Measurement Plan

My policy is to measure the most interesting binaries on at least five nights each year. Some such as Castor

(alpha Gem) tend to get more than this because the star is so bright it can be seen in twilight and observing can start earlier when the seeing can be rather good. The standard pairs used also tend to be rather bright for the same reason. Relatively close pairs (at around 1'') which are measured occasionally because they are slow-moving get four nights and any other pairs (usually wide) get three nights. As for the number of settings made on each individual star this tends to depend on the difficulty of the pair. In the summer of 1999, for instance, the fine binary zeta Her which consists of stars of magnitude 2.9 and 5.8 was separated by just under an arcsecond. This meant that measuring the companion depended very much on sufficiently good seeing but, even so, setting the position angle wire resulted in values which scattered by as much as 15 or 20°. In this case, I make up to eight settings in position angle. For wider pairs, where the separation is perhaps 20 or 30'', the agreement between individual angle settings is usually better than one degree and four measures are deemed sufficient.

It is very useful to mark up the target stars on the star atlas because another time-consuming activity is moving the dome by hand. By concentrating on a number of pairs in the same region of sky not only can these be observed more quickly but a comfortable observing position need not be disturbed too often. Having said that, trying to see stars near the zenith with a long-focus refractor requires the ability of a contortionist and I tend to avoid stars which are too high in the sky. There is no doubt that comfort is a significant advantage in securing better measures.

The pairs to be measured will depend on several factors, the prime one being the seeing. If the seeing turns out to be particularly good then I tend to concentrate on the closest pairs. If seeing is poor then wider pairs can be tried. It is very rare in Cambridge that stars of 1'' separation cannot be measured so it is clear that the city environment is not necessarily a bad one even though the sky is usually rather bright. Another factor may be the number of observations left for a particular pair. It is better although not necessary to try and get sufficient measures for a mean during the same season. For wider pairs which are slow moving it may be three or four years before I get sufficient measures for a mean.

A red torch is used throughout: for examining the star atlas for the location of the next pair, looking at the verniers on the micrometer and writing down the settings in the observing sheet. A simple hand-held torch with a button to allow the light to be flashed on and off is most efficient. Rechargeable batteries soon recoup the initial outlay.

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