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View Full Version : Nebular filters vs. laser pointers



Tog
2007-Dec-22, 04:30 PM
Feel free to move to another section.

In answering a question on using a laser to blind a camera operator, and whether that light would shoe up on film I had an anti-intuitive moment.

I was going from the view that a laser would only show up if the beam were of a wavelength that allowed it to pass through the glass and be recorded by the film. A perfect example (I thought) would be to shine a red laser pointer through an O3 filter. My laser should be about 650 to 670 nm (it's a 3 dollar one built into a pen). The filter show band pass at 496 nm (90%), 501 nm (94%) and 486 nm (0.8%). The laser goes through it with no detectable loss of intensity.

Why?

RickJ
2007-Dec-22, 09:05 PM
Many OIII filters also pass H-alpha as well though they rarely advertise this fact. H-alpha is 6562.8 or 656 nm, about the same as your laser pen. Though even in those that don't, a light as bright as a laser pen will surely get through and still be bright. My ancient OIII from Lumicon (one of the very first they produced) passes no visible H-alpha. If I look at the sun through my Daystar H-alpha filter using it I see no hint of the sun. Yet a laser pen goes through very easily. It is reduced but unless you are looking for it you'd hardly notice. Then I tried a year newer one that does pass H-alpha but yet works better visually as an OIII than the original and the difference was unnoticable even in a side by side comparison. The light is way too bright for any measurement by my CCD, even with the blue filter and a .001" exposure.

In any case this is normal. I'd expect a bit more supression in one made for narrow band imaging but still the laser would get through and be very bright I'd expect. Not having one I can't test it.

Also my Lumicon H-Beta passes H-alpha very well

Rick

Tog
2007-Dec-27, 08:30 AM
Great. Thanks for the info.