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Are Light Pollution Filters Obsolete

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#1 D.T.

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Posted 16 June 2020 - 06:43 PM

I have a backyard location for my telescope.  I live in a suburb of Portland Oregon.  My location has typical suburban light pollution.

 

I have been looking at a light pollution filter for the SpaceCat 51.  It seems that all of the light pollution filters, on the market, were designed assuming that most skyglow is from Sodium or Mercury streetlights.  My city is rapidly changing it's streetlights to white LED.  Since these LEDs emit broadband, are all of the filters currently marketed as light pollution rapidly becoming useless? Don't waste the money?

 

Opinions?

 

Dana

 



#2 havasman

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Posted 16 June 2020 - 07:48 PM

For visual the answer is yes indeed. But with that scope you're likely doing AP and IDK about that.



#3 N3p

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Posted 16 June 2020 - 07:52 PM

If you drive to a decent spot where you can see the milky way easily, the filters will be useful. A good UHC/NPB and or OIII, they still work if the region is good. I tried my NPB 2" last Sunday in a yellow region.. it worked very very well on the NA nebula and the Veil nebula.

 

In my opinion, a good UHC/NPB or OIII is not a waste of money. For the cheaper "light pollution filters" .. I don't know, I was told to use quality filters.

 

As for the LEDS street lamps.. all the new lights I see are pretty, Yellow, they are not pure white no more. 


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#4 D.T.

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Posted 16 June 2020 - 08:10 PM

The question is posed as pertinent to my backyard only.  Also, I am asking WRT the Williams SpaceCat.  The camera that I use on the SpaceCat is a ZWO ASI1600mm (non-cooled).  I was wondering if I was to put an Astronomik CLS CCD Light Pollution Filter in front of the camera would it actually reduce light pollution and increase contrast.  I was thinking the answer would be: "well it would have 5 years ago.  But today forgetaboutit."

 

That is what I have in mind currently.  But I am also interested in the broader question of "are light pollution filters dead?".



#5 Arcamigo

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Posted 16 June 2020 - 08:18 PM

I have the 2" Orion SkyGlow Astrophotography Filter. A good test is to hold up the filter in front of various lights to see how well it attenuates it. The attenuation in front of a white LED flashlight is noticeable, but it isn't significant.

 

However, Portland, Hillsboro, McMinnville and other cities around here still have a yellowish dome above them, so this filter still has some benefits around here today. I've seen a slight improvement with fainter DSOs like the California Nebula.



#6 D.T.

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Posted 16 June 2020 - 09:00 PM

I have the 2" Orion SkyGlow Astrophotography Filter. A good test is to hold up the filter in front of various lights to see how well it attenuates it. The attenuation in front of a white LED flashlight is noticeable, but it isn't significant.

 

However, Portland, Hillsboro, McMinnville and other cities around here still have a yellowish dome above them, so this filter still has some benefits around here today. I've seen a slight improvement with fainter DSOs like the California Nebula.

Well, since I don't have the filter, I am only thinking of buying it, I can't exactly hold it up in front of various lights.

 

And even if it does have some limited utility today, if it's going to be useless in 2 or 3 years, I won't buy it.



#7 ButterFly

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Posted 16 June 2020 - 09:26 PM

Get the narrowest bands possible.  I used both a Lumicon UHC and a Baader UHC-S in NYC after the LED switch.  The Baader UHC-S has very broad bands, similar to Lumicons's DeepSky.  The Lumicon worked well, but the Baader was too broad for much contrast boost.  In darker skies, the Baader gives brighter images becuase of the broader band.

 

For visual only on the WO, don't bother - it's a tiny aperture.  The 8" will still do okay.  For AP, get the narrowest bands you can afford.  Nothing can bring back broadband sources llike galaxies or refelction nebulae.



#8 sg6

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Posted 17 June 2020 - 03:57 AM

The question is posed as pertinent to my backyard only.  Also, I am asking WRT the Williams SpaceCat.  The camera that I use on the SpaceCat is a ZWO ASI1600mm (non-cooled).  I was wondering if I was to put an Astronomik CLS CCD Light Pollution Filter in front of the camera would it actually reduce light pollution and increase contrast.  I was thinking the answer would be: "well it would have 5 years ago.  But today forgetaboutit."

 

That is what I have in mind currently.  But I am also interested in the broader question of "are light pollution filters dead?".

As you have a mono camera and so will have filters, usually narrow or very narrow then those will in efect act as a LP filter when used. Well until some evil lighting engineer inserts LED's that are at OIII and Hb wavelength. Then you are as they say "Stuffed!"

 

For the L aspect I wouldn't like to say, but for mono imaging at the narrow wave bands used then the filters themselves will block the light pollution aspect.

 

But in general the old LP filters were for Na and Hg and are somewhat irrelevant. Only some as those lights will still be in use and will contribute to the general LP.

 

Many CLS filters seem to rather simply just cut the whole middle section of green/yellow out. So color balance is thrown. It is a somewhat heavy handed approach to being an LP filter.

 

For a color camera I can see that the future will be one of the tri-band filter which would pass just the narrow sections of R, G, B to the camera. So allowing color imageing but blocking the non-defined wavelengths out whish would be the LP aspect.

 

As said worst aspect is someone inserts OIII or Hb wavelength emitting LED's in to a lamp array.


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#9 D.T.

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Posted 17 June 2020 - 11:22 AM

"Many CLS filters seem to rather simply just cut the whole middle section of green/yellow out. So color balance is thrown. It is a somewhat heavy handed approach to being an LP filter."

 

Ah, but does it work?  When I consider what to do from my suburban light polluted backyard, I fully understand that I have to give up something.  I might be happy with a tradeoff whereby I lose color balance but gain better contrast between stars and galaxy's against the background skyglow.  Does the CLS filter do that?



#10 Arcamigo

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Posted 17 June 2020 - 01:05 PM

The problem in the Portland area is that the UGB keeps expanding, resulting in development and lighting growing like a cancer. Filters might work today, but in five to ten years, nothing might.

 

The filter I mentioned is a comb filter, transmitting the Hb (486.1nm), OIII (500.7nm), Ha (656.3nm), NII (658.4nm), and SII (672.4nm) wavelengths, while removing UV (<410nm), 440nm, 550nm, 590nm (LPS), and 635nm. The spectral output of LED street lighting has a significant blue LED component at 440nm, that is blocked by this filter. LED lighting also has energy centered at about 550nm, which is also blocked by the above filter, and while not a peak like the blue LED, is spread with energy comparable to the blue, towards 500nm and 650nm, fortunately with a null near OIII.

 

So, given all of this, the most important question is will it work for you? All, I can say is maybe and it depends on what you want to see. I suspect, based on my experience, that it will help, and will continue to help even with LED street lighting, but it really depends on the overall luminance in your area.

 

I'm not promoting the Orion filter. There might be better ones out there. I would buy another one with similar comb characteristics if I knew of one.



#11 ButterFly

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Posted 17 June 2020 - 03:31 PM

I might be happy with a tradeoff whereby I lose color balance but gain better contrast between stars and galaxy's against the background skyglow.  Does the CLS filter do that?

No.  Stars and galaxies are broadband emitters.  Street lamp LEDs are as well.  Any attempt to reduce the broadband emissions of LEDs will also reduce the broadband emissions of stars and galaxies.



#12 barbarosa

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Posted 17 June 2020 - 04:15 PM

I live in the burbs. Most of the street lighting, nearly all, is now "white" LED.  You would be hard pressed to find the Milky Way. I no longer use an eyepiece and most nights are EAA nights. I have some filters that get at least of some use with a color camera.

 

These filters are-

  1. unbranded "Moon and Sky Glow" filter, possibly GSO or Zhummel and probably for visual use.
  2. IR/UV blocking filter. 
  3. ZWO or Optolong IR long pass filter used for the moon, Venus and Mercury to reduce the effect of turbulence.
  4. Celestron filter for the RASA which seems to work with both galaxies and nebulae, and of course requires some red boost  to get color balance. I've not yet tried the RASA with any other filter.
  5. Optolong L Enhance I like it for some targets but again color balance takes a hit.

I see too that some people are quite pleased with using one of the IDAS filters or one of the triple filters.

 

No personal experience with filters at a dark site.


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