A 6 inch refracting transit telescope made by Troughton and Simms.

"I gave the order for a new transit circle with 6-inch telescope to Mr. Simms, of Charlton, near London, requesting him to make certain alterations in the usual form, which experience has shown to be desirable. The instrument will combine all the most recent improvements and some that are used for the first time. I believe it will be one of the finest instruments extant." H. C. Russell, 1875.

There are two main types of telescopes. One uses a curved reflecting mirror to capture an image of the astral bodies the other uses a refracting lens to magnify the image.

This telescope, ordered from Troughton and Simms in 1875 arrived in Australia in 1877. It is one of the most significant of the museum's astronomical instruments because it was used to measure Sydney time and also do most of the observational work. Upon its arrival it replaced the old transit circle, which was thought to be deficient, and Russell used it to acquire accurate positions of stars for the trigonometrical survey of New South Wales.

One of the new innovations included in its design was the casting of circle from solid metal circles rather than in parts or leaving areas open. This was done to ensure the effects of temperature were spread more evenly across the circles to decrease distortion. Russell believed this telescope was the first to be cast in this way by Troughton and Simms.

This telescope also has a number of accessories made by Troughton and Simms which aided in the setting up and use of the instrument including: a level trier (H9957); testing equipment (H9976); lifting jack (H10005) and pivot apparatus (H10202) & (H10306). There is also a specially designed observation chair (H9901) which was probably locally made.

Another important job was its use as a support instrument in the mapping the stars project where it was used to acquire accurate positions of reference stars. The project was initiated in 1887 at a meeting of the International Congress of Astronomers in Paris. Henry Russell from The Sydney Observatory attended and was one of the many who accepted the proposal put forward by Dr. David Gill to map of the stars by photography. The importance placed on this project can be gauged by the fact that it dominated the activities of The Sydney Observatory into the 1960s.