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1)what nebula filters are and how they work:
Nebula filters are optical interference filters created by
thin film processes where super thin multiple layers of dielectric materials
are deposited on substrates (usually glass, quartz, or other optically transmissive
materials) according to predetermined computer generated designs. Desired
parameters such as bandwidth, central wavelength, and transmission can be
accurately predicted with various optical thin film design software programs
and designs. These filters work by a principal known as optical wave
interference where designs will specifically block (by reflection) most of the
wavelengths that cause the night sky to glow, while passing beneficial
wavelengths where nebula are known to have strong emission lines. Because most
light pollution wavelengths are rejected, and the emission wavelengths
transmitted, the contrast of the nebula can often be significantly, even
dramatically increased. Because most modern Optical Thin filters feature
transmission well over 90% the nebula will not be dimmed by more than a few
percent, yet the background sky at the eyepiece is dimmed very significantly.
This increases contrast and makes the nebula more visible.
2)the best magnification ranges and darkness in the field:
Since extended objects get dimmer as they are magnified, and
the background gets darker, there is a maximum magnification at which these
filters provide adequate contrast enhancement.
A good rule of thumb is to keep magnifications under 10 times per inch
of aperture when using them, though very small nebulae may need more
magnification to see them as non-stellar.
Since the bandwidth is narrowed, stars get significantly dimmed along
with the background sky, and this will cause an overall darkening of the
field. Since the goal is to view the
nebula and details within it, this is only an aesthetic problem if you want to
view the stars in the field along with the nebula. In that case, a broader bandwidth filter may
provide the desired view, but at a sacrifice of some contrast enhancement. At higher magnifications, a broader bandwidth may prove useful.
3)the different kinds of nebula filters:
Type 1: Broadband or Light Pollution Reduction filters:
These are essentially a filter that allows the transmission
of all wavelengths of light except for a wide swath in the green, yellow and
orange, which is filtered out. The
effect is to eliminate some of the sky glow that happens because of sodium
vapor and mercury vapor lights. The
effect is to turn up the contrast by darkening the night sky without
significantly dimming anything in the field of view.
The disadvantages are that many nighttime light sources are
now full spectrum (i.e. sky glow is there at wavelengths passed by the filter)
and many deep-sky objects are also full spectrum, so are dimmed by the
filters. And, alas, severe light
pollution results in internal reflections between the two sides of the glass,
causing a lot of light scatter.
Ironically, the effect of broadband filters is best when the sky is
already quite dark and not light-polluted.
Nonetheless, broadband filters have their place and do have
enough effect to make them useful in many suburban and exurban
environments. Though they aren't the
optimum filter choice for viewing nebulae, the fact they work a bit on many
targets makes them still useful to the astronomer. Due to the wider bandwidth, they can also be
used at higher magnifications, an important consideration if one doesn’t desire
as much darkening of the field. Many
also work on bluish reflection nebulae, which the narrower filters do not work
There is a sub-class of these filters that uses
"notches" in the spectrum to reduce light pollution while barely
dimming the objects. These
"notch" or "contrast enhancement" filters can be used on
many different kinds of deep-sky objects. Contrast enhancement is, of course, also reduced.
Type 2: Mediumband nebula filters (with bandwidths in between
the broadband and narrowband filters):
These are filters that pass the Hydrogen Beta and Oxygen III
lines emitted by most nebulae (though width bandwidths wider than narrowband
filters) and (often) the red wavelengths of Hydrogen alpha and other deep red
wavelengths. Contrast enhancement isn't
as great as the narrowband filters, but stars aren't as diminished in
brightness either, so the overall view remains brighter and may be more
These filters work best in extremely dark skies or in very
large scopes and on some nebulae with multiple lines of emission at a variety
Type 3: Narrowband Nebula Filters (sometimes called the
These produce prodigious improvements in contrast when
viewing emission nebulae, both bright nebulae and planetary nebulae. How they work is to restrict the transmission
wavelengths to primarily the 486nm Hydrogen-Beta (blue) and 496nm and 501nm
Oxygen-III (blue-green) lines in the spectrum, where the emission of most
nebulae is strong and our eyes' sensitivity to light is greatest. Most nebulae emit even more light at the
Hydrogen-Alpha (deep red) line in the spectrum, but our eyes are particularly
insensitive to light at that wavelength (which is why deep red LED flashlights
don't destroy our night visions). Some narrowband filters also transmit the
deep red of Hydrogen-Alpha, but they have their greatest effect with either a
really big scope or an eye unusually sensitive to red to make use of the deep
red transmission. The overall field,
however, is not quite as precipitously dimmed, so many may find the view more
The disadvantages of a narrowband nebula filter are that, in
restricting the transmission so greatly, the stars are significantly reduced in
brightness and many if not most of them disappear from view. And the overall image is somewhat
darker. The nebula, though, is hardly
dimmed at all, while the rest of the field of view goes to a dark inky
black. That improves contrast immensely,
and makes the nebulae appear a lot larger and brighter relative to the
field. Many have said it is the
equivalent of getting a larger scope.
If you get just one nebula filter, this type you should
get. All the varieties of emission
nebulae are helped. There is a wide range of bandwidths in this category, with wider ones providing less contrast, but also brighter images.
Type 4: Line, or specific spectral line filters (H-Beta, or
These are super narrow band filters that only transmit
specific lines of the spectrum. Visual
observers might want a Hydrogen-Beta filter (486nm, blue) for nebulae with
nearly all their emission energies in hydrogen wavelengths, or an Oxygen-III
filter for those nebulae (like a lot of planetary nebulae) that emit a lot of
their energies at the two O-III wavelengths of 496nm and 501nm. There are other line filters for
astrophotographers, but they are used in wavelengths to which our eyes are
relatively insensitive (such as the aforementioned Hydrogen-Alpha line in the
The disadvantages to these nebula filters are their
specificity to certain lines in the spectrum.
A particular nebula might work well with one and not the other. But that is also their strengths, because the
contrast enhancement is the greatest if the filter matches the emission from
the nebula. There are far more objects in
the sky that benefit from an O-III filter than an H-Beta, so the O-III filter
is the more commonly purchased "second nebula filter". And, of course, in making the bandwidth even
narrower than the UHC type, the overall image is even darker and even fewer
stars show in the field. But, the
enhancement of the nebula that emits in the line(s) of the filter is profound. A few O-III filters have wider bandwidths that also encompass the C2 lines common in the gaseous tails of comets. If comets are a commonly viewed type of object, these may be of use.
The best filters for observing nebulae have bandwidths that
are very narrow and cover the necessary wavelengths and very little else.
Better (read: usually more expensive) nebula filters have narrower bandwidths,
higher transmission percentages at those wavelengths, better mechanical
construction, larger clear apertures, and anti-reflection coatings to aid
transmission at the chosen wavelengths.
Whether they have transmission in red wavelengths is a matter of
personal preference. Some filters have
red transmission; others don't. It may
be worthwhile to try both types before you settle on one.
5)why one might want more than one kind: aesthetics versus
maximum contrast; red versus no red:
If one observes under very dark skies, or has a large
aperture scope, most nebulae are visible with no filter at all, so choosing a
slightly wider filter in a category may make sense.
The view will have more stars and the overall field view will be
brighter. Yes, contrast enhancement
won't be maximized, but sometimes aesthetics wins out over performance. In the big scopes, or with observers who see
red better than most, having a transmission in the deep red can enhance the
color presentation in some of the brighter nebulae. Some viewers don't like the red transmission
because it isn't in focus at exactly the same time as the green-blue image we
instinctively focus on, leaving a slight reddish "fuzz" around the
stars. If the object has a strong
Hydrogen alpha transmission, though, having a red transmission can help see red
color if viewing through a larger scope, and yield a somewhat brighter field
without sacrificing contrast, even in a smaller scope.
For maximum contrast, the narrower bandwidth filter wins
out, so there can be reason enough to have more than one filter to view
6)the scientific way of choosing one for a specific object:
Charts showing the relative transmission strengths at each
of the spectral lines exist for most nebulae.
If the nebula doesn't emit any O-III lines, there is no reason to use a
filter that has a broad enough bandwidth to pass those wavelengths. And vice versa. That means one can be scientific about the
choice of a nebula filter and why line filters like the O-III and Hydrogen-Beta
exist for the visual observer.
So what really helps the visibility of full-spectrum,
non-nebular, deep-sky objects in the severely light-polluted skies of a big
city? Gasoline. You put it in your car and drive the
telescope to where the skies are darker.
That doesn't mean you can do no deep-sky observing in a brighter sky,
but certainly the reward of using your telescope under darker skies is worth
the trouble of a trip every now and then. A broadband or Notch filter might help a little bit, but don't expect miracles.
And, of course, these filters really don't help the bluish
reflection nebulae much, or the dark nebulae.
For the best views of these, dark skies are necessary. Some people note that filters that have a
wide transmission bandwidth in the blue end of the spectrum may be useful on
bluish reflection nebulae, such as parts of M20 (the Trifid Nebula), or M78.
8)Some final notes:
The enhancement these filters provide is the same in dark
skies, so the performance enhancement is worthwhile regardless of sky
conditions. And last, though expensive (they have up to seventy (and more) layers on
glass to accomplish the necessary filtration), they will last a lifetime if
taken care of, and a filter usable on one scope will also be usable in any scope
purchased in the future.
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