So I found this little filter....

I found a green filter at a surplus shop.
It's a sandwich of some kind, gold coated
inside one side, silver on the other.
A very deep green maybe 2x that of a standard
#58. But here's the interesting thing;
It almost completely extincts high pressure
sodium. It doesn't work on incandescents
or mercury, but even when looking at a
HPS close by, it's almost invisible.
Given that 90% of the light pollution I
experience is now due to them, this might
make a good nebula filter for a larger
scope. It fit in standard filter housing
and I'll try it out, if it ever clears
up here.
-Rich

Do you know any very deep green nebulae?
Subtraction is always subtraction.
No matter how you look at it.
Welding goggles for M42? Anybody?

Chris.B

On 11 Dec 2004 00:12:36 -0800, wrote:

Do you know any very deep green nebulae?
Subtraction is always subtraction.
No matter how you look at it.
Welding goggles for M42? Anybody?

Chris.B

You don't seem to understand. Regular
nebula filters (OIII, UHC types)
reduce the nebular light by substantial
portion as well. The only reason you
see the nebula "better" is due to the increased
contrast between the nebula and the sky
background which has been dimmed far more.
That's why on small scopes, some nebula filters
do more harm than good; They reduce the nebula
light too much so the contrast effect becomes
almost secondary. I figure the green filter
will dim the nebula quite a bit, which is why
I suggested it for a large telescope that
will still have enough light grasp to provide
a good nebula light throughput. My curiosity
was because this filter dims HPS much better
than my OIII or UHC filters so the overall
effect in a large scope might be positive.
-Rich

My own night sky is already dark enough thankyou.
My nebul=E6 bright enough for any man.
With a modest instrument and a dark adapted eye.
* # * *=20
* :^) *
Chris.B

RichA posted:

You don't seem to understand. Regular
nebula filters (OIII, UHC types)
reduce the nebular light by substantial
portion as well. The only reason you
see the nebula "better" is due to the increased
contrast between the nebula and the sky
background which has been dimmed far more.
That's why on small scopes, some nebula filters
do more harm than good; They reduce the nebula
light too much so the contrast effect becomes
almost secondary.


Well, I'm afraid that you may not understand things completely either.
Emission nebulae have emission lines. Most narrow-band "nebula" filters
(Lumicon UHC for example) transmit around 90% of the light which comes from
these spectral lines (the OIII doublet at 4959 and 5007 Angstroms, and the
H-Beta line at 4861 Angstroms). The "dimming" of these emission lines would
be very difficult to detect visually (about 0.11 magnitudes). At the same
time, these filters will remove a lot of the skyglow from wavelengths which
are not needed, thus improving the contrast. When used *at the proper
magnification*, (between 3.7x and 9.9x per inch of aperture) these filters can
be successfully used even in some relatively small apertures. Indeed, I have
used them in scopes as small as 2 inches on large and bright nebulae. The
only thing which small scopes tend to do is have a more limited access to
objects which these filters might be able to help, due to their smaller
aperture (ie: its not the filter's fault). Thus, this "filters diming things
too much on small apertures" is pretty much a myth. I have even used them in
my 80mm f/5 refractor to see some fairly *faint* targets like the supernova
remnant IC 443 in Gemini (UHC filter) or Barnard's Loop (H-Beta filter). A
few weeks ago, I laid my 2" OIII filter in the dewcap of my 8x50 finder to see
both sides of the Veil quite nicely, so again, if you use the right power (and
understand the limitations of the *aperture*) you can get a lot out of filters
even with some smaller telescopes. 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 *
**********************************************

On Sun, 12 Dec 2004 01:05:39 -0600, David Knisely
wrote:

RichA posted:

You don't seem to understand. Regular
nebula filters (OIII, UHC types)
reduce the nebular light by substantial
portion as well. The only reason you
see the nebula "better" is due to the increased
contrast between the nebula and the sky
background which has been dimmed far more.
That's why on small scopes, some nebula filters
do more harm than good; They reduce the nebula
light too much so the contrast effect becomes
almost secondary.


Well, I'm afraid that you may not understand things completely either.
Emission nebulae have emission lines. Most narrow-band "nebula" filters
(Lumicon UHC for example) transmit around 90% of the light which comes from
these spectral lines (the OIII doublet at 4959 and 5007 Angstroms, and the
H-Beta line at 4861 Angstroms). The "dimming" of these emission lines would
be very difficult to detect visually (about 0.11 magnitudes). At the same
time, these filters will remove a lot of the skyglow from wavelengths which
are not needed, thus improving the contrast. When used *at the proper
magnification*, (between 3.7x and 9.9x per inch of aperture) these filters can
be successfully used even in some relatively small apertures.

If there is no perceptable dimming of the nebular light with these
filters, then why is it not recommended to use them, especially the
narrow band models when imaging?
-Rich

In article ,
says...
too much on small apertures" is pretty much a myth. I have even used them in
my 80mm f/5 refractor to see some fairly *faint* targets like the supernova
remnant IC 443 in Gemini (UHC filter) or Barnard's Loop (H-Beta filter). A

I'll have to try that - I was just thinking last Friday night about
Barnard's Loop. I have an ST80 and an H-BETA. Maybe I'll try it.
Maybe my Genesis, as well.

Have you been successful visually on the Witchhead Nebula in any
size/power/filter range?

--

-- Len Philpot - --
------ ----- http://philpot.org/ --

RichA posted:

If there is no perceptable dimming of the nebular light with these
filters, then why is it not recommended to use them, especially the
narrow band models when imaging?

Because imaging is *not* viewing. The requirements for photography are often
*vastly* different than for viewing with the unaided eye. Some color films
in particular are not very sensitive to the OIII doublet (it falls in a sort
of "dip" in the film's spectral sensitivity curve). Thus, using a nebula
filter like the OIII for photography in color would not result in much of a
response on the film at all. However, the OIII doublet is near the maximum
sensitivity region for the eye, so most people have no trouble in seeing this
wavelength. For other photographic purposes the requirements may mean the use
of a different filter or set of filters, rather than those which are best
suited for visual use. Broadband "light pollution" filters are often used for
photography, and it is here that they tend to work the best (even a bit better
than visual use). In fact, one of the most useful filters for black and white
imaging of emission nebulae is the H-alpha filter, a filter which is almost
useless for night visual work. 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 *
**********************************************

Len Philpot wrote:

Have you been successful visually on the Witchhead Nebula in any
size/power/filter range?

I can just get a hint of its glow in my 10x60 binoculars, but that's about it.
It is mostly a reflection nebula, so other than broadband filters, most
don't seem to bring it out all that well. 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 *
**********************************************

On Mon, 13 Dec 2004 01:13:45 -0600, David Knisely
wrote:

RichA posted:

If there is no perceptable dimming of the nebular light with these
filters, then why is it not recommended to use them, especially the
narrow band models when imaging?

Because imaging is *not* viewing. The requirements for photography are often
*vastly* different than for viewing with the unaided eye. Some color films
in particular are not very sensitive to the OIII doublet (it falls in a sort
of "dip" in the film's spectral sensitivity curve). Thus, using a nebula
filter like the OIII for photography in color would not result in much of a
response on the film at all.

How about the average CCD? Is it's response that much different
(aside from infrared sensitivity) to the human eyes?
-Rich