On 19 Feb 2007 "Starboard" wrote:

I was using a standard #58 (24% light transmission)

The #56 Green has 53% transmission. The #11 Yellow-Green has 62%
transmission. I have both of these as well as the #58. I considered
trying all three (and others) on M42/43.

Unfortunately an apology of sorts is called for. I tried none of the
above filters last night. I started the night with a somewhat
meticulous observation and sketch of M79 at 155x. After a break I
went to work on M42/43 at 61x. By the time I was finished clouds were
becoming a problem.

If you look at the transmission spectra of a broadband light pollution
filter, its bandpass seems to be approx mid green through mid blue
(450 - 525nm) and most of red (650). The #58 green has a whole 24%
light transmission, which is pretty wide. Apparently wide enough to
pass the whole green band (of which the nebulae's emissions are part
of) while blocking the extreme blues and yellows associated with city
light pollution. Red is blocked as well. The broadband passes red, the
UltraBlock does not.

Let me know if you agree.

From what little I know the blocking and transmission properties of
color filters are subtler than those of the more expensive
interference (multi-layered) filters. The blocking tends to be not as
thorough. The transmission tends to be not as high; and the
transition in between tends to be not as sharp (not as steep).

What matters when observing an object is the relative colors of
neighboring regions and whether or not a given filter will darken one
region noticeably more than it does a neighboring region. Any filter
that makes a difference is worth using!
--
Bill
Celestial Journeys
http://cejour.blogspot.com