Omega's New Offering Of "Galaxy Filters"...Anyone used them?

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Dave Gede

Posted: Tue Jan 30, 2007 9:14 am

Guest

Hi All,

Looks like Omega's Galaxy Filters (GCE) are now being offered to the
public. They are built by D&G optics.

I dug around for some reviews by they are few and far between.
Anyone out there have any experience with these filters?

Here is a link to the product:

http://www.omegafiltersebuyer.com/servlet/the-GCE-Filters/Categories

Any insights would be appreciated.

Thanks,
Dave

Ed T

Posted: Tue Jan 30, 2007 9:50 am

Guest

The transmission spectrum is "proprietary data"! Trust us....

By the way, the manufacturer is DGM Optics, not D&G.

There are a number of other manufacturers who make similar filters that
block the main wavelengths emitted by sodium vapor lamps AND will let you
peek at their transmission spectra, LOL.

Ed T

"Dave Gede" <czar7@cgi101.com> wrote in message
news:1170162874.199492.98940@j27g2000cwj.googlegroups.com...

Quote:

AstroApp

Posted: Tue Jan 30, 2007 3:23 pm

Guest

On 30 Jan 2007 05:14:34 -0800, "Dave Gede" <czar7@cgi101.com> wrote:

Quote:

Ok...here's whatever insight I can offer, after more than 20 years of
pondering the use of filters.

Galaxies may be slightly enhanced by a standard LPR filter -- in MY
opinion. That opinion is not at all corroborated by other very
experienced amateurs, who don't think that they are effective. I've
argued this here before and conclude that it all comes down to an
individual's own personal experience and not an absolute assertion of
concrete "truth".

One of the first makes of such filters to come on the market was the
Lumicon brand, created by my old friend Jack B. Marling, who founded
the Lumicon Company after he left Livermore Labs, where he had been an
optical physicist.

The "Deep Sky" filter he designed was actually intended to help
perceive detail and contrast in galaxies, which is now (it would
appear) quite forgotten by most people interested in filters, who
focus on the nebular type.

Jack wrote a paper for the Riverside Telescope Makers Conference,
RTMC80, pp. 56-81. It describes the theory behind the filter, and
explains why it can give enhanced views of both nebulae AND galaxies.
He gave me an edited version of this paper and allowed me to publish
it in my software program for telescope users, which at one time was
sold by Lumicon under the name "Lumiview". I have put this paper on
my website here:
http://home.earthlink.net/~astro-app/horsehead/Marling.htm

Recently I've undertaken a study of faint, sometimes very obscure
objects that are usually viewed and photographed from very dark sky
rural sites. But, I'm using a slightly light polluted site near San
Jose, in order to determine if the best modern techniques and
equipment can make something of them. One of the first things I
verified was that, indeed, the filters based on the "LPR" type design
-- having a bandstop only around the frequencies of sodium vapor
streetlights -- will improve views of CERTAIN faint galaxies. Dust
lanes are more apparent; sometimes faint arms that are otherwise lost
when they fade into the background light may stand out a little bit
better; and extremely diffuse local group galaxies might become
visible, whereas without the filter, one could pass over them without
perception. I'd barely used my own LPR type filters for anything
other than photography until making this finding, which I'be been able
to repeat, time after time.

Since as someone else has pointed out that the filters you asked about
don't give disclosure of their bandpass characteristics, one can only
assume that the makers are attempting to create a completely new type,
somewhere between the LPR (anti-streetlight) filter, and the narrower
band nebular filter (of course, there is always the possibility that
they actually AREN'T doing so, but are merely creating the impression
by means of advertising.) It would seem to me to be very easy to
determine the response curve of those filters, if any reviewer with
access to a spectroscope could test them. My own double blind
multi-party tests of filters, conducted in the late 1980s, illustrated
the fact that often we could not get a consensus about which one gave
the "best" view, or the most detailed information, no two users quite
agreeing on every aspect of what they saw, or preferred to see.
Therefore, even the slightest difference in bandstop, "Q", and
transmission peak will alter what one perceives; its possible to
fine-tune some particular detail according to a specific goal.
Perhaps these filters have tried to do that.

Therefore, all we can do is either buy and try them, or wait for a
trusted reviewer to do it and let us know more about them.

Meanwhile the various makers of filters that I have experience with
all seem to have done a very good job, although careful A-B testing
will show variations. Once again: my testing showed that these things
were hard to discern, and hard to HOLD in mind while changing filters.
Using a "slide bar" to switch them back and forth, immediately, with
one given eyepiece is not always the proper test, as different filters
are designed to work best with different exit pupils, which would mean
changing magnification to achieve highest contrast.

Therefore, a test of filters with a slide bar should ideally be done
with only competing brands of IDENTICAL TYPE filters: i. e., with
SkyGlow versus Deep Sky, or Lumicon UHC versus Orion UltraBlock.
Then, the same exit pupil will be used on each identical bandpass type
filter, with respect to the wavelengths they are designed to
suppress/transmit.

To my knowledge, the filter tests that have been published in
magazines and on the net seem not to have addressed this issue. A
truly comprehensive test report really *should* include a full
accounting of the magnification/exit pupil used for each filter, and
for each observation, to determine if the optimal result has been
achieved. Just saying that "OIII is better than UHC for Nebula X" is
not quite enough information, especially if the determination has been
made merely by switching them with a filter slider bar at the same
magnification.

What this all boils down to is that, really, everyone who wants to use
filters should do their OWN tests. No matter what an "authority" or
"tester" says, based on whatever methodology they used, what's really
going to matter is what YOU think.

AstroApp

David Knisely

Posted: Tue Jan 30, 2007 5:18 pm

Guest

The DGM Optics GCE filter is a broadband filter which is an attempt to
just reduce the effects of the main lines and band of light pollution
while letting in as much of the rest of the spectrum as possible. I am
testing one right now for later review on Cloudynights.com, and so far,
it shows a slight improvement on *some* galaxies under *certain*
conditions. Its passband is somewhat less aggressive than something
like the Lumicon Deep-Sky filter, so it isn't quite as effective at
fully notching out the light pollution lines. Still, on at least a few
galaxies, I noted a bit of an improvement over the view in the Deep-sky,
although this is not universally so. The research will continue....
Clear skies to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely

Posted: Tue Jan 30, 2007 5:47 pm

Guest

AstroApp posted:

Quote:

Therefore, a test of filters with a slide bar should ideally be done
with only competing brands of IDENTICAL TYPE filters: i. e., with
SkyGlow versus Deep Sky, or Lumicon UHC versus Orion UltraBlock.
Then, the same exit pupil will be used on each identical bandpass type
filter, with respect to the wavelengths they are designed to
suppress/transmit.

To my knowledge, the filter tests that have been published in
magazines and on the net seem not to have addressed this issue. A
truly comprehensive test report really *should* include a full
accounting of the magnification/exit pupil used for each filter, and
for each observation, to determine if the optimal result has been
achieved. Just saying that "OIII is better than UHC for Nebula X" is
not quite enough information, especially if the determination has been
made merely by switching them with a filter slider bar at the same
magnification.

I did this for the Lumicon series of filters in the article FILTER
PERFORMANCE COMPARISONS FOR SOME COMMON NEBULAE, which can be found at:

http://www.cloudynights.com/item.php?item_id=1520

It is an extended survey of 93 emission and planetary nebulae which used
the Lumicon Multi-filter selector for instant comparison of the views.
It is intended mainly to give the filter user a rough idea of what to
expect when various filters are used on some of the more prominent
nebulae, and which filters *may* be more appropriate for use on the
object in question. Powers and instruments where changed to see at what
level an optimal view was obtained, and then the filters were compared
at the same power level. Generally, certain trends began to emerge
which did guide the choice on power and instrument for the different
classes of filter (i.e. broadband, narrowband, or line). This is where
the recommendation that filters for deep-sky are best used at between
3.5x and 9.9x per inch of aperture was arrived at. However, with
different personal tastes or requirements, the choice of filter to use
on a given object may never be exactly "cut and dried", as I mention in
the text of the article.

As for comparing the same design of filter from differing manufacturers,
I do use the Multi-filter selector and some detailed comparisons when I
review them for Cloudynights. The latest review of the DGM Optics NPB
narrowband filter on Cloudynights was done in this way. However,
differences in performance are sometimes not exactly "in your face", and
this is proving to be the case as I work with the DGM GCE "galaxy"
filter. Like you, I can see a *mild* improvement in the contrast for
*some* galaxies under *certain* conditions, but some people may not feel
that the level of improvement is worth the cost. Clear skies to you.

--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Guest

Posted: Sat Feb 03, 2007 10:00 am

On Jan 30, 4:18 pm, David Knisely <KA0...@navix.net> wrote:

Quote:

Its passband is somewhat less aggressive than something
like the Lumicon Deep-Sky filter, so it isn't quite as effective at
fully notching out the light pollution lines.

I'd seen a review some time back. Either in S&T or maybe Deep Sky
discussing filters. The article said the Orion Skyglow was "less
agressive" than the lumicon Deep Sky filter. Currently on sale by the
way. The article is probably ten years old though and don't know if
Orion may have changed the filter.

Alvan Clark

Rich

Posted: Sat Feb 03, 2007 2:19 pm

Guest

On Jan 30, 8:14 am, "Dave Gede" <c...@cgi101.com> wrote:

Quote:

Scam. Only discrete frequency objects are noticeably enhanced by the
use of filters, not galaxies, despite what some people wanting to
justify their expenditure think.

David Knisely

Posted: Sat Feb 03, 2007 3:46 pm

Guest

Rich posted:

Quote:

Scam. Only discrete frequency objects are noticeably enhanced by the
use of filters, not galaxies, despite what some people wanting to
justify their expenditure think.

No scam, just maybe not providing as much of a contrast boost as the
narrowband and line filters do on emission nebulae. These broadband
filters do "notch out" some of the common emission lines and bands found
with light pollution while letting through as much of the rest of the
visual spectrum as possible at a high level of transmission. They do
improve the contrast to a mild degree, especially for imaging. For
visual use, they may be disappointing to some, but others find the small
but definite boost in contrast for things like reflection nebulae or
even some galaxies enough to justify the cost. Clear skies to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

Rich

Posted: Sat Feb 03, 2007 6:56 pm

Guest

On Feb 3, 2:46 pm, David Knisely <KA0...@navix.net> wrote:

Quote:

Rich posted:

Scam. Only discrete frequency objects are noticeably enhanced by the
use of filters, not galaxies, despite what some people wanting to
justify their expenditure think.

No scam, just maybe not providing as much of a contrast boost as the
narrowband and line filters do on emission nebulae. These broadband
filters do "notch out" some of the common emission lines and bands found
with light pollution while letting through as much of the rest of the
visual spectrum as possible at a high level of transmission. They do
improve the contrast to a mild degree, especially for imaging. For
visual use, they may be disappointing to some, but others find the small
but definite boost in contrast for things like reflection nebulae or
even some galaxies enough to justify the cost. Clear skies to you.
--

The boost is so modest, you can hardly see it, even with large scopes.

People who invest in things like this, eyepieces that cost hundreds of
dollars are better off
(IMO) investing in more scope "inches" of aperture. Having said that,
narrow band nebula filters
really do work well.

David Knisely

Posted: Sun Feb 04, 2007 2:23 am

Guest

Rich posted:

Quote:

The boost is so modest, you can hardly see it, even with large scopes.

I can easily see the mild increase in contrast in a 100mm f/6 refractor
at about 20x using the Lumicon Deep-Sky filter on objects like the
Andromeda Galaxy or M33. From my driveway, the Deep-sky filter makes
the two dust lanes in M31 come out with somewhat more contrast than they
would without a filter. The Deep-sky filter makes the mottling in NGC
253 more prominent in an 8 or 10 inch at low to moderate powers than
without a filter, even from a dark sky site. On some reflection nebulae
like NGC 1999, NGC 7023 or the Merope Nebula, the Deep-sky filter
consistently provides a mild but fairly noticeable boost in contrast.
The filter does not work for all objects, but it often does have at
least some beneficial effect. Clear skies to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely

Posted: Mon Feb 05, 2007 1:43 am

Guest

Rich posted:

Quote:

This is what calls their efficacy into question: "The filter does not
work for all objects."
With nebula filters on emission nebula, there is no question they work
on all of those kind of objects, because the contrast effect of
cutting down the sodium and mercury emmissions versus the nebula's
light is apparent. The fact the deepsky or now the so-called galaxy
filters don't seem to work on polychromatic sources like galaxies
equally seems to hint that maybe their effect is more placebo than
real.

Well, not all nebula filters work for all nebulae either! The OIII
filter is totally useless on the Horsehead Nebula, and the H-Beta filter
nearly kills many if not most planetary nebulae. Thus, *these* filters
do not work for all objects. The selection of which filter to use will
depend on both the particular target and the observing conditions. As
for how they work, from a dark sky site, nebula filters work as well or
even better than they do under urban or suburban lighting because they
are blocking *more* than just man-made light pollution. They block the
common airglow lines from the atmosphere itself, scattered sunlight from
interplanetary dust, and the dim glow from the huge number of faint
unresolved stars in the background. This provides a noticeable increase
in the contrast even when man-made light pollution is non-existent.
Most seasoned observers who view at some of the major "pristine site"
star parties still use their filters on many emission nebulae for this
reason.

As for the Deep-sky filter, the GCE filter is *not* the same as the
Deep-sky. As mentioned earlier, the Deep-sky filter has two deep
steep-sided notches which prevent the common Mercury and Sodium Vapor
lines as well as the airglow lines from getting in. The GCE is somewhat
broader with a less aggressive sloping-sided notch that does not
completely extinguish these lines, but merely reduces them in strength.
From in-town, I found that the Deep-sky filter seemed to provide just
a hair more contrast than the GCE on the galaxies which I have been
testing it on so far. This would seem to support the initial conclusion
that the GCE is not as an effective "light pollution" filter as the
Deep-sky is.

For galaxies, these broadband filters generally work best on the larger
and more diffuse galaxies, although again, the effect can be different
for different galaxies, as some are bluer or more yellow than others.
Many competent observers have reported the gain in contrast, so it
definitely isn't a "placebo" effect. Indeed, photography with the
broadband filters shows the gain in contrast a bit more easily than it
is seen visually. With smaller galaxies, increasing the power can have
a somewhat similar effect as the filters sometimes produce, as the image
scale is higher to help compensate for the poor resolution of averted
vision, and the brightness of the sky background can be somewhat
reduced. Whether an observer thinks the gain in contrast with broadband
filters on galaxies is worth the cost or not is a subject worth
discussion, but the effect of these filters is indeed real and not just
a case of wishful thinking. Clear skies to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

W. H. Greer

Posted: Mon Feb 05, 2007 2:59 pm

Guest

After following this thread for a while I got to wondering: Does
anyone make a filter that passes everything except natural sky glow?
Such a filter might be a good general purpose deep sky filter for use
under skies that do not suffer from light pollution.

Such a filter would not replace an OIII or H-Beta for the special
classes of objects for which those filters cater to. Nevertheless, a
filter that blocks only natural sky glow ought to provide *some*
enhancement to most, if not all deep sky objects for observers who
observe under dark skies. Basically, I'm asking for a gentler filter
than most, if not all that are currently on the market.

The Baader Moon-SkyGlow filter seems to come close to the type of
filter I'm thinking about. I wonder, would the Baader filter show
galaxies better (when used under a dark sky) than the galaxy filter
being discussed here?

So, what filters are available now (or in the near future) for
*general* deep sky use under a dark (such as magnitude 6.5) sky? It
seems that most filters that approach this area are strongly tuned to
block artificial lights -- and that tends to weaken the light from
stars and galaxies as well.
--
Bill
Celestial Journeys
http://cejour.blogspot.com

David Knisely

Posted: Mon Feb 05, 2007 3:41 pm

Guest

W.H. Greer wrote:

Quote:

Natural light pollution comes in two varieties: Emission and continuum.
The common emission lines from airglow are the O[I] lines at 5577,
6300 and 6364 Angstroms, as well as the faint Sodium "D" lines at 5890
and 5896 Angstroms. To block these effectively requires just a standard
"broadband" LPR filter like the Lumicon Deep-sky, Orion Skyglow, or the
IDAS LPS filters. The DGM GCE filter will attenuate some of these lines
as well, although not as completely as the other filters I mentioned.
The continuum light is basically sunlight scattered off interplanetary
dust, as well as a fainter component from extremely faint unresolved
background starlight, so that is more difficult to filter out. Clear
skies to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

W. H. Greer

Posted: Tue Feb 06, 2007 8:55 pm

Guest

On Mon, 05 Feb 2007 13:41:12 -0600, David Knisely <KA0CZC@navix.net>
wrote:

Quote:

Those filters block enough additional light to be detrimental when it
comes to visual observation of many, if not most stars and galaxies. I
was more curious about a filter for visual use that selectively
blocked the natural air glow emission lines (5577, 5893 and 6300)
while allowing most of what remains to pass through largely
undiminished; basically a *dark sky* air glow filter designed for
*visual* use in areas either completely or mostly free of man-made
glows.

Such a filter, used visually under a dark sky, should provide natural
looking views of stars, galaxies, etc. against a somewhat darkened sky
background.
--
Bill
Celestial Journeys
http://cejour.blogspot.com

Ioannis

Posted: Tue Feb 06, 2007 10:04 pm

Guest

"W. H. Greer" <sendnomail@tome.net> wrote in message
news:9v6is2pgpi319fi4um31nuumg8qrmivmd9@4ax.com...

Quote:

On Mon, 05 Feb 2007 13:41:12 -0600, David Knisely <KA0CZC@navix.net
wrote:

Natural light pollution comes in two varieties: Emission and continuum.
The common emission lines from airglow are the O[I] lines at 5577,
6300 and 6364 Angstroms, as well as the faint Sodium "D" lines at 5890
and 5896 Angstroms. To block these effectively requires just a standard
"broadband" LPR filter like the Lumicon Deep-sky, Orion Skyglow, or the
IDAS LPS filters.

Those filters block enough additional light to be detrimental when it
comes to visual observation of many, if not most stars and galaxies. I
was more curious about a filter for visual use that selectively
blocked the natural air glow emission lines (5577, 5893 and 6300)
while allowing most of what remains to pass through largely
undiminished; basically a *dark sky* air glow filter designed for
*visual* use in areas either completely or mostly free of man-made
glows.
Such a filter, used visually under a dark sky, should provide natural
looking views of stars, galaxies, etc. against a somewhat darkened sky
background.

I am not sure if such a filter will work. At least as far as the sodium D line
is concerned, the mechanism of light production in high pressure sodium (HPS)
lamps around this line is extremely complicated.

The actual sodium D lines ARE blocked already. Basically what happens is HPS
lamps exhibit self-reversal because the D lines are resonance lines, so what
you get is lots of light AROUND the actual D lines themselves and no light
exactly AT D. Here's a rough description of the phenomenon for the Sodium D
line and for the Mercury blue line (which is much less noticeable):

http://ioannis.virtualcomposer2000.com/spectroscope/absorption.html

The light at the "sides" of the D lines is called "the wings" of the D line,
and it's pretty much the light which is actually responsible for HPS lighting
and light pollution. You can see a picture of the entire spectrum of an HPS
(second to last pic) on this page:

http://ioannis.virtualcomposer2000.com/spectroscope/Na.html

As a result of the D lines having "wings", a substancial area of the spectrum
has to be killed if the filter is to make any sense. Unfortunately, the area
that needs killing includes useful red, yellow and green light which
contributes to faint visibility.

To make things worse, all the rest of the lines of Sodium exhibit thermal
broadening, which forces the lines to emit an entire band around them (lines
to the right on the pic), so these cannot be blocked very efficiently either,
unless a whole chunk is killed along with the central wavelength.

The situation is far better with low pressure sodium (LPS) and high pressure
mercury (HPM) lamps, because these lamps do not exhibit self-reversal. Well,
they do, but it's not noticeable. A filter which blocks the EXACT frequency on
these lamps works beautifuly. With HPS lamps the situation is a mess.

All this mess was essentially unavoidable, because one of the main reasons for
the extreme popularity of the HPS lamps is PRECISELY their ability to reverse
the D line and broaden the rest of the Sodium lines, so that engineers could
solve the annoying problem of the monochromaticity of the LPS lamps.

Which, if you ask me, doesn't bother me at all.

Quote:

--
Bill
--

I.N. Galidakis
http://ioannis.virtualcomposer2000.com/

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