Expert: Tom Whiting - 10/20/2004

Question
How do filters (such as light pollution and colored filters) only let one frequency group through and block out all/most others? How are filters made?

Answer
Hi Bridget,
Well, I'm not into the manufacturing field, but I've read that
the so-called light pollution and nebular filters (like the O-III
nebular filter which I have one) are made by depositing
various die-electric compounds in different layers, onto
the filter, (a piece of clear glass)  thus each allowing only certain wavelengths of light to pass through the filter.  In the case of the O-III filter,
only blue-green light at 5007 Angstroms is allowed to
pass through the filter, and since that is the primary light
of all planetary nebula, it makes for a great filter to visually
see the neblosity thru your scope.  (Of course, it takes a
toll on the central star in the planetary nebula, so if you
desire to view the central star, don't use the filter!)
I don't know what the compounds are that they use....I'm not even sure that they publish that information, as it may be proprietary (industrial secret) information.  PLUS, its probably
compounds that have to be vacuum deposited (vaporized) in a hot oven under very strict conditions of temperature and pressure, so even if we did know what compounds they use, you probably don't have the required equipment to successfully perform the process.

But color filters for enhancement of planetary features are
simply dyes, and much cheaper to make, as there is no real
light rejection as with the other two types mentioned above.

You can tell this by the cost.....a Lumicon O-III nebular filter
will cost you between $100 and $200, but they really
enhance the nebulosity on all planetary nebulae, most
emission nebulae, and even some other objects.....so well
worth the cost.

Simple colored filters are much cheaper, about $10 per filter,
but they don't reject light, they simply amplify that particular
color, so it enhances certain planetary details....as that's
about all you use your colored filters for....on planets only.

There is one other super light rejection filter on the market,
the Hydrogen Beta Filter, for also around $100, but its only
good for visually seeing the Horsehead Nebula in Orion
and the California Nebula in Perseus...and that's it.....in fact,
its even called (nicknamed) the Horsehead Filter....but to
me, its not worth the money for only 2 objects.....better to
get an Oxygen-III (O-III) filter for thousands of objects.

Oh, standard light pollution filters only reject some wavelengths of light, so their cost falls somewhere between
the two mentioned above...around $30 or $40 for a good
light pollution filter....these are designed to reject the light
of a few desired wavelengths, say yellow sodium vapor lights, and the blue of Mercury Vapor lights, and that's about
it.  The downside is, you sacrifice about one whole magnitude number on the objects you are attempting to observe....and one whole magnitude means you are dimming
down the objects in the Field Of View by 2.5 times....and
sometimes its just not worth it....I don 't use light pollution
filters...I just travel to very dark skies, like Cherry Springs
State Park in N. Central Pennsylvania, where a lot of us
travel to, each new moon weekend for all night observing.
BTW, its free camping to all astronomers at that PA state park, but no white light permitted at night....zero white light allowed.  And green laser pointers are outlawed there too.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie PA

PS...please visit our astronomy club website to see some
fine picts of my new 30 inch portable scope....largest portable
scope in Pennsylvania, and perhaps E. of the Mississippi...
http://www.velocity.net/~bwhiting/