Deep Sky Section Meeting 2002 - Meeting Report

Held at the Humfrey Rooms, Castilian Terrace, Northampton 2002 March 9th


At this years meeting (left to right), Tom Boles, Bob Marriot,
Guy Hurst, Gordon Rogers, Nick Hewitt, Nik Szymanek, and Roger Pickard
(photo George Bowler)

The 2002 meeting of the Deep Sky Section enjoyed a large turnout at the Northampton venue. Section Director Nick Hewitt opened the meeting with his own report on the Section's activities. The Section was in a healthy state, with some 120 members, although publications had been thin on the ground due to lack of material. The Section's supernova hunters were continuing to make discoveries: Mark Armstrong had discovered over 30 supernovae, Tom Boles was on his nineteenth and Ron Arbour had bagged his fifth.

Dr Hewitt gave the first of the main talks, on the Section's archive of visual observations. Since the formation of the Section in 1982 some 51 contributors had sent in approximately 500 drawings of clusters, nebulae and galaxies. Dr Hewitt was currently engaged in scanning the best observations into digital form, as some of them were showing signs of age and he was considering making a CD-ROM of the best material sent in over the years.

The 1980s were the heyday of visual deep-sky observing, as CCD cameras had not yet come onto the amateur market. The majority of observers were using Dobsonians, the Schmidt-Cassegrain being less popular than it was later to become. David Graham (now the Saturn Section Director), Alan Dowdell, John Lewis and many other observers sent in a large volume of pencil drawings made at the eyepiece. John Lewis was one of the first UK observers to use large telescopes in conjunction with nebula filters. In the 1990s fewer drawings were submitted, but the quality increased. Planetary nebulae became a favourite target for many observers. The main contributors were Stewart Moore, Lee Macdonald, Faith Jordan (who travelled to Australia to make drawings of southern objects) and Mario Frassati.

At the end of the talk Dr Hewitt asked for the views of those present on what to do with the visual archive. There was strong support for the production of a CD-ROM, although it was pointed out that such digital media quickly became out of date as technology changed and that it might not be possible to read a CD-ROM in 50 years' time!

The second talk, by Gordon Rogers, was on the more high-tech theme of CCD imaging. Mr Rogers' career as a serious amateur astronomer began in 1994, when he observed what looked like a transient lunar phenomenon (TLP) on the Moon through a small telescope. He contacted several big names in astronomy about his observation, but found difficulty in getting people to believe him. He therefore decided to begin photographing the night sky, so that he would have photographic evidence to back up any unusual observations he made.

More recently he obtained a 16-inch Meade SCT and an ST-8 CCD camera by Santa Barbara Instruments Group. To house his equipment he built an unusually ambitious observatory, with a dome on the roof of his house supported by 3 8-metre long piers built into the ground, for maximum stability. Planning permission for the project had to be sought and he had to plant trees in his garden to prevent the observatory interfering with the landscape.

Mr Rogers showed many excellent images of deep-sky objects taken at his observatory, plus some images taken by other amateurs. Of particular interest were two images of the galaxy M33 in Triangulum, taken on separate nights under what seemed like identical observing conditions. The image taken on the later night was much fainter, showing that conditions are not always what they seem.

After lunch, Dr. Jon Loveday (University of Sussex) gave a talk on the Sloan Digital Sky Survey. This is a new survey of about a quarter of the sky with the aim of making a definitive map of the 'local' universe. The observations for the (mostly US-led) project are being made using a 2.5-metre Ritchey-Chrétien telescope at Apache Point Observatory in New Mexico, USA. The sky is scanned in strips using an array of large-format CCD detectors covering a total area of 2˝° of sky. In addition to taking images, the project will secure spectra of 1 million galaxies.

In the first two years of its existence, the survey has scanned 41% of the planned area of sky. The data obtained is being made available to the public via the World Wide Web, the first release appearing in June 2001, containing spectroscopic data for 55,000 objects and photometry (brightness) data for 10 million. At the website (http://skyserver.fnal.gov), users can search for data on an object, and by entering an object's name and/or its co-ordinates can download a 5-colour image or a spectrum. It is also possible to browse an area of sky as imaged by the survey and zoom in on an object of interest to obtain data on it.

Results from the survey so far show that the galaxies tend to clump together, a feature which is useful for testing cosmological models. The distribution of galaxies thins out beyond a redshift of 0.1, but this is an effect caused by the survey's limiting magnitude - further out, only the most luminous galaxies show up on the Sloan images. The survey has, however, discovered some very remote quasars with high redshifts, one of which is the most distant quasar known, with a redshift of 6.2. Sloan has also discovered a methane dwarf star - a type of star that is cool enough to contain organic compounds in its atmosphere.

After a short break, Nik Szymanek gave a live demonstration of image processing, using the Northamptonshire Natural History Society's impressive computerised projection facilities. Some of the images he showed were his own, taken with amateur-sized telescopes and commercial CCD cameras. Others were obtained with the Isaac Newton and William Herschel telescopes on La Palma and sent to him for processing.

A 'raw' CCD image fresh from the camera does not look impressive. One such image he showed, a 5-minute exposure of the Trifid Nebula M20, showed what looked like a multitude of faint stars. This was, in fact, thermal noise, which could be eliminated by taking an exposure of the same duration with the lens cap on - a 'dark frame' and then subtracting this from the image using image processing software. It is also necessary to take a 'flat frame' - an identical exposure of some evenly-illuminated surface, to eliminate optical defects such as that caused by the telescope's central obstruction.

Image processing can be done using widely-available packages such as Adobe PhotoShop, or with programs specifically for CCD imaging. One dedicated CCD package, Maxim DL, has a particularly useful feature enabling the user to extract detail from the core of a galaxy which has been 'burnt out' by a long exposure. Using PhotoShop, Mr Szymanek showed how to assemble three monochrome images taken through red, green and blue filters into a single colour image. The software can be used to correct for the drift of stars between exposures caused by guiding errors. Doughnut-like artefacts caused by dust can also be removed using the 'clone' tool, which 'paints' the doughnut out with the colour of an adjacent area of sky. However, Mr Szymanek warned that, while improving the aesthetics of an image, such retouching is not scientifically valid!

One of the most spectacular images shown by Mr Szymanek was of the Whirlpool Galaxy, M51. The image was originally taken by the William Herschel Telescope in order to test a new camera. Using the techniques described above plus unsharp masking (famously pioneered by David Malin in the days of emulsion photography), Mr Szymanek produced a magnificent image, showing intricate detail to the core of the galaxy. The picture was reproduced in the June 2001 issue of Sky and Telescope. Mr Szymanek also showed how a detailed monochrome image of M43 (an extension of the Orion Nebula, M42) taken by the Isaac Newton Telescope could be turned into a colour image by overlaying it with one of his own colour images, taken with an SBIG ST-6 CCD camera.

The final talk of the day was given by Variable Star Section Director Roger Pickard, on the subject of variable star photometry with CCD cameras. It is difficult for a visual observer to estimate a variable star's brightness to better than ± 0.3 magnitude and professional researchers require more accurate data than this. In the 1980s the introduction of photoelectric photometry began allowing amateurs to get better results, and CCD cameras nowadays make it even easier to obtain accurate measurements. CCD photometry does not even need a telescope - a CCD camera fitted with a 135mm lens on an undriven camera tripod is quite adequate. As for ordinary imaging, it is necessary to calibrate with dark and flat frames, and it is also important to use filters for consistent results. The brightness of the variable star on the resulting image is then measured using software.

One software package that Mr Pickard has found particularly useful for CCD photometry is AIP for Windows, which accompanies The Handbook of Astronomical Image Processing by Richard Berry and James Burnell, published by Willmann-Bell Inc. To estimate a variable star using this program, you select the variable, a comparison star and a third star, known as a 'check star', to further ensure the accuracy of the result. The software then automatically measures the brightness of the variable relative to the other two stars. Inserting the magnitude of the comparison star then gives you the measured magnitude of the variable. In his conclusion, Mr Pickard pointed out that a deep-sky CCD image could contain lots of useful data on variable stars within the field.

Nick Hewitt rounded off the meeting by thanking all the speakers for their interesting and well-presented talks - and, above all, the members of the Northamptonshire Natural History Society for organising the event and working hard during the day to provide the refreshments.

Lee Macdonald
Secretary