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Why Light Pollution Filters can be a bad idea.

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#1 Madratter

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Posted 26 January 2020 - 11:29 AM

Without getting heavy into the math, light pollution is a signal, not noise. Your target is also signal.

 

Lets say that you get 100 points of signal (ADU) from the light pollution. And let us say that the pure target signal looks like this on part of the imaging chip.

 

113, 110, 114, 117, 116

 

With light pollution it would be

 

203, 210, 214, 217, 216

 

Can we recover the signal from the target? Yeah, no problem, just subtract 100 from everything. We are back to

 

113, 110, 114, 117, 116

 

It is NOT the signal portion of light pollution that causes a problem. It is the noise portion. The noise goes as the SQRT of the signal. So if the signal from the light pollution is 100, then the average noise is 10.

 

Now where am I going with this?

 

Light pollution filters work by blocking part of the light spectrum where you expect to get some of your light pollution (low pressure sodium, mercury, etc.). But to do so, they are also blocking some of the signal from your target. And to make matters worse, much of the light pollution is not in the portion that gets blocked (incandescent lights, LED lighting, etc.)

 

It is often the case that the signal you lose from your target, by going with a light pollution filter, is more than the noise you incur by allowing the light pollution, (which you won't be totally blocking anyway).

 

And so by using a light pollution filter, you can actually decrease your S/N ratio.

 

Now there does come a point with faint enough targets in bad enough light pollution that the light pollution filter does actually help. But you won't know that point without doing some experimentation.

 

I have heard some people mention around here that they thought that using a light pollution filter actually hurt their images, and they were very possibly right. And these are people that aren't imaging in Bortle 3 and 4 zones with minimal light pollution.

 

Manufacturers of light pollution filters are unlikely to explain this. They are out to sell you filters.

 

Now that said, there are targets out there that will almost certainly benefit from them. These are the SAME targets that are used for narrow band imaging. Much of their signal is in very narrow bands and when they design the light pollution filters, they avoid notching out those bands. So for example, if you are taking a picture of the North American Nebula, or the Veil Nebula, you will want that light pollution filter on.

 

But if you are taking pictures of Galaxies or reflection nebulae like the Pleiades, they may or may not do you any good, and in fact could be harmful.

 

Moral of the story... Don't assume it is going to help unless you are on a narrow band target. Try it and see if it is actually making a positive difference.


Edited by Madratter, 26 January 2020 - 12:22 PM.

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#2 Becomart

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Posted 26 January 2020 - 12:15 PM

Great post. I think any on this forum who have been around a while know that LP filters can do as much harm as good. Whilst I’ve never used one, the images I’ve seen from them stack up to what you’re saying. The problem is, the recent influx of astro related you tube channels have made them a more popular choice recently as people tend to watch more than they will read. If the LP is so bad that you need a filter, I would think it’s better to go to a darker site. 


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#3 PilotAstronomy

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Posted 01 February 2020 - 09:55 PM

I used to use one w my OSC. But I stopped and just use my uv/ir lum filter now and processing is much easier.
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#4 jgraham

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Posted 01 February 2020 - 10:15 PM

I do all of my imaging from my Bortle 8 backyard. I spent a fair amount of time imaging with and without a light pollution filter. Without the light pollution filter I used either a custom white balance using an image of a relatively blank section of sky as the reference frame or the automatic color balance function in Nebulosity. With a light pollution filter I used either a Hutech IDAS LPS2 or an Orion Imaging Skyglow filter combined with the automatic color balance function in Nebulosity. Both approaches with and without a light pollution filter gave good results, though the images taken with a light pollution filter showed a tad better contrast. Soooo, for now I always use a light pollution filter. That will likely change as LED light pollution replaces vapor lamp light pollution. It is good to know that I have a Plan-B as our light pollution is likely only going to get worse.

Ugh.
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#5 WoodlandsAstronomer

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Posted 01 February 2020 - 11:30 PM

Some LP filters are great for use in the city when viewing or imaging emission nebulae. Optolong L-enhance dual band pass pretty much only passes H alpha, H beta, and OIII wavelengths which is pretty much signal for emission nebula. I give it a lot of credit with being able to take decent images in a severely light polluted large city! Ymmv
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#6 Peregrinatum

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Posted 02 February 2020 - 12:18 AM

Good read Madratter, very clearly explained waytogo.gif



#7 RogeZ

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Posted 02 February 2020 - 02:09 AM

Without getting heavy into the math, light pollution is a signal, not noise. Your target is also signal. This is true.

 

Lets say that you get 100 points of signal (ADU) from the light pollution. And let us say that the pure target signal looks like this on part of the imaging chip.

 

113, 110, 114, 117, 116

 

With light pollution it would be

 

203 (213), 210, 214, 217, 216. But this is not true; LP is a signal and therefore also a Poisson distribution. Therefore you don't get exactly 100 each time, you get approx 205, 220, 225, 207, 214 (etc)

 

Can we recover the signal from the target? Yeah, no problem, just subtract 100 from everything. We are back to: No you can't. You don't know that the real value of the light pollution is 100, therefore removing any ADU value introduces noise.

 

113, 110, 114, 117, 116 This is the idea of gradient removal, but this is not a noiseless process as you describe here. In other words because you don't know the value of the signal you can't perfectly remove it.

 

It is NOT the signal portion of light pollution that causes a problem. It is the noise portion. The noise goes as the SQRT of the signal. So if the signal from the light pollution is 100, then the average noise is 10.

 

Now where am I going with this?

 

Light pollution filters work by blocking part of the light spectrum where you expect to get some of your light pollution (low pressure sodium, mercury, etc.). But to do so, they are also blocking some of the signal from your target. And to make matters worse, much of the light pollution is not in the portion that gets blocked (incandescent lights, LED lighting, etc.) This is a stretch. LP filters are designed to eliminate peak emission lines of common LP sources like metal halides and sodium high vapor. Even if the target has any signal in this wavelengths, you just cant compete in photon count.

 

It is often the case that the signal you lose from your target, by going with a light pollution filter, is more than the noise you incur by allowing the light pollution, (which you won't be totally blocking anyway). I would like to see a study that proves this.

 

And so by using a light pollution filter, you can actually decrease your S/N ratio. Since I'm only interested in target S/N, blocking as much of the signal from LP is ideal. Getting signal from LP introduces noise, remember the sensor can't tell the difference.

 

Now there does come a point with faint enough targets in bad enough light pollution that the light pollution filter does actually help. But you won't know that point without doing some experimentation. 

 

I have heard some people mention around here that they thought that using a light pollution filter actually hurt their images, and they were very possibly right. And these are people that aren't imaging in Bortle 3 and 4 zones with minimal light pollution.

 

Manufacturers of light pollution filters are unlikely to explain this. They are out to sell you filters.

 

Now that said, there are targets out there that will almost certainly benefit from them. These are the SAME targets that are used for narrow band imaging. Much of their signal is in very narrow bands and when they design the light pollution filters, they avoid notching out those bands. So for example, if you are taking a picture of the North American Nebula, or the Veil Nebula, you will want that light pollution filter on.

 

But if you are taking pictures of Galaxies or reflection nebulae like the Pleiades, they may or may not do you any good, and in fact could be harmful. Except for the fact that blocking LP allows you to expose longer and capture more the target light by protecting your pixel wells, there is just so much you can capture before you saturate.

 

Moral of the story... Don't assume it is going to help unless you are on a narrow band target. Try it and see if it is actually making a positive difference. You have to remember you are competing against another signal source that emits maybe 30 or 300 times more signal than the target, its easy to lose the target in the noise of the LP signal.

I will let some of the experts on the topic chime in (where is Jon Rista when you need him flowerred.gif ) but I think you have a few incorrect concepts/ideas on the your discussion above. See my comments on top.


Edited by RogeZ, 02 February 2020 - 02:09 AM.


#8 John Tucker

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Posted 02 February 2020 - 02:58 AM

I'd be inclined to agree with RogeZ.  Unwanted signals can be "noise" that increases with the square root of exposure time or it can be real signal that increases linearly with exposure time just like the desired target signal. 

 

If I use a microphone to make a recording of my favorite song playing while an air horn continuously blasts in the background, it doesn't matter how many recordings I make and average, I won't average the air horn sounds out.  They aren't noise, they are actual signal.



#9 sharkmelley

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Posted 02 February 2020 - 04:45 AM

Is it signal? Or is it noise?

Cue another discussion on the dual nature of light pollution wink.gif 

 

Mark


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#10 Madratter

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Posted 02 February 2020 - 08:09 AM

I simplified by not including the noise component. But the bottom line is the signal component is rather easily estimated and fairly accurately subtracted. Of course it does leave the noise behind. But the point is that the signal portion of the light pollution is almost entirely subtracted out.


Edited by Madratter, 02 February 2020 - 12:06 PM.


#11 Eric Seavey

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Posted 02 February 2020 - 01:50 PM

Using a light pollution with an OSC camera can have certain uses.  I only use light pollution filters for emission nebulae, where my goal is to preserve the H-alpha, SII, OIII, and/or H-beta lines, while reducing the sky brightness, and reducing star brightness.

I never use light pollution filters for galaxies or reflection nebulae, instead I rely on gradient reduction software.


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#12 freestar8n

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Posted 02 February 2020 - 06:27 PM

Be careful taking square roots to find shot noise.  Only do it after converting ADU to electrons.  The shot noise in a 100 ADU signal could be 10 ADU, or it could be 1 ADU or even 5 ADU.  It needs to be converted to electrons before the square root.

 

As for light pollution filters - they only have a chance to work if the pollution is primarily in certain emission bands.  My light pollution is very broadband, so there is no way to differentiate it from the light from a galaxy.  Any time you cut down the signal you want just as much as the signal you don't want, you will lose SNR.

 

Frank


Edited by freestar8n, 03 February 2020 - 12:29 AM.

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#13 Madratter

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Posted 02 February 2020 - 06:33 PM

Good point Frank on converting to electrons. I did miss that. It doesn't alter the argument much, but does alter the details.

 

So to give an example. My camera has a gain of .38 (measured).

 

100 ADU = 38 electrons.

Square root of 38 electrons is 6.16 electrons.

In ADU that is 6.16/.38 = 16.22 ADU not the 10 if you just took the squareroot right away.

 

I stand rightly corrected on that detail.

 

Thanks!



#14 TelescopeGreg

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Posted 02 February 2020 - 06:59 PM

As for light pollution filters - they only have a chance to work if the pollution is primarily in certain emission bands.  My light pollution is very broadband, so there is no way to differentiate it from the light from a galaxy.  Any time you cut down the signal you want just as much as the signal you don't want, you will lose SNR.

 

Frank

So, how does one know what sort of LP one has?  Any way to measure it?
 



#15 freestar8n

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Posted 02 February 2020 - 07:45 PM

So, how does one know what sort of LP one has?  Any way to measure it?
 

In my case I already had an IDAS LP filter - and I just compared a galaxy signal vs. the sky background with and without the filter - and found they both got reduced about the same amount - which means there won't be an SNR win.

 

If you have any indication the lights in your area have been converted to LED then LP filters probably won't work.

 

But if you happen to know it is mostly mercury vapor and/or sodium then they have a chance to work.

 

I don't know a simple way to measure the spectrum of the sky glow without a spectrometer.

 

Frank



#16 RogeZ

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Posted 02 February 2020 - 08:19 PM

Good point Frank on converting to electrons. I did miss that. It doesn't alter the argument much, but does alter the details.

 

So to give an example. My camera has a gain of .38 (measured).

 

100 ADU = 38 electrons.

Square root of 38 electrons is 6.16 electrons.

In ADU that is 6.16/.38 = 16.22 ADU not the 10 if you just took the squareroot right away.

 

I stand rightly corrected on that detail.

 

Thanks!

Of course not Madratter... you were just doing unity gain flowerred.gif


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#17 Jon Rista

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Posted 02 February 2020 - 11:36 PM

I will let some of the experts on the topic chime in (where is Jon Rista when you need him flowerred.gif ) but I think you have a few incorrect concepts/ideas on the your discussion above. See my comments on top.

Actually, I agree with Madratter. I've actually been saying the same thing for years...that LP filters are not necessarily beneficial, and in fact can quite often be detrimental to SNR in the long run. Keep in mind...the BEST LP filters cut out HALF the visible spectrum. These are the multi-pass (or multi-notch, depending on how you look at it) filters. There are other LP filters that cut out nearly 2/3rds of the visible spectrum. That loss of light can indeed hurt SNR. If your SOLE purpose is to enhance the Ha and OIII signals, then you might as well cut out the vast majority of the spectrum and get a duo/triad type filter. 

 

In fact, most of the people who advocate LP filters are those just interested in the OIII and Ha emissions, and are generally unconcerned with other emissions. For at least a couple of years now, I've advocated only for the narrow bandpass multi-band LP filters that enhance this very kind of narrow band emission object, and otherwise don't recommend any LP filters, or recommend just using RGB filters with a mono camera. 

 

But LP filters are not necessarily a win, and IMO are often more detrimental than they are helpful. (In agreement with Madratter.) 


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#18 freestar8n

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Posted 03 February 2020 - 12:52 AM

Light pollution filters certainly can work - if the emission is in a narrow wavelength range.  The argument of the OP is that you can't take away light pollution without also taking away signal from the object you are imaging - but it is in fact possible if most of the emission is in a narrow band, so you can remove nearly all of it while only removing part of the object signal.

 

I think in the past they had a good chance or working in many backyards - as long as the main light was caused by mercury vapor or sodium.

 

So it is certainly possible for a filter to remove 90% of the light pollution while only removing 10% of a galaxy signal - and that would be a net boost in SNR.  The sky background and its corresponding shot noise would be greatly reduced while the object signal remains about the same.

 

But unfortunately LED's will probably take over and the pollution will tend toward continuum emission - and there won't be any way to selectively remove it as there is with narrowband emission.

 

Astro color filters have a gap in them intended to help with light pollution - but nowadays all it does is hurt when imaging things like galaxies.

 

Frank



#19 bobzeq25

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Posted 03 February 2020 - 01:01 AM

Adding to the chorus.

 

Broadband light pollution filters are relatively crude devices that work by ripping out parts of the spectrum.   The idea is that the parts ripped out are more likely to be light pollution, the parts passed through are more likely to be wanted signal from the target.

 

This can be true, if the target is an emission nebulae, and the light pollution is mostly from metal vapor lights.   Things go downhill from there.  By the time you get to galaxies and LED lights, they do more harm than good.

 

One reason to prefer gradient reduction is that it operates spatially, not by spectrum.  The computer looks for the characteristic pattern from light pollution (it increases as you go closer to the horizon), and tries to subtract that out.  It works surprisingly well, samples below.  Bortle 7, one shot color.  The final version added in data from a narrowband Ha filter, that's here.

 

https://www.astrobin.com/384117/G/

 

The bottom line is that pretty near all experienced imagers use gradient reduction.  Some add a broadband light pollution filter, some don't.  Many who do will only use one on emission nebulae.

 

Narrowband filters work, and work well.  On emission nebulae.  They need long exposures, so good tracking.  Since almost all of the spectrum is gone, color is largely or entirely artificial.

 

There's been a number of filters introduced lately that try to be in between  They often go by names like duo band, and are specifically designed for one shot color cameras.  A lot of people like those.  On emission nebulae.  <smile>.

 

ABE exampl before.jpg

 

ABE example after.jpg


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#20 freestar8n

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Posted 03 February 2020 - 01:59 AM

I am not sure how gradient removal got mixed up in a discussion of light pollution filters.  Gradient removal subtracts the light pollution signal but leaves its noise at full strength, whereas light pollution filters reduce both the sky background and its noise - and any gradient it might have.

 

You can have strong light pollution but no gradient at all if it is uniform across the image - and as part of processing you would just do an overall background removal of the light pollution signal.  But again its noise would still be present.

 

You always have some sky background signal present, even with 3nm narrowband imaging.  But you just subtract it away when you set the background value for the image as you stretch it.  And its noise, although small, will be there.

Frank



#21 Jon Rista

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Posted 03 February 2020 - 02:19 AM

Light pollution filters certainly can work - if the emission is in a narrow wavelength range.  The argument of the OP is that you can't take away light pollution without also taking away signal from the object you are imaging - but it is in fact possible if most of the emission is in a narrow band, so you can remove nearly all of it while only removing part of the object signal.

 

I think in the past they had a good chance or working in many backyards - as long as the main light was caused by mercury vapor or sodium.

 

So it is certainly possible for a filter to remove 90% of the light pollution while only removing 10% of a galaxy signal - and that would be a net boost in SNR.  The sky background and its corresponding shot noise would be greatly reduced while the object signal remains about the same.

 

But unfortunately LED's will probably take over and the pollution will tend toward continuum emission - and there won't be any way to selectively remove it as there is with narrowband emission.

 

Astro color filters have a gap in them intended to help with light pollution - but nowadays all it does is hurt when imaging things like galaxies.

 

Frank

I think it is highly unlikely for LP filters to only remove 10% of a galaxy signal. Most galaxies emit strongly across the full spectrum, so you are generally going to be looking at somewhere around a 40-50% reduction in signal from the galaxy. I mean, galaxies are dense collections of stars, after all, which emit strongly across the spectrum. Regardless of their color. 

 

==========

 

I recommend people be wary of LP filters. 

 

The only time LP filters might be useful is when emission nebula are the target. But even then...Bob's example with Horsehead nebula...that region is actually packed with very faint blue and blue-white reflection nebula that often gets nuked by LP filters. So even if emissions nebula are primary contributors to a scene, they are not always the only contributors. Not to mention Flame nebula, which is orangeish-yellow in color (although there are also a fair amount of blueish reflections over it as well, which are rarely revealed, never revealed if an LP filter is used), and it often turns more reddish or pinkish yellow with LP filters. 

 

A3prAnA.jpg

 

LP filters decimate signals. It's what they are designed to do, and they do not really discriminate whether a signal is actually a pollutant, or from an object. 


Edited by Jon Rista, 03 February 2020 - 02:20 AM.

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#22 freestar8n

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Posted 03 February 2020 - 02:53 AM

I think it is highly unlikely for LP filters to only remove 10% of a galaxy signal. Most galaxies emit strongly across the full spectrum, so you are generally going to be looking at somewhere around a 40-50% reduction in signal from the galaxy. I mean, galaxies are dense collections of stars, after all, which emit strongly across the spectrum. Regardless of their color.


Much of this discussion is moot because light pollution is increasingly wide spectrum and there isn't any way to improve wide band imaging with a filter.  (Of course, narrowband imaging of nebula emission lines still works great.)

 

But in the past, and in certain situations, the right filter could be very effective.  Here is a discussion of low pressure sodium light pollution:

 

http://www.flagstaff...ight-pollution/

 

"A nearly monochromatic yellow-orange"

 

The primary emission is in a very narrow band that could be notched out easily, with little impact on the rest of the spectrum.

 

So my point is that there is nothing fundamental preventing a filter from having huge benefit in boosting SNR across a wide spectrum - because certain streetlights have very narrow emission bands that can be blocked.  And it is all separate from gradient removal.

 

But in practice, nowadays and in the future - I doubt that anyone's backyard imaging can be helped by a light pollution filter.

 

Frank


Edited by freestar8n, 03 February 2020 - 02:53 AM.


#23 bobzeq25

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Posted 03 February 2020 - 09:59 AM

I am not sure how gradient removal got mixed up in a discussion of light pollution filters.  Gradient removal subtracts the light pollution signal but leaves its noise at full strength, whereas light pollution filters reduce both the sky background and its noise - and any gradient it might have.

Because, as a practical matter, GR is an excellent way to reduce the impact of light pollution for those of us who image from light polluted skies.

 

Light pollution is always a gradient, because of the atmosphere.  Physics.  How well that gradient is detected and removed is variable according to the tools used and the skill in how they're wielded.

 

As you point out, GR does not remove the random component of light pollution, but that is much smaller than the fixed component.  The inefficiency of broadband light pollution filters, in many (most?) cases, is worse.  As a practical matter, removing more of the most significant component (GR) can be more effective than removing less of both components (filter).

 

The thing that bothers me the most is beginners who think a broadband light pollution filter is _the_ answer (the terminology aids that misconception) to the issue, while having no knowledge of GR and no intent to do it.  Using both is not silly (some imagers do that), but omitting GR is a mistake.


Edited by bobzeq25, 03 February 2020 - 10:01 AM.


#24 AKHalea

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Posted 03 February 2020 - 11:01 AM

Much of this discussion is moot because light pollution is increasingly wide spectrum and there isn't any way to improve wide band imaging with a filter.  (Of course, narrowband imaging of nebula emission lines still works great.)

 

But in the past, and in certain situations, the right filter could be very effective.  Here is a discussion of low pressure sodium light pollution:

 

http://www.flagstaff...ight-pollution/

 

"A nearly monochromatic yellow-orange"

 

The primary emission is in a very narrow band that could be notched out easily, with little impact on the rest of the spectrum.

 

So my point is that there is nothing fundamental preventing a filter from having huge benefit in boosting SNR across a wide spectrum - because certain streetlights have very narrow emission bands that can be blocked.  And it is all separate from gradient removal.

 

But in practice, nowadays and in the future - I doubt that anyone's backyard imaging can be helped by a light pollution filter.

 

Frank

+1

 

From my experience, the LP filters have worked for me for galaxies primarily because the LP in my backyard (Bortle 7/8 zone) is still vapor lamp emission lines, not broadband LEDs. It may change in future as we use more LEDs, but so far, it has worked. So, despite strong disagreements from other experts, I fully agree with Frank that LP filters work to improve SNR when LP is from the vapor lamps. Cheers ..... Anil


Edited by AKHalea, 03 February 2020 - 11:06 AM.


#25 Jon Rista

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Posted 03 February 2020 - 05:12 PM

Much of this discussion is moot because light pollution is increasingly wide spectrum and there isn't any way to improve wide band imaging with a filter.  (Of course, narrowband imaging of nebula emission lines still works great.)

 

But in the past, and in certain situations, the right filter could be very effective.  Here is a discussion of low pressure sodium light pollution:

 

http://www.flagstaff...ight-pollution/

 

"A nearly monochromatic yellow-orange"

 

The primary emission is in a very narrow band that could be notched out easily, with little impact on the rest of the spectrum.

 

So my point is that there is nothing fundamental preventing a filter from having huge benefit in boosting SNR across a wide spectrum - because certain streetlights have very narrow emission bands that can be blocked.  And it is all separate from gradient removal.

 

But in practice, nowadays and in the future - I doubt that anyone's backyard imaging can be helped by a light pollution filter.

 

Frank

 

 

 

+1

 

From my experience, the LP filters have worked for me for galaxies primarily because the LP in my backyard (Bortle 7/8 zone) is still vapor lamp emission lines, not broadband LEDs. It may change in future as we use more LEDs, but so far, it has worked. So, despite strong disagreements from other experts, I fully agree with Frank that LP filters work to improve SNR when LP is from the vapor lamps. Cheers ..... Anil

What Frank is saying is, an LP filter COULD be made that ONLY filtered out the yellow-orange low pressure sodium vapor emissions, and that IF such a filter were made, then maybe only about 10% or so of the overall spectrum would need to be blocked. Thing is, such a filter does not exist, not to my knowledge at least. Even if it did exist, there is so much LP outside the LPSV emission lines nowadays that such a filter would largely be useless...one might as well simply image without filters at all.

 

Which, in fact, is my recommendation to you. Try imaging without the filters, and see just how much the LP filter is affecting the signal. I can simply guess that you will notice a marked change in the color of the objects, but it is also likely that you won't really notice much of a change in overall SNR of the object with or without the filter. 




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