photo:light_pollution
light pollution
light pollution
if the air around us was completely clean and pure, free of all dust, pollutants and light, this issue wouldn't be so crucial. But that's sadly not the case. All the dust and pollution that is suspended in the air scatters light in all directions.
the primary sources of light are street/city lights (eg. outdoor building lighting) and the moon (full moon being the worst).
light pollution reduces the detail and brightness of sky objects.
light pollution can be easily seen by the lighting up of clouds at night
also included in the table is the approximate exposure in a digital camera (after dark frame subtraction) at f/4 800ISO pointed at the zenith that results in a luminosity histogram peak at 50% (ie. mean value is 128 in 8 bit channel).
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limiting visual magnitude | zenith sky brightness | Unaided eye capabilities |
3.5 (urban) | 25-50x | Milky Way is completely invisible; 0.6 minutes exposure; |
4.5-5.0 (suburban) | 7-10x | Milky Way and Zodiacal light invisible. Typical conditions found in suburbs of major cities. Passing clouds are easily seen due to being lit up from surrounding lights. 1.5-2.4 minutes exposure; |
5.1-5.5 | 5-7x | The indistinct Milky Way faintly visible only near the zenith. Zodiacal light invisible. M31, the Andromeda Galaxy, is barely discernible. 2.4-3.8 minutes exposure; |
5.6-6.0 (outskirts) | 4-5x | clouds are brighter than the sky because they are lit from below. The Milky Way is now more easily seen, but lacks detail. M13, the Great Hercules globular star cluster can now be just glimpsed when near the zenith. The Zodiacal light is still invisible. The Milky Way from Auriga through Orion still invisible. 3.8-6.0 minutes exposure; |
6.1-6.5 (rural) | 2-3x | The Milky Way is now obvious and some detail can be glimpsed. The Zodiacal light is now barely visible, but not obvious. The Milky Way from Auriga through Orion is faintly visible. There is still noticeable sky-glow along the horizon due to distant towns and cities. NB. the Sydney sky-glow is visible even 700km inland! eg. Eastern/central Oregon; 6.0-9.5 minutes exposure; eg. Clonbinane, 68km north of Melbourne |
6.5-7.0 (dark sky rural) | 1-2x | the sky is packed with stars, the Milky Way is a mass of swirling, jumbled detail and any clouds appear blacker than the sky itself. Sky brightness mainly due to natural sky glow. Much structure is visible in the Milky Way. The Zodiacal light is an obvious cone of light. The major constellations are less obvious due to “noise” caused by the large number of faint stars now visible. Passing clouds appear as dark moving masses as they block the natural skyglow or the Milky Way. A few sources of sky-glow are still visible. eg. western USA, New Mexico, 9.5-15 minutes exposure; eg. in Victoria, Australia: Hall's Gap, Horsham, Nagambie |
>7.1(darkest skies) | 1x | Incredible! The Milky Way contains an enormous amount of structure all the way to the horizon and you can easily see your way around by it's light. The Zodiacal light now encircles the entire ecliptic. There are no sources of sky-glow along any part of the horizon. Many meteors are visible. >15 minutes exposure; eg. in Victoria, Australia: Murchison, Ouyen, Heathcote |
light pollution and distance from urban centres:
natural sky glow
The Australian Outback, the coast northwest of Perth, the Chilean observatory sites, and isolated places in the US Southwest, plus many others have sky brightness negligibly different from the natural background, which sets a fundamental (and more-or-less inescapable) limit on how dark a site can be.
The moonless night sky at a remote location far from any man-made light pollution is, however, still not completely black. To most people who are fully dark adapted, it appears a dark gray, but it may also have some faint color.
The dark night sky is illuminated by a natural skyglow that is composed of four parts:
Airglow is the brightest component and is caused by oxygen atoms glowing in the upper atmosphere which are excited by solar ultraviolet radiation. Airglow gets worse at solar maximum. Airglow can add a faint green or red color to the sky background. The color may be vivid if there is a strong
aurora occurring.
Interplanetary dust particles reflect and scatter sunlight and make up the zodiacal light and gegenschein.
At night starlight is scattered by the atmosphere, just as sunlight is during the daytime. Air molecules scatter short blue wavelengths more, which is why the daytime sky is blue. The night sky also has a very faint blue component from scattered starlight.
Countless stars and nebulae in our own galaxy also contribute to the brightness of the night sky, most easily seen in the form of the Milky Way.
Despite the fact that many folks have not seen the zodiacal light, much less the gegenschein or zodiacal band, it is the main contribution to the natural sky brightness even the ecliptic poles.
The night-airglow varies considerably due to solar activity on the time scale of minutes/hours as well as over the 11-year solar cycle, and can greatly compromise the darkness at a site on any particular night.
The zodiacal light, zodiacal band, and gegenschein are prominent features of the night sky at true-dark sites. They are not tests of visual acuity, but of sky brightness.
The night-airglow is also easy to see at dark sites, at least where there is little scattered light from atmospheric dust and aerosols. There are many reported visual sightings of the rippled structure in this phenomenon, looking like banded very thin altocumulus clouds. This light is visible mostly from a forbidden line of ground-state oxygen which emits at 5577A, where most light-pollution filters have their red cutoff.
The widely accepted value for sky brightness at the zenith at a site completely free of man-made light sources and near solar activity minimum is V mag. 22.0 per square arcsecond = mag. 13 per square arcminute. In other words, a perfect site has a sky brightness equivalent to having a mag. 22 star in every square arcsecond box (hardly bigger than the star image itself) over the entire sky.
Where there's no light pollution the limiting magnitude is usually assumed to be 6.5, though some people can see fainter. Under such conditions, the sky is packed with stars, the Milky Way is a mass of swirling, jumbled detail and any clouds appear blacker than the sky itself.
astrophotography in areas of light pollution:
light pollution spectra:
high pressure sodium lamps have narrow peaks between 570, 583, 600, and 617nm
coiled fluorescent lamps have mercury peaks at 365, 405, 436, 546, 577, and 617nm
high intensity tungsten-halogen outdoor lamps often have a broad spectrum esp. from 500-750nm:
3000K filament has peak at 900nm rising steadily from 300nm thus main impact is > 600nm
4000K filament has peak at 700nm rising steadily from 250nm thus main impact is > 500nm
5000K filament has peak at 480nm thus main impact is 400-600nm
neon peak at ~640nm
natural airglow occurs at 558 and more weakly at 630nm.
car headlights are broadband and cannot be blocked
full moon:
nebula emission spectra:
OIII at 496 & 501nm producing a teal blue-green color but is transmitted in both blue & green filters (esp. planetary nebulae)
H-alpha at 656-658nm producing a red color
H-beta at 486nm, hence blue
cyanogen at 511 & 514nm (esp. comet gas tails)
light pollution filters (these only enhance nebulae and make stars dimmer!):
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problems:
results will not be as good as going to a dark sky site!
expensive $US185 upwards
do not block all light pollution
may not be as effective for visualising broadband objects such as stars
a filter that substantially improves views of star clusters, galaxies, or reflection nebulae does not exist as these reflect light across the spectrum and filters blocking light pollution will necessarily make these dimmer.
may require increased exposure times
some filters are not available any larger than 2“ - see Pertti's tests using 48mm (2”) filters on Canon lenses via step-down rings
here
may have problems with digital cameras as most block H-alpha regions so using a filter that primarily only allows H-alpha will result in very long exposure times
require high quality multicoated optic systems to minimise internal reflections they produce as they reflect non-transmitted light rather than absorbing it
cause colour shifts towards the edge of image in wide field images as curved lenses refract light differently as narrowband filters are tuned to specific wavelengths by carefully controlling the thickness of layers of dielectric material on glass. Because the effective thickness changes when the filter is tilted, the wavelength tuning will also change.
if using a front-mounted filter which sits on the camera body (usually a Canon), cannot use EF-S lenses.
SBIG CFW-8 & CFW5C filter wheels:
Assume the dominant light pollution is from high pressure sodium street lamps. Design a “gap” between the red and green filters to eliminate sodium light pollution as much as possible. Keep the blue and green filters as efficient as possible.
ie. both the red & green filters have a more narrow spectrum than usual to create a gap coinciding with the sodium lamps, however, the strongest emission at 570nm is allowed to pass at 80% transmission via the green filter and does little to block the mercury emissions.
the main problem with RGB filter sets is the poor transmission of 480-550nm & thus the important OIII line is not captured well, hence many are migrating to CMY filter sets for “true color” or Ha-OIII-? filter sets for dramatic false colour images
combining this with the IDAS filter:
blue filter now blocks the 430nm mercury line
green filter now blocks the 540nm mercury line but unfortunately still lets in the strong 570nm sodium line
red filter still allows, although at lower transmission, the 620nm mercury & sodium emissions
IDAS filter by Hutech:
works well with mercury-vapor lights
can be used in front of the RGB filters but will also affect the luminance channel exposure
it blocks narrow spectral lines from mercury, sodium and neon lamps, leaving reasonable color balance among the RGB filters, but at the expense of nearly twice the imaging time.
transmits 400-430nm, 450-540nm, 570-590nm, 600-620nm, 640-700nm and thus blocks most of the mercury, sodium and neon emissions.
many prefer the IDAS-LPS because the color balance is almost unchanged. It is less efficient in heavy light pollution but this is the price to pay for color fidelity. Color balance, it can be managed easily in Photoshop with the grey pipet or doing a custom with balance with a DSLR using a piece of white paper. But if a color is dropped by the filter (like h-alpha for example), no color balance will get it back.
2“ (48mm) filter can be used at prime focus or on the front of lenses via step-down rings
Astronomik UHC:
narrow band Ha & OIII filters which are good for the emission nebulae, but better for visual than photo use:
require at least a 4” telescope
H-alpha filters:
no good for most digital cameras as they have IR filter that blocks H-alpha
3nm bandwidth - only for f/ratios slower than f/4 else S/N reduces near the edge of image
10nm bandwidth - allows more stars to be imaged
OIII filters:
Many emission nebulae and most planetary nebulae will look remarkably better through an OIII filter.
as the OIII filter discriminates against all other wavelengths, the background image is considerably dimmer, even more than a narrow-band filter, thus it is more suitable for telescopes 6-inches and larger.
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good for visual astronomy of nebulae in light polluted areas
does a better job enhancing planetary nebula and some emission nebula than the Ultrablock or UHC
bandpass 11nm; optimum exit pupil 2-5mm light polluted & 3-7mm dark sky
Thousand Oaks LP-3 Oxygen III
OIII + Hb filters “narrowband”:
most useful for observing emission or planetary nebulae.
the background field of view becomes rather dim, but target objects still stand out well and actually appear brighter because of the added contrast.
Lumicon UHC (the latest version) and Orion UltraBloc:
poor color balance (because color spectrum is strongly filtered) thus greens become blue
suitable for visual usage of emission nebula (better contrast) such as Orion, Lagoon
filter h-alpha
very narrow h-beta (less dark blue)
not recommended as a light pollution filter but great for viewing nebulae
bandpass 24nm; optimum exit pupil 1-4mm light polluted & 2-6mm dark sky
Thousand Oaks Lp-2 narrowband
OIII + cyanogen filters:
H-beta filters:
OIII+H-alpha+H-beta+NII filters:
Hutech IDAS NBN-PV:
Hutech IDAS LPS-V3:
broadband filters:
The real use for broadband filters is to enhance some reflection nebulas, HII regions (nebula) in galaxies such as M101 and M33 and astrophotography of nebulas only. It doesn’t produce dramatic results but can help. You will not use a broadband filter much, a narrowband filter is better for nebulae.
can be used on any size telescope
Meade broadband #511B:
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blocks < 450nm, 540-650nm
bandpass 90nm; optimum exit pupil 0.2-2mm light polluted & 1-4mm dark sky
better color balance, suitable for astrophoto on any objects, allow h-alpha band & wider h-beta band(blue)
similar to the Astronomic UHC
Celestron LPR:
works well for hi-pressure sodium lights
The transmittance graphic of the Celestron LPR is very similar to an Astronomik UHC filter. It is suitable for photographic use in severe light polluted skies.
thus greens become more blue but not as much as for the Lumicon UHC
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filters out < 480nm, 500-530nm, 570-620nm, 730-750nm
good for light pollution but allows neon, tungsten-halogen;
good for reducing blue scatter & when used with Baader IR cut filter removes all unfocused light which is essential for good digital images
used also as a moon filter & in solar astronomy with a sun filter
good for viewing Red Spot on Jupiter & details on Mars
reduces chromatic aberration in refractors
Orion Skyglow Broadband:
cheap ($US60) filters out 300 – 480 nm and 530 – 630nm
but reduces light transmission and makes viewing of 6th magnitude star fields difficult in an 8“ telescope
“The detail this simple filter affords when viewing emission nebulae is worth the money alone. Normal viewing of the Orion Nebula without the filter is fascinating, but viewing this nebula with the filter in-place is truly astonishing. You can spend hours studying its inner detail, the filaments and brightness contrasts. The Orion Nebula comes alive and you feel fortunate that is marvelous sight is within our astronomical backyard. ”
Thousand Oaks Lp-1 broadband
Wratten 44 photographic filter (cyan or “minus red”) or B+W 470:
filters out 200-430nm and 560-730nm
may only be available in gelatin
used photographically to reproduce spectral response of very old orthochromatic B&W film
as these are absorption filters, they are not as efficient at transmitting light (40% vs 90%) as interference filters as used above
NB. As filter bandpass decreases, optimum exit pupil size tends to increase. To determine the best eyepiece focal length to use with a given filter, simply multiply the optimum Exit Pupil value shown above by your telescope's focal ratio
NB. The narrower the bandpass, the higher the rejection of light pollution and the blacker the skies. However, a narrower bandpass also means fainter star images.
see also:
monochrome CCD cameras with 3 step filter imaging
stacked, short exposure images
image processing to minimise light pollution effects:
best to do your gradient removal on the individual (combined) channels prior to any other processing. For example, create your master “red”, green, blue, etc…remove gradients..than normalize..then do the color combine.
suggestion by DeHaven good for stars but this will not help show nebulae or comet trails:
a) take original image and make a copy of it
b) take copy and paste it into original image
c) take second layer and:
use a median filter to get rid of the stars (If you don't, a bright star will cause a hump in your gaussian filtered
image that is difficult to get rid of. One can then use a less broad gaussian)
use Gaussian Blur to COMPLETELY blur out any and all details
d) combine these layers using the difference mode and transparency at 100%
e) save file
f) repeat with all images shot
g) combine and align all edited images using free transform and
linear burn
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photo/light_pollution.txt · Last modified: 2016/06/12 13:00 by gary1