Overview

Deep Sky Filter

• Intended for viewing nebulae from light-polluted skies.
• Blocks all mercury vapor and high & low pressure sodium vapor lamp light, neon lights and airglow, while transmitting the rest of the visible spectrum.
• The best all-around visual light pollution filter for use in urban skies.
• This filter also provides high-contrast views of the Martian polar caps

Description

The Deep Sky Filters are the result of 20 years of steady design improvements, and continue to deliver the highest performance of all anti-light pollution filters obtainable today. The following information recommends which filter to use on which celestial objects, and explains how filter transmissions differ.

The 1.25" Deep Sky is Lumicon's most popular nebula filter. It, along with the 2" and Cassegrain 2" versions block all high & low pressure mercury and sodium vapor lamp light, neon lights and airglow, while transmitting the rest of the visible spectrum. Visually, the Deep Sky Filter enhances contrast of nebula under both light-polluted and dark skies. The Deep Sky Filter also passes infrared light, making it an excellent filter for photographing deep space objects through light-polluted skies.

The best all-purpose nebula filter on the market today, the Lumicon Cassegrain 2" Deep Sky Filter threads onto your telescope's SCT rear-cell, and presents male SCT threads for accessory attachment. The Cassegrain filter cell allows you to change eyepieces without threading/unthreading the filter, and is also easier to change-out than the standard 2" filter cell with gloves on, for cold-weather observing.

To ensure that your Lumicon filter remains the World's Best, the strictest quality control standards are employed throughout the production process. Each Lumicon Deep Sky Filter is individually inspected and scanned with the percentage of light transmittance of the H-Alpha, H-Beta and OIII emission lines.

Specifications

Exit Pupil Specifications:
The exit pupil of a telescope is a measure of specific magnification, which differs from absolute magnification, and which determines the surface brightness of an extended object image. Exit pupil diameter may be expressed as the quotient of eyepiece focal length divided by the telescope's focal ratio. For example, a 32mm eyepiece used on an f/10 telescope will have a 3.2mm exit pupil. Each Lumicon filter has an optimum eyepiece exit pupil range shown below.

Notice:
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 Exit Pupil value shown above by your telescope's focal ratio. For example, if you are using the Lumicon H-Beta filter at a dark site and your telescope has an f/6 focal ratio, the best range of eyepiece focal lengths to use with this filter is [(4 to 7) x 6] = 24mm to 42mm.

Filter Construction:
Lumicon nebula filters are made using thin-film dielectric coatings on optically flat glass. These exclusively designed dielectric coatings consist of over 30 alternating layers of several different materials. Each layer is about a wavelength of light thick and has a thickness accurate to 2 - 3
angstroms. The Deep Sky Filters use very hard electron-beam deposited coatings on one side of the glass substrate, and delicate anti-reflection coatings on the other. Except for the Deep Sky Filter, all Lumicon filter coatings are very hard, and may be cleaned carefully with alcohol. The UHC, OIII, and HBeta filters consist of two elements, sandwiched coatings, and anti-reflection coatings on all surfaces to prevent ghosting.

Mechanical Design:
These filters thread directly into most eyepieces and telescope accessories. Threads are standard for 1¼" filters. 48mm filters are standard for 2" O.D. eyepieces.

Bandpass:
These Lumicon filters reject man-made and natural light pollution. Mercury light pollution occurs at 365, 405, 436, 546, 577, and 617nm. High-pressure sodium streetlights emit at 570, 583, 600, and 617nm. Natural airglow occurs at 558 and more weakly at 630nm. There is a window of greatly reduced light pollution from 440nm (blue) to 540nm (green). The Lumicon Deep Sky Filter has a wide 90-100nm bandpass for most of this range (441-535nm) to yield maximum transmission of light from stars and
galaxies. The UHC Filter has a narrow 22nm bandpass through 484-506nm. The OIII Filter has a very narrow 11nm bandpass for 495-501nm, and the H-beta Filter has the narrowest bandpass of all - only 8nm centered at 486nm. 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. Nevertheless, the Deep Sky Filter has high transmission for the photographic red nebula emission lines.

Nebula Emission Lines:
The main visible radiation from emission nebulae consists of doubly ionized oxygen near the wavelength of 500nm. There is also weaker emission due to hydrogen-beta at 486nm. The invisible but photographically important emission of red hydrogen-alpha and ionized nitrogen occur near 657nm.