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Science Forum Index » Astro - Amateur Forum » Using a green filter to combat light pollution
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| Starboard |
 Posted: Mon Feb 19, 2007 5:42 pm |
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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 |
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| W. H. Greer |
| Posted: Mon Feb 19, 2007 9:19 pm |
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On 19 Feb 2007 "Starboard" <errol_winn@yahoo.com> wrote:
Quote: 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.
Quote: 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?
Quote: 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 |
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| Starboard |
| Posted: Mon Feb 19, 2007 10:01 pm |
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Quote: 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.
Quote: 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
Quote: 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)
Quote: 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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| Martin Brown |
| Posted: Tue Feb 20, 2007 5:07 am |
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On Feb 19, 9:42 pm, "Starboard" <errol_w...@yahoo.com> wrote:
Quote: 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/astronomyhks/chapter/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.
Quote:
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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| Starboard |
| Posted: Tue Feb 20, 2007 8:51 am |
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Quote: 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.
Quote: 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.
Quote: 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.
Quote: 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 |
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| W. H. Greer |
| Posted: Tue Feb 20, 2007 1:06 pm |
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On 19 Feb 2007 "Starboard" <errol_winn@yahoo.com> wrote:
Quote: 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.
Quote: 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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