Meeting of the Section

Northampton : Saturday, 15 May 1999

R.J. Neville, Journal of the British Astronomical Association, 109 (5), 292 (1999)

This was the fifth annual meeting of the Section and, as in previous years, proved an unqualified success. Bob Marriott’s formula of eleven short talks separated by extensive lunch and tea breaks again proved very popular with the audience, which included a wide range of professional and amateur engineers, scientists and observers.

The first speaker of the day was Stuart Cooper, who first described and exhibited his beautifully engineered Crayford eyepiece mounts. He then continued with a detailed explanation of the manufacture and use of his own laser collimator. Incorporating a miniature laser pointer (purchased in bulk from a market in Hungary), this precision device fits into the eyepiece mount and enables swift and accurate alignment of Newtonian telescope optics. Successful supernova patroller Tom Boles described how he upgraded his observatory equipment with a Byers robotic mount, a 14-inch Schmidt–Cassegrain telescope and a new CCD camera – and then promptly discovered another supernova! The

Three of Stuart Cooper's Crayford eyepiece mounts

computer-controlled precision mount allows rapid finding and accurate tracking during the 30-s exposure required to reach magnitude 18 with the sensitive detection system. As supernova detection depends on sampling numerous images, this system is highly effective.

Graham Salmon (who, as in previous years, had travelled up from Truro) described some of the techniques of CCD photometry and the importance of dark fields. Although a precise and exacting operation, measuring stellar brightnesses and their variations lends itself to the application of modern computer techniques, which were well demonstrated.

There then followed a long and leisurely lunch-break, allowing much discussion and useful exchange of ideas. Richard Miles (well known for his pioneering photometric work in cooperation with Andy Hollis, and author of a recent definitive paper on the subject) began the early afternoon session by discussing a series of CCD images stored in his computer, which were rendered easily visible to all by means of a video projector. He described the importance of acquiring many averaged, flat, dark and bias fields, and showed how they improved the quality of the data. Amazingly, very ‘spotty’ raw images, caused by dust motes on the camera faceplate, were processed to magnitude–time curves having an accuracy in the order of a hundredth of a magnitude. (Richard is to be thanked for bringing along this delicate and bulky equipment in order to present this revealing demonstration, and also for his donation of a Starlight Xpress framestore CCD system to the Association’s instrument collection.) Jim Hysom (professional optician and director of AE Optics) described the production of a 36-inch mirror to replace a deteriorating original at the Cambridge Observatory. A 37-inch diameter, low-expansion blank had to be reduced in size and ground flat before serious work could begin. The many slides he showed illustrated the problems encountered (and overcome) in handling this very heavy and delicate object. Grinding and polishing was all performed with the surface facing upwards and the mirror being rotated around its optical axis. In general, sub-diameter tools were used, with frequent checking of the radius of curvature by means of a large spherometer. This method enabled a match in focal length to the original mirror of as little as a ¼-inch. After installation of the new mirror the telescope continued to produce high-accuracy radial velocity measurements. Steve Evans ended the first afternoon session with ‘What’s new in the wonderful realm of meteors?’. Over many years he has worked unceasingly – inventing equipment and applying new techniques to the production of reliable and accurate data to determine atmospheric flight paths and meteor orbits. Steve has worked in collaboration with Andrew Elliott and his low-light-level TV imaging system to greatly extend our knowledge of these elusive objects.

Tom Boles relaxes after a light lunch

After another long and useful break for tea and discussion, Maurice Gavin impressed us with yet another improvement of his pioneering technique of recording the spectra of very faint objects such as quasars and supernovae. Formerly, physical limits to the distance of the CCD chip from the diffraction grating produced a wavelength dispersion that was larger than desirable, and which reduced the image brightness and therefore extended exposure times more than necessary. The introduction of a relay lens allowed a shorter and brighter spectrum to be produced as close to this lens as necessary, and then be projected at unit magnification, or even less, onto the CCD chip. It is clearly a much more flexible system for producing various dispersion spectra than that of changing the grating or re-engineering the camera, and Maurice is to be congratulated for his ingenuity.

Bob Neville then described how an Excel spreadsheet running on a personal computer can be used to eradicate the tedium of calculating mirror surface profiles during figuring. The method merely automates and slightly refines Texereau’s method of assessing wave-front errors during this critical and often taxing stage of mirror production. Using Richard Miles’ computer equipment and video projector, a real-time demonstration was presented, showing how knife-edge measurements of zonal radii of curvature can be instantly transformed into a graph of surface error across the mirror. This method produces a reliable indication of where to continue polishing to reduce raised zones in the quest for the correct paraboloid. However, it must be used in conjunction with the Dall null test and a Ronchi grating, or possibly a double-pass null test using a large flat in order to ensure a smooth and accurate surface figure.

Finally, Bob Marriott briefly described and showed slides of some ‘real telescopes’. These were instruments (mostly refractors) important to the history of optics and astronomy throughout the nineteenth and twentieth century. Some of the more famous owners and users were included in the photographs, and interesting anecdotes and snippets of information greatly enhanced the presentation, which was given in Bob’s inimitable style.

The meeting continued slightly longer than intended, but most of the 55 participants stayed until the end. As in previous years, many then headed for The Fish, where lively discussion continued for several more hours. The emphasis of this year’s meeting was instruments, techniques and data reduction, and during the entire day not a single picture of an astronomical object was thrown onto the screen. However, the overt enthusiasm and interest of everyone present demonstrates that ATM continues to thrive.

Our thanks are again extended to Bob Marriott, who organised everything to run as smoothly and successfully as ever.