9 anomalies, the usual situation is that the focus is maintained at a

nearly constant value when the twperature is falling fast, while a slow temperature change produces a more rapid fall in focus reading. In some cases the focus reading actually rose to begin with, even thou^ the temperature of the telescope fell.

These phenomena can he explained as the result of theimal lag

within the telescope structure.

In the notation of Pig. 1 the distance from the primary vertex to the focus,is

X * "s'«*p

■ rp - Tg _ 2a

where we have neglected the small influ^ice of the corrector plate on the focus position. Facial differentiation gives for the two

telescopes

SLT JGT

V'i i-g -0.40 -0.33

> 3/1 rp +1.79 +1:64

3 /3 a -4,58 -4.28

While the construction materials of the SLT are not known exactly, we note that the coefficient of thermal expansion of steel is about 3 times larger than that of Hysil glass, and taken with the large value of a it appears that the separation of the mirrors is the crucial factor.

The observed behaviour can be explained in terms of the action of the bimetallic compensator which is built into the secondary mirror mount# This acts to maintain the separation of the mirrors as the temperature falls, giving a nearly constant focus for some time.

10.

(Walandf 19^)# It effectively provides a negative thermal coefficient.

It does not cool so rapidly as the tube, which protects it from the cold outside, but its effect predominates %dien equilibrium is attained and the overall temperature coefficient has the opposite sign to that which would result if the secondary mirror were simply attached to the tube. The initial rise in focus reading %diich has sometimes been noticed is due to the tube cooling before any marked change occurs in the temperature of the compensator.

It is clear that the bimetallic compensator cannot be relied upon to meet the accuracy required for in-focus photometry*

5* Pocussing for Photometric Observations.

The accuracy attained by the knife edge method meets the requirements of in-focus photometry# It is only necessary to determine the focus

once each hour or so to keep the focus within the tolerance of + 1 thou at the centre of the plate.

The different plateholders were adjusted relative to a standard so that differential tilts across the field did not exceed + ÿ thou; the standard plateholder was squared on relative to the focal plane to a simi­ lar tolerance. The adjustment of plateholders was done easily using an arrangement suggested by Dr# V.C. Reddish* The plateholder was mounted face down in a replica of the plateholder mount which was attached to a tripod with adjustable legs. A dial test indicator on a steel block could be moved freely on a polished granite slab with its probe in contact with a plate in the holder and the legs adjusted to give the same reading all over the plate. A second plateholder, with the same plate inserted, then replaced the first, and its

XI. adjusting screws were set to give the same reading overall.

Finlay-Freuadlich and Waland (1953) reported that the secondary

mirror could tilt sli^tly as the telescope*s orientation was altered, leading to a change in image quality and a tilt of the focal surface relative to the plate. No effect of this nature was noticed in the

present study; the appearance of the astigmatic Off-axis images,

which is veiy sensitive to focus changes, remains unchanged over a wide range of telescope attitude.

The corrections required for the UBF filters were detenained from several knife edge tests and focus plates to an estimated + thou. In practice each filter was used with a particular plateholder and the appearance of the images far from the optical axis used to deduce small corrections from operational plates. The focus error for a particular plateholder + filter combination should not exceed^ ^ thou.

The test gear described above was used to detemine the profile of unexposed photographic plates* It W£^ found that plates are noimally concave on the emulsion side, in accordance with earlier investigations by others. The depth of the concavity between the plate centre and the mean of the three support points was measured for 3 different plate batches :

103a3) 9 plates 0.2 thou.

OaO 7 plates 0*5 thou*

OaJ 12 plates 1.0 thou.

Individual plates may depart considerably from the mean; it mi^t be desirable to press the plates flat or to use specially selected flat glass plates.

12* .

It is considered that the required accuracy of focus is attainable

with the Scott Lang Telescope#

6m Focussing Problem with the Janas Gregory Telescope*

The methods used to study the focus problems and thermal properties of the Scott Lang Telescope were applied to the James Gregory Telescope during I965# The softness and complexity of the images reduces the accuracy of focus determinations from focus plates; it is only with difficulty that the focus can be judged to the nearest 2 thou# The sjyherical aberration means that there is no position of the knife edge for which the whole aperture goes dark at once, so that it is necessary to estimate the focus for the same aperture zone each time# Individual knife edge and focus plate estimates of the focus difjfered by up to 6 thou, with differences greater than 2 thou in 40^ of the cases, and even larger errors were sometimes deduced from the image quality of intended photometric plates set on a knife edge focus* Figure 3 shows a histogram of the difference between the focal positions derived from the two methods applied in quick succession#