When it comes to stargazing in the city, we are at the mercy of light pollution – a side effect of our collective reliance on artificial light. In earlier centuries, even densely populated cities were significantly darker at night than today. Victorian Londoners would stand outside and admire the Milky Way from Trafalgar Square – today an almost unimaginable experience that will probably never happen again, except during a catastrophic power outage.
Technology has undoubtedly improved nightlife in the city, making outdoor spaces safer and more accessible after dark, but it’s a double-edged sword. This same advancement has eroded the beauty of the sky. However, technology exists to solve problems, even the problems it causes. So, could special astronomy technology reduce the effects of light pollution and help us reclaim the night? It turns out that out-of-the-box equipment can help you make the most of the clear city air. Here’s what you need to know.
Bringing back the contrast
Because the cumulative glow of our outdoor lighting is reflected off the atmosphere, the sky acts like a dull mirror on the ground. As a result, the apparent contrast of celestial bodies is reduced, making them less visible to our eyes. So, can you see stars in a light-polluted sky? We need to use equipment that helps us restore this perceived contrast to make the stars and other objects stand out against the too bright background. Fortunately, there are two ways to increase contrast that, especially when combined, can produce surprisingly impressive results.
Binoculars and telescopes
Let’s give our eyes a technological boost. Binoculars and telescopes work by collecting a large amount of light and forming an image. This is then enlarged and focused into a cone that fits into the pupil of the eye. The light-gathering power of a lens or mirror, even a small one, is generally many times greater than that of the naked eye.
This means that faint objects far below the eye’s sensitivity can be seen. Unfortunately, these instruments cannot tell the difference between starlight and light pollution. They simply collect all the visible light in the field of view, but our experience can tell us otherwise, and that’s all that matters.
Some have argued that magnifying an image of the sky with binoculars or a telescope increases the contrast, but this is not true and is the result of a misunderstanding of optical theory. The contrast of an image remains unchanged regardless of magnification, as both the stars and the surrounding sky are modified by the same factor.
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However, the perceived contrast is improved. If you look at the night sky with some magnification, the background appears to darken in relation to the objects of interest, making them appear more conspicuous. Why is this? There are a number of mechanisms at play.
First, to enlarge an image, its light must be spread over a wider area. Binoculars and telescopes catch a lot of incoming light, but only some of it is used by the eyepiece before it enters your eye. That little monster from the center of the image is blown up, scattering the light and making the final image darker. Light pollution is then spread over a larger area, reducing the density.
Stars, on the other hand, appear so infinitely small, no matter how far we zoom in, that they are always seen as point sources. Their light is also spread out, but the effect is imperceptible to the eye and so we perceive more contrast. As such, we see many more and much fainter stars, just as we would under an ideal sky.
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Second, when the contrast is very low, for example when looking at a diffuse object such as a nebula or galaxy, the apparent size of the object plays an important role in how we see differences in brightness. Since binoculars and telescopes make these objects appear larger to us, they are more likely to meet the low-contrast sensitivity threshold determined by our eyes.
This means that both stars and deep sky objects stand out more when using binoculars and telescopes, and as such, these tools dramatically improve our experience of the urban night sky. Indeed, the effect often surprises people, because the gap between the naked eye of stars in cities and the countryside is very large. But with optical aid, this gap narrows.
Filters allow us to really enhance the contrast of images formed in the eyepiece of our telescopes, by rejecting the light we don’t want to see and letting the light through. These small, cost-effective components screw into the eyepiece barrel and filter the incoming light before it enters the eyepiece. Two types of filters target light pollution.
Filters to reduce light pollution
Light Pollution Reduction (LPR) filters are broadband filters that gently attenuate the light passing through them to remove wavelengths or colors generally associated with light pollution. They operate on the principle that most outdoor lighting relies on low-pressure sodium vapor lamps, which give off a rather warm yellowish hue.
LPR filters reject the colors emitted by these lamps, allowing the rest of the visible spectrum to pass through. Light pollution consists largely of reflected sodium vapor emissions, so LPR filters selectively reduce the impact on image contrast. Of course, these filters also cut some of the starlight, but in the end most of it gets through and very little artificial light gets through. Of course, this changes the color balance of the image, making it appear more bluish, but that’s a small price to pay for a noticeable increase in contrast.
Ultra High Contrast Filters
Ultra High Contrast (UHC) filters, true to their name, dramatically increase contrast, but there’s a caveat. These are narrowband filters that block most of the light that passes through the telescope. They are designed to transmit only certain narrow wavelengths of light associated with the emission of gases such as hydrogen and oxygen – prominent gases in nebulae, which are excited by the energy of nearby stars, causing them to glow.
UHC filters are essentially nebula filters, designed to widen the gap in clarity between nebulae and the sky. On paper they seem too good to be true, but they are generally not suitable for viewing the sky with the eye. Rather, they are aimed at astrophotographers looking to improve their deep sky shots. Experienced telescope users sometimes use these filters for visual work, but it’s best to start with a general purpose LPR filter.
It is important to note that these filters only work by reducing the total light passing through the telescope assembly. This always results in darker overall images. We sacrifice some brightness to get the contrast back. In general, such filters are usually not suitable for very small telescopes and perform much better with larger ones. The UHC filters require large openings for good results. They are sold in two sizes: 1.25″ and 2″ to match industry standard eyepiece barrel diameters. Make sure to double check which one you need.
So if you want to boost your urban astronomy, let technology be your companion. Collect more light, spread it out and throw away what you don’t need until the natural wonders of the night sky shine through.
Editor’s Note: If you take a great urban stargazing photo and want to share it with the readers of Space.com, please send your photo(s), comments, and your name and location to firstname.lastname@example.org.
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