Telescope Reviews: Getting my first OIII Filter. What DSOs to see?

We're in agreement. Colors can reveal a shared illusion.
Some colors are seen by all observers and seem to be enhanced by using larger scopes. Those colors are likely to be real. For instance, I have seen, in M42 and M43:
--a dusty rose color inside the "ring" on one side
--a peach-gray color inside the ring on the other side
--the Trapezium region as greenish-gray
--M43 as bluish-gray, definitely bluer than the Trap. region
--pink to faint red in the "arches" that lead away from the Trap. region
Are the colors real?
Well,
--I have seen them in scopes from 9 cm to 81 cm.
--The colors appear stronger and less subtle in larger scopes
--The colors are more apparent with larger exit pupils
--The colors match the tints seen in color photographs.
--The colors are easiest without a filter of any kind.
--The blue-green colors near the center are MORE apparent with a nebula filter, but the other colors seem to disappear.
--Where I observe, this nebula, in a 12.5" scope, will damage your night vision due to its brightness, so there is a high likelihood of mesopic vision when viewing the nebula--especially at lower magnifications.
--Last, other viewers see the same colors and agree on where they show.

Does this PROVE the colors are not an optical illusion common to the human eye/brain perceptual instrument? No. But they certainly indicate that color descriptions have some validity in the commonality of perception in human observers. That is, if I say there is a pink tint, and you also see a pink tint, then another viewer may see the same pink tint. Even if the colors are an optical illusion, if we all see the same thing then we have a common shared experience about which we can ponder.

As to using an O-III filter on a nebula where the UHC bandwidth may show more nebula, David would agree there is some validity in doing so. Small knots and details that are somewhat submerged in bright nebulosity come to the forefront, and seeing these details can give you a better idea of structure. For example: NGC6888 in Cygnus. The UHC shows it very well, and shows a large extent of nebulosity. The striations and "ropy waves" of nebulosity within the nebula are more visible, however, with an O-III filter. Both filters show different aspects, and studying the nebula with both reveals different aspects.

As to M17, on a particularly clear and transparent night not many months ago, I was studying M17 with a new O-III filter (over 97% transmission at both 496 and 501nm), and noticed the sky around M17 had a "ropy" character as if there were waves of nebulosity filling the field of view, well outside the central nebulosity (which looked like a large oval with the "Swan" only a tiny part). I followed this ropy nebulosity a few fields of view until another bright nebula swan into the field of view. It was M16!. For the one and only time in my life I had visually seen that M16 and M17 are merely bright points in the same nebula. Would I have seen that with a UHC filter? I'm not sure. But following the nebula where it leads can be very revealing.

For instance, M42/M43, and NGC1977 can be seen to be bright parts of the same nebula. M8 can be seen to be part of the same nebula as M20, connected by NGC 6526.

Good filters can be wonderful tools for investigating nebulae.

--------------------
Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member