Light Pollution Filters

A didymium glass "color enhancer" ("Tiffen color enhancer") will work like a
weak broadband nebula filter and will give you some idea of what to expect.

A didymium glass "color enhancer" ("Tiffen color enhancer") will work like a
weak broadband nebula filter and will give you some idea of what to expect.

A didymium glass "color enhancer" ("Tiffen color enhancer") will work like a
weak broadband nebula filter and will give you some idea of what to expect.

Matt posted:

Will these light pollution filters make a huge difference?

Filters can be useful for improving the view of some objects both under light
pollution and (especially) under dark skies. However, they will not eliminate
the effects of skyglow caused by strong light pollution. Here is some
information about these filters:

USEFUL FILTERS FOR VIEWING DEEP-SKY OBJECTS
by David Knisely
Prairie Astronomy Club

There are a number of different filters available on the market today for
improving the views of various Deep-Sky objects, with most coming in one of
three classes: 1. Broad-Band "light pollution" filters, 2. Narrow-Band
"Nebula" filters, and 3: Line filters.
BROAD-BAND "LIGHT-POLLUTION" FILTERS
The broad-band "Light-pollution Reduction (LPR) filters are designed to
improve the visibility of a variety of Deep-Sky objects by blocking out the
common Mercury vapor, Sodium, and some other emission lines from man-made or
natural sources which contribute to light pollution, while letting through a
broad range of other more useful wavelengths. Since the eye is mainly a
"contrast detector", this selective screening out of some of the background
skyglow increases the contrast and helps Deep-sky objects stand out more
noticably. While these broad-band filters do not eliminate the effects of
light pollution or make the objects brighter, in many cases, these filters
can improve the visibility of some deep-sky objects to at least some degree.
The greatest improvement in the overall view is often found with emission
nebulae, but broadband filters can also give a slight contrast boost to some
reflection nebulae and a few of the larger more diffuse galaxies. In
addition, larger versions of these broadband filters which fit over camera
lenses can be somewhat useful for photography of wide star fields when some
skyglow is present.
Some available broad-band filters are the Lumicon Deep-Sky, the Meade
Series 4000 Broadband, the Celestron LPR, the Thousand Oaks Type 1, and the
Orion SkyGlow. The broadband filters can offer a noticable boost in contrast
and visibility of the fainter outer detail in emisson nebulae over non-
filtered views for objects like the Orion Nebula (M42), the Lagoon Nebula
(M8), the Merope Nebula, the Trifid (M20), and a number of others. However,
the improvement is not as noticable on star clusters or galaxies. I have
found that using the filter on larger and more diffuse galaxies like M33,
M81, M101, NGC 253 and NGC 2403 in my 10 inch when weak skyglow is present
will help boost the visibility of the detail, but the effect is fairly mild.
On star clusters, there is even less of an effect, since some of their
emission falls in the portions of the spectrum blocked by these filters. In
that case it may be better to use slightly higher power on some of the
smaller objects to dilute the light pollution effect a bit. Since some light
is blocked by the filters, there can be times when a few objects may even
look fainter from a dark sky site when using a broad-band filter than without
one. Severe levels of light pollution may also be too much for the broad-
band filters to handle effectively, so you still want to find as dark an
observing site as you can and use averted vision. The broadband filter has
an additional bonus, as it does work fairly well as a blue filter for
observing Jupiter and for bringing out the white clouds and polar caps of
Mars. In summary, the broad band "light pollution" filter can be useful in
compensating for some light pollution, but may not be the most impressive
filter intended for deep-sky use.
NARROW-BAND "NEBULA" FILTERS
Narrow-band "Nebula" filters, as the name implies, are mainly designed
for viewing many emission nebulae. These filters allow only the bright pair
of emission lines of Oxygen III, the Hydrogen Beta emission line, and
wavelengths between H-beta and the OIII lines to get through. Narrow-band
filters darken the background skyglow significantly without hurting the
nebula, and are often of considerable help when observing in mild to moderate
light pollution. The filter's improvement of the view of emission nebulae
is usually superior to that of the broadband filters, as many faint nebular
objects become much easier to see (without the filter, some may not be
visible at all!). Even the more prominent nebulae which are visible without
filters gain considerable detail and contrast with the narrow band units.
However, you still need to use proper dark adaptation, averted vision and low
to moderate powers (3.7x to 14x per inch of aperture) to get the most out of
these filters.
Some available narrow-band filters are the Lumicon UHC, Astronomik UHC,
Meade Series 4000 Narrowband, Thousand Oaks Type 2, and Orion Ultrablock.
The UHC and Meade Narrowband also have a deep-red passband for the Hydrogen
Alpha line. Both the UHC and Ultrablock will, for example, often show the
Rosette Nebula TO THE UNAIDED EYE when you look through them from a dark sky
site. Even under a really dark sky, the contrast and detail improvements are
impressive, and most observers continue to use their narrow-band filters at
such dark-sky sites. One neat trick for finding tiny planetary nebulae is to
"blink" the objects by holding a narrow-band filter between the eyepiece and
the eye. The stars in the field will dim somewhat, but the planetary nebula
will remain undimmed, thus standing out from the background stars.
In comparison, the UHC and Ultrablock have very similar characteristics,
although the UHC has a slightly higher light transmission factor in its
primary passband than the Ultrablock, which may be helpful for viewing faint
nebulae. The Meade Narrowband filter has a passband quite similar to that
of the UHC. Spectroscopic comparison of the two filters reveals that the
Ultrablock's passband is more rounded and slightly narrower than the more
flat-topped UHC, with falloffs in light transmission towards the passband
edges, especially towards the H-beta side. The UHC also shows a red "leak"
passband including the H-alpha line (the Ultrablock doesn't have one),
which may contribute to the image brightness with larger apertures. The
Ultrablock's more rounded and slightly narrower passband may be reasons why
some observers have reported a bit darker field and slightly higher contrast
under light pollution with some objects using the Ultrablock. At times the
Ultrablock has also been slightly less expensive than the UHC, but when not
sale priced, the two filters are of similar cost. Both will perform very
well, and the overall difference between them is very slight. However, these
"nebula" filters usually slightly reduce the brightness of most star
clusters, reflection nebulae, and galaxies, although in moderate light
pollution, a narrowband filter may still be of some use on these objects with
larger apertures. Photographic use of these narrow band filters is also not
recommended.
LINE FILTERS
Line Filters are very narrow passband specialty units which are designed
to let in only one or two spectral lines from emission nebulae, such as the
close pair of Oxygen III lines or the Hydrogen-Beta line. In the line filter
category, the Oxygen III (OIII) filter is the real standout. Its very narrow
bandwidth allows only the pair of emission lines of Oxygen to get to the eye,
and for many planetary and some diffuse emission nebulae, the boost in
contrast has to be seen to be believed! The Veil and Helix Nebulae look like
photographs in a 10" with the OIII filter, and some of the "green box"
emission nebulae in SKY ATLAS 2000.0 jump out at you. You may even see some
nebulae which are not shown on some atlases. This filter is often the best
one for many planetary nebulae, with the "blinking" technique becoming vastly
more effective, as the stars nearly vanish, leaving the planetary standing
out like a sore thumb. However, since the bandwidth of the OIII filter is so
narrow, it may hurt some nebulae with significant H-beta emission somewhat,
like the nebulae around Gamma Cygni or the Horsehead. Differences between
this filter and Broad-band filters like the Lumicon UHC are mainly in nebula
visibility and contrast. Many nebulae show a slightly larger area of
nebulosity in the UHC filter with slightly higher brightness, but in the OIII
filter, they will often have more contrast and dark detail. However, the
OIII filter really dims the view of star clusters and galaxies even more than
the narrow band filters do, although observers with large telescopes may find
the OIII useful for bringing out a few emission nebulae in other galaxies,
like the HII regions in M33. The Lumicon and Meade models also have a
substantial red passband, and on bright emission nebulae like M42 and M8,
weak red color in parts of the nebulae have been reported visually using
moderate to large apertures. Recently, Thousand Oaks has produced its
"Type-3" Oxygen III filter, as has Astronomik. It doesn't seems to have the
tiny "red-ghosting" secondary star images that the Lumicon model does,
yielding more point-like star images, although its overall performance in
enhancing nebulae is quite similar to the Lumicon model. Meade has also
introduced its own OIII filter.
Another somewhat less-used line filter is the H-Beta. As the name
indicates, the filter only lets through the H-Beta emission line of Hydrogen,
and is best known for its effect on the Horsehead Nebula, the California
Nebula, the Coccoon Nebula, and a few others. On an 8" to 10" scope, the
Horsehead Nebula goes from near invisibility to visibility, and the
California Nebula becomes fairly easy, gaining a great deal of contrast and
filamentary detail. An improvement over non-filter use for additional
objects like M42/43, the North America Nebula, and a few others can also be
noted, but in many cases, these other objects can appear somewhat better
overall in the UHC or OIII filters. The H-beta can also be used to observe
some of the structural details of some brighter nebulae by comparing the
H-beta view with that in other filters. However, the H-beta does not usually
work well on most planetary nebulae, as it nearly wipes out some of them and
greatly dims most of the rest. The total number of emission nebulae which
the H-beta will significantly improve is somewhat limited. Many of these
"H-beta" objects tend to be fairly faint to begin with (like the Horsehead)
and require larger apertures for decent views even with the filter. Unless
you REALLY like looking at these faint H-beta targets, you may be able to do
without the H-Beta filter. Thousand Oaks also makes their "Type-4" version
of the H-beta filter, as does Astronomik.
For recommendations, if you can afford only one filter, get a narrowband
filter like the Lumicon UHC, Meade 4000 Series Narrowband, or Orion
Ultrablock (whichever is least expensive at the time). If you can afford to
get two filters, the OIII makes a good companion filter to a narrowband one
but remember to use them with an eye that is properly dark adapted and employ
averted vision. Filters won't make the objects brighter, but in many cases,
they can make many of them a lot easier to see. Have fun!

David Knisely, Prairie Astronomy Club


--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely's filter reviews are invaluable! I would like to add that I
have found that the Lumicon and Thousand Oaks OIII filters work better for
me on scopes greater than 8", as on scopes 8" and less they seem too dark.
For these smaller scopes I like the UHC better. I consider my Lumicon 2"
OIII and UHC filters among my best astro purchases, and I will someday get a
2" H-Beta. Also, I think that a filter slide would be a terrific add-on,
allowing easy and quick switching between them. Note that I live in a rural
area with dark skies.

Dennis

Hi there Dennis. You posted:

David Knisely's filter reviews are invaluable!

Thank you! It was a labor of love, but it has proved useful.

I would like to add that I
have found that the Lumicon and Thousand Oaks OIII filters work better for
me on scopes greater than 8", as on scopes 8" and less they seem too dark.

Well, I have successfully used the OIII on scopes as small as 60mm as long as
the power range is kept low (4x to 10x per inch of aperture). Indeed, I have
often used the OIII on M42, M8, the Veil, the North America (NGC 7000), the
Helix (NGC 7293), and a few other targets with my little 80mm f/5 refractor.
However, the reason that most people think the OIII doesn't work as well for
smaller apertures is because the number of objects which those apertures can
easily reach (and which the OIII is capable of helping) is somewhat limited.
It does produce a dark field, but with proper dark adapation, a benefit can
still be gotten from the OIII in smaller apertures (and amateurs will still
want to have the OIII around when they upgrade to a larger scope).

For these smaller scopes I like the UHC better. I consider my Lumicon 2"
OIII and UHC filters among my best astro purchases, and I will someday get a
2" H-Beta.

I often recommend that if someone can only afford to purchase one filter, it
should be a narrow-band one like the UHC, as it seems to be the most useful
for the greatest number of nebular objects.

Also, I think that a filter slide would be a terrific add-on,
allowing easy and quick switching between them.

The Lumicon slide may be OK for some telescopes, but it has a number of
problems as well (see the review of it on the cloudynights web page:
http://www.cloudynights.com). It works OK on SCT's, but it introduces a long
eyepiece position which may make it difficult or impossible to achieve focus
on some Newtonians and refractors. Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

I would like to add that I
have found that the Lumicon and Thousand Oaks OIII filters work better
for
me on scopes greater than 8", as on scopes 8" and less they seem too
dark.

Well, I have successfully used the OIII on scopes as small as 60mm as long
as
the power range is kept low (4x to 10x per inch of aperture). Indeed, I
have
often used the OIII on M42, M8, the Veil, the North America (NGC 7000),
the
Helix (NGC 7293), and a few other targets with my little 80mm f/5
refractor.
However, the reason that most people think the OIII doesn't work as well
for
smaller apertures is because the number of objects which those apertures
can
easily reach (and which the OIII is capable of helping) is somewhat
limited.
It does produce a dark field, but with proper dark adapation, a benefit
can
still be gotten from the OIII in smaller apertures (and amateurs will
still
want to have the OIII around when they upgrade to a larger scope).


I see your point about the OIII and smaller apertures. Maybe for me it is
not so much the the OIII is too dark, but that it dims stars. For instance,
I really like how the open cluster NGC6530 looks in M8, and I find that in
anything smaller than 10" the OIII pretty much kills it. I really like the
OIII in the 10", and in this scope I find I use it more than the UHC.

Dennis

I would like to add that I
have found that the Lumicon and Thousand Oaks OIII filters work better
for
me on scopes greater than 8", as on scopes 8" and less they seem too
dark.

Well, I have successfully used the OIII on scopes as small as 60mm as long
as
the power range is kept low (4x to 10x per inch of aperture). Indeed, I
have
often used the OIII on M42, M8, the Veil, the North America (NGC 7000),
the
Helix (NGC 7293), and a few other targets with my little 80mm f/5
refractor.
However, the reason that most people think the OIII doesn't work as well
for
smaller apertures is because the number of objects which those apertures
can
easily reach (and which the OIII is capable of helping) is somewhat
limited.
It does produce a dark field, but with proper dark adapation, a benefit
can
still be gotten from the OIII in smaller apertures (and amateurs will
still
want to have the OIII around when they upgrade to a larger scope).


I see your point about the OIII and smaller apertures. Maybe for me it is
not so much the the OIII is too dark, but that it dims stars. For instance,
I really like how the open cluster NGC6530 looks in M8, and I find that in
anything smaller than 10" the OIII pretty much kills it. I really like the
OIII in the 10", and in this scope I find I use it more than the UHC.

Dennis

I would like to add that I
have found that the Lumicon and Thousand Oaks OIII filters work better
for
me on scopes greater than 8", as on scopes 8" and less they seem too
dark.

Well, I have successfully used the OIII on scopes as small as 60mm as long
as
the power range is kept low (4x to 10x per inch of aperture). Indeed, I
have
often used the OIII on M42, M8, the Veil, the North America (NGC 7000),
the
Helix (NGC 7293), and a few other targets with my little 80mm f/5
refractor.
However, the reason that most people think the OIII doesn't work as well
for
smaller apertures is because the number of objects which those apertures
can
easily reach (and which the OIII is capable of helping) is somewhat
limited.
It does produce a dark field, but with proper dark adapation, a benefit
can
still be gotten from the OIII in smaller apertures (and amateurs will
still
want to have the OIII around when they upgrade to a larger scope).


I see your point about the OIII and smaller apertures. Maybe for me it is
not so much the the OIII is too dark, but that it dims stars. For instance,
I really like how the open cluster NGC6530 looks in M8, and I find that in
anything smaller than 10" the OIII pretty much kills it. I really like the
OIII in the 10", and in this scope I find I use it more than the UHC.

Dennis

Hi there Dennis. You posted:

David Knisely's filter reviews are invaluable!

Thank you! It was a labor of love, but it has proved useful.

I would like to add that I
have found that the Lumicon and Thousand Oaks OIII filters work better for
me on scopes greater than 8", as on scopes 8" and less they seem too dark.

Well, I have successfully used the OIII on scopes as small as 60mm as long as
the power range is kept low (4x to 10x per inch of aperture). Indeed, I have
often used the OIII on M42, M8, the Veil, the North America (NGC 7000), the
Helix (NGC 7293), and a few other targets with my little 80mm f/5 refractor.
However, the reason that most people think the OIII doesn't work as well for
smaller apertures is because the number of objects which those apertures can
easily reach (and which the OIII is capable of helping) is somewhat limited.
It does produce a dark field, but with proper dark adapation, a benefit can
still be gotten from the OIII in smaller apertures (and amateurs will still
want to have the OIII around when they upgrade to a larger scope).

For these smaller scopes I like the UHC better. I consider my Lumicon 2"
OIII and UHC filters among my best astro purchases, and I will someday get a
2" H-Beta.

I often recommend that if someone can only afford to purchase one filter, it
should be a narrow-band one like the UHC, as it seems to be the most useful
for the greatest number of nebular objects.

Also, I think that a filter slide would be a terrific add-on,
allowing easy and quick switching between them.

The Lumicon slide may be OK for some telescopes, but it has a number of
problems as well (see the review of it on the cloudynights web page:
http://www.cloudynights.com). It works OK on SCT's, but it introduces a long
eyepiece position which may make it difficult or impossible to achieve focus
on some Newtonians and refractors. Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************