Light Pollution

In this section - Birmingham's Night Sky - Coping with Light Pollution - Seeing Faint (Drift method & Averted Vision)

Birmingham's Night Sky

Birmingham is the UK's second largest city, with an immediate population of 2 million, and a catchment area of some 7 million people. Observing from this massive metropolitan area can be difficult, with it being impossible to escape the localised and general light pollution present. Even driving into the country doesn't help, as the glows from the massive industrial area of the Midlands are always present on the horizon.

Here are a couple of images which show just how bad light pollution gets in Birmingham and the surrounding area, and the problems experienced with localised light pollution caused by street and security lighting.

This is Birmingham at night - or at least the part which is 3 miles east of my home. The city centre, which lies just 5 miles to the south of my location, produces far more light pollution than this photograph shows.

A view of Birmingham from the South of the city. This photograph was taken some 12 miles distant at the Clent Hills, a "darkish" site where I have made many Meteor watches over the years. This short exposure was made under very transparent skies, with no haze present!

This is the view of the steet-lighting in my road, and directly outside my house. These Sodium lights replaced old fashioned Mercury lamps in 2003. The road is now well illuminated after dark- as is the night sky!

Surrounded

Security Lights

This image taken from my back garden shows the problem I have with local light pollution. Each light is on a passive IR sensor, and remain on for several minutes when activated. The two lights in the foreground present the biggest problem, as they shine directly into my garden. Both remain on for 5 minutes each time they become activated, whereas the others stay on for around three minutes. Although my observatory protects me to a certain degree from the direct glare of these lights, the sky becomes saturated with stray light whenever they are triggered. This is the view looking North. Houses to the South are similarly equipped, as are the houses to the East. To the West is street lighting as shown in the images above.

The best way to illustrate the problems that light pollution cause to the visual observer is through photographs of the night sky. These two images were taken in September 2005. Left we see Mars near to the Pleaides, whilst right is Orion rising in the South Eastern sky. Both images were taken with a digital camera set at ISO 200, and 10 seconds unguided exposure. The sky was very transparent on the night, with no haze present at all. On both occasions, there were no local security lights active. The orange sky is from general light pollution of the area.

Coping with Light Pollution

The vast majority of us who want to enjoy and observe the night sky have to do so from area's where there is light pollution to various degrees of severity. You should not let this discourage you from going outside with a telescope or binoculars and making those observations. There is very little we can do about the problem if we live in a town or city, so learn to live with it and try to adapt to your local conditions. Never make light pollution an excuse for not observing!

It is possible to do certain things which will help you see those faint variables (or tricky nebulae if the Deep Sky is your thing) visually from even the brightest of skies. It is absolutely untrue to say that visual observations of faint stars cannot be carried out in light polluted skies - as I hope the light curves within these pages will show! Here are a few ideas which I have employed to help me observe from bright skies for over fourty years...

Stray Light:

To counteract the problem of security lighting, bedroom lights etc. shining directly into the observatory, I have placed matt black screening in strategic positions around the garden. Although this does little in preventing the light from polluting the sky, it does help to keep the observatory and observer reasonably dark. These are very simple timber constructions which have lasted for many years, and require very little maintenance.

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Occasionally stray light will still get to you, despite the precautions you take. This is especially true if you use a moderate/large reflector and have to stand on a ladder to get to the eyepiece, placing your line of vision above that of your screens. A good tip is to add a screen to your telescope (see below).

This is a piece of heavy duty black card (purchased from any art & craft shop) attached to the side of the telescope with velcro pads opposite the eyepiece. It does not need to go completely around the telescope. It stops stray light from entering the draw tube hole (which brightens the field when viewed through the eyepiece) and protects the observer's eye whilst observing. It also allows the observer to get closer to the Moon to observe, without Moonlight falling into the light path reflected from the secondary. The velcro pads allow the shield to be easily removed in windy conditions. It's light in weight, and won't cause balancing problems if you use a Dobsonian. Very cheap and very effective!

One final solution is to use an observing hood. This goes over the head of the observer as well as over the eyepiece, thus keeping out all stray light. It does cause problems in the winter, as it tends to cause the eyepiece to dew up very easily. You can buy these from various astro-suppliers for silly money, but why bother? Use a black bin-liner (or two if they are very thin like Birmingham City Council provide). They work just as well, and cost next to nothing. Be prepared for some strange looks if your neighbour looks out of the window and spies you wearing a black bin bag on your head, and always remember that your wearing it if you have surprise visitors. It can be embarrassing!

Mirrors & Eyepieces:

Regardless of the precautions taken to minimise stray light, to see near to your telescopes limit in urban skies you need to have clean optics - as pristine as you can get them. Regular washing of both mirrors in a reflector is an absolute must, at least every 6 months.

Use distilled water, pure cotton wool and a plastic container used only for this purpose. Always clean the mirror whilst under the water, and never use a detergent which contains lanolin, as this will leave white marks on the mirror surface. Various solutions containing near pure alcohol can also be used, but are very difficult to obtain in the UK. If you don't know what you are doing, seek advice from someone who does. This is not an exercise for the novice or faint hearted! It's also a good idea to have your mirrors re-aluminised every two-three years to enable you to get to those faint limits. Can be expensive, but well worth the cost! I used to have my 40cm reflector aluminised every two years during the June Full Moon period, so as not to miss too much observing.

Attaching a heater system to your primary and secondary mirrors is also a good idea, as dew will cause big problems if allowed to settle on the mirror. Remember that for someone living in an urban area, dew isn't just clean water, it's full of pollutents - which are only too eager to damage that beloved coating. Also remember that a pristine mirror will only perform well if it's collimated. A properly collimated mirror could mean an extra 0.5-1.0 mag gain. Spend some time on this, it will pay dividends!

It has never ceased to amaze me that some people spend several thousand pounds on a telescope, and a very small fraction of that cost on an eyepiece. A good eyepiece will make an average telescope perform well, but a poor quality eyepiece will surely make a good (and expensive) telescope perfom badly. This is especially true if your struggling with light pollution. I personally use Teleview Naglers & Radians. Absolutely superb eyepieces, which I'm sure give me an extra half mag or more in my light polluted skies. Yes they are expensive, but you only need to buy them once! Give lots of thought to the eyepieces you are going to buy. You only get what you pay for!

Seeing faint:

Apart from protecting ouselves from stray light and treating our telescopes like a much loved pet, what can we do as an observer to help in seeing faint objects in bright urban skies? Well there are two methods which I use (and many others use too I should add) that I can absolutely recommend in trying to capture every photon of light collected by the telescope. The drift method, and averted vision.

Drift Method

The eye is very sensitive to movement. Place the object (or where you think the object is if it's invisible) on the eastern side of the eyepiece field, switch off the drive and let the stars drift slowly. Whilst staring at the centre of the field, the object under observation will become visible as it reaches this central point. It will be obvious that this method employs the use of averted vision (see below) as well as the movement of the star in helping to detect faint objects. With practice you will be able to make estimates of very faint stars using this method, but remember that you must always note it as a glimpse if you only see it for a second or two. Gently tapping the side of the telescope can also help in detecting faint objects. As the telescope dampens the shake, the eye can and will pick up faint - previously unseen - stars. Practice, practice, practice...

The Drift Method

Averted Vision:

As astronomers we know that by glancing to the side of an object and letting photons fall on the more sensitive parts of our eye, we can see things that are not normally visible. This is termed 'Averted Vision', as the most optically sensitive part of the retina is off set from the centre of the eye. By allowing light to fall upon these sensitive area's, we improve our chances of seeing faint stars, structure in Nebulae and fine Planetary detail. However merely averting the eye away from the object under observation is not enough. There is a right and wrong way to use averted vision. The retina has two well defined area's which we will call for arguments sake the sweet spot and blind spot. To use averted vision correctly, the light from the object under observation must fall upon the sweet spot of the retina. For right eyed observers, this means averting the eye to the right and slightly upwards - left eyed observers would do the opposite - so that the object lies to the nose side of vision. Obviously we are all different in our physiological makeup, so you will have to decide just how far to the left or right & upwards you need to avert your eyes to get maximum effect. The drawing below shows a general area where this should fall, but all it takes is practice to find your best averted position.

Averted Vision

The blind spot of the eye refers to the area where the optic nerve actually connects to the retina. This lies opposite the sweet spot, so that a right eyed observer who averts his eye to the left will actually see less than if averted vision was not used at all! It therefore becomes obvious that a casual gaze around an eyepiece field with your eye in search of something faint is counterproductive and will only result in frustration!

What about the use of binoculars or binocular viewers? It seems that by using two eyes averted vision cannot be used at all, as both sweet spot & blind spot will be used at the same time. Well the trick is to simply look upwards. This allows the light to fall on a slighlty less sensitive part of the retina, but miss the blind spot of both eyes completely. The benefits of using both eyes to observe will far outweigh the disadvantage of finding the perfect averted sweet spot to make your observation.