what size telescope would be required to see magnitude 18 stars
Akira Fujii
How dark is your sky? The relationship between light pollution and astronomy is a concern amateurs and professional astronomers alike. Are your skies dark enough? A precise answer to this question is useful for comparing observing sites and, more important, for determining whether a site is nighttime enough to let you button your eyes, telescope, or camera to their theoretical limits. As well, you need accurate criteria for judging sky weather when documenting unusual or borderline observations, such as an extremely long comet tail, a faint aurora, or subtle features in galaxies.
Calorie-free Pollution and Astronomy
On Internet bulletin boards and newsgroups I see many postings from beginners (and sometimes more experienced observers) wondering how to evaluate the quality of their skies with a keen interest in light pollution and astronomy. Unfortunately, nearly of today's stargazers have never observed under a truly dark sky, so they lack a frame of reference for gauging local conditions. Many describe observations made at "very nighttime" sites, merely from the descriptions it's clear that the heaven must take been only moderately nighttime. Nigh amateurs today cannot get to a truly dark location within reasonable driving distance. Thus, upon finding a semirural observing site where stars of magnitude 6.0 to vi.3 are marginally apparent to the unaided eye, they believe they have located an observing Nirvana!
Thirty years ago 1 could find truly dark skies within an hr'due south drive of major population centers. Today you lot ofttimes need to travel 150 miles or more than. In my own observing career I accept watched the extent to which ever-growing calorie-free pollution has sullied the heavens. In years long past I witnessed nearly pristine skies from parts of the highly urbanized northeastern United States. This is no longer possible.
Limiting Magnitude Isn't Plenty
Amateur astronomers unremarkably judge their skies by noting the magnitude of the faintest star visible to the naked eye. However, naked-center limiting magnitude (NELM) is a poor criterion. It depends too much on a person's visual acuity (sharpness of eyesight), as well as on the fourth dimension and effort expended to see the faintest possible stars. One person'southward "5.5-magnitude sky" is another'south "6.three-magnitude sky." Moreover, deep-heaven observers need to assess the visibility of both stellar and nonstellar objects. A modest amount of light pollution degrades diffuse objects such as comets, nebulae, and galaxies far more than than stars.
To help observers judge the truthful darkness of a site, I have created a ix-level scale. Information technology is based on nearly l years of observing experience. I hope information technology will prove both enlightening and useful to observers — though it may stun or even horrify some! Should information technology come into broad utilise, it would provide a consistent standard for comparison observations with light pollution. Researchers would also exist better able to assess the plausibility of an unusual or marginal observation. All around, it could be a boon to those of us who regularly scan the heavens.
How Nighttime Is Information technology? Rate Your Skies
The aforementioned constellation panorama in an urban, Class viii or ix sky.
John Bianchi
Class 1: Excellent Nighttime-heaven Site
To the unaided eye the limiting magnitude is 7.6 to viii.0 (with try), though the presence of Jupiter or Venus in the heaven seems to dethrone nighttime adaptation. The zodiacal light, gegenschein, and zodiacal band are all visible — the zodiacal calorie-free to a striking degree, and the zodiacal band spans the unabridged sky. The milky way M33 is an obvious naked-eye object even with direct vision. The Scorpius and Sagittarius regions of the Milky way cast obvious diffuse shadows on the ground. Airglow, a very faint, naturally occurring glow nearly evident within virtually 15° of the horizon, is readily apparent.
With a 32-centimeter (12½-inch) telescopic, stars to magnitude 17.5 can be detected with effort, while a 50-cm (20-inch) instrument used with moderate magnification will reach 19th magnitude. If you are observing on a grass-covered field bordered by copse, your telescope, companions, and vehicle are about totally invisible. This is an observer's Nirvana!
Grade two: Typical Truly Dark Site
Airglow may be weakly apparent along the horizon. M33 is rather easily seen with direct vision. The summer Milky Way is highly structured to the unaided eye, and its brightest parts look like veined marble when viewed with ordinary binoculars. The zodiacal light is still bright plenty to cast weak shadows just before dawn and after sunset, and its colour can be seen as distinctly yellowish when compared with the blue-white of the Milky way. Any clouds in the sky are visible simply every bit dark holes or voids in the starry groundwork. Y'all can see your telescope and surroundings only vaguely, except where they project against the sky. Many of the Messier globular clusters are distinct naked-eye objects. The limiting naked-heart magnitude is as faint equally 7.1 to vii.5, while a 32-cm telescope reaches to magnitude 16 or 17.
Grade 3: Rural Sky
Some indication of lite pollution is evident along the horizon. Clouds may appear faintly illuminated in the brightest parts of the sky about the horizon but are night overhead. The Milky way still appears complex, and globular clusters such as M4, M5, M15, and M22 are all distinct naked-eye objects. M33 is piece of cake to run into with averted vision. The zodiacal low-cal is striking in bound and fall (when it extends 60° to a higher place the horizon afterwards sunset and before dawn) and its colour is at least weakly indicated. Your telescope is vaguely apparent at a distance of twenty or thirty anxiety. The naked-middle limiting magnitude is six.vi to 7.0, and a 32-cm reflector will achieve to 16th magnitude.
Form 4: Rural/Suburban Transition
Wintertime constellations in a suburban or rural-suburban transition heaven, with the winter Galaxy visible only not dramatically then. Such a sky, adequately good past many people'due south standards, might rate 4 or 5 on Bortle's calibration. Many fainter stars than are depicted here would be visible with close scrutiny.
Fairly obvious low-cal-pollution domes are credible over population centers in several directions. The zodiacal calorie-free is clearly axiomatic but doesn't even extend halfway to the zenith at the starting time or end of twilight. The Galaxy well above the horizon is even so impressive merely lacks all but the virtually obvious structure. M33 is a difficult averted-vision object and is detectable only when at an altitude higher than l°. Clouds in the direction of light-pollution sources are illuminated but just slightly so, and are still dark overhead. You tin can make out your telescope rather clearly at a distance. The maximum naked-eye limiting magnitude is 6.1 to 6.5, and a 32-cm reflector used with moderate magnification will reveal stars of magnitude 15.v.
Form 5: Suburban Heaven
Only hints of the zodiacal light are seen on the all-time spring and autumn nights. The Milky way is very weak or invisible most the horizon and looks rather washed out overhead. Light sources are axiomatic in most if not all directions. Over nigh or all of the sky, clouds are quite noticeably brighter than the heaven itself. The naked-eye limit is around 5.six to 6.0, and a 32-cm reflector will reach almost magnitude 14.5 to 15.
Class 6: Bright Suburban Sky
No trace of the zodiacal light can exist seen, fifty-fifty on the best nights. Any indications of the Milky way are credible only toward the zenith. The sky within 35° of the horizon glows grayish white. Clouds anywhere in the sky announced fairly bright. You have no trouble seeing eyepieces and telescope accessories on an observing tabular array. M33 is incommunicable to see without binoculars, and M31 is only modestly apparent to the unaided eye. The naked-eye limit is about 5.5, and a 32-cm telescope used at moderate powers will testify stars at magnitude fourteen.0 to fourteen.five.
Class 7: Suburban/Urban Transition
The entire sky background has a vague, grayish white hue. Strong light sources are axiomatic in all directions. The Milky way is totally invisible or virtually and then. M44 or M31 may be glimpsed with the unaided center but are very indistinct. Clouds are brilliantly lit. Even in moderate-size telescopes, the brightest Messier objects are pale ghosts of their true selves. The naked-heart limiting magnitude is 5.0 if you really endeavour, and a 32-cm reflector will barely achieve 14th magnitude.
Course 8: Urban center sky
The sky glows whitish gray or orangish, and you tin read newspaper headlines without difficulty. M31 and M44 may be barely glimpsed by an experienced observer on good nights, and only the bright Messier objects are detectable with a modest-size telescope. Some of the stars making up the familiar constellation patterns are hard to see or are absent-minded entirely. The naked eye can pick out stars down to magnitude four.5 at all-time, if you know merely where to expect, and the stellar limit for a 32-cm reflector is footling better than magnitude 13.
Grade nine: Inner-city Sky
The entire sky is brightly lit, even at the zenith. Many stars making up familiar constellation figures are invisible, and dim constellations such as Cancer and Pisces are not seen at all. Aside from mayhap the Pleiades, no Messier objects are visible to the unaided eye. The only celestial objects that actually provide pleasing telescopic views are the Moon, the planets, and a few of the brightest star clusters (if you can discover them). The naked-eye limiting magnitude is 4.0 or less.
Editors' annotation: This commodity on calorie-free pollution and astronomy appeared in the Feb 2001 issue of Heaven & Telescope.
Source: https://skyandtelescope.org/astronomy-resources/light-pollution-and-astronomy-the-bortle-dark-sky-scale/
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