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Does LP filter work for me? The answer is:

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#1 freestar8n

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Posted 15 April 2019 - 04:03 AM

I have a Hutech IDAS LPS-P2 light pollution filter and I was never sure if it worked for my location or not.  So I did some tests and made some measurements.

 

Here is one test - based on 3x5m exposures with the LP filter and with a C filter on EdgeHD11 f/7 with ASI1600.

 

The images were calibrated and averaged with no rejection:

 

ngc2442_c.jpg


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#2 freestar8n

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Posted 15 April 2019 - 04:04 AM

Here is the other:

 

ngc2442_lp.jpg

 

Frank


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#3 freestar8n

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Posted 15 April 2019 - 04:08 AM

I guess if you look at the file names you can tell which is which.  I just did a stretch in PS as I normally would - to bring out the faint details until noise shows.  There may be more sophisticated ways to do the comparison - but I think the difference shows pretty clearly.  To me the first one is better - and that is the one with the C filter.

 

So - for me if I want to go deep with a luminance image of a faint field of galaxies - I'm better off just using a C filter because the reduction of light pollution - which does happen - is not enough to compensate for the loss of galaxy signal - which also happens.

 

Frank


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#4 freestar8n

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Posted 15 April 2019 - 04:11 AM

I made some raw ADU measurements of the images with Maxim - to get a since of how bright sections of the galaxy are above the background - and I also measured the sigma of the background.  You can do this directly from the calibrated image and you don't even need to convert to electrons.  It will give a direct idea of the SNR.

 

For the C filter the nucleus is about 13700 above background and the sigma of the background is 210.

 

For the LP, the nucleus is about 5950 above background and the sigma of the background is 155.

 

So - the background noise does go down - but not as much as the actual galaxy signal.

 

And a part of the galaxy arm is 820 for C and 390 for LP.  Again way too much loss of signal.

 

Frank


Edited by freestar8n, 15 April 2019 - 04:12 AM.


#5 freestar8n

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Posted 15 April 2019 - 04:23 AM

I also used my Sloan filters to measure the sky background across each filter - which is nice with Sloan because there are no gaps.

 

The results are:

 

C:   3.47 e/s

i' :   0.19

r':    1.64

g':   1.48

LP:  1.66

Ha:  0.0188

 

As a check, the sum of i' r' g' is 3.31 - which is a bit less than C.  And that's what I would expect since the three filters span from deep blue to the IR - which is most but not all of what C captures.

 

At the same time, the LP filter cuts the sky background about in half - which means the noise only goes down by 30% - because of the square root involved.  If the light pollution is basically whitish like the galaxy, then the signal itself is cut down by half - and there is a net loss of SNR.

 

Another interesting thing is how small the Ha sky background is with 3nm.

 

The r' filter spans 140nm and includes the Ha line - so a simple guess would be that the Ha sky background would be 3/140 * 1.64 = 0.035 e/s.  But instead the real value is about half that.  That just means that most of the light pollution is in a different part of the spectrum - possibly more toward the green from the Ha line.

 

That's why it's hard to tell from a sky magnitude value how much background there is in a narrowband filter.

 

Anyway - I wasn't really sure until I made these measurements that my LP filter just isn't a win.  Maybe another would be better.  I think my pollution is a mix of mercury and high pressure sodium but I'm not sure.

 

I think they will soon be switching the neighborhood all to white LED - at which point I think all light pollution filters will stop working - and a study like this won't even be needed.

 

Frank


Edited by freestar8n, 15 April 2019 - 04:25 AM.

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#6 james7ca

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Posted 15 April 2019 - 07:05 AM

Frank, thanks for doing this study. I've never tried to measure or even make a direct comparison between images using a luminance or light-pollution filter, but I gave up on using so-called LP filters several years ago when imaging broadband targets (galaxies or simple star fields and clusters, since it seemed that the filters didn't provide any benefits under my red/orange zone skies). That said, I suspect that they still work on emission nebula since those objects are bright in wavelengths that aren't being blocked by the filter. So, the true object signal remains largely unaffected but the sky background is still darkened.



#7 freestar8n

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Posted 15 April 2019 - 08:20 AM

Hi James. For emission nebula you can use filters tuned to the exact emission lines - but the lp filter I have is trying to be tuned to block specific lp lines. And I guess my lp is just too broadband. It might work well even for galaxies if I mainly had low pressure sodium and mercury lines. But I guess my lp is broader.

Well I thought it had a chance to work for me but I guess not. It may work for others though - and what I show above is one way to find out.

Frank

Edited by freestar8n, 15 April 2019 - 08:20 AM.


#8 OldManSky

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Posted 15 April 2019 - 08:36 AM

Please pardon my ignorance, but...what does a "C" filter filter?


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#9 james7ca

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Posted 15 April 2019 - 08:39 AM

Yes, but some LP filters are tuned to pass the common emission lines so on an emission nebula they should still work for luminance or even when using a one-shot-color camera. However, I don't think I have used a LP filter for several years running. 



#10 freestar8n

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Posted 15 April 2019 - 08:46 AM

Yes here I’m focused on galaxies like in my example. Continuum sources. The filter I’m using is trying to help with galaxies but the fact that the light pollution is also continuum means it won’t work.

Other filters might help for me with emission nebulae and my filter may help others with narrow lp lines. So I’m not saying such filters will never work.

In my example it does reduce the sky background a lot. But it also cuts out too much of the object.

Frank

#11 jhayes_tucson

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Posted 15 April 2019 - 08:51 AM

Frank,

That's a good measurement.  Years ago I did something similar but I've lost the results.  I concluded pretty much the same thing and I haven't touched that LPS filter ever since.

 

John



#12 Peter in Reno

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Posted 15 April 2019 - 09:00 AM

I never liked LP filter because it messes up color balancing (i.e. too many stars were too bluish) so I rely on processing to remove or reduce gradients with PixInsight and works well.

 

Peter



#13 bobzeq25

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Posted 15 April 2019 - 09:36 AM

I never liked LP filter because it messes up color balancing (i.e. too many stars were too bluish) so I rely on processing to remove or reduce gradients with PixInsight and works well.

 

Peter

+1.  I've gone round and round with Jerry Lodriguss on this one.  A CLS gathers dust on my shelf.  Red Zone, Bortle 7, mag per arc sec squared low 18s.

 

I agree with Frank that it depends on the spectrum of the target and of the light pollution.  That's simple.  Also agree that for emission nebulae, where they work best, narrowband is a preferable solution.  The new dual band, tri band, quad band LP filters are an intermediate approach, narrower than older broadband LP filters, much wider than narrowband.

 

The killer is beginners who are not (hopefully, just not yet) using gradient reduction, and think that a broadband light pollution filter will solve their problem.  After all, it's a light pollution filter.  Sigh.


Edited by bobzeq25, 15 April 2019 - 09:44 AM.

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#14 fetoma

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Posted 15 April 2019 - 09:52 AM

Please pardon my ignorance, but...what does a "C" filter filter?

I believe he means a "clear" or "luminance" filter.



#15 jpbutler

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Posted 15 April 2019 - 10:10 AM

I live in a Bortle 8 to 9 zone mag/" of 17.5 to 18 (new moon) and have been imaging in the last year with the asi071 on an edgehd 11 when it isn't overcast. 

Just started using the idas lps d1 filter and as silly as it sounds, the best thing about it is the fact that now I can do 30 second subs instead of 10 second subs.

I shot 750 30 second subs for my last galaxy.

At 10sec per sub it would just be too much data to deal with.

 

John


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#16 44maurer

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Posted 15 April 2019 - 10:15 AM

This may be a true statement from your location. I only mention this, so others reading this will take that into account.
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#17 Jon Rista

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Posted 15 April 2019 - 10:21 AM

Great test, Frank. The differences are pretty clear in your first two images. 

 

I dropped LP filters a few years ago, after finding they often hurt SNR in as many cases as they may have helped. Even with nebular objects, while yes emission line nebula only emit light at certain narrow bands, there are usually other broadband objects around. Stars for one, reflection nebula, dust reemitting light they absorb, etc. If you have particularly bad LP (red/white zone) a lot of these other broadband objects mixed in with emission nebula are often difficult if not impossible to see unless you have significant integration. If you are in an orange or yellow zone and certainly anything darker, then these other broadband objects can add a lot of interesting detail to emission nebula images.

 

LP filters have their place, but they are not always guaranteed to improve results. 


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#18 OldManSky

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Posted 15 April 2019 - 10:37 AM

I believe he means a "clear" or "luminance" filter.

Ah, OK.  Thanks.



#19 bobzeq25

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Posted 15 April 2019 - 12:24 PM

Great test, Frank. The differences are pretty clear in your first two images. 

 

I dropped LP filters a few years ago, after finding they often hurt SNR in as many cases as they may have helped. Even with nebular objects, while yes emission line nebula only emit light at certain narrow bands, there are usually other broadband objects around. Stars for one, reflection nebula, dust reemitting light they absorb, etc. If you have particularly bad LP (red/white zone) a lot of these other broadband objects mixed in with emission nebula are often difficult if not impossible to see unless you have significant integration. If you are in an orange or yellow zone and certainly anything darker, then these other broadband objects can add a lot of interesting detail to emission nebula images.

 

LP filters have their place, but they are not always guaranteed to improve results. 

Pretty much agree.  The strategy I often use is to mix Ha with RGB.  The processing is often difficult, but it can lead to images in light polluted skies that show amazingly dim features.  Example (not that it's a great image), which even uses OSC for the RGB.

 

https://www.astrobin...4117/G/?nc=user

 

The concept of "noise" here is interesting.  I think it's most illuminating to consider the light pollution gradient as "noise" (after all, it interferes with your image), just operating on a different (large) scale.


Edited by bobzeq25, 15 April 2019 - 12:27 PM.


#20 Jon Rista

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Posted 15 April 2019 - 12:42 PM

Pretty much agree.  The strategy I often use is to mix Ha with RGB.  The processing is often difficult, but it can lead to images in light polluted skies that show amazingly dim features.  Example (not that it's a great image), which even uses OSC for the RGB.

 

https://www.astrobin...4117/G/?nc=user

 

The concept of "noise" here is interesting.  I think it's most illuminating to consider the light pollution gradient as "noise" (after all, it interferes with your image), just operating on a different (large) scale.

Don't lose sight of the fact that LP adds both a spatially varying offset (the "gradient") as well as additional shot noise. The additional shot noise remains after the gradient is removed, and the only way to deal with that is either to integrate more data (and since noise grows at half the rate of the signal, you need to quadruple integration to reduce noise by half...so the amount of time required to deal with increased shot noise from LP can grow significantly), or to apply more aggressive NR. 

 

One of the reasons why eliminating an LP filter may be better than using one is doing so can allow more signal through (and depending on the object, potentially twice as much or more signal, depending on where the bulk of the objects signal is emitted), meaning the signal grows even faster. That can help overcome shot noise from LP. 

 

Gradient reduction is an essential step in processing any light polluted image, with or without an LP filter...but it only takes care of one part of the degrading aspects of LP.


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#21 Lead_Weight

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Posted 15 April 2019 - 12:52 PM

All my light pollution filters have made their way over to my guide scopes. I stopped using them on my main imagers, but they can help with picking out stars in heavy LP areas. Just a little bit extra contrast that isn't always there. I'm not sure it makes a huge difference, but those filters were just sitting there taking up space in my junk box. Haha.


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#22 Jon Rista

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Posted 15 April 2019 - 01:14 PM

All my light pollution filters have made their way over to my guide scopes. I stopped using them on my main imagers, but they can help with picking out stars in heavy LP areas. Just a little bit extra contrast that isn't always there. I'm not sure it makes a huge difference, but those filters were just sitting there taking up space in my junk box. Haha.

I did the same thing for a while. I had problems with my guide cameras and bloated stars, or with very large IR halos around the stars. I ended up just sticking a normal L filter on them in the end, to just cut off the NIR and eliminate the halos and deep-red bloat. 


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#23 bobzeq25

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Posted 15 April 2019 - 01:18 PM

Don't lose sight of the fact that LP adds both a spatially varying offset (the "gradient") as well as additional shot noise. The additional shot noise remains after the gradient is removed, and the only way to deal with that is either to integrate more data (and since noise grows at half the rate of the signal, you need to quadruple integration to reduce noise by half...so the amount of time required to deal with increased shot noise from LP can grow significantly), or to apply more aggressive NR. 

 

One of the reasons why eliminating an LP filter may be better than using one is doing so can allow more signal through (and depending on the object, potentially twice as much or more signal, depending on where the bulk of the objects signal is emitted), meaning the signal grows even faster. That can help overcome shot noise from LP. 

 

Gradient reduction is an essential step in processing any light polluted image, with or without an LP filter...but it only takes care of one part of the degrading aspects of LP.

Agreed.  But I'll note the shot noise is _approximately_ the square root of the gradient.  Good bar graph in The Astrophotography Manual that illustrates the relative amounts.  And, as you say, noise reduction in processing also can reduce the effects of the light pollution shot noise.

 

What infuriates me about Jerry is that he often pretends the small scale shot noise is the only noise worthy of consideration.  Rhetoric that masks the reality.


Edited by bobzeq25, 15 April 2019 - 01:19 PM.


#24 Jon Rista

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Posted 15 April 2019 - 01:38 PM

Agreed.  But I'll note the shot noise is _approximately_ the square root of the gradient.  Good bar graph in The Astrophotography Manual that illustrates the relative amounts.  And, as you say, noise reduction in processing also can reduce the effects of the light pollution shot noise.

 

What infuriates me about Jerry is that he often pretends the small scale shot noise is the only noise worthy of consideration.  Rhetoric that masks the reality.

Jerry is right, as small scale shot noise is the only true noise that actually affects the image. The gradient is technically (from a data/processing standpoint) just an offset, not a true noise (random time- and/or spatially-varying unknown deviations in the signal). The gradient is also pretty easy to correct as well unless you have a very complex field (i.e. almost anywhere in Cygnus.)

 

Shot noise is not easy to correct. Shot noise reduction also only affects the noise, and will not increase signal, and there is not necessarily a guaranteed improvement in SNR with NR...NR may simply improve the appearance of the image, but otherwise have no impact on whether faint details are visible or not.

 

Further, shot noise from LP is often by far the most significant source of noise in the image overall (depending on the sensor temp, dark current may be the only potential contender to LP for the dominant noise term.) LP alone can be dozens of electrons per second or more, can be tens to even hundreds of times stronger than object and background sky signals, so it has a huge impact on SNR. The gradient hardly matters in comparison. For the most part, the gradient can be dealt with via a change in offset, and there are many ways to model and remove gradients with various programs for more optimal correction. The loss of SNR due to LP is much more difficult to deal with. Noise reduction can smooth the noise profile out, however it will not actually increase the object signal, so there are specific limits as to what NR can do to help the SNR of a heavily polluted image. 

 

What Frank is sharing here is that an LP filter may not actually improve SNR, and in fact may hurt SNR. For broadband objects, most LP filters (even the multi-pass type Frank is talking about) will cut out ~50% of the visible spectrum, if not more. That loss of light across the spectrum can result in a 50% (or greater) reduction in object signal, but only a ~30% reduction in noise. That hurts SNR instead of helping it. As much as signal grows faster than noise, if you cut out spectrum signal can also drop faster than noise. Depending on the kind of object, imaging without an LP filter may be better for SNR, because you keep more object photons, thus increasing SNR. 

 

All of this really doesn't have much of anything to do with gradients these days, though. An LP filter does not eliminate gradients. Gradient reduction is usually necessary whether you are using an LP filter or not, so I would not call it an alternative to using an LP filter. And I definitely wouldn't trivialize the impact of LP shot noise, which if LP is present is usually the single most significant source of shot noise in the image overall, and thus has the greatest impact on SNR overall. 

 

nz8iRnv.jpg

 

The only difference between these two images is the amount of LP. Same exposure, different locations. One is a red zone, ~18.5mag/sq" or around there, the other a green zone, ~21.3mag/sq" or around there. The SOLE difference between the two is how much LP each image has. Once the additional offset is subtracted away from the red zone image, the difference in noise is readily apparent:

 

CnZCEz3.jpg

 

The only difference here is LP. The gradient was easy to deal with. The remaining noise, however...was very difficult to deal with. Even after nearly 10 hours of exposure, the image remained quite dirty compared to dark site data:

 

tjWY8CD.jpg


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#25 Jon Rista

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Posted 15 April 2019 - 01:42 PM

The final image (IIRC, ~9.8 hours) had a lot of color noise and large scale background blotchiness that I could never eliminate:

 

zEm7HZr.jpg


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