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Using a green filter to combat light pollution
Last night as the frost was setting in, I was out taking advantage of
some really good weather. Although a little chilly, the transparency was excellent (with the exception of some really high - thin and badly fragmented - cirrus/stratus type clouds) and seeing was 8 out of 10. No surface wind. And of course, no moon. As I studied Saturn, I wondered if I might get a little extra contrast using one of the color filters that I almost forgot I had (I hardly use them at all). Although I do think that the filters improve contrast of certain planetary features, e.g. color bands, I also think that the *overall* view is best without any filtration. JMO. But I swung the scope over at M42 (forgetting that the green filter was still in), and found that there was a little contrast improvement. Not as much as provided by the SkyGlow, but noticeable nonetheless. The green filter seemed to help out with the light pollution. Why it never dawned on me before - to try the green on the Orion nebulae - beats me. Makes perfect sense when I think about it. Anyway, thought I would throw it out there for anyone observing through light polluted skies whom do not own a light pollution filter to try. Errol pasnola.org |
#2
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Using a green filter to combat light pollution
On 19 Feb 2007 "Starboard" wrote:
As I studied Saturn, I wondered if I might get a little extra contrast using one of the color filters that I almost forgot I had (I hardly use them at all). Although I do think that the filters improve contrast of certain planetary features, e.g. color bands, I also think that the *overall* view is best without any filtration. JMO. I've usually observed Saturn without filters; but this was prior to getting a 12 inch scope. More light could make things a bit more interesting . . . I've used filters a bit more often on Jupiter -- and even *more* often (as in most of the time) on Mars. But I swung the scope over at M42 (forgetting that the green filter was still in), and found that there was a little contrast improvement. Not as much as provided by the SkyGlow, but noticeable nonetheless. The green filter seemed to help out with the light pollution. That's an interesting observation! My 12 inch is outside cooling right now; but my sky isn't expected to be very good tonight. Depending on the nature of the night I may try out a few filters under my relatively dark sky. Do you know which green filter you were using? Anyway, thought I would throw it out there for anyone observing through light polluted skies whom do not own a light pollution filter to try. Color filters might be interesting under a dark sky as well ;-) -- Bill Celestial Journeys http://cejour.blogspot.com |
#3
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Using a green filter to combat light pollution
I've usually observed Saturn without filters; but this was prior to
getting a 12 inch scope. More light could make things a bit more interesting . . . My thoughts exactly. With the 8", I could not afford to loose 25 - 30% light in a filter. I've used filters a bit more often on Jupiter -- and even *more* often (as in most of the time) on Mars. Regretfully, I did not own a scope when Mars made it's close pass in Aug 2003. I have yet to see a satisfying view of Mars. I have lots to look forward to in this hobby Depending on the nature of the night I may try out a few filters under my relatively dark sky. Do you know which green filter you were using? I was using a standard #58 (24% light transmission) Color filters might be interesting under a dark sky as well ;-) Bill If you look at the transmission spectra of a broadband light pollution filter, its bandpass seems to be approx mid green through mid blue (450 - 525nm) and most of red (650). The #58 green has a whole 24% light transmission, which is pretty wide. Apparently wide enough to pass the whole green band (of which the nebulae's emissions are part of) while blocking the extreme blues and yellows associated with city light pollution. Red is blocked as well. The broadband passes red, the UltraBlock does not. Let me know if you agree. Errol pasnola.org |
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Using a green filter to combat light pollution
On Feb 19, 9:42 pm, "Starboard" wrote:
Last night as the frost was setting in, I was out taking advantage of some really good weather. Although a little chilly, the transparency was excellent (with the exception of some really high - thin and badly fragmented - cirrus/stratus type clouds) and seeing was 8 out of 10. No surface wind. And of course, no moon. As I studied Saturn, I wondered if I might get a little extra contrast using one of the color filters that I almost forgot I had (I hardly use them at all). Although I do think that the filters improve contrast of certain planetary features, e.g. color bands, I also think that the *overall* view is best without any filtration. JMO. But I swung the scope over at M42 (forgetting that the green filter was still in), and found that there was a little contrast improvement. Not as much as provided by the SkyGlow, but noticeable nonetheless. The green filter seemed to help out with the light pollution. It might help a bit with sodium lights, but it is not so good against the bright green mercury line ar 546nm and at best the in band transmission of the dye based filter is about 50% so you are paying for the increased contrast with a huge loss of light. See for example: http://www.oreilly.com/catalog/astro...ter/hack58.pdf A wratten 44 cyan filter can also be used in a similar way. Although if you have the choice a dedicated light pollution filter using interference in thin films has much higher transmission in the passband and sharped cutoff. These days coloured glass filters are perhaps underused. Why it never dawned on me before - to try the green on the Orion nebulae - beats me. Makes perfect sense when I think about it. You would still be better off with a proper light pollution filter. But gel filters are certainly a lot cheaper. One thing that gel filters will do that an interference filter doesn't like is sit in front of a wide angle lens to allow long wide field exposures without excessive sky fogging. The interference filter passband shifts when the light passes through at oblique angles leading to very odd chormatic effects at the edge of frame. Regards, Martin Brown |
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Using a green filter to combat light pollution
It might help a bit with sodium lights, but it is not so good against the bright green mercury line ar 546nm and at best the in band transmission of the dye based filter is about 50% so you are paying for the increased contrast with a huge loss of light. See for example: Thanks for posting the book excerpt. I wasn't sure about the 546nm band at all; being green. Anyway, I just thought I would mention that I thought the green filter ($9) could possibly double as a poor man's LP filter. And yes, you are correct that it attenuates allot of light, as I too stated. I did state that the transmission was a "whole 24%" across the wide bandpass; average that is. Sarcasm would have been more pronounced had I said "a whopping 24%." ;-) I guess I could have been more clear about that and the trade off. These days coloured glass filters are perhaps underused. When I was through enjoying the splendid view of Saturn, as it was from the New Orleans area late Saturday night, I decided to try the el- cheapo green glass filter for the heck of it. I remember not liking them much. However, this time was different in a way. I was satisfied with the subtle increase in contrast of certain features at the loss of the overall view. But more so unexpected was to find that it was useful, however slight, on nebulae. I thought a nebulae would be barely visible in the ep. Far from it. Although a dimmer, it was still fairly bright and I detected a contrast increase. The view might be considered by some to be better. I thought it was nice. You would still be better off with a proper light pollution filter. True. I hope the post didn't come off sounding like I thought the green filter was a substitute for an LP filter. But gel filters are certainly a lot cheaper. That was the ultimate point of the post. Thanks again for bringing details to the tread. Errol pasnola.org |
#6
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Using a green filter to combat light pollution
On 19 Feb 2007 "Starboard" wrote:
I was using a standard #58 (24% light transmission) The #56 Green has 53% transmission. The #11 Yellow-Green has 62% transmission. I have both of these as well as the #58. I considered trying all three (and others) on M42/43. Unfortunately an apology of sorts is called for. I tried none of the above filters last night. I started the night with a somewhat meticulous observation and sketch of M79 at 155x. After a break I went to work on M42/43 at 61x. By the time I was finished clouds were becoming a problem. If you look at the transmission spectra of a broadband light pollution filter, its bandpass seems to be approx mid green through mid blue (450 - 525nm) and most of red (650). The #58 green has a whole 24% light transmission, which is pretty wide. Apparently wide enough to pass the whole green band (of which the nebulae's emissions are part of) while blocking the extreme blues and yellows associated with city light pollution. Red is blocked as well. The broadband passes red, the UltraBlock does not. Let me know if you agree. From what little I know the blocking and transmission properties of color filters are subtler than those of the more expensive interference (multi-layered) filters. The blocking tends to be not as thorough. The transmission tends to be not as high; and the transition in between tends to be not as sharp (not as steep). What matters when observing an object is the relative colors of neighboring regions and whether or not a given filter will darken one region noticeably more than it does a neighboring region. Any filter that makes a difference is worth using! -- Bill Celestial Journeys http://cejour.blogspot.com |
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