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TELESCOPE FILTERS

  Scope City's Optics Crash Course
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PART V: TELESCOPE FILTERS

There are many kinds of filters that can be used with a telescope. What all filters have in common is that they enhance the visibility of a particular object (or type of objects) in the sky, and make observing more rewarding. Let's talk about them:

Planetary Filters: These are typically made of optically-ground and polished colored glass. They can be attached to the eyepiece or star diagonal, and are sorted in color according to the Wratten filter scale, which originally came from industrial use. Different colors enhance different details on planets, and a simple guide to the most popular colors follows:

  • #80A blue or 82A light blue-enhances Jupiter's belts, belts on Saturn's disc, and clouds on Mars
  • #15 deep yellow or 8 light yellow-enhances the sharpness of Saturn's rings, the Moon, and Mercury
  • #21 orange, 23 red-orange, or 25 red-enhances dark markings on Mars

Lunar Filters: It doesn't take much looking at the Moon through a telescope until you realize that the Moon is very bright-so bright as to be slightly painful-especially when the Moon is more than ? full. A simple filter that cuts the brightness makes all the difference in the world. Here's what we recommend:

  • Neutral Density Filter-filtering from 50% to 87% of the light (but without changing color), this is the simplest way (and least expensive) to reduce the brightness to a comfortable level. Made of optically-ground and polished glass, these are attached to the eyepiece or star diagonal.
  • Variable Polarizing Filter-containing two polarized filters stacked together, these can be rotated relative to one another to produce nearly any degree of brightness desired. This is the best Lunar filter because it can be adjusted for the exact phase of the Moon that night-not too dark for the slim little crescent, or too bright for the full Moon.

Nebula or Light Pollution Reduction filters: By selectively filtering out light in the sky caused by outdoor lighting or simply the glow of the night sky itself, while admitting the light from nebulae relatively untouched, the background sky in the eyepiece is darkened significantly while the nebula is not dimmed. This enhances contrast, and makes fainter nebulae or details in nebulae more visible. These have revolutionized amateur astronomers' views of the skies. It's an easy way of making your telescope perform as if it were a bigger scope. Made of multiple layers of dielectric coating materials applied to optical glass, they are difficult to manufacture, so they can be expensive.

Of course, any object that has a continuous spectrum (like a star or a galaxy) will have its light filtered out by these filters, so their primary usage is to enhance the visibility of nebulae (many of which are some of the most spectacular objects in the sky). Additionally, they can help reduce the negative effects of light pollution in the urban sky, and make many more objects visible. Best used with magnifications of between 3X and 10X per inch of aperture (example: 3¡± telescope = 9 power to 30 power) so the image doesn't become too dark.

There are 3 basic types:

  • Broadband filter-this is the most subtle of the filters, filtering out the least amount of light (concentrating on light generated by most outdoor lighting). This type of filter works best in dark skies, but produces some enhancement for the urban dweller. The standard of excellence here: Lumicon Deep-Sky Filter.
  • Narrowband filter-this is the one to get if you only have one filter. It filters out nearly all of the light from the sky, merely passing the light at the 3 or 4 frequencies emitted by most nebulae. This type of filter produces astounding enhancement of nebular images, even for the urban dweller. It works exceptionally well in dark skies, as well. Both the size and the amount of detail in each nebula will increase dramatically. The standard of excellence here is the Lumicon UHC (Ultra-High Contrast).
  • Line filter-providing an even narrower bandwidth, these filters pass only certain frequencies of light, creating the maximum contrast possible. The trick is to match the filter with the type of object being viewed. Most objects respond well to the O-III filter, but only a handful responds well to the H ¦Â (Hydrogen Beta) filter. The standard of excellence here, especially for that class of nebulae called
    Planetary Nebulae, as well as a lot of bright nebulae is the Lumicon O-III (Oxygen 3).

Comet Filter: Passing only the frequencies of light emitted by most comets, this specialty filter makes comets seen many times brighter, and enhances detail in their tails substantially. Most comets don't get bright enough to see with the naked eye, yet still display rich details through the telescope. This filter helps tremendously. The standard of excellence in this filter (sometimes called the SWAN band) is the Lumicon Comet Filter.

Solar Filters:

Special note: For safety, DO NOT USE a solar filter that threads onto the eyepiece! These are very dangerous. They can heat up and crack at anytime, letting vision-damaging solar rays through. The heat buildup can also damage the telescope and its parts. You don't see many of these around anymore, but there may be still some remaining on dealer's shelves.

The only safe solar filters are the ones that attach securely to the outside end of the telescope, covering the objective lens or mirror. When using these, remember to cap or cover any finder scope to prevent accidental damage to the finder scope (or your ear!).
Do not leave a telescope unattended during a solar observing session, especially when children are around.

Our Daytime star (the Sun, of course) is an exciting, dynamic place when you can safely look at it through the telescope. The Sun's surface dissolves into tiny details, like sunspots, faculae, prominences, flares, plages, etc. Though you've heard all your life about how unsafe it is to look at the sun with a telescope, with the appropriate filter it becomes the one celestial object you can look at in the daytime. There are 2 basic types of solar filters available:

White Light Solar Filter-made of thin Mylar film or metal plating on glass, these safely reduce the light of the Sun by a factor of nearly a million to make viewing the Sun's surface, or photosphere, safe. You'll see sunspots (with their surrounding penumbras, or aureolae), bright spots called faculae, and, when the atmosphere is steadiest, an actual granularity to the Sun's surface. These filters fit over the front of the telescope. The standard of excellence is from Thousand Oaks.

Hydrogen- a (Alpha) Solar Filter-though expensive, these filters reduce the light of the Sun to only one frequency deep in the red where the Sun emits a tremendous amount of energy. This is the frequency where the atmosphere of the Sun above the photosphere emits most of its energy. Additionally, this is the frequency of light emitted by huge streamers of gas (called prominences) that are expelled from the Sun along magnetic field lines that protrude from the surface of the Sun. You'll see streams of gas flying away from
the Sun, with activity changing minute-by-minute. You'll see clouds of gas floating above the Sun, and more surface details than you thought possible. The standard of excellence, here, is from Coronado Instruments. [Coronado also makes excellent Solar Telescopes!]

Photographic Filters: The camera shows it all-all the flaws of the scope, and all the frequencies of light that come through the camera. Accordingly, it is often necessary to filter the light that comes into the camera to have the camera operate at its best. There are three basic filters commonly used in camera photography:

  • Minus Violet Filter-Though unnecessary in reflectors, this filter can reduce or eliminate the Violet haze that is often visible in photographs taken through Achromatic Refractors. Additionally, they improve visual resolution in objects viewed, as well. Lumicon and William Optics make excellent ones.
  • Infrared Filter-Every digital camera's chip is sensitive to infrared light. Unfortunately, our environment is full of it, and it all gets recorded in the image. But it doesn't enhance the image, so it needs to be filtered out. Lumicon makes a superb one that attaches to most photographic adapters. Some digital astronomical camera systems have the IR filter built in.
  • Hydrogen- a (Alpha) Filter-Many objects in the night sky emit a lot of light at this deep-red frequency. Taking a long shot at this frequency to combine with a long, black and white shot (often called Luminance) can produce that wonderful reddish color common to nebula photographs. There is often more detail visible at this frequency than at any other.
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LUMICON TELESCOPE FILTERS

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Lumicon 1.25 inch Filter Selector Focuser Combo Large Base
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Lumicon 1.25 inch Filter Selector Focuser Combo Large Base
Item No. 735-86005
Manuf. No. Lumicon LF5005

1.25" Filter Selector- Low Profile Focuser Combo for Newtonian on 3.5" Base is a combination of our LH2020
MSRP: $195.00
Our Price: $129.95
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Lumicon 1.25 inch Filter Selector Focuser Combo 3 inch Base
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Lumicon 1.25 inch Filter Selector Focuser Combo 3 inch Base
Item No. 735-86000
Manuf. No. Lumicon LF5000

1.25" Filter Selector- Low Profile Focuser Combo for Newtonian on 3" Base is a combination of our LH2000
MSRP: $195.00
Our Price: $129.95
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Lumicon Expansion Kit for 2 inch Multiple Filter Selector, with 2nd Filter Tray
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Lumicon Expansion Kit for 2 inch Multiple Filter Selector, with 2nd Filter Tray
Item No. 735-85032
Manuf. No. Lumicon LF5032

The kit converts the standard Lumicon 2" Multiple Filter Selector with 4 filter holders into a two-tray system, with a total capacity of 8 filters.
MSRP: $120.00
Our Price: $79.95
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Lumicon 1.25 inch Multiple Filter Selector slide
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Lumicon 1.25 inch Multiple Filter Selector slide
Item No. 735-90001
Manuf. No. Lumicon LF5012

LUMICON Multiple Filter Selectors allow you to switch back and forth between filters without having to thread and unthread them from your eyepieces or diagonal
MSRP: $195.00
Our Price: $129.95
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Lumicon 2 inch Lunar & Planetary Filter Set (4)
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Lumicon 2" Lunar & Planetary Filter Set (4)
Item No. 735-22100
Manuf. No. Lumicon LF5050

2 inch Lunar & Planetary Filter Set (4)
MSRP: $152.00
Our Price: $104.95
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Lumicon 1.25in Lunar & Planetary Filter Set (4)
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Lumicon 1.25in Lunar & Planetary Filter Set (4)
Item No. 735-21100
Manuf. No. Lumicon LF5045

1.25in Lunar & Planetary Filter Set (4)
MSRP: $105.00
Our Price: $69.95
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Lumicon Filter Selector Diag-MFS Combo 2 inch LumiBrite for Refractors
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Lumicon Filter Selector Diag-MFS Combo 2" LumiBrite for Refractors
Item No. 735-83025
Manuf. No. Lumicon LD3025

Lumicon 2" Filter Selector - LumiBrite Diagonal Combo for Refractors
MSRP: $510.00
Our Price: $359.95
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Lumicon Filter Selector Diag-MFS Combo 2 inch Enhanced for Refractors
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Lumicon Filter Selector Diag-MFS Combo 2" Enhanced for Refractors
Item No. 735-83015
Manuf. No. Lumicon LD3015

Lumicon 2" Filter Selector - Enhanced Diagonal Combo for Refractors
MSRP: $432.00
Our Price: $269.95
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Lumicon Expansion Kit for 1.25 inch Multiple Filter Selector, with 2nd Filter Tray
Lumicon Expansion Kit for 1.25" Multiple Filter Selector, with 2nd Filter Tray
Item No. 735-85030
Manuf. No. Lumicon LF5030

Lumicon 1.25" Multiple Filter Selector Expansion Kit, with 2nd Filter Tray
MSRP: $52.00
Our Price: $34.95
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Lumicon 2 inch Multiple Filter Selector with T-Threads For Photography
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Lumicon 2" Multiple Filter Selector with T-Threads For Photography
Item No. 735-82000
Manuf. No. Lumicon LF5055

2" Multiple Filter Selector with T-Threads For Photography
MSRP: $225.00
Our Price: $159.95
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Lumicon Infrared 1.25 inch Filter
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Lumicon Infrared 1.25" Filter
Item No. 735-20700
Manuf. No. Lumicon LF3140

Infrared 1.25" Lumicon Filter
MSRP: $98.00
Our Price: $69.95
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Lumicon Minus Violet 1.25 inch
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Lumicon Minus Violet 1.25"
Item No. 735-20645
Manuf. No. Lumicon LF3120

Minus Violet 1.25"
MSRP: $85.00
Our Price: $59.95
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Lumicon Night Sky Hydrogen-Alpha (H-Alpha) 72mm Filter
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Lumicon Night Sky Hydrogen-Alpha (H-Alpha) 72mm Filter
Item No. 735-20572
Manuf. No. Lumicon LF3105

For dramatic contrast increase in black and white photography and CCD imaging..
MSRP: $170.00
Our Price: $119.95
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Lumicon Night Sky Hydrogen - Alpha 1.25 inch
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Lumicon Night Sky Hydrogen - Alpha 1.25"
Item No. 735-20500
Manuf. No. Lumicon LF3085

For dramatic contrast increase in black and white photography and CCD imaging..
MSRP: $112.00
Our Price: $79.95
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Lumicon Multiple Filter Selector 2 inch
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Lumicon Multiple Filter Selector 2"
Item No. 735-90015
Manuf. No. Lumicon LF5015

LUMICON Multiple Filter Selectors allow you to switch back and forth between filters without having to thread and unthread them from your eyepieces or diagonal.
MSRP: $240.00
Our Price: $169.95
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