Re: [Utah-astronomy] How do colored planetary filters work?
Chuck wrote -
I'd have to think on that definition, and test it at the eyepiece... I don't think that they actually lighten similarly-colored tones-filters should darken them also; just not as much. Contrast is still increased.
Here's an easy demonstration of the similar-color-lightening effect. I did it this evening after work. It appears the similar-color-lightening and opposite-color-darkening effects are a basic photographic filter principle used by terresterial photographers. Maybe some of you camera buffs can chime in. Take your box of astronomy filters outside before sundown. Look for an neighborhood area that has lots of different colored flowers in it. Spring is good time of the year to do this. Fortunately, my neighbor has a green thumb and I had a good selection right outside my door. Look at a red flower through your dark red #25 filter. The flower will appear white while the green leaves will be several tones darker. Through a #15 Deep Yellow, a yellow flower looks white while the green leaves will appear darker. Look at a green leafy tree through your #58 Green filter - it will look lighter, not darker. Use a #80 Blue on a blue flower - it will appear light and white while the green will appear nearly black. My Purple filter makes green trees appear jet black, blue flowers disappear; yellows remain visible. When applied to planets, these terresterial effects probably are all dependent on getting enough light into the eye to trigger your color vision - hence the need for big light-bucket apeture. While at the U of U star party last night, I tried the one green #58 filter that they had on Saturn under poor seeing through their 14" Meade. A disk band just barely detectable in white light. The band was clearly enhanced with the green filter. The enhancement was subtle and not earth-shattering - but definitely was there.
Feel like a testing session this weekend, if I can clear it with the boss?
Good idea. Maybe John Lennon had something going with those rose colored glasses that he used to wear? Think I'll get me a pair. -:) Clear skies - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net
I don't doubt at all that colors similar to the filter *appear* lighter, BUT- It's a contrast illusion. The similarly-colored areas appear lighter in comparison to dissimilarly-colored areas, but there is actually less light getting to the eye from all areas, even the similarly-colored regions. The filter does not increase flux regardless of color; glass absorption actually reduces throughput of the filter color slightly. I verified the issue with my light meter. In no case does the reading increase when a filter is introduced. But the effect of contrast enhancement is nonetheless incredibly useful at the eyepiece; the brain's ability to interpret a contrast enhancement as a brightening works in our favor. The same thing happens with narrowband LPR filters. Overall, the entire image is darkened slightly, even the object glowing at the filters bandpass wavelength. But since the background (analagous to non-similarly colored regions with colored filters) darkens much more, the increased contrast is interpreted by the brain as a brightening of the object. On 5/10/07, Kurt Fisher <fisherka@csolutions.net> wrote:
Here's an easy demonstration of the similar-color-lightening effect.
participants (2)
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Chuck Hards -
Kurt Fisher