One of the puzzles of visual astronomy that many beginning amateur
astronomers get caught up in is the use of filters. Which ones to
purchase, and which ones are useful for what types of observing are the
most frequently asked questions. Here, then, is a summary of most of the
filters in current production and their uses, with a few hints thrown in
from my own personal observations.
First of all, let's start with the question of why one uses a filter at
all. Simply put, a filter can greatly enhance the human eye's perception
of small details on solar system and deep sky objects. There are those
amateur observers who maintain that they never use filters and that one
doesn't need them. To this argument, I have only this to say; no pair of
human eyes is perfect. If a filter helps you get better use out of yours,
then use them. Judicious use of photo-visual filters can greatly enhance
an observing session.
Filters work by blocking a specific part of the color spectrum, thus
significantly enhancing the remaining wavelengths. Colored filters work by
absorption/transmission, and instantly tell you which part of the spectrum
they are reflecting, and therefore transmitting. The so-called
light-pollution reduction and nebulae filters are very selective in the
wavelengths they transmit. For these it is best to refer to the
manufacturer's specifications on a given filter.
Colored filters are referred to by their Wratten numbers. The Wratten
system was developed by Kodak in 1909 and has been the standard ever
since. Filters used for photography, astronomy, and other applications all
use this same standard. So here, then, is a summary of the filters most
commonly used in astronomy.
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#8 Light
Yellow (83% transmission) This is used for
enhancing the detail in red and orange features in the belts of Jupiter.
It is also useful in increasing the contrast in the maria on Mars, and
increasing the resolution of detail on Uranus and Neptune in telescopes
with 10" or more of aperture. This is a great filter for enhancing lunar
detail, too, particularly on telescopes with 8" of aperture or less.
#11
Yellow-Green (78% transmission) This filter is great for
bringing out surface details on Jupiter, and to some degree, Saturn. It
darkens the maria on Mars and does slightly improve visual detail on
Uranus and Neptune, again in telescopes with 10" of aperture or more. This
filter is what I used primarily for observations of Jupiter after it was
pummeled by Comet Shoemaker-Levy in 1994. It brought out the impact areas
in excruciating detail.
#12
Yellow (74% transmission) Enhances red and orange features
on Jupiter and Saturn, while blocking blue and green wavelengths. It also
lightens the red and orange features on Mars, while reducing, or blocking,
the transmission of blue and green areas; this increases the contrast
between the two. It also enhances the blue clouds in the Martian
atmosphere. This is one of my favorite Mars filters for that reason. Very
nice for increasing contrast in lunar features also, in telescopes of 6"
of aperture and above.
#15 Deep
Yellow (67% transmission) This filter is used to bring out
Martian surface features, and the polar ice caps. It can also be used to
enhance the orange and red features, bands and festoons, on Jupiter and
Saturn, and for low-contrast cloud detail on Venus. Try it also on lunar
surfaces; it works nicely to improve the contrast. Grins and Giggles: Try this one for daylight
observation of Venus and Mercury.
#21
Orange (46% transmission) The #21 orange reduces
transmission of blue and green wavelengths, thus increasing the contrast
between these areas and red or yellow or orange areas. It is great on Mars
because of this. It sharpens the boundaries between these areas on the
planet's surface. I also use it on Jupiter to sharpen the contrast in the
belts and to bring out the Great Red Spot. It will also slightly increase
surface details on Saturn. This one behaves very similarly to the #15 but
gives slightly more contrast.
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#23A Light
Red (25% transmission) This is another great filter for
use on Mars, Jupiter, and Saturn, but because of lowered light
transmission, probably shouldn't be used on a scope of 6" of aperture or
smaller. It performs many of the same functions as the #21 and the #15,
but again, with more contrast than do either of these. It is also a great
one to try for daylight observations of Mercury and Venus because it
increases the contrast between these planets and the bright blue sky.
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#25A Red
(14% transmission) The #25A filter strongly blocks the
transmission of blue and green wavelengths, which result in very sharply
defined contrast between the cloud formations and the lighter-toned
surface features on Jupiter. This filter is also quite useful for
definition of the Martian polar ice caps and maria. However, because of
the reduced light transmission, the #25A should probably only be used on
telescopes with 8" of aperture, or more. Grins and
Giggles: Try this one on Venus. Not only does it reduce the light
glare, it really does some interesting things to the clouded Venusian
atmosphere.
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#38A Dark
Blue (17% transmission) The #38A is very good for use on
Jupiter because it strongly rejects red and orange wavelengths in the
belts and in the Great Red Spot, thus increasing the contrast. It works
well on Martian surface phenomena, like dust storms, and increases the
contrast in the rings of Saturn. This is a good one to use on Venus, too,
because of its low light transmission; it really increases the contrast of
subtle cloud markings. The #38A should only be used on telescopes of 8" of
aperture or more, because of the reduced light transmission.
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#47
Violet (3% transmission) This filter strongly rejects red,
yellow, and green wavelengths, making it a good one to use on the Martian
polar ice caps. It is THE filter of choice for observations of Venus
because of its low light transmission and its ability to enhance upper
atmosphere phenomena. It is also touted as being useful for providing
contrast in the ring system of Saturn, but I have not found it to be
particularly useful for this purpose. Great for enhancing lunar detail,
also. Grins and Giggles: Try this one on
Jupiter and the Galilean moons. The planet is electric purple, the moons
hot pink. A psychedelic trip that's LEGAL!!
#56 Light
Green (53% transmission) This filter is excellent for the
observation of Martian polar ice caps and for the yellow tinted dust
storms on the planet's surface. It also increases the contrast of the red
and blue regions in Jupiter's atmosphere and cloud belts. Another one that
is great for lunar observing also.
#58 Green
(24% transmission) This filter strongly rejects red and blue
wavelengths and increases their contrast on the lighter parts of the
surface of Jupiter. It is also useful for enhancing the cloud belts and
polar regions on Saturn. It does a fantastic job on increasing the
contrast in Mars polar ice caps and also does a reasonable job of
increasing the contrast of atmospheric features on Venus. Again, because
of lower levels of light transmission, this filter probably shouldn't be
used on telescopes of less than 8" of aperture.
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#80A Blue
(30% transmission) Many people say that if you can only buy one
filter, this should be it. This is the one of two that come out of my
filter case the most often. This filter is the best and most popular for
the study of detail on Jupiter and Saturn. It enhances the contrast of
rills and festoons in Jupiter's cloud belts, as well as details of the
Great Red Spot. It also brings out detail in Saturn's belts and polar
features. This filter is also very useful for lunar observing. Grins and Giggles: Try this filter to split
Antares. It works very well for this purpose, especially when the
two stars are at their maximum separation.
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#82A Light
Blue (73% transmission) This is the second filter that,
along with the #80A, comes out of my filter case the most often. It works
well on Jupiter, Mars, Saturn and the moon. Its pale blue color enhances
areas of low contrast and avoids significant reduction of overall light
level at the same time. I find this filter extremely useful. Grins and Giggles: Try this one on bright galaxies,
particularly face-on spirals. I accidentally left it in an eyepiece when I
swung over to take a look at M51. The detail in the spiral arms was quite
pronounced over what I was used to seeing. It took me some time to figure
out why. I've tried it on a number of bright galaxies since then and it
really does a great job in increasing the detail in galactic
structure. Also, try this one to split Antares. Either the 80A
or the 82A works quite well for this.
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ND96 Neutral
Density (0.9 density, 13% transmission) The neutral
density filter transmits light uniformly across the entire visible
spectrum. Because of this, it is an excellent filter to use for glare
reduction, particularly while observing the moon with any telescope 4" of
aperture and larger. Some people also use it to split difficult double
stars, particularly those in which one member of the pair is significantly
brighter than the other. I have tried this myself, but so far with limited
success.
SPECIALTY
FILTERS
BROADBAND, OR LIGHT POLLUTION
REDUCTION FILTERS
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Orion
SkyGlow This is one of a relatively new class of filters known as
light pollution reduction, or LPR, filters. It is designed to darken the
background sky by blocking mercury vapor light transmission and enhancing
transmission in the hydrogen beta, doubly ionized oxygen (OIII) and
hydrogen alpha regions of the spectrum. What this means to the layman is
that the filter increases the contrast of deep-sky objects, emission
nebulae in particular, with the background light-polluted sky. This filter
doesn't work particularly well on other types of objects, but does a fine
job with emission and planetary nebulae, because they emit light in the
hydrogen alpha, hydrogen beta, and doubly ionized oxygen wavelengths.
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Meade
Broadband This is another type of LPR filter; the spectrum of
wavelengths passed by the Meade Broadband is nearly identical to that of
the SkyGlow. Since I very seldom observe from light polluted skies, I am
probably not getting the maximum benefit from my Broadband, but even under
dark skies it is quite effective in improving the contrast of emission and
planetary nebula. One thing I have found out in observing planetary
nebulae, though ? use of these filters on M1 under even a fairly
light-polluted backyard sky can help bring out such diffuse nebula as
M1. Under a VERY dark site. M1 will reveal detail that is not quite
as clear without the filter.
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Celestron
LPR Celestron recently came out with this filter. It also enhances
the transmission of light in the hydrogen alpha, hydrogen beta, and doubly
ionized oxygen wavelengths. I have never used it, so have little to say on
its performance, but I would expect it to react very similarly to both the
SkyGlow and the Broadband.
NARROWBAND
FILTERS
Lumicon Ultra
High Contrast (UHC) This is probably the most focused of narrowband
filters. This filter performs equally well under skies with some light
pollution, or a dark sky site. It provides great contrast on emission and
planetary nebulae. Of the three in this category, this is my favorite. It
does a great job in bringing out detail on diffuse nebulae such as M1.
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Orion
UltraBlock This filter optimizes transmission of light in the
hydrogen beta and doubly ionized oxygen wavelengths, as well as strongly
blocking (99.9%) mercury and sodium emission bands. Put simply, it blocks
all types of light pollution, from incandescent to fluorescent lighting,
which broadband filters do not stop. It provides great contrast on
planetary and emission nebulae, even from heavily light polluted
areas. The Ultrablock also has very similar bandpass and performance
as the Lumicon UHC, and the Thousand Oaks Type II. It does, however,
have a more rounded and very slightly narrower bandpass than the
UHC. It also doesn't have the deep-red bandpass for hydrogen-alpha
that the UHC has. Sirius also makes a narrowband filter that is
"tunable" in bandpass.
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Meade
Narrowband This filter also optimizes transmission of light in the
hydrogen beta and double ionized oxygen wavelengths, but unlike the
UltraBlock, it also transmits hydrogen alpha emissions as well. The
difference between the BroadBand and the NarrowBand is that the NarrowBand
is more focused. This is THE best filter for use on M27, the Dumbbell.
Nothing else comes close. This filter's characteristics, in terms of
bandpass width, are almost identical to the Lumicon UHC.
OTHER SPECIALTY FILTERS
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Lumicon
OxyIII This filter is quite specialized in that it allows light
transmission only in the doubly ionized oxygen wavelengths. This is the
filter of choice for the Veil and does amazing things with other emission
nebulae whose predominant emissions are in the doubly ionized oxygen
range. Grins and Giggles: Having difficulty
splitting Antares? Try the OIII on it. It turns the primary star into what
looks like a red LED, and the companion into a green one. Pretty cool! You
can try it on other doubles too, where the primary star is much brighter
than the companion.
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Lumicon H
Beta This filter is so specialized, it is really only good for two
objects; IC 434, the emission nebula surrounding the Horsehead Nebula in
Orion, and the California Nebula. Both of these objects emit strongly in
the hydrogen beta portion of the spectrum. While it is possible, just
barely, to catch a glimpse of the Horsehead without the H Beta, the view
with it is far more stunning. The filter emphasizes the glow of both of
these nebulae, turning them red. In the case of IC 434, the red outline
around the actual Horsehead itself (B33) is what makes it visible. The
California Nebula is similar in that it is just barely visible without the
H Beta. With it, the California "shape" becomes clearly visible.
This filter can also be used for viewing some nebulae; unfortunately most
of them are quite faint.
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Solar
Filters These fall into two basic types: optical mylar and coated
glass, often referred to as a Type II. Both Thousand Oaks and Orion make
these filters, which are an absolute must for solar observing. The solar
image through mylar is a pale blue; through the glass filters it is
orange. Sunspots and faculae are readily visible. They can either be
purchased for the full aperture of the scope, or smaller and used
off-axis. The glass filters are more expensive, but more durable. If
scratches or pinholes appear on these filters, you can simply cover them
over with a black waterproof felt-tip pen. This will in no way diminish
light transmission.
Hydrogen
Alpha These filters are also used strictly for solar observing.
They transmit strongly in the hydrogen alpha portion of the spectrum, thus
revealing solar prominences and flares. They come in three basic types; as
heated interference filters ? the DayStar type, or as "T=Scanners", which
are small filters that can be tilted in front of the eyepiece to tune the
wavelength and are low transmission sub-angstrom bandwidth solar filters.
The third type is a low transmission 1.5 angstrom bandwidth solar
prominence filter. DayStar filters (multi-element sandwich
construction monochromators) are expensive, from $1500-$3500 for the
amateur on up to $8000 for university grade. They employ an 80 layer
interference filter in an "oven" which maintains a constant temperature.
They are graded by how sensitive they are in their resolution, in
angstroms; .9, .8, .7, .6, and .5, with the .5 being the most
sensitive. Unheated versions can be purchased for as little as
$800. The Coronado line falls into the category of a T-scanner.
The T-scanners are relatively
inexpensive. They look like a nebula filter in a large holder that goes in
front of the eyepiece and can manually tilted to tune the frequency of the
light. Both types, the DayStar an the T-scanner, require a red "energy
rejection filter" over the objective and an f/30 optical system ? usually
achieved by using an off-axis sub-aperture mask and filter. The f/30 focal
ratio also gives very high magnification.
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Polarizing
Filters These filters adjust the brightness of images to a more
optimal level for observing. They consist of two polarizing layers mounted
in a rotating cell. They vary the light transmission from 3% to 40% and
are most frequently used for lunar observing. The light transmission can
be varied depending on the phase of the moon.
There are probably other types of filters out there that are in use by
some astronomers, but this list covers those that are most commonly used.
You can obtain a set of 100 colored filters from companies like Edmund
Scientific; these filters are acrylic, and very inexpensive. They come in
two sizes and can be held in front of the eyepiece to see the effects. The
smaller size, 1.5" by 3.25" is extremely convenient for eyepiece cases.
The larger size, 3" x 5", can be cut into four pieces and mounted in
cardboard slide mounts, thus making four sets of very inexpensive filters.
The slides can then be held over the eyepiece for observing. Each of these
sets comes with the transmission wavelength graph for each filter.
Filters can also be stacked on top of each other to obtain the
characteristics of each at the same time. Keep in mind that the more
filters you stack, the more you reduce light transmission. Most people,
when stacking filters, will stick to the lighter colors. Also, keep in
mind that if you stack a red, green, and blue filter together, you have
essentially created a neutral density filter which will block all
wavelengths. For some objects this would be a total disaster.
So, which ones should you buy? You may wish to order an inexpensive
acrylic set and try them out before you purchase the more expensive glass
filters, at least for the colored filter categories. As for other LPR and
nebulae filters, it is best to try them out with your own telescope, your
viewing conditions, and your own eyes, if possible, before making a
decision. It is also a good idea to educate yourself on the use of filters
in general. Filters improperly used can make for less than optimal
observing; but they can greatly enhance an observing session if the
observer knows in advance what they can, and cannot, do.
As of April, 2003, Parks Optical purchased what was left of
Lumicon. They have promised to produce the same filters with the
same quality as Lumicon did when it was in business. Parks has also
announced that they will produce a line of colored planetary filters that
will be of far better quality than those that are already on the
market.
With much thanks to Robert Haler, David Knisely, and Chuck Hards
for pointing out errors contained in my assessments.
Copyright 1998 Pulcherrima Productions. No part of this
article may be reproduced without the consent of Pulcherrima Productions
or the author.
Hope you enjoyed this one! |