Why the "petrol filter" is the fix for light pollution

[Lokifish]

For some of you, light pollution is an annoyance. For some of you, light pollution is a nightmare. You look at longer hoods, light blocking panels, and the ever so popular light pollution filter. You see overly orange "before" shots, and gorgeous "after" shots and wonder why most will tell you "don't bother", or use a "petrol filter". Well here's why.

In most marketing you see those before and after shots. You will also you see a spectrograph of what the filter blocks vs full spectrum light. What you won't see is a spectrographic image of the filter vs actual light pollution sources. Well I'm gonna fix that, and show why we say what we do when it come to LP filters. Each image will show the unfiltered light on the top, and the filtered light on the bottom. So let start.

Your typical Neodymium based LPS filters vs

Full spectrum (almost but not quite, it's the moon)

Good ol' halogen floods found on many homes and buildings

A nice unshielded Sodium street light

Mercury Vapor street/barn lamp (most of my neighbors have these as porch lights on the back of their house)

It looks like multiple images made the forum unhappy. So see the following posts

Now while I do not show fluorescent strip mall lights, and some of the other types, you get the idea. I also did not show what far more aggressive LPS filters look like. Those cut large sections out of the spectrum and cause a very noticeable drop in brightness and color shift. The make for poor viewing filter and poor imaging filters. So when you read "use a petrol filter", or "don't bother", there's a good reason for this. About the only pro filter comment you do tend to read is "takes the edge off", which is true.

Gear Used
Canon T3
660 grating (My apologizes to the spectra folks. I know you guys can do far better, but all I had was stripped CD spectrograph I made for educational/fun purposes)
A mix of LPS filters from cheap to $$$. Not going to say what's what, as the results where not significantly different when run through the grating. About the only big difference was AR coating or not, and the price.

[Lokifish]

Full spectrum (almost but not quite, it's the moon)

[Lokifish]

Good ol' halogen floods found on many homes and buildings

[Lokifish]

A nice unshielded Sodium street light

[Lokifish]

Mercury Vapor street/barn lamp (most of my neighbors have these as porch lights on the back of their house)

[JayTee]

Hi Russ,

What exactly am I looking at? To me, the bottom image source light doesn't look that much different than the top image for that light source. Is this showing the "ineffectiveness" of the filter in question?

JT

[Lokifish]

You're correct, it's not that different. The bulk of LP filters block the common orange/yellow region and a touch of the green. Some will also block a very small segment in the blue/violet.

Edit
I also shot the sequence with four different filters and could barely tell the difference between them. A better grating and rig might show it, but the real world results would be about the same.

[JayTee]

Ahhhh! Got it. Very informative and illuminating.

Cheers,
JT

[Altocumulus]

Illuminating.

I don't night observe (yet) - largely because I have a mass of city lights to the south and southeast - Aberdeen Scotland - annoying!

[helicon]

Thanks Lokifish. I am currently working on building a collection of filters for various purposes to try to beat my LP and find this pretty helpful. Of course, the best solution is dark skies!

[UlteriorModem]

Very nicely done and presented.

But you did not address LED lighting.

In the past the vast majority of light pollution was from HPS or 'sodium' lighting. Filters were pretty effective at notching that spectrum out. Just look at your spectrum for the sodium, the blue/green bands are all but missing.

Now that the older technology is being replaced with LED type lights with a much broader spectrum, filters are no longer really effective at all. In fact the LED's output spectrum is right in the wheelhouse of the 'typical' DSO so if you did filter out the LED source, you would also filter out the DSO.

[Lokifish]

Thanks Lokifish. I am currently working on building a collection of filters for various purposes to try to beat my LP and find this pretty helpful. Of course, the best solution is dark skies!

Some LP filters are little better at it than others, but that's mostly dealing with AR coatings and the glass itself. Beyond that, the examples show just much of the spectrum is affected by LP sources even with filters.

Very nicely done and presented.

But you did not address LED lighting.

In the past the vast majority of light pollution was from HPS or 'sodium' lighting. Filters were pretty effective at notching that spectrum out. Just look at your spectrum for the sodium, the blue/green bands are all but missing.

Now that the older technology is being replaced with LED type lights with a much broader spectrum, filters are no longer really effective at all. In fact the LED's output spectrum is right in the wheelhouse of the 'typical' DSO so if you did filter out the LED source, you would also filter out the DSO.

You're right, I did miss LEDs. The moon shot could be used in that case as both are full spectrum. So trying to filter out LEDs is like trying to block the moon, stars, and DSOs. May as well leave the cap on at that point. It's sad really as it doesn't have to be that way. Narrow band RGB LEDs would provide "white" light and have transmission lines that could be filtered out with very little affect on visual and imaging. The lighting industry has even suggested fairly narrow band warm LEDs as a solution. They cited health, safety, and even astronomy as reasons to use them, but most places are ignoring the recommendation.

The type of older tech used is more a regional/local thing from what I've seen. In the tri-city area where I live, it's almost based on what city you happen to be in. I do agree that LP filters do notch out the more annoying part of the spectrum, it's just what's left over that kills you in truly heavy LP areas (High Bortle 7 and worse).

[helicon]

They are starting to replace the old sodium vapor lights where I live with LED's. I have urged the city to put dampers on them so the light shines down only and not out into the sky, so far the powers that be have thanked me for my input, but the ongoing resolution of this potential problem is unknown at this point.

[chicagorandy]

"the powers that be have thanked me for my input"

Which here in Chicago is 'political speak' for "We don't care, shut up and go away." lol

[Lokifish]

Where I live, light pollution statutes is pretty much limited to "do not point bright lights at the sky" and you need a permit to use one of those carbon arc aerial search lights. So you can imagine what kind of response I received regarding light trespass, and LED transition concerns.

[UlteriorModem]

Indeed full cut off is a good step, it is the only thing I will specify. Another advantage of LED lights is that good ones have very precise lighting patterns and can be tailored to keep trespass light down to .1 fc believe it or not.

But the market is flooded with 'cheap' led area lights in the 4,000K spectrum. These cheap lights do not have such precise control. In fact many of then are just blobs.

9 times out of 10 I will specify LED area lights that are full cut off and good output control only to have the owner / contractor "Value Engineer" the job and submit E-Econolite fixtures. Yes that's a thing. We try to discourage it but when the owner sees a $100.00 fixture vs a $300.00 fixture it is very hard to convince them otherwise. After all its just a street light right?

Also LED sources are not particularly 'broad band' but its broad enough. Just google LED 4,000K spectrum. As I said before the spectrum is right in the wheel house of most DSO unfortunately.

[STEVE333]

I would like to bring some more SUNSHINE and HOPE to this topic.

MY SITUATION:
I live 1/2 mile from the local shopping mall and 1/2 mile from the main street in town with all the street lights. The LP is Bortle 5. I image with a Canon T3 (modified).

MY SOLUTION:
For Galaxies and Star Clusters I've been using an IDAS LPS-D1 filter which helps some. Even with this filter I am limited to 3 min exposures because of the LP. Because galaxies and star clusters are both broadband sources the LP filters help some, but, not a lot as already mentioned.
However, for Nebulae the story is very different!! I have used both the Opt Triad Tri-Band filter ($775) and the STC Astro Duo-Narrowband filter ($399). Both prices are for the 2" version. Either of these filters screw into the internal threads of the Baader Protective T-Ring for Canon EOS (Item No.: 2958550) which I use to connect the camera to the telescope. Four of the images in my Signature (California Nebula, Thor's Helmet, Horsehead & Flame Nebulae, and, Rosette Nebula) were taken with the Triad or Opt filter. I was able to use 9 min exposures and could have used even longer. Below is my Horsehead Nebula (taken with the Triad Tri-Band Filter). As you can see, these filters can be very effective for suppressing Light Pollution.



Below are links to each of the images on Flickr.

California Nebula https://www.flickr.com/photos/125271061 ... ateposted/
Thor's Helmet on Flickr https://www.flickr.com/photos/125271061 ... ateposted/
Horsehead on Flickr https://www.flickr.com/photos/125271061 ... ateposted/
Rosette Nebula https://www.flickr.com/photos/125271061 ... ateposted/

I hope this will be encouraging to some of you fighting Light Pollution. These filters have opened up a whole new world of Nebula targets for me.

Steve

[UlteriorModem]

Yep the tri band or duo band filters do a pretty good job. But oh those prices!

[STEVE333]

Yep the tri band or duo band filters do a pretty good job. But oh those prices!

I totally agree about the costs UM - For me, if I'm going to put in all the effort to carry everything out (takes me 5 trips up and down the mobile home stairs to get everything out), set it up, do the alignment, take the pictures (sometimes in the cold), break it down (always in the cold), and bring it all back in (another 5 trips) late at night or early in the morning, and miss out on sleep time, I want the best results I can get! Plus, either one (especially the Duo-Narrowband) is way cheaper than buying a mono camera + 2 or 3 NB filters + filter holder + filter holder motor.

Like you said, either one of these filters will do the job. I believe the Duo-Narrowband filter may provide better data for HOO processing because of the narrower passband at the Oiii wavelengths, however, still need more testing to confirm this.

Steve

[Lokifish]

I'm jealous, I have to drive about 30 or 40 miles to get to skies like yours.