Introduction to Astronomy – Week 4
Week 4 of the Introduction to Astronomy course at the IOA in Cambridge saw us having a talk from Carolin Crawford entitled “The Stars, Gas and Dust of the Milky Way”.
Carolin’s talk was one of the most interesting and complicated that we have had so far on the course. Carolin’s talk really centered around the gas, dust and dark matter that exists in our universe.
The talk began by describing the size of our universe and our place in it. It was described to us that our bulge in our universe is 6kpc wide (kiloparsec) which is about 19.5 lightyears. With the stellar disc being about 30kpc, the outer stellar halo being 50kpc and the dark matter halo being outside of this. The dark matter halo makes up 90% of our galaxy by mass.
It was also pointed out that hydrogen is the most common element in our solar system.
We were shown that we are located in the Orion arm of the galaxy between the Sagittarius and Perseus arm.
It was good to be shown some amazing images of nebulae from the Hubble telescope, as it allowed me to record some names of nebulae worth looking out for, which included Cats Eye Nebula, Eagle Nebula, Rosette Nebula and the Iris Nebula.
At the end of the talk we went to view the moon through another of the Institutes Telescopes. This time it was the 36 inch telescope.
The 36 Inch Telescope
The 36 inch telescope was built in 1951-55 by the now-defunct firm of Sir Howard Grubb, Parsons & Co. at Newcastle-upon Tyne. It replaced a much older telescope of the same aperture, which was brought to Cambridge from South Kensington when the Solar Physics Observatory moved there in 1913. That telescope was returned to its owners (The Science Museum) before the new one was installed; the Director of the Observatories at the time (Professor R.O. Redman), who in his youth had made substantial use of the old telescope, always averred that it should never have left the Museum!
The 36-inch, which is thought to be the largest telescope in the country, has three possible focal stations. There is a prime focus with a focal ratio of f/4.5; the primary mirror is a paraboloid, so no corrector is needed to obtain good images on the optical axis. In practice the prime focus has been little used: the telescope is large enough to make access to the focus difficult from the side of the tube. The other possible foci are coude, with a choice of two focal ratios, f/18 and f/30. The coude arrangement is unusual inasmuch as the light beam is directed UP the polar axis rather than downwards: that permits the shorter focal ratio to be exceptionally short for a coude, and results in a focus at a level near to that of the telescope, which is somewhat convenient for a lone observer who needs to operate both the telescope and whatever auxiliary equipment is placed at the focus.
On the other hand, the arrangement lacks part of the advantage of a conventional coude focus, which is often in a basement that enjoys good passive thermal stability (and, from the point of view of the observer personally, protection from wind and extremes of cold!).
Until recently the f/18 focus has been the favoured option, but new equipment that for the first time utilizes the f/30 arrangement has now been brought into use.
The f/30 focus is just within the dome, high up to the north of the telescope, and its use involves a further reflection. In the present application, that reflection takes place close to the focus, and the beam is turned vertically downwards by successive internal reflections within two right-angle quartz prisms cemented together.
The initial image is re-imaged at a focal ratio of f/14.5 at the position required for the auxiliary equipment. A simple plano-convex quartz field lens is cemented to the exit face of the quartz-prism assembly to image the telescope aperture upon the re-imaging lens.
In the early years of its operation, the telescope was used to send starlight into a spectrometer where the light intensities in several wavelength regions. which were accurately defined by masks in the focal plane of the spectrum, could be inter-compared.
The view through this scope was one of the moons craters, although it was not exactly clear, I think a view of the full moon would have looked more impressive.
The telescope itself is very impressive and very large, it sits on a classic Alt type mount just like my LX200 but this also have a fixed concrete wedge underneath it as well. The telescope itself does track, but unfortunately most of the domes require manual rotation.