Urban Observing: Challenges and Techniques

Deep-sky observing is challenging, especially when pursued under lit-up skies. Some of the challenges are unique to the urban or suburban environment, but most of them are experienced to some degree even at pristine rural sites.

The three main factors determining whether you can see any given object, and how detailed it will appear if you do see it, are the brightness of your skies, the aperture of your telescope, and your skill.

The degree of skyglow is overwhelmingly important for viewing so-called faint fuzzies — that is, galaxies and nebulae. Most novice stargazers greatly underestimate the importance of this factor — and many, especially in suburbs, greatly underestimate just how bright their skies are.

Even at a pristine site with no artificial light pollution, natural sources such as the Milky Way, zodiacal light, and airglow provide enough light to walk comfortably in an open field without a flashlight. Your hand will appear quite dark by contrast if you hold it up against the sky. Clouds appear as strikingly black patches devoid of stars, but the clouds themselves are invisible, since there’s nothing below them to light them up.

Surface Brightness

Anywhere near a settled area with outdoor lights, the sky appears much brighter still — bright enough to swamp the light from faint galaxies. Just how much the skyglow hurts any particular object depends on the intensity of the object’s light, as explained in my page about Surface Brightness.

Aperture

The most important feature of any telescope is its aperture: the width of its main mirror or lens. That determines how much light the telescope gathers and how well it can resolve faint or small objects. The telescope’s design (refractor, reflector, or catadioptric) is of secondary importance. To a good first approximation, all 4-inch telescopes are the same, whether they’re refractors with 4-inch lenses or reflectors with 4-inch mirrors.

In almost all cases, telescopes with larger apertures show objects better than ones with smaller apertures. The only exceptions are objects that are so big that they don’t fit in the field of view of large telescopes. You may sometimes see it claimed that big telescopes are useless — or even counterproductive — in the presence of light pollution. There is essentially no truth to this claim, as explained in my page about Aperture Versus Light Pollution.

Filters

The term “light pollution filter” is a misnomer; no filter can do much to reduce the ill effects of skyglow. That’s particularly true when viewing galaxies and star clusters, which emit their light over a full range of wavelengths. Any filter that blocks a significant amount of light pollution also blocks much of the light from a cluster or galaxy. Some people find broadband filters to be marginally helpful on these objects, but most find them useless or counterproductive.

It’s a completely different story when it comes to nebulas — in particular, emission nebulas such as M8, M17, or M42 (the Lagoon, Swan or Omega, and Great Orion) and planetary nebulassuch as M27 or M97 (the Dumbbell and Eskimo). These nebulas emit most of their light in a few narrow wavelengths, and filters that block most light while letting these wavelengths pass through can spectacularly improve their appearance. That’s true both under dark skies and in the presence of light pollution.

Magnification

It’s extremely important to use appropriate magnification when viewing your target. There is no rule that covers all cases, so it’s always wise to experiment. I usually start at the lowest magnification available, then increase it slowly as long as the view continues to improve. Often, different magnifications show different aspects of the same object.

There are some general guidelines, which are best expressed in terms of your telescope’s aperture. 100X is high magnification for my little 70-mm refractor, medium for my 7-inch Dob, and low on a 20-inch Dob. I find it most convenient to think in terms of exit pupil, which is your telescope’s aperture divided by the magnification. So 100X yields an 0.7-mm exit pupil on my 70-mm refractor, a 1.78-mm exit pupil on my 7-inch (178-mm) Dob, and a 5.08-mm exit pupil on a 20-inch (508-mm) Dob.

Anything over a 3-mm exit pupil counts as low power. These magnifications are usually used to maximize the field of view, but usually show less detail than higher magnifications — with a few striking exceptions!

Exit pupils between 1 and 2 mm are best for most deep-sky objects, depending partly on the object and partly on individual taste. Smaller exit pupils (higher magnifications) are useful mostly for resolving individual stars in globular clusters and for viewing details in small, bright planetary nebulas.

Finding Objects

I star-hop to all of my targets using a Telrad on my 7-inch Dob and a red-dot finder on my 70-mm refractor. For my Messier project I sometimes mounted the refractor on the Dob so that I only had to find each object once. I mostly used custom star-charts printed by a planetarium program, but I sometimes used Sky Atlas 2000.0 or Uranometria. In general, Sky Atlas 2000.0 does not show enough stars to locate the more difficult Messier objects under heavy light pollution, and even Uranometria is only marginally adequate.

Finderscopes are somewhat harder to master than Telrads or red-dot finders, but once the necessary skill has been acquired, they are very useful under heavy light pollution, where relatively few stars are visible to the naked eye. Both of my scopes have unusually wide maximum fields of view, allowing them to serve as their own finderscopes, but I strongly recommend a finderscope for any city dweller whose scope’s maximum field of view is less than 2 degrees.

Some kind of mechanical finding aid, such as setting circles, digital setting circles, or electronic Go To capability can certainly be useful in any conditions, and particularly so under heavy light pollution. I do not get much joy from such equipment; I prefer spending my time looking at charts and looking at the sky to interacting with machinery, but I have no trouble understanding people who feel differently.

Regardless of whether you use mechanical aids or star-hop, it’s essential in the case of the fainter objects to know precisely where your scope is pointed, and where to start looking for your target. It is not sufficient to aim it vaguely in the right direction with a Telrad and hope that the object will show up in the eyepiece. That technique works fine for objects that are immediately obvious as soon as they enter the field of view, but to locate a faint galaxy against a bright background, you need to know precisely what quadrant of the eyepiece to start scanning with averted vision. For faint objects, I generally prefer star-hopping from a bright star to using the point- and-shoot method with a Telrad. The essence of star-hopping is that you always know exactly where you are every step of the way. I find that easier than matching up what I see through the eyepiece with my charts after my Telrad has gotten me within half a degree of the target.

Likewise with setting circles and Go To drive. Unless these are accurate to a few arcminutes, so that you can be sure that the target is centered in a high-power field each and every time, you will still need to consult your charts and match up star fields to know where in your eyepiece to start looking for your target.

Averted Vision

Averted vision — looking a little away from your target instead of directly at it — is essential for finding faint targets. It soon becomes second nature to any deep-sky observer; I actually have to make a conscious effort when I want to view and object with direct rather than averted vision.

Averted vision works because the central part of your retina is crowded full of cones, which sense color but work only in bright light. That leaves less room for the rods, which are sensitive to low light levels. So you want to view faint subjects with peripheral vision and avoid using the central part of your retina.

In evolutionary terms, peripheral vision is meant for detecting moving prey and defense against moving predators, and indeed, many faint targets show up best if you sweep the scope slowly over their location, but are much harder to see when they stay in the same place. Conscious control of breathing can be very useful for seeing faint objects, as it is for most difficult physical tasks. In particular, there is a natural tendency to hold one’s breath when concentrating hard, and this is almost always counterproductive.

Learning to Trust Your Eyes

The most important skill for deep-sky observing — and the one that’s hardest to master — is learning to trust your eyes. Yes, that faint smudge that appears in your peripheral vision really is there, despite the fact that it disappears when you look straight at it. Learning to discriminate between marginal observations and optical illusions is challenging, and comes only with experience. It’s easy to go too far in the opposite direction; the history of astronomy is full of observations of objects that weren’t really there, from William Herschel seeing forests on the Moon to Percival Lowell seeing canals on Mars.

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