Washed-out Astronomy

Planetary nebulas generally make great targets for washed-out astronomers. Most of them standout well even in light polluted skies because they are usually rather small and therefore have relatively high surface brightness. Their small size also means they are best seen using relatively high magnification which helps to darken the background and boost contrast. Finally, planetary nebulas respond very well to narrowband filters, even when observing in urban environments. (Image: Glenn Hitchcock and Bob Cowart/Adam Block/AURA/NOAO/NSF)

Famous planetaries like the Dumbbell Nebula (M27) and the Ring Nebula (M57) are well known to most amateur astronomers and are spectacular objects even in light-polluted skies. But regardless of season there are always planetary nebulas well positioned in the sky for urban observers. Here is a list of good planetaries for washed-out astronomers…

Kruger 60 is a particularly interesting binary star system in Cepheus. With a short period of only 44.7 years, you can easily see Kruger 60’s PA change about 8 degrees per year. There’s even a convenient nearby reference star that makes the change in PA obvious. Only 13 light years away, Kruger 60 is also one of Earth’s nearest neighbors. Both components are low-mass red dwarfs, but with only 0.18 solar masses, Kruger 60 B is one of the lowest mass stars known. Finally, Kruger 60 B is also a flare star, irregularly doubling in brightness for periods lasting about 5 to 10 minutes. When it flares, it can match or exceed Kruger 60 A in brightness.

Kruger 60, however, is a challenging target for observers in light polluted environments. If you want to take on this challenge, I have more information for you…

Globular clusters generally look great even in washed-out skies. Many globulars—especially the Messier globulars—are bright enough to be relatively easy to find even in heavy light pollution. And when you use high magnification to try to resolve the globular, you also improve the contrast by darkening the background. In washed-out skies it will certainly be harder to resolve some of the globulars, and you won’t see the fainter stars so you will loose a bit of star density. But any of the major globular clusters still look spectacular. When I have guests at the telescope, I first turn the telescope to any visible planets. But the very next thing I turn the telescope to is a globular cluster: I can count on it to generate plenty of “wows” from my audience.

Now there are a lot of globular clusters, so you might wonder which ones look best in an urban environment. Everyone has their favorites, but here are mine, all proven performers observing from washed-out skies…

Cygnus X-1 is almost certainly a black hole. It was discovered in 1964 as a strong X-ray source, and has ever since been the object of intense study. Cygnus X-1 turns out to be too compact to be any known kind of object besides a black hole. It has a mass of about 8.7 solar masses (which exceeds the theoretical maximum mass of a neutron star of about 3 solar masses), but based on how quickly its x-ray intensity fluctuates, Cygnus X-1 has to be less than about 60 km wide.  Assuming Cygnus X-1 is a black hole, its event horizon is currently estimated to have a radius of about 26 km.

Most interestingly, Cygnus X-1 orbits the blue supergiant star HDE 226868 at a separation of about 0.2 AU. Cygnus X-1 has distorted HDE 226868 into a tear-drop shape and is eating it away (although whether X-1 is actively stripping away 226868’s outer layers or simply sucking up 226868’s solar wind is unclear; the edge of material gravitationally bound to 226868 is close to the star’s surface). For washed-out astronomers, the most interesting aspect is that HDE 226868 is a 9^th magnitude star, making it an easy target for even small telescopes in urban environments. And while you can’t actually see Cygnus X-1 itself, it’s still pretty cool to be looking at a star that’s being eaten alive by a black hole. (ESA/Hubble illustration)

 Here’s how to find HDE 226868 (and Cygnus X-1)…

In urban environments, local lights shining on your telescope can be an even bigger problem than the washed-out skies. Any stray light that finds its way into your telescope tube (or more precisely, into your eyepiece) is a contrast killer, making it even harder to see anything but the brightest objects. While there is little you can do to block general light pollution, you can keep stray light from entering your telescope with a simple tube extension that serves as a light shield.

A tube extension light shield is simply a tube that extends the front of your telescope, making it harder for off-axis light to get into your telescope’s light path. These light shields are particularly effective on Newtonian telescopes because the focuser is too close to the end of the tube on many Newts (especially on many commercial models). Dew shields (such as those commonly used on Cassegrains) also make effective light shields. You can buy dew shields/light shields from various vendors. But you can also make one pretty easily for under $5. All you need is need is some black poster board paper, white glue, and packing tape. It may sound flimsy, but it only needs to stop photons and be stiff enough keep its shape. Here is how I made one for my 10 inch (250mm) Dob …