(Anti) nebular filter?

Finally got around to trying my Meade 2" Nebular (sic) filter the
other night. Performance was underwhelming for visual use. I expected
a subtle effect, but I think the drawbacks outweigh the surprisingly
slight contrast enhancement gained. I'm not keen on the "bluish" tint
it gives to everything (an aesthetic judgment, I know), and the
faintest stars disappear (seems to lose at least a magnitude at the
faint end). Definitely the least bang/buck so far! No matter. I'll put
it aside until I get into serious CCD work - I'm sure it will get a
lot more use there. I assume I'd get the same (limited) effect with
O-III or other narrow-band filters - seems they're better suited to
photographic use.

Losing the faintest stars raised another question. For the VISUAL
observer, is an "anti nebular" filter possible? I want to see more
faint stars buried in nebulosity (e.g. the E, F and G components of
the trapezium in M42). I know IR is useful for seeing through dust and
gas - but my IR sensitive retinas are on back order :-). Since nebulae
are largely reflected starlight, I guess it's a tall order to filter
them and leave stars behind - but maybe...?

Cheers
Beats

"justbeats" wrote in message
m...
Finally got around to trying my Meade 2" Nebular (sic) filter the
other night. Performance was underwhelming for visual use. I expected
a subtle effect, but I think the drawbacks outweigh the surprisingly
slight contrast enhancement gained. I'm not keen on the "bluish" tint
it gives to everything (an aesthetic judgment, I know), and the
faintest stars disappear (seems to lose at least a magnitude at the
faint end).

That's what you're paying for it to do -- block white light so you can see
the nebula against a darker background. It cuts not only skyglow but also
stars.

Keep at it; you'll find that there are situations in which it's really
helpful. NGC 281 in Cassiopeia is a good nebula to observe with and without
the filter.

it aside until I get into serious CCD work - I'm sure it will get a
lot more use there. I assume I'd get the same (limited) effect with
O-III or other narrow-band filters - seems they're better suited to
photographic use.

No; narrowband nebula filters cut the deep red emissions from nebulae that
film and CCDs pick up very well. I do not normally use a nebula filter
photographically at all. Sometimes I use a broadband nebula filter for film
exposures under heavy light pollution.

Losing the faintest stars raised another question. For the VISUAL
observer, is an "anti nebular" filter possible? I want to see more
faint stars buried in nebulosity (e.g. the E, F and G components of
the trapezium in M42). I know IR is useful for seeing through dust and
gas - but my IR sensitive retinas are on back order :-). Since nebulae
are largely reflected starlight, I guess it's a tall order to filter
them and leave stars behind - but maybe...?

Sure. You might try a pale red or orange filter for starters... I'd have to
look at spectra and work out what would work best, but experimentation would
be faster. Interesting idea!


--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope

"justbeats" wrote in message
m...
Losing the faintest stars raised another question. For the VISUAL
observer, is an "anti nebular" filter possible? I want to see more
faint stars buried in nebulosity (e.g. the E, F and G components of
the trapezium in M42). I know IR is useful for seeing through dust and
gas - but my IR sensitive retinas are on back order :-). Since nebulae
are largely reflected starlight, I guess it's a tall order to filter
them and leave stars behind - but maybe...?

As you point out, reflection nebula have largely the same spectrum as the
stars. But emission nebula are different. There it would be easy to cut off
the nebula and leave the star. This would be perfect for hunting down
central stars in planetary. And, it would help bring out the trapezium.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

"Michael A. Covington"
wrote

It cuts not only skyglow but also stars.
Sure, I know it does that by design – I was trying to gauge the amount
of cut-off from a broad-band filter. I guessed a magnitude, but (based
on one of your later answers) it will depend on individual stars
spectra. I hadn't thought of that.

I should point out that I only tested on nebulae I'm familiar with
(i.e. I've already been able to detect structure in them). By
definition those are placed away from light pollution. I'll have
another go closer to the light domes on the horizon. I bet sky-glow
reduction is more useful then, but I've just not felt a need to look
into the murk yet; still too much new and wondrous stuff to see within
45 degrees of zenith :-)

NGC 281 in Cassiopeia is a good nebula to observe with and without…
Thanks for this suggestion. It has some very interesting dust blobs
and is currently well placed. I'll give it a try and report back
(cloud permitting).

No; narrowband nebula filters cut the deep red emissions from nebulae that
film and CCDs pick up very well.
Hang on! Just because CCD picks up deep red very well, doesn't mean I
should limit my CCD imaging to that area of the spectrum, does it? An
O-III image (for example) will show distinctly different structure to
one taken in deep red. Sure it will take longer exposure (i.e. less
convenient than unfiltered imaging – perhaps this is what you are
alluding to), but it must yield a more interesting image than
unfiltered; especially when mixed with LRGB channels in a final
composite.

Sure. You might try a pale red or orange filter for starters...
Of course! I was thinking of stars collectively (white), not as
individuals with their own spectra. It's obvious that a red star
against a blue nebula could be "contrast enhanced" using a red filter.
Tougher with mixed spectra doubles though...

One final question (since I'll be experimenting with filters now :-).
How to do this "blinking" trick? I can't seem to get enough space
between my face and the EPs to comfortably slide a filter back and
forth. Just resting the filter on the eyepiece isn't good either (too
easy to scuzz it up with skin oils). The Meade nebula filter won't
screw into Nagler 2" EPs (why?) so I mount it on the diagonal; but
this makes it difficult to compare effects as there's delay involved
in the switch over. Can I get a filter wheel or slide compatible with
10" SCT, Meade focuser, Meade 2" diagonal, Nagler EPs and 2" filters?
Will it add much length to the optical train (I'm worried about
swing-through)?

Recommendations welcome.

Cheers
Beats

Beats posted:

Finally got around to trying my Meade 2" Nebular (sic) filter the
other night. Performance was underwhelming for visual use.

This is surprising, but may indicate one of several things may be wrong.
First, if a person is not well dark-adapted, the effects will be basically as
you described, since most of the major enhancement takes place for the fainter
nebular details (I suspect you are observing in less than decent ambient
"darkness"). Second, the magnification needs to be in the 4x to 10x per inch
range, with the best performance closer to the 4x figure. Third, the filter
should either be a "narrow-band" or a "line" filter, as the more broad-band
filters have only a limited rejection capability. As for the OIII, get ready,
as it just about "kills" the stars leaving only the brighter ones. For this
reason, the "blinking" technique for locating small planetary nebulae in
star-rich fields is one of the strong-points of the OIII filter. These
filters (other than the broadband ones) are also not designed for photography,
but merely visual use). 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 *
**********************************************

Chuck Taylor wrote:
"justbeats" wrote in message
m...
Losing the faintest stars raised another question. For the VISUAL
observer, is an "anti nebular" filter possible? I want to see more
faint stars buried in nebulosity (e.g. the E, F and G components of
the trapezium in M42). I know IR is useful for seeing through dust and
gas - but my IR sensitive retinas are on back order :-). Since nebulae
are largely reflected starlight, I guess it's a tall order to filter
them and leave stars behind - but maybe...?

As you point out, reflection nebula have largely the same spectrum as the
stars. But emission nebula are different. There it would be easy to cut off

Actually, reflection nebulae are considerably bluer than the illuminating
stars (since the particles are almost all sub-micron in size,
scattering the blue light more effectively). So the bluest filter
you can still get enough light through will enhance genuine
reflectoin nebulosity (and maybe help distinguish them from
emission nebulae in combinations such as the Trifid). Also,
for CCD imaging, a deep enough red (i,.e. near-IR) filter
can reducetheir intensiy, since there is a gap between H-alpha and
about 9000 A for strong emission lines. This has been used to bring out
such features as the Oroin nebula cluster's fainter stars.

Bill Keel

In message 1066672981.392981@cache1, David Knisely
writes
Beats posted:

Finally got around to trying my Meade 2" Nebular (sic) filter the
other night. Performance was underwhelming for visual use.

This is surprising, but may indicate one of several things may be
wrong. First, if a person is not well dark-adapted, the effects will be
basically as you described, since most of the major enhancement takes
place for the fainter nebular details (I suspect you are observing in
less than decent ambient "darkness"). Second, the magnification needs
to be in the 4x to 10x per inch range, with the best performance closer
to the 4x figure. Third, the filter should either be a "narrow-band" or
a "line" filter, as the more broad-band filters have only a limited
rejection capability.

But he said he had their broadband Nebula filter. Not a great first
choice of filter to buy for visual use and expensive in the 2" size.
Seems to be a common first buy mistake...

But I had assumed that the US made broadband filters are optimised for
mercury street lighting so that they would work visually a bit better in
the USA than in the UK (almost no mercury lights). Is this in fact not
the case?

It is a strange thing that even broad band filters that have high
transmission and strong rejection of LPS light pollution don't seem to
help that much visually. A few objects are helped but faint stars are
always lost.

Photographically is another story but you can't increase the eye's
integration time - so visually they are disappointing.

As for the OIII, get ready, as it just about "kills" the stars leaving
only the brighter ones. For this reason, the "blinking" technique for
locating small planetary nebulae in star-rich fields is one of the
strong-points of the OIII

UHC is probably as good as any for a first filter to buy.

Regards,
--
Martin Brown

In message , justbeats
writes
Finally got around to trying my Meade 2" Nebular (sic) filter the
other night. Performance was underwhelming for visual use. I expected
a subtle effect, but I think the drawbacks outweigh the surprisingly
slight contrast enhancement gained. I'm not keen on the "bluish" tint
it gives to everything (an aesthetic judgment, I know),

Narrower band filters give things even stronger tints. But they do at
least have the merit of providing stunning contrast enhancements - but
they necessarily wipe out a lot of continuum starlight in the process.

Losing the faintest stars raised another question. For the VISUAL
observer, is an "anti nebular" filter possible?

Possible but probably not very interesting. The normal incidence
reflection off an OIII filter will be an anti nebula filter. But I
wouldn't bother.

I want to see more
faint stars buried in nebulosity (e.g. the E, F and G components of
the trapezium in M42). I know IR is useful for seeing through dust and
gas - but my IR sensitive retinas are on back order :-).

CCD cameras are surprisingly sensitive to near IR and you can get low
pass glass filters.

Since nebulae
are largely reflected starlight, I guess it's a tall order to filter
them and leave stars behind - but maybe...?

Comparatively few bright nebulae are reflected starlight. UV from bright
stars excites the rarefied gases in the nebulae and they emit light at
specific wavelengths in much the same way that neon signs do.
OIII, H-alpha, Hbeta being amongst the brightest.

Regards,
--
Martin Brown

Hi,
I can make a notch filter that will block from about 490nm-510nm which
will eliminate the nebula.

Our VFS(Variable Filter) has the ability to reject the nebula. Its
interesting how it sharpens the Trapezium.

I may consider making a 'Nebula emininator', though I'm not sure what
the demand for such a limited filter would be.

Al M



Martin Brown wrote in message ...
In message , justbeats
writes
Finally got around to trying my Meade 2" Nebular (sic) filter the
other night. Performance was underwhelming for visual use. I expected
a subtle effect, but I think the drawbacks outweigh the surprisingly
slight contrast enhancement gained. I'm not keen on the "bluish" tint
it gives to everything (an aesthetic judgment, I know),

Narrower band filters give things even stronger tints. But they do at
least have the merit of providing stunning contrast enhancements - but
they necessarily wipe out a lot of continuum starlight in the process.

Losing the faintest stars raised another question. For the VISUAL
observer, is an "anti nebular" filter possible?

Possible but probably not very interesting. The normal incidence
reflection off an OIII filter will be an anti nebula filter. But I
wouldn't bother.

I want to see more
faint stars buried in nebulosity (e.g. the E, F and G components of
the trapezium in M42). I know IR is useful for seeing through dust and
gas - but my IR sensitive retinas are on back order :-).

CCD cameras are surprisingly sensitive to near IR and you can get low
pass glass filters.

Since nebulae
are largely reflected starlight, I guess it's a tall order to filter
them and leave stars behind - but maybe...?

Comparatively few bright nebulae are reflected starlight. UV from bright
stars excites the rarefied gases in the nebulae and they emit light at
specific wavelengths in much the same way that neon signs do.
OIII, H-alpha, Hbeta being amongst the brightest.

Regards,

Martin Brown wrote:

But he said he had their broadband Nebula filter. Not a great first
choice of filter to buy for visual use and expensive in the 2" size.
Seems to be a common first buy mistake...

Actually, what he said was "Finally got around to trying my Meade 2"
Nebular (sic) filter the
other night.", so no mention was made of it being broadband or
narrow-band. In a later posting, he does mention a broad-band filter,
but is not specific on the one he used. Meade makes 3-types
(broad-band, narrow-band, and OIII), although calling their broadband
filter a "nebula" filter is a bit of an overstatement, as the broadband
filters are designed mainly to get rid of the emission lines from
man-made and natural sources of skyglow (they are more properly referred
to as "light pollution filters"). Narrow-band filters are designed
specifically to let through only the emission lines from nebulae
(H-alpha, OIII, and H-Beta) while rejecting almost everything else.
Broadband filters are not necessarily a mistake, as they can help boost
contrast on a number of objects (not necessarily nebulae), but the
amount of improvement they yield is often somewhat less than some people
expect. 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 *
**********************************************