. Of especial importance is Figure 10d which indicates the topography 
of the platen when the normal operating vacuum is applied. It is something 
close to this geometry that existed when the photographs of the Vermont and 
Atlanta Projects were taken. One should appreciate, however, that the 
laboratory measurements may not fully apply to the platen under operating 
conditions in the camera, for to some extent the figure of the platen may 
benefit from edge flattening resulting from pressure against the frame 
defining the focal plane. 
The experiments with the DBA Reseau Platen naturally raised 
related questions concerning the operational stability of the Zeiss platen 
normally used in the camera. Measurements before and after the application 
of the vacuum were accordingly made on this platen also. The results given 
in Figure 11 show that the Zeiss platen is subject to similar deformation 
from the vacuum. The Zeiss platen seems also to have acquired a concave 
set from long term use. As can be seen from comparison of Figures 10d and 
11b, the contours of the Zeiss platen are more circular than those of the 
DBA platen which tend to be rather square with rounded corners. These 
differences reflect the respective geometries of the re-enforcing ribs on 
the backsides of the two platens — a centrally radiating 'spiderweb' 
pattern in the case of the Zeiss platen and a 3x4 'box cell’ pattern in 
the case of the DBA platen. Because of the radial symmetry of the ribs 
of the Zeiss platen, it turns out that a second degree polynomial can 
provide an excellent analytical representation of the surface, whereas, 
as can be seen from Table 3, a fourth degree polynomial is needed for a 
comparable fit to the DBA platen. In the process of polynomial fitting, 
it appears that terms involving even powers of x and y are dominant (note, 
for instance, with the DBA platen the trivial improvement of the third 
degree fit as opposed to the marked improvement with the fourth degree 
fit). This result is in conformance with what is to be expected from the 
series solution of the partial differential equation governing the defor- 
mation of evenly loaded plates (Sechler (1968)). This consideration is 
reflected in a revised version of the error model currently used at DBA, 
a topic to be taken up later. 
«D5.