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NoNaD filter
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NoNaD - A useful filter for Sodium light

It has long been know in the glass blowing industry that glass doped with certain rare earth salt mixtures has the unusual and useful property of filtering out the bright orange-yellow Sodium D-line emission almost completely.
This type of doped glass is widely used in the glass blowing industry to enable the workers to see into the flame when making intricate designs in sodium borosilicate glass. These glasses give a very high attenuation of the sodium D-line of around a 100000 times. The most commonly available version of this is available as spectacle grade lens blanks using the trade name of Didynium glass filters.

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Light-pollution filter
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The Baader Planetarium Moon & Skyglow (Neodymium) filter brings an entirely unique approach to contrast enhancement and light pollution reduction. Based on earlier research at Carl Zeiss, Baader has developed a new filter that uses Neodymium glass which filters out a few specific wavelengths from streetlights as well as skyglow from the Moon. The unique spectral characteristics of Neodymium also boost color contrasts by isolating the Red, Green, and Blue regions (so-called RGB enhancement). The result is a filter that leaves natural colors mostly intact, but enhanced. Lunar and Planetary views are improved in all scopes, reflectors, refractors (APO or Achro).
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Baader Moon & Skyglow Filter - 2"

Price: $89.95

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The Baader Planetarium Neodymium Moon & Skyglow Filter is basically a light pollution filter, with unfortunately, the same drawbacks that all light pollution filters have; they actually remove the light from the observed object as well... But for general observing, these types of filters are a 'must have' for anyone living under light polluted skies.

Price-wise these filters are expensive, with prices almost double that of cheaper alternatives, (although, the prices are coming down).

Performance-wise, it clearly removes most of the light pollution. However, my comparison tests with the usual stellar and planetary suspects, against cheaper alternatives (namely†the Sky Watcher anti-light-pollution filter) don't show vastly noticeable improvements, to visual observing, to justify the price increase. †
Shockingly for some, for price/performance i would actually recommend the Sky Watcher 1.25" anti-light-pollution.†

Also, despite what the adverts say, it should be noted that the Baader Planetarium Neodymium Moon & Skyglow Filters are recommended for Reflectors; Refractor owners are advised to purchases the Baader Contrast Booster Filter†(which has the same filter coatings, but also a chromatic aberration correction filter).

Baader Neodymium and IR Cut Filter 1.25 in: £34.72
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Sky Watcher 1.25" anti-light-pollution nebula filter (LPR)

These contrast-enhancing anodised aluminium, optical glass filters are designed to block out the wavelengths of light emitted by mercury-vapour light and other common causes of light pollution, whilst providing higher transmission at critical hydrogen-alpha and hydrogen-beta lines than competing filters. Bright, light-polluted skies appear much darker, and the contrast between object and sky is improved significantly. This contrast-enhancement effect is particularly apparent on nebulae. Unlike stars, emission nebulae give off light in a very narrow range of wavelengths. These filters allow maximum transmission of the important wavelengths of H-alpha, H-beta and doubly ionised oxygen - the ones most commonly emitted by nebulae. Views of galaxies and star clusters are also enhanced, but to a lesser degree. Also improves contrast on reddish planetary detail. Works well in telescopes of all apertures but is highly recommended for use on smaller telescopes up to 114mm diameter, where conventional filters swallow up too much light.

£19.99

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Ed ~ This glass filter cuts out urban light pollution and darkens the sky background. However, the filter (even the most expensive alternatives) will also block out very faint objects (deepsky objects) and likely lessen the resolution of the scope. These broadband (wideband) filters are for observing nebulae, and therefore not recommended for faint galaxies, comets, or close double star observing. If you suffer from severe light pollution and want to observe emission and planetary nebulae then this is really a must have item.
The filter is suited for visual use and is available in 1.25" and 2" fittings.

It should also be noted that the filters will impart a slight colour cast on the image.

For fast telescopes the filter will work best with 4-15mm focal length eyepieces, in light polluted city skies; and 10-30mm at dark sky sites. Longer focal length telescopes will allow for lower magnifications in the light polluted skies.

Owners of telescopes with large apertures (over 100mm) may wish to consider narrowband filters such as the SkyWatcher Ultra High Contrast Filter for improved nebulae viewing.



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