How to Avoid Light Pollution While Using a Telescope

Even though the United Kingdom has the biggest total area of dark skies in Europe, light pollution remains a serious issue that plagues both amateur and professional astronomers, as can be seen from the image below. Nonetheless, amateur observers have a distinct advantage over professionals in that they can move their equipment to avoid the worst light polluted areas, but for those who can’t move to dark observing sites, here are some practical tips to get the best possible views of the heavens from light-polluted areas. First things first though- what is light pollution exactly?

Image credit: Astronomy Now

Image credit: Astronomy Now

Light pollution explained

In its simplest form, light pollution is light that is directed to where it is not wanted or needed, and it can occur even in the smallest villages. From an astronomer’s perspective however, light pollution is any light that spoils his view of the night sky.

This spoilage can be in the form of “sky glow”, which is the cumulative effect of all misdirected light, such as streetlights, light from parking areas, shop fronts and sports fields, and badly designed security/outdoor lighting from the millions of homes in any large city. Additionally, and more pertinent to an individual observer, is line-of-sight light pollution, which is the light that shines directly into his eyes from say, the security/outdoor lighting from his neighbour’s houses.

Making the most of a bad situation

Given the fact that light pollution is here to stay for at least the near future, there are some things the average amateur observer can do to minimise its effects on his observing experiences, and the best place for any beginner observer to begin is to understand his equipment, and the need to adapt his vision to low light conditions.

Dark adaptation

Human vision work in two “modes”- photopic mode for vision in high light conditions, and scotopic mode, for vision in low light conditions. In photopic mode, the cones in the eye detect bright light and colour, and in this mode, the rods in the eye that detect low light levels are deactivated.

However, high light pollution levels partially prevent the activation of the rods that are designed to detect faint light while keeping the cones partially active, with the result that dim objects remain invisible, or difficult to see. Thus, to get the best views under light polluted skies, an observer needs to adapt his vision to low light conditions as fully as he can.

Under “normal” conditions, where the transition from light to dark is steep and sudden, this process can take anything from 30-, to 60 minutes. Sadly, though, the dark adaptation process can take as long as 24 hours or more when the observer’s eyes were exposed to extremely bright sunlight during the day or even longer if the observer had been exposed to sunlight reflecting off very bright surfaces such as snow on the ground during the day.

In practice, this means that an observer can expect to lose as much as 0.75 magnitudes off his threshold magnitude because his vision is not fully dark-adapted on the succeeding night. Moreover, even a momentary exposure to a bright light will destroy whatever dark adaptation he had accomplished, meaning that the adaption process must start again from scratch.

Light pollution and aperture

One common myth holds that since large aperture UK telescopes gather more light than small aperture instruments do, the larger instrument must gather a correspondingly larger amount of light pollution as well, which according to the myth, means that the view in the larger instrument must be washed out.

This is simply not true. The fact is that a large telescope will gather more starlight than a small one will, irrespective of where the large telescope is used. Regardless of the level of light pollution in any given area, a large-aperture instrument will consistently reveal more stars than a smaller instrument.

The moral of this story should be obvious: large apertures work better than small apertures in areas with high levels of light pollution; however, large aperture instruments (especially reflecting telescopes) are more prone to image distortion caused by heat currents in the tube, but this is easily corrected by the installation of small cooling/circulation fans at various locations in the tube to stabilise the air in the tube.

More tips and tricks

There are many common-sense tips and tricks any observer can follow to maximise his or her viewing experience under light polluted skies. Here are a few that won’t cost an arm and leg-

Optimise your telescope

“Optimisation” simply means adding a tube extension to the front of the instrument. To achieve the best results, the extension must be at least one tube diameter long, especially on reflecting telescopes. On instruments of this type, the focuser is often so close to the top of the tube that stray light from the side can spill over the top of the tube- straight into the focuser or even onto the secondary mirror. The image below shows a homemade tube extension on a reflecting telescope.

Image credit: Minx.cc

Image credit: Minx.cc

A tube extension prevents this from happening, but be sure to paint the inside of the extension matte black to prevent reflections from the inside of the extension. Another option is to glue some black velvet to the inside of the extension to absorb stray light from the side. Matte black paint and velvet work equally well to suppress stray light, and to improve contrast in the image.

Use the right eyepiece

Under light polluted skies, the exit pupil of an eyepiece is much more important than the magnification that eyepiece delivers. The exit pupil can be described as the diameter of the bundle of light that exits the eyepiece, and using the correct exit pupil can greatly enhance an image under light polluted skies.

The object of using the correct exit pupil under light polluted skies is to maximise the amount of light from different kinds of objects that reaches an observer through the eyepiece. If the exit pupil is too large, some of the light exiting the eyepiece will fall onto the observers’ retina, instead of “into” his pupil, meaning that much of the contrast (and quality) of the image is lost. Conversely, if the exit pupil is too small, the image may be dimmed to the point where it becomes impossible to see, or to focus properly. The table below gives some examples of recommended exit pupils to use in less-than-ideal conditions, such as heavy light pollution.

Object viewed Exit pupil (mm)
Extended open star clusters, complete lunar disc 3 to 5
Deep sky objects such as planetary nebulae and smaller galaxies, binary stars, lunar detail, and planets in poor seeing conditions 2 to 4
Binary stars, lunar detail, and planets on night of good seeing 0.5 to 2

Source: Astronomy.com

Note that eyepieces meant for use under light polluted skies must be fitted with full eyecups to prevent stray light from the side from entering your eye.

Build a light shield

If all else fails, constructing a shield to protect yourself from particularly troublesome low-level light sources is a viable option. The images below show detail of a DIY shield that can be constructed cheaply from materials (PVC tubing and fittings) that are available from any hardware store. Note however that it may be necessary to anchor this shield in windy conditions.

Image source: Chris Bayus

Image source: Chris Bayus

One more thing

Under severely light-polluted skies, investing in a GO-TO telescope mount might be of some use, especially if there are so few reference stars visible that star hopping to a desired target becomes impossible. However, the downside of GO-TO telescopes involves the fact that there is often a trade-off between tracking ability and the quality of the optics, meaning that even if the instrument manages to find and track an object, the less-than-perfect optics might render the object almost invisible in heavily light polluted skies.

Novice observers in light polluted areas who are considering investing in a GO-TO instrument are therefore strongly advised to only deal with reputable dealers in optical instruments on the one hand, and to join an astronomy club in the area to gain as much experience and knowledge of observing in less-than-ideal conditions before purchasing a telescope, on the other.