A Solution for the Problem of Light Pollution ??

Introduction

As we approach the millenium, most amateur astronomers are acutely aware of the problems of light pollution and the difficulties associated with observing the night sky in heavily populated areas. They also probably feel that things will only get worse.

Light pollution is a rather all-encompassing expression describing the light which hinders observation. It has several sources, including natural ones such as the light generated by the bombardment of the upper atmosphere by solar radiation. However, the most pervasive source of light pollution is the glare associated with the street lighting in built up areas. Short of moving house, there is little the individual urban amateur astronomer can do to eliminate this problem. There is some hope, however. Filtering the light pollution and viewing only the wavelengths of light of interest is one way of achieving good results.

A recent lecture at Manchester Astronomical Society illustrated the manufacturing technology which is used to make filters to reject the unwanted light. This brief paper describes the efforts under way in Manchester to make routine light pollution filtering a realistic goal.

Getting the light you want

The common or garden incandescent lamp emits a continuous spectrum - ie a wide range of wavelengths in the visible and near infra red. Most street lights use sodium vapour at either high or low pressure. The low pressure lamps emit a bright narrow spectrum of light at around 590nm. The high pressure types emit a continuous broad visible spectrum, but with bright peaks between 550nm and 630nm. Mercury vapour lamps, used in security lighting, emit a broad spectrum between 540nm and 630nm with some content in the blue region.

These wavelengths are the 'enemy'. It very much depends on the type of object you want to observe and the way you are observing it (by eye, film or possibly CCD), but knowing where the pollution lies and the required observation wavelengths, it is possible to design and make a filter to reject the 'enemy'.

As an example, deep-sky emission nebulae glow most brightly at wavelengths as follows,

• Oxygen III, or O-III, emitting at 495nm and 500nm
• Hydrogen alpha, or H-alpha, emitting at 656nm
• Hydrogen beta, or H-beta, emitting at 486nm

A filter which cuts out all wavelengths apart from these will give good results when observing these types of bodies.

Such filters are made in a similar fashion to the way mirrors are metallised. They rely on the wave nature of light and the interference effects which can be generated in very thin solid layers of transparent material. They are called interference filters, and find application in all sorts of devices.

Probably the biggest application of these interference filters is on spectacle lenses. A bare surface of glass has a 4.2% reflection loss. This means that 8.4% of light is lost from two surfaces in a spectacle lens. This loss gives rise to glare and a loss in brightness. It is possible using interference filter technology to reduce the reflectance of a surface to less than 0.5%, thus eliminating glare and associated problems.

MAS and work on filters

There are manufacturers of light pollution filters in the US and their products are available on the UK market. The biggest barrier to their wide scale application is their cost. An industry standard sized machine which has the capability of making these filters costs in the region of £350,000 to buy new. On the second hand market, prices do not dip much below £150,000 to £200,000. This is why these filters cost around £200 each.

Manchester Astronomical Society is currently working closely with a North of England manufacturer, with the aim of developing suitable filters and bulk buying to get the price down to perhaps a third of that currently charged for American made filters.

Watch this space!

Maintained by Michael Oates
Page modified 18 January, 2005