P^2 - Paul
Filters vs. skyglow (update- CORRECTED!)
And the winner is... Skyglow.
From my back yard this evening: the highest dewpoint night of the year so far, so lots of skyglow lighting up the place. An ideal night to see how good various filters are at nuking the pestilence.
These are exposures through three different filters (Hoya FL-Day, Hoya Intensifier, B+W 090 light red, plus no filter). All exposures are ISO200, 5 minutes, and aperture was adjusted to compensate for the filter factor: f/5.6 for no filter, f/2.8 for the red, and f/4 for the FL-D and intensifier. White balance was fixed to daylight for all four shots.
Most of the skyglow here is high pressure sodium vapour street lamps, plus some incandescent from the nearby houses. The yellow-red cast is obvious in the (top) unfiltered exposure.
Surprisingly, the didymium intensifier filter, with its big spectral notch in the yellow does basically nada. Clearly the sodium is redder than the light this filter blocks. correctedThe intensifier DOES block much of the sodium ickyness.
The FL-D filter, who's intent is to convert "daylight" fluorescent lamp spectrum to something actually approaching daylight (by filtering out the green light from the mercury), seems to do a better job than the Intensifier filter. Doesn't do a good job blocking the redness.
Caveat: there's always the possibility I mixed up the filters in the gloom. <Mea culpa I did - sorry!)
The red filter (bottom) shows there is mostly red in that spectrum.
So why use filters at all? There are some perfectly credible filters right on that fancy CCD in the camera! The right side shows the red, green, blue channels from the no-filter exposure (top). The red channel obviously has most of the contamination. The green is better. But there is still some in the blue. The CCD filters are pretty broadband. If the contaminating light is very narrowband (like the mercury green line, or the low-pressure sodium orange line), then dedicated filters can nuke just those colours of light and leave most of the rest of the spectrum unscathed. I guess that's what Orion and Baader must do for their skyglow filters.
Filters vs. skyglow (update- CORRECTED!)
And the winner is... Skyglow.
From my back yard this evening: the highest dewpoint night of the year so far, so lots of skyglow lighting up the place. An ideal night to see how good various filters are at nuking the pestilence.
These are exposures through three different filters (Hoya FL-Day, Hoya Intensifier, B+W 090 light red, plus no filter). All exposures are ISO200, 5 minutes, and aperture was adjusted to compensate for the filter factor: f/5.6 for no filter, f/2.8 for the red, and f/4 for the FL-D and intensifier. White balance was fixed to daylight for all four shots.
Most of the skyglow here is high pressure sodium vapour street lamps, plus some incandescent from the nearby houses. The yellow-red cast is obvious in the (top) unfiltered exposure.
Surprisingly, the didymium intensifier filter, with its big spectral notch in the yellow does basically nada. Clearly the sodium is redder than the light this filter blocks. correctedThe intensifier DOES block much of the sodium ickyness.
The FL-D filter, who's intent is to convert "daylight" fluorescent lamp spectrum to something actually approaching daylight (by filtering out the green light from the mercury), seems to do a better job than the Intensifier filter. Doesn't do a good job blocking the redness.
Caveat: there's always the possibility I mixed up the filters in the gloom. <Mea culpa I did - sorry!)
The red filter (bottom) shows there is mostly red in that spectrum.
So why use filters at all? There are some perfectly credible filters right on that fancy CCD in the camera! The right side shows the red, green, blue channels from the no-filter exposure (top). The red channel obviously has most of the contamination. The green is better. But there is still some in the blue. The CCD filters are pretty broadband. If the contaminating light is very narrowband (like the mercury green line, or the low-pressure sodium orange line), then dedicated filters can nuke just those colours of light and leave most of the rest of the spectrum unscathed. I guess that's what Orion and Baader must do for their skyglow filters.