Filters, filters which to choose …

Filter Overview

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Filters are used to enhance the sharpness and contrast at the eyepiece or in an image. All filters perform in the same way and some may be solely for visual use while others can be used for imaging as well. No matter the type, they all serve to block certain specific wavelengths while passing others in order to reveal more disernadely certain details or objects in the sky. Some filters can be used for both visual and imaging astronomy such as the narrowband and the line filters. On the other hand planetary filters are used solely for visual astronomy. Astrophotography, using a monochrome camera, requires specially dedicated RGB filters. Achromatic refractors can benefit from the use of the colored filters of green, deep yellow and orange which reduce chromatic aberration.

There are a number of specialized filters which fall under the following categories as described below:

Planetary Filters
Colored filters are useful for observing the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Color telescope filters and camera filters are described using Wratten numbers. 

Planetary color filters come in two sizes 1.25 and 2 inches. They are designed to screw onto the eyepieces with similar diameters. Some people buy the 2 inch filters although more expensive because they can screw the filter onto the 2 inch adapter and view with a 2 inch eyepiece then they simply add the 1.25 to 2 inch adapter and put in the 1.25 eyepiece instead, without having to change the filter. Optimally it has been said however that the filter functions best screwed onto the eyepiece itself.

Each color eyepiece filter passes its characteristic color of light while blocking complementary colors. To bring out a particular feature of a planet choose a color filter of the color complement to that planetary feature.

Consider the aperture of the telescope you will be using with the filter. The smaller the aperture, the lighter the color you should use. As a general rule, dark colored filters should be reserved for use with apertures of 8″ or larger.  Generally for an 8 inch telescope it would be better to use filters having a transmission of 40 percent or greater. Filters can be stacked, giving many possible combinations; but the limiting factor would be increasingly less light transmission (VLT) and hence diminished views. For this reason stacking of filters lends itself better to telescopes of larger apertures than 8 inches. Choose quality filters for best results. It is best to try before you buy…at a star party would be the perfect time.

Here are a few filters that you may want to consider buying at first which have a comparatively high transmission and would definitely render good views in a telescope of 8 to 10 inches.

#8 Light Yellow (83% transmission)
Moon: Feature Contrast
Mars: Maria
Jupiter: Belts
Uranus: Dusky Detail (in telescopes of 10 inches or greater)
Neptune: Dusky Detail (in telescopes of 10 inches or greater)

#12 Yellow
Moon: Feature  Contrast
Mars: Blue-Green Areas
Jupiter: Red-Orange Features
Saturn: Clouds, Red-Orange Features

#11 Yellow-Green
Mars: Maria
Jupiter: Clouds, Red-Blue Contrast
Saturn: Clouds, Cassini Division, Red-Blue Contrast

#82A Light Blue
Moon: Low Contrast Features
Mars: Low Contrast Features
Jupiter: Low Contrast Features
Saturn: Low Contrast Features

Broadband Filters
These filters also known as LPR or Light Pollution Reduction filters can be used for visual and imaging purposes to reduce the effects of sky glow and artificial lights such as mercury vapor and sodium lamps. The usefulness is mainly the improvement of the overall view with emission nebulae, the slight contrast boost to some reflection nebulae, the larger more diffuse galaxies and as acting as a blue filter for viewing Jupiter and viewing the clouds of Mars. In general, the broad band “light pollution” filters can be described as only somewhat useful in compensating for some light pollution.

Narrowband Filters
These filters are best used to view emission nebulae. Only the bright pair of nebular emission lines of Oxygen III (4959 angstroms and 5007 angstroms wavelength), the Hydrogen Beta emission line (4861 angstroms), and wavelengths between H-beta and the OIII lines to get through. Narrow-band filters darken the background sky glow significantly without affecting the nebula improving contrast greatly. The view of the emission nebulae is greatly improved as compared to that using a broadband filter. This filter is thus useful in mild to moderately polluted skies and even more so under dark skies.

As an extra note:  I have also used this type of filter (Orion Ultra Block) for lunar observation and found it to give impressive views. The detail was greatly improved, particularly in the regions farther from the terminator. Actually, I found that the views of the Moon using this filter to be far better than that with either the moon filter or the polarizing filter.

Line Filters
Line Filters are designed to pass through only one or two spectral lines from emission nebulae, such as the close pair of Oxygen III lines with the OIII filter or the hydrogen-beta line with the H-Beta filter.  In the line filter category, the Oxygen III (OIII) filter allows for a significant boost in contrast of many planetary and some emission nebulae. Although a narrowband filer shows more nebulosity than an OIII filter…an OIII filter will have more contrast and show finer detail.

The “blinking” technique is used to easily find planetary nebula and works best using the OIII filter. Simply hold the filter between your eye and the eyepiece then remove it and repeat. The stars will be dimmer with the filter but many planetary nebula will show as detectibly brighter.

The H-Beta filter line filter is less used but is best known for its effectiveness on the Horsehead Nebula, the California Nebula and the Coccoon Nebula. It will definitely require at least an 8 to 10 inch telescope aperture.

Moon Filters
 Also called Neutral Density Filters, block out typically 80% of the light intensity, reducing the glare thus allowing for greater detail to be seen.

Polarizing Filters
Better purchased as a Variable Polarizing Filter, these type of filters allow you to adjust the brightness reducing the glare. Bright light can damage the eyes. I would highly recommend these filters from my experience and especially with telescopes of 10 inch aperture or greater where the brightness reflected from the lunar surface or planet such as Jupiter or Venus can be overwhelming. Also these filters can serve to help resolve double stars.

Solar Filters 

• Never look directly at the Sun with or without a telescope or binoculars without protective specialized solar filters as permanent eye damage will occur.

• Do not attempt to observe the partial or annular phases of any eclipse with the naked eye. Failure to use appropriate filtration may result in permanent eye damage or blindness.

• Always supervise children with telescopes.

• Educate newcomers at star parties on proper safety when viewing the Sun.

• Avoid pointing a telescope which has no protective filters at the Sun as this will heat the inside of the tube causing damage to the scope.

There are two types of Solar Filters, in metal coated mylar film or specialized glass and added to the front of the telescope for viewing the Sun safely. Due to the possibility that tiny un-noticeable cracks could develop over time in the mylar film sheet which could allow eye damaging solar light to enter the telescope, I would recommend the specialized glass instead. Also the metal coating which is applied to mylar film makes the Sun appear blue or neutral-white. The coating on glass solar filters yields a more realistic yellow image.

Please note that the black glass solar filters that screw into the eyepiece and that may come with department store telescopes are dangerous and should not be used to view the Sun. Discard those filters immediately.

UV-IR Cut Filters
These filters are useful in only the full spectrum, the modified DSLR cameras and UV / IR sensitive webcams. Since ultra violet (UV) light and Infrared (IR) light does not come to the same focus as visual light a degradation of the image occurs. The UV-IR Cut filter improves the sharpness of the image by blocking that part of the  spectrum which is UV and IR while letting only the visual part of the spectrum will go through to the sensor of the camera. Since the human eye cannot see either UV or IR light it is of no use in improving the view.

IR Pass Filters
The atmosphere causes turbulence, light scattering due to dust, refraction of light and color fringing. The purpose of the IR Pass Filter is to allow imaging to take place in a region of the spectrum which is far less susceptible to these adverse atmospheric effects. Since the human eye cannot see in infrared it is of no use in visual astronomy. However, astrophotography with this filter will result in a much sharper image. In fact this filter can be combined with the RGB filters in processing as a luminosity channel to create a significantly sharper image using cameras with monochrome sensors. It can also be used as a single filter with the appropriate IR sensitive camera for lunar and planetary, greatly improving detail. Using these filters to image DSO will entail star bloat and very long exposure times as compared to visible spectrum imaging.

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