Light pollution suppression filters are often the
topic for heated discussions among amateur astronomers because
of some common misconceptions:
Some limitations of light pollution suppression filters
should also be kept in mind:
- Filters make nebulae brighter.
This is incorrect. Filters subtract light,
making objects dimmer. In the case of filters
designed for light pollution suppression,
wavelengths from certain types of artificial
lights are blocked, but other wavelengths
(particular the nebula emission lines) are allowed
through, thus improving contrast. This
allows longer astrophoto exposures to be made
before the sky background begins to wash out the
image desired. Visually, the contrast increase may
be perceived to be an increase in brightness.
- Filters make nebulae easier to see.
This is true only if properly used. If used in an
urban environment, there are often sources of
light (including the sky itself) which prevent
full dark adaptation of the eye (about 30 minutes
in full darkness is normally required).
- Filters work on all objects.
This is not true of all light pollution
suppression filters to the same degree. In
general, broadband sources (galaxies, clusters,
stars, or reflections of stars) will not
experience as great a contrast gain as emission
line objects. Filters designed for visual use or
to select specific narrow emission lines (e.g.
OIII) may suppress broad parts of the spectrum,
decreasing the light from broadband sources even
more than filters designed for balanced color
- Filters will block all light pollution.
This is not true. Many light sources such as car
headlights are still broadband sources and will
not be effectively blocked.
- Filters increase exposure times.
While light pollution suppression filters will
slightly increase the exposure times of emission
line objects in their bandpass and affect
broadband sources somewhat more, they do not
require increasing the length of an
exposure. However, because they increase the
contrast of emission line objects compared to the
sky background, it is usually desirable to
increase exposure times to make better use of the
imaging media's dynamic range.
- Filters cause internal reflections.
Interference filters such as the IDAS LPS filters
work by passing virtually all of the selected
wavelengths through the filter layer and
reflecting all other wavelengths, so they
inherently do reflect the rejected light (unlike
However, this is not a problem in
a high quality telescope with fully multicoated
optics (all surfaces, including internal surfaces
multicoated) as the reflected components should be
passed back out the scope. Low cost optics
which typically omit anti-reflection coatings on
internal surfaces may reflect enough light from
bright objects back into the filter to cause
reflections to be visible at wavelengths near the
band edges. Reflections from curved optical
surfaces in the telescope may also alter filter
surface incidence angles enough to shift reflected
components into the filter's bandpass. The
solutions are to use high quality optics both ahead
and behind the filter or, if possible, place the
filter as far as allowed from other optical
components such as a flattener or compressor.
The IDAS filters themselves are high quality
optical elements and are anti-reflection
multicoated on the front surface to prevent
internal reflections from being a problem.
- For photographic use, some color shift will occur
because the filters do not block light in
infinitely narrow bands. This affects the ratio
of light in the three color bands being sampled,
causing a shift in the perceived colors. The
degree to which this occurs may depend not only on
the filter being used, but also the type of color
film used. Visually, color shifts are usually
inconsequential since most extended objects will
be below the eye's threshold for color detection.
- Narrowband filters are tuned to specific
wavelengths by carefully controlling the thickness
of layers of dielectric material on glass.
Because the effective thickness changes when the
filter is tilted, the wavelength tuning will also
change. In practice, this means that wide-angle
views will show a perceptible color shift between
the center and edges of a photo.
Who needs a light pollution suppression
This section of a panorama of the Milky Way
is a composite of
photographs taken over one evening in
September 1999 from the site of the
Orange County Astronomers, located near
Mt. Palomar. The bright glow at the bottom
is from San Diego city lights. No filter
was used for this photograph.