Filters Used In Astronomy
By: Jeff Thrush
www.thrushobservatory.org
ND = 0.3 means 50% transmission
Minor Planets
Electromagnetic Spectrum
Visible Light
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The wavelengths our eyes can detect is only a small portion of the
electromagnetic spectrum. We call this the visible light spectrum.
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If the visible portion of the light spectrum is divided into thirds, the
predominant colors are red, green and blue. These three colors are
considered the primary colors of the visible light spectrum.
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All the other colors we can see in nature are found somewhere along
the spectrum between blue and red.
Primary / Secondary Colors
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Primary colors can be arranged in a circle, commonly referred to as a color
wheel. Red, green and blue (RGB) form a triangle on the color wheel. In
between the primary colors are the secondary colors, cyan, magenta and
yellow (CMY), which form another triangle.
Color Reproduction
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There are only two basic ways used to reproduce color:
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Additive Color System (RGB)
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Subractive Color System (CMY)
Additive Color System
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The additive color system involves light emitted directly from a source,
before an object reflects the light.
The additive reproduction process mixes various amounts of red, green
and blue light to produce other colors.
Combining one of these additive primary colors with another produces
the secondary primary colors cyan, magenta, yellow.
Combining all three primary colors produces white.
All image capture devices utilize the additive color system to gather the
information needed to reproduce a color image. These devices include
digital cameras, flatbed scanners, drum scanners, and video cameras.
Subtractive Color System
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The subtractive system creates color by subtracting or absorbing certain
wavelengths of color while reflecting other wavelengths back to the viewer.
When combined in equal amounts, pure subtractive primary colors produce
the appearance of black.
Two subtractive primary colors are required to produce one additive primary
color.
A Better Understanding
Characteristics of a red filter
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White light is composed of
three primary colors: red,
green and blue.
A filter of a primary color will
transmit its own color and
absorb the other two
Characteristics of a yellow
filter.
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Secondary colors are
mixtures of primary colors.
Yellow, for example, is a
combination of red and
green.
Because a filter passes its
own color and absorbs
others, a yellow filter passes
red and green and absorbs
blue
Glass Filters
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These filters are nothing more than
colored glass or plastic.
Have lower light transmission levels
requiring longer exposure times.
Cost less and are easier to
manufacture than dichroic filters.
They can be purchased in a wide
range of sizes and shapes.
Filters
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Yellow Filter: A Wratten #8, 12, or 15 can improve the markings in the clouds
of Venus and enhance dust storms on Mars.
Orange Filter: A Wratten #21 is one of the most useful filters. It brings out
details on Mars and enhances some zonal details on Jupiter, also darkens the
blue sky so daytime observations of Jupiter, Venus and the Moon are much
improved.
Red Filter: A Wratten #23A, 25, or 25A are used to enhance contrast on Mars,
Jupiter, and Saturn. A red filter, however, is fairly dark, so it works best on larger
aperture telescopes that give brighter images. Switching between red and blue
filters can sometimes bring out subtle coloration on the surface of the Moon.
Green Filter: A Wratten #58 allows you to see more clearly the edges of the
Martian polar caps and enhances the belts and the Great Red Spot in the clouds
of Jupiter.
Filters
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Blue Filter: A Wratten #44A, 47B or 80A is used to detect high
altitude clouds on Mars, white ovals and spots in the belts of Jupiter,
and the zones of the clouds of Saturn, and to reduce the glare of the
bright Moon. The 80A is the filter to have if you only buy one filter.
Polarizing Filter: Cuts down the glare when observing a nearly full
Moon, making it easier to see ray structure. It will also cut down
daytime glare. A “variable” Moon filter will allow you to dial in a specific
amount of neutral density light reduction. For terrestrial photography
with your telescope, this filter will reduce sky glare early in the morning
and the late afternoon.
Filters
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Solar Hydrogen Alpha Filter: These expensive filters are essential if
you want to observe and photograph solar prominences. This special
filter transmits a very narrow slice of the spectrum, typically 1/5000 to
1/3000 of the visible spectrum's full width. The transmission window of
these ultra-narrowband filters is generally centered on the bright
hydrogen-alpha (H-alpha) line at a wavelength of 656.3 nanometers
(6563 angstroms) in the red.
Wide Band H-Alpha Filters
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Sharp-cut-off deep-sky photography
filter with zero transmission below
630nm, 90% at Hydrogen-Alpha
656.3nm, and 90-98% from 657nm to
the near infrared.
Filter Factor - Exposures will be
increased 2-10 times, depending on
the faintness of the nebula.
Hypersensitized Kodak Technical
Pan 2415 is the only recommended
film for use with this H-Alpha Filter;
do not use Tri-X or any other B&W
film.
Can be used with CCD Cameras
Not suitable for visual use.
Dichroic Filters
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Dichroic filters are amazing in their
simplicity and efficiency.
These filters are made of thin-film
coatings applied to a glass
substrate.
The coatings can range from as few
as three or five layers to as many as
30 or 40 layers.
Each film layer is approximately one
one-thousandth of a millimeter
thick.
The process is carried out in a
vacuum chamber and is similar to
the process used to make
semiconductor chips.
Dichroic or interference filters pass
certain precise wavelengths and
reflect all others.
LPR Filter
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LPR filters are designed to
darken the background sky by
blocking light pollution.
Enhances transmission in the
hydrogen beta, doubly ionized
oxygen (OIII) and hydrogen
alpha regions of the spectrum.
The filter increases the contrast
of deep-sky objects, emission
nebulae in particular.
Ultra-High Contrast Filter
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The UHC filter is a narrowband
LPR filters.
The filter is used to enhance
the contrast of diffuse nebula.
This filter performs equally well
under light polluted or dark
skies.
Allows the transmission of
O-III, Hydrogen Beta and
Hydrogen Alpha light.
Great for the Swan Nebula.
O-III Filter
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An O-III filter is used to
enhance the contrast of diffuse
nebula by allowing only the
doubly ionized oxygen band
through.
Great for the Ring Nebula, Veil
Nebula, and the North
American Nebula.
H-Beta Filter
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This filter is so specialized, it is
really only good for two
objects; IC 434, the emission
nebula surrounding the
Horsehead Nebula, and the
California Nebula.
This filter can also be used for
viewing some nebulae;
unfortunately most of them are
quite faint.
Astronomik Ha Filter
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Transmissions as high as 97%
13 nm bandwidth
Total blocking of all unwanted
light from light-pollution
Ideal for planetary nebulas and
supernova remnants
Completely resistant against
high humidity and scratches
Can be used on every clear
night regardless of the Moon
phase.
10nm vs 3nm H-Alpha Filter
CMY Filters
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The CMY method to restore the
true color image is more
complex than using traditional
RGB imagery.
These equations are applied to
each pixel in an image taken
using CMY filters.
R=Y+M-C
G=Y+C-M
B=C+M-Y
LRGB & Photometric Filters
Astronomik RGB Filters
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Manufactured in Germany and
distributed by Adirondak Video
Astronomy in the U.S.
Highest transmission areas of any
RBG filters.
Very broad red filter.
Offers both NIR blocking and nonblocking luminance filters.
NIR blocking filter reasonably
matches RGB but lets additional UV
in.
OIII, H-b and H-a emission filters
are parfocal with RGB. Dichroic.
Blue:Green crossover - 495 nm
At OIII - B=42%; G=58%
Custom Scientific RGB Filters
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Manufactured by Custom
Scientific
Generally provided with SBIG
filter wheels.
Luminance filter is not UV or
near-IR blocking.
Lower blue transmission than
Astronomik or Hutech.
Narrow red filter with high
transmission. "Gap" placed
between green and red filters
centered at 590 nm - the sodium
line - as a light pollution
measure. Dichroic.
Blue:Green crossover - 493 nm
At OIII - B=44%; G=93%
Meade Pictar RGB Filters
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These filters have exceptionally high
transmissions, and a broad blue filter
to compensate for lower blue CCD
efficiency.
The O[III] transmission is high in
BOTH the blue and green filters
making this filter set optimal for
acquiring nebulae signals.
The clear filter is not NIR-blocking,
and does not match the low
wavelength edge of the blue filter.
These are included in the Meade
Color Filter Wheel and cannot be
purchased separately at this time.
Blue:Green crossover = 499 nm
At OIII - B = 85%; G = 89%
Optec RGB Filters
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Unusual RBG filter set.
Red filter is similar to the Schuler.
Optec has also placed a "gap" for
sodium light pollution between the
green and red filters.
The blue filter is very wide and
extends below 350 nm.
Good match between the luminance
filter and the RGBs.
RGB filters do have some UV
transmission below 400 nm that will
be detected.
Red filter transmits considerable
NIR.
Blue:Green crossover - 503 nm
At OIII - B=90%; G=39%
True Technology RGB Filters
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A 5-piece filter set requiring the use
of a separate NIR-blocking filter
(shown in black) to block the red
filter.
High filter transmission.
There is a "gap" between the red and
green filters, but it is not centered on
the sodium light pollution.
Green filter has violet transmission.
Clear filter has high transmission.
Blue:Green crossover - 502 nm
At OIII - B=83%; G=68%
SBIG STV & ST-237
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These small filters are included in
SBIG's CFW5C filter wheel used
with their STV and ST-237 CCD
cameras.
They are similar to the 1.25" RGB
filters used in filter wheels for their
ST-7/8/9/10 CCD cameras.
The clear filter is not NIR-blocking,
whereas the RGBs are.
Narrow red filter with high
transmission. "Gap" placed between
green and red filters centered at 590
nm - the sodium line - as a light
pollution measure. Dichroic.
Blue:Green crossover - 500 nm
At OIII - B=72%; G=72%
Schuler RGB Filters
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Colored glass filters. Twice as thick
as the dichroics.
Lower transmission than dichroics
but broader filters making them as
efficient as many of the dichroics for
galaxies and globulars.
Will need about 20% more time than
dichroics for H-alpha and OIII
signals from nebulae.
Good NIR match for luminance filter,
but it lets in considerable UV.
Red filter transmits considerable
NIR.
Designed for superior color balance.
Blue:Green crossover - 496 nm At
OIII - B=34%; G=56%
Schuler UBVRI Filters
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These filter are used to take
photometric measurements using
a CCD camera.
To learn to do photometric work,
you only need the V Filter.
Once your proficiency increases,
you can use the whole range of
UV, Bu, V, R, I filters.
The Schuler filters are in use by
many AAVSO members.
You do not need to spend 500.00
per filter to achieve high quality
repeatable scientific results
RGB Filter Calibration
Results of star HIP 116740
Filter
ADU
Coefficient
R
367042
1.000
G
399871
0.917
B
262750
1.396
Creating a TriColor Image
Questions