Orion "Ultra Block Filter"

[Rob72 (Robert)]

Hi, I recently purchased a 2" Orion Ultra Block filter, after reading/researching a multitude of filter reviews. I wanted a filter to enhance views of nebula's like Orion, Crab, etc through my 115mm apo refractor. Indeed the clarity & detail has been improved, although all targets are now green. Is this the norm?
Rob.

[barx1963 (Malcolm)]

The Ultra Block filter as I understand it is a narrowband filter designed to reduce the effects of light pollution. It allows ionised oxygen (presumably OIII primarily) and Hydrogen beta light through and blocks everything else(according to the Orion website). Our eyes detect OIII as green or greenish blue and Hydrogen beta as red, but our eyes are much less sensitive to red, so yes it will cause the nebula to look a bit green.
Looking on the Orion website there is not a lot of info but it does suggest this is for areas that experience significant light pollution, so Brisbane is probably good spot to try it!

Did it come with a transmission graph showing exactly what the transmission of light is like?

Malcolm

[Renato1 (Renato)]

Yes - if you mean aqua green.

If viewing from a light polluted backyard, you may want to try block stray light entering eye from the side. Your hand may do the trick, or a big piece of preferabbly black cloth thrown thrown over your head (though anything cam be improvised).
Regards,
Renato

[Don Pensack]

Nebulae emit light at a variety of wavelengths, but most are outside the visible band or they are in the deep red, where our eyes have very little sensitivity at night.
But they also tend to emit light at wavelengths we can see at night, and see quite well:
486nm Hydrogen Beta--blue
496nm Oxygen III (doubly ionized oxygen)--blue green
501nm Oxygen III--blue green.
So here is how the filter works:
Our eye sees fainter features and nebulae when there is more contrast with the backgound.
By filtering out almost all light other than the 3 wavelengths I mention, the sky gets 90% darker but the nebula is barely touched in terms of its brightness.
As a result, we see much more nebula and the background in the eyepiece goes to jet inky black.
Of course, stars are also dimmed a lot since they emit a lot of light at wavelengths now filtered out.

If you are seeing color, it is a sign your eyes have not yet dark adapted, because our scotopic, night, vision is color-blind and can only see shades of gray.
It is possible for a nebula (M42 comes to mind) to be so bright in a telescope that it starts turning your color vision back on. We refer to this as mesopic because a limited color response is possible.

Night vision is most sensitive at night around 500nm (blue green), while daytime (photopic) vision is most sensitive at 550nm (green).

[glend (Glen)]

Don your posts here are always informative - thanks.

[ZeroID (Brent)]

+1 what he said.
Thanks Don, interesting to say the least.