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
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
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.