you just got your
new telescope - you carefully opened the box and followed all the instructions.
that sun just will not go down fast enough. finally, darkness falls. the
first thing a new telescope owner will do is point it up at the moon and
look into the eyepiece. wow.
but if you can imagine,
the moon actually can appear better than that when properly filtered. telescopes
have the ability to attach filters for many different purposes - some of
the most common filters are for looking at the moon, planets and sun.
are an invaluable aid in lunar and planetary observing. they reduce glare
and light scattering, increase contrast through selective filtration, increase
definition and resolution, reduce irradiation and lessen eye fatigue.
a moon filter will
thread directly onto the bottom of your eyepiece. nearly all eyepieces are
threaded for filters. think of a moon filter like sunglasses for your telescope.
moon filters will cut down glare and bring out much more surface detail
and give you better contrast.
[top of page]
work by blocking out certain colors in the visible spectrum of light. a
red filter, for example, will block out all but the red wavelength of light.
if you look at an object that is primarily red while using a red filter,
the object will appear very bright. areas which are not red will appear
more clearly because they contrast with the wavelength of light which is
being passed by the filter.
when using filters,
make note of the visible light transmission (vlt) of the filter you would
like to use. the vlt is a number, which describes the overall amount of
light that is allowed to pass through the filter. the lower the vlt number,
the dimmer an image will appear. filters with a vlt less than 40% are not
recommended for use on telescopes with an objective aperture of less than
6 inches due to the decreased image brightness.
filters are sorted
by the kodak wratten numbering system. each filter is listed by its color
and wratten number. the wratten numbers will help to ensure similar results
between different filters. the image should appear the same when viewed
through any #82a light blue filter, for example.
here is a list of
some of the most commonly used astronomical color filters and some suggested
uses for each of them.
light yellow - 83% vlt
a light yellow filter
helps to increase the detail in the maria on mars, enhance detail in the
belts on jupiter, increase resolution of detail in large telescope when
viewing neptune and uranus, and enhance detail on the moon in smaller scopes
yellow green - 78% vlt
to bring out dark surface detail on jupiter and saturn, darkens the maria
on mars, and improves visual detail when viewing neptune and uranus through
yellow - 74% vlt
yellow filters help
greatly in viewing mars by bringing out the polar ice caps, enhancing blue
clouds in the atmosphere, increasing contrast, and brightening desert regions.
yellow also enhances red and orange features on jupiter and saturn and darkens
the blue festoons near jupiter's equator.
orange - 46% vlt
an orange filter
helps increase contrast between light and dark areas, penetrates clouds,
and assists in detecting dust storms on mars. orange also helps to bring
out the great red spot and sharpen contrast on jupiter.
light red - 25% vlt
light red filters
help to make mercury and venus stand out from the blue sky when viewed during
the day. used in large telescopes, light red sharpens boundaries and increases
contrast on mars, sharpens belt contrast on jupiter, and brings out surface
detail on saturn.
red - 14% vlt
red provides maximum
contrast of surface features and enhances surface detail, polar ice caps,
and dust clouds on mars. red also reduces light glare when looking at venus.
in large telescopes, a red filter sharply defines differences between clouds
and surface features on jupiter and adds definition to polar caps and maria
[top of page]
dark blue - 17% vlt
dark blue provides
detail in atmospheric clouds, brings out surface phenomena, and darkens
red areas when viewing mars. dark blue also increases contrast on venus,
saturn, and jupiter in large scopes.
violet - 3% vlt
violet is recommended
only for use on large telescopes. a violet filter enhances lunar detail,
provides contrast in saturn's rings, darkens jupiter's belts reduces glare
on venus, and brings out the polar ice caps on mars.
light green - 53% vlt
light green enhances
frost patches, surface fogs, and polar projections on mars, the ring system
on saturn, belts on jupiter and works as a great general-purpose filter
when viewing the moon.
green - 24% vlt
dark green increases
contrast on lighter parts of jupiter's surface, venutian atmospheric features,
and polar ice caps on mars. dark green will also help bring out the cloud
belts and polar regions of saturn.
blue - 30% vlt
a blue filter provides
detail in atmospheric clouds on mars, increases contrast on the moon, brings
out detail in belts and polar features on saturn, enhances contrast on jupiter's
bright areas and cloud boundaries. a blue filter is also useful in helping
to split the binary star antares when at maximum separation.
light blue - 73% vlt
light blue functions
much the same as #80a blue while maintaining overall image brightness. light
blue will also help to increase structure detail when looking at galaxies.
other kinds of
pollution reduction filters
(lpr) are designed to selectively reduce the
transmission of certain wavelengths of light, specifically those produced
by artificial light. this includes mercury and both high and low pressure
sodium vapor lights. in addition, they block unwanted natural light caused
by neutral oxygen emission in our atmosphere. as a result, lpr filters darken
the background sky, making deep-sky observation and photography of nebulae,
star clusters and galaxies possible from urban areas. lpr filters and not
sued for lunar, planetary or terrestrial photography.
[top of page]
credits - "telescope 101"