Washed-out Astronomy

If you have a Dobsonian or Newtonian reflector, flocking is a great way to reduce stray light and improve the contrast. What is flocking? It simply means lining or painting the inside of your telescope with something that absorbs light—ideally something that absorbs close to 100% of visible light. Most telescope insides are painted flat black, but that usually doesn’t do a particularly good job of absorbing incident light. If you don’t believe me, look down the inside of your telescope tube in the daylight: is it truly pitch black or just dark gray?

Flocking is particularly helpful in light-polluted environments because there is so much more stray light to eliminate. Think of it this way: if you are in a really, really dark site, flocking doesn’t matter much because there is essentially no stray light. In fact, in really dark skies, you don’t even need a light shroud around a truss-tube telescope—there is just no light to leak into your light path. In contrast, your typical urban or sub-urban environment has plenty of stray light sources: the neighbor’s porch light or bedroom light, the nearby street light, the sky-glow from downtown, and even the light-gray background of the sky itself. If any of this light gets into your eyepiece, it contributes to brightening the background and killing the contrast.

Now flocking is pretty easy and one of the secrets is that you only need to flock relatively small portions of your telescope to get essentially all of the benefits...

In a previous story I covered some galaxies that you can see reliably even under washed-out skies. Unfortunately some relatively bright galaxies that amateurs everywhere know and love are particularly susceptible to the effects of even modest light pollution. If you live in urban or suburban areas, you can spend hours looking for these without success. To save you this frustration, I’ve prepared a list of famous galaxies that are very hard to see in washed-out skies. You can treat this as a list of galaxies to avoid, or if you prefer, consider it a challenge list for observing under washed-out skies....
(Photo of M33 by Filip Lolić.)

For urban astronomers, galaxies are consistently disappointing. Galaxies that are stunning from a dark-site can be stubbornly invisible in even mildly washed-out skies. A very slight loss of contrast is enough to hide spiral arms, dark lanes and any other structural details. What makes galaxies even more frustrating is that none of the usual tricks for enhancing contrast work on them. Because they are extended objects with low surface brightness, increasing magnification just makes their already faint and diffuse light even more diffuse and fainter. Because galaxies radiate primarily stellar light, narrowband filters which can be very effective on emission and planetary nebulae don’t help at all with galaxies. So what’s an urban astronomer to do? Just give up on galaxies? Well, luckily there are a few galaxies that can be reliably seen under washed-out skies...

(Photo of M31 by Richard Arendt.)

This review of Sky Atlas 2000.0 is the second in a series of atlas reviews to help you decide which star atlas is right for you. My first review covered S&T’s Pocket Sky Atlas. Sky Atlas 2000.0 is a large-scale atlas beautifully printed in a large-page format. It has stars down to magnitude 8.5 and deep sky objects down to around magnitude 13. Sky Atlas 2000.0 comes in three editions: Desk (black stars on white background), Field (white stars on black background), and Deluxe (color-coded deep-sky objects on white background), and each edition comes in a paper version and a laminated version. All versions include close-up charts of crowded areas such as the Virgo cluster, as well as a transparent plastic coordinate-grid overlay for determining positions accurately.

Regardless of the edition, this is clearly a star atlas for serious star-hopping which will help you find objects well beyond the Messier catalog. It is a fantastic atlas, but for observers in urban (and even sub-urban) areas, Sky Atlas 2000.0 is probably overkill: most of the deep-sky objects won’t be visible in washed-out skies, even with relatively large amateur telescopes. Although having more stars plotted compared to Pocket Star Atlas or Norton’s Star Atlas can be an advantage for star hopping if you have a 50mm or better finder scope, I haven’t found the difference significant when star-hopping in light-polluted skies. However, if you regularly go to a dark-sky site in addition to observing from urban areas, Sky Atlas 2000.0 makes sense as an atlas that will serve you well in both locations....

In my earlier story on 3C 273, I promised to cover some other quasars that are within reach of urban astronomers. In fact, there are four quasars brighter than about magnitude 14. While this may sound pretty faint, because quasars are star-like, you can use high magnification to darken the background and get enough contrast to see them, even in washed-out skies. Under favorable conditions you should be able to see all four of them with a 10 inch (250mm) telescope from your front yard. (NASA artist's impression of a quasar.)

The brightest of the four at magnitude 12.8 is 3C 273. The next brightest is Mrk 421 in Ursa Major at magnitude 13.3. Both of these are within reach of a 6 inch (150mm) telescope, and easily visible with an 8 inch (200mm) telescope, even in urban skies. The other two are more challenging: Mrk 501 in Hercules at magnitude 13.9 and OJ 287 in Cancer at magnitude 14.2. Three of these, Mrk 421, Mrk 501, and OJ 287, are not only quasars, but blazars, which means they are highly variable in brightness over relatively short time periods. This is particularly true for Mrk 421 and OJ 287.

Quasars are cool objects in and of themselves. But there are other reasons that make these fascinating objects. One is the tremendous distances involved: 3C 273 is 2.5 billion light years away and OJ 287 is over 3.5 billion light years away. They are also powered by massive black holes: Mrk 421 has a black hole of about a billion solar masses; OJ 287 has a binary pair of black holes, the primary weighing in at 18 billion solar masses—the largest black hole known.

Now finding specific 13th and 14th magnitude "stars" can be a challenge, so if you are interested I have a full set of finder charts for all of these....