The Lake County Astronomical Society

Limits are Meant to be Broken

Jack Kramer

Newcomers to observing sometimes puzzle over lists of celestial objects, wondering which ones they'll be able to see with their telescopes. However, old hands at observing normally regard an object's given magnitude as a very general guideline. Factors such as transparency of the sky, the object's size, and local light pollution all dictate what you can see on any given night. Other factors include the design of the telescope and quality of the optics, as well as the experience of the observer. On top of that, not all sources agree on each object's exact magnitude. Typically, magnitudes are determined based on photographic plates that have low sensitivity to the blue end of the spectrum - a range at which many deep sky objects radiate strongly and at which the human eye is more sensitive. Thus the magnitude may be shown fainter than at visual wavelengths. In other cases, the photographic magnitude might be brighter than at visual wavelengths. Even magnitudes referred-to as "visual" actually may be based on readings from instruments.

(Un)limiting Magnitude

The table below lists how faint a star is visible in instruments of given sizes.

This is just one such list; checking other sources will usually show some disagreement. For example, a different source that I checked gives a magnitude a few tenths fainter for each telescope size. So don't take this as gospel. It's also worth noting that this list is based on stars - point sources - but extended objects of the same magnitude, such as galaxies and nebulae, would normally prove slightly harder to see.

Your 8-inch scope probably is not going to pick up a 15^th magnitude galaxy. But depending on circumstances, it's possible to better the previous estimates by a half magnitude or even more. It's often possible to detect a faint and small object, especially if it's adjacent to a brighter object. This happens frequently in the case of galaxy clusters where very faint galaxies lie near brighter ones that serve as "beacons". If the faint ones were out by themselves, you'd probably never notice them at all. The NGC galaxies 4820, 4990, 5046, and 5373 in Virgo lie near brighter galaxies. All four have magnitudes listed from 15 to 15.3, yet I saw them in my 10-inch scope - something the rulebook says I shouldn't have been able to do. In addition to their position relative to other objects, I had the advantage of observing in a very dark sky and employed averted vision in order to see them. On the other end of the scale, higher magnitude objects sometimes don't appear nearly as bright as you'd expect. The huge spiral galaxy M101 in Ursa Major is a classic example. At 9^th magnitude, you'd think it would be easy to see. Yet it's a difficult object because its light is spread across roughly 25 arc minutes of sky. Published magnitudes are integrated; that is, treated as though the objects were point sources, when in fact some are quite extended.

Dawes' So-Called Limit

Hand-in-hand with limiting magnitude is the Dawes Limit. This stipulates how much separation in arc seconds between stars can be detected by each size telescope. This is referred-to as "resolution" or "resolving power". The larger the telescope, the closer the stars can lie with respect to each other and still be detected as discrete objects. The classic formula for resolution is 5.0" (arcseconds) divided by the aperture of the telescope in inches. Thus, a 4-inch scope should resolve a double star whose components are separated by no less than about 1.25". This formula also is applied as a guideline to indicate a telescope's ability to show minute details on extended objects like planets.

As with other rules, it's a generalization that doesn't always apply. Dawes based his formula on actual observations of stars of equal brightness. But double stars are rarely of equal brightness, and the less equal they are the harder they become to split. Objects like the Moon and planets also don't follow the formula. High contrast features are much easier to spot below the theoretical limit than the Dawes criterion supposes. This is the case with rilles on the moon under a low angle of illumination. And the shadows of Jupiter's moons when they transit the face of Jupiter are clearly visible, even though their diameters are well below the Dawes limit. On the other hand, the Dawes limit is based on refracting telescopes; obstructed systems (reflectors) have a harder time clearly resolving minute features, unless the mirrors are of exceptional quality.

Magnification

Using higher magnification increases your ability to see fainter magnitudes by providing a darker background. According to Bradley Schaefer of the University of Texas, going from 100x to 300x on an 8-inch telescope will increase your "light grasp" by about a quarter magnitude. The increases are even more dramatic on larger scopes. Of course, atmospheric conditions and overall optical quality must be good enough to sustain the higher magnification. Given the right conditions and equipment, the 50x-per-inch-of-aperture limit can often be exceeded on some objects.

Eyepiece Designs

Those with un-driven Dobsonian scopes eventually gravitate toward wide field (and expensive) eyepieces. One reason is that an eyepiece with a fairly narrow field of view makes it harder to locate a faint object. But just as important is the fact that the longer you can keep a field in view, the better chance you will have of catching whatever objects are there and seeing details in an object once it's acquired. Each time you have to nudge the telescope along to follow an object, you break off your concentration on the object, plus you have to re-acquire it in your view. So while Orthoscopic and Plossl eyepieces pass the maximum amount of light and provide extremely good contrast, I feel you'll get a better view of faint objects with wider field designs such as Naglers, Panoptics, and Pentax XLs.

The comfort factor also comes into play. Wide field eyepieces have the advantage of being easier to use because of the larger size of the eye lens and longer eye relief. (Eye relief is the distance to the point behind the eyepiece where you must position your eye so you can see the entire field of view.) This is particularly important when using high magnification in planetary observing. If you have to squint through a tiny peephole lens and position your eye right on top of the eyepiece so your lashes are brushing it, you won't feel like lingering there very long. This is true even if your scope has a motor drive. But if the eyepiece is easy to view through, you'll tend to observe the object longer and probably see a lot more detail in it. Experienced planetary observers usually agree that the simpler designs such as the Plossl and Orthoscopic are just about the best eyepieces in terms of image quality. But as a practical matter you'll probably see more detail in something like a Radian or Pentax XL because you're enjoying the experience more.

Other Tricks

There are a few other ways to coax a faint object into view. One of the most common is averted vision - looking slightly to one side of where an object lies. How this works is that the faint light falls on rods around the periphery of your inner eye, rather than on the cones in the center, which are less sensitive. If you stare straight at an object, the light is focused on the cones, which have higher resolution but less sensitivity to faint light.

Motion is another trick. Many observers have found that slightly jiggling the telescope makes very faint objects just discernible. I've found this most useful with faint nebulosity where you catch the edge of a large nebula as you cruise by it. Then when you stop moving the telescope and stare directly at the area, the nebulosity is nowhere to be seen. Experienced deep sky observers frequently move the scope slowly back and forth across the field where the faint object lies in order to catch that fleeting glimpse.

Getting back to the comfort issue, if you're less tense, you have a better chance of improving your visual acuity. So in addition to using a wider field eyepiece, one way to be more relaxed is to remain seated while observing. For that reason, the adjustable "observer's chairs" are a popular item.

You, the Observer

Despite the title of this article, limits can't always be broken. In fact, it's worth repeating that quite often we come up empty-handed on objects that are expected to be well within the range of our telescopes. But conditions vary, so trying again is the name of the game.

There's no substitute for knowing what to look for and how to look for it. Guests at a public star party will frequently have a difficult time seeing a deep sky object in your scope that is perfectly obvious to you. Just so, a skilled, experienced observer will generally see objects better than will a newcomer to the hobby. And the experienced observer will have a pretty good idea whether he or she has a shot at seeing a certain faint object or detail. This is not a skill learned in one evening's observing, but something that's mastered over time ... as you break some of the so-called limits.