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Filter to remove Northern lights?

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#1 Greensky

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Posted 01 December 2020 - 04:27 PM

First post here!
So I am considering trying out some deep sky imaging, but before I start spending any money, I need to know if I can overcome one, potentially big problem.

I live right under the so-called "aurora oval" at 69 degrees north here in Norway.
Clear skies without any northern light are rare, even at solar minimum that we have now.
One thing that may help is that NL is not very static and doesn't necessarily cover all the sky or last all night, so I assume I could go for more exposures and just pull the "greenest" pics out of the stack?
But I still think some sort of filter would be beneficial for me, but what should I be looking for? Wavelength etc?

 

PS, I am aware some may think it's inane to call the beautiful NL a "problem"
And I can assure anyone that I love to observe NL, and also take pictures of it, but now I just want to try out something new :)


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#2 happylimpet

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Posted 01 December 2020 - 04:42 PM

What a fantastic problem! First step will be to find out the dominant wavelengths of aurora, and then see if any filters match it. I doubt there is anything specifically intended....



#3 ismosi

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Posted 01 December 2020 - 05:23 PM

If you had enough subs, wouldn't the NL go away with rejection?



#4 Der_Pit

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Posted 01 December 2020 - 06:07 PM

I'd assume it's mostly the green ones?  Those are 557.7nm (red is 630nm).  But I wouldn't expect that you can easily get inverse notch filters that take that range out....

But yes, it is atomic line emission, and therefore extremely narrowband and in theory easy to suppress....



#5 Alex McConahay

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Posted 01 December 2020 - 06:08 PM

Yes, it is odd that you are trying so hard to remove something that many of us go far out of our way to see. 

 

And it is hard to believe that the aurora borealis is really light pollution to some. 

 

But.....before we speculate----what kind of camera are you using? One Shot Color or LRGB/Mono?

 

Alex



#6 Huangdi

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Posted 01 December 2020 - 06:18 PM

If you had enough subs, wouldn't the NL go away with rejection?


Unlikely. I have had an issue in the past where I'd have inconsistent color-gradients in my images and I just couldn't figure out what it was. Flats didn't help, no matter often I tried different setting during stacking or tried to remove the gradients in the linear stage, it didn't work.

When I finally went through my subs, it all made sense. The light of my head lamp, laptop, phone and whatnot all shone into the truss tube of the newtonian I was imaging with and ruined a whole bunch of subs. Once they were deleted, the problem was gone.

#7 klaussius

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Posted 01 December 2020 - 06:34 PM

Aren't the NL basically airglow in spectrum?

 

In that sense, I think any LP filter would get rid of most of them.



#8 Greensky

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Posted 01 December 2020 - 06:35 PM

Thanks for all the replies!

First, notch filter sound interesting. I did find somthing that might work here, https://www.thorlabs...ctgroup_id=3880 NF561-18 and NF633-25 Perhaps they would do the trick? However, there is no way I am (now)going to spend $1000 on two filters before I even have a decent telescope crazy.gif

 

Edit: Klaussius, LP filter may just be an overall better option. I will look into that.

---------

Alex, I have a Nikon D5100 I was planning to start with.


Edited by Greensky, 01 December 2020 - 06:55 PM.


#9 Alex McConahay

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Posted 01 December 2020 - 07:50 PM

With a OSC camera (the Nikon) , you would need a filter that lets in all light EXCEPT green (assuming most of your aurorae are green, as most are, I believe.....but not all.) 

 

I do not believe such a filter exists. 

 

Were you using a Mono camera, you would be having no problems with the red and blue filters. Perhaps you could do some trickery changing exposures with the green. But, that is if you had a mono camera!

 

Having said that.....I think if you just take a picture, and turn down the green in processing, and perhaps judicious use of color balancing, you may get more than you expect. YOur green channel will be weaker in detail. But, that may not matter much. 

 

 

ALex



#10 DRK73

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Posted 01 December 2020 - 08:38 PM

Sorry to threadjack a little, but I live in a part of the world where dollar bills grow on trees - and it's really annoying. Can someone recommend a good weed killer? 


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#11 Greensky

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Posted 01 December 2020 - 09:36 PM

With a OSC camera (the Nikon) , you would need a filter that lets in all light EXCEPT green (assuming most of your aurorae are green, as most are, I believe.....but not all.)

Thanks for the info, but why won't a standard light pollution filter that suppress the 560-ish wavelength (and a bit more) work with the camera I have? I'm green (smirk.gif) so please fill me in.

 

 

Sorry to threadjack a little, but I live in a part of the world where dollar bills grow on trees - and it's really annoying. Can someone recommend a good weed killer? 

lol.gif That's funny, but here is the thing, NL pictures have become so easy to capture, you can even get ok results with a decent phone now days. That also mean regional social media and photosharing sites are flooded with NL pictures, I even see grandmothers post the stuff. I am aware it's not a common sight everywhere, and I know people pay a lot of money to travel to places just to see it, and I understand why, it's very, very beautiful. But I'm looking for something new and I love technical challenges. Btw, if you envy my location for the NL, just remember that it also comes with another, perhaps even bigger "problem" and price to pay in this hobby: Midnight sun from May to late July. fingertap.gif


Edited by Greensky, 01 December 2020 - 09:39 PM.

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#12 SteveInNZ

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Posted 01 December 2020 - 10:37 PM

It looks as though the green emission (558nm) sits right between two dominant Mercury emission lines. So the filters that are designed as broadband, light pollution filters pass what you want to block and block either side of it. That leaves you with the rather blunt instrument of the CLS filter which blocks about 100nm of the spectrum.

 

It might be worth having a look on eBay for a filter. There are lots of interference filters used in biology and chemistry and there might be something there that just happens to do what you need.

 

Steve.



#13 Alex McConahay

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Posted 01 December 2020 - 11:21 PM

>>>>>>Thanks for the info, but why won't a standard light pollution filter that suppress the 560-ish wavelength (and a bit more) work with the camera I have? I'm green (smirk.gif) so please fill me in

 

I would have to compare the bandpass of the filter you are thinking of to the wavelength of the aurora. 

 

I am not going to pretend I am an expert on the color of the northern lights. But, my limited experience is that they are not one color of green, but depending on where the solar stuff hits in the atmosphere, and what gas is there, the aurora can be anything from red yellow green blue and things in between. I have seen mostly green, but even then, it is not just one green. It is a range of greens with touches of yellow and blue. I do not think there is a single filter that can take out that range and leave the rest. But, you know, I may be wrong there. 

 

When I think of LPR filters, I think of bandpasses that identify the chief light pollution bands around popular artificial terrestrial lighting sources and block them. BUt they are very narrow. They leave lots of green and yellow through.

 

But, really, I think it is worth a try to just go out and take a picture. Certainly in a wide field, the green  or other color will be obvious. If you take a picture of the whole sky, you will see the aurora contrasted against the sky and landscape. But, in a narrow field of half to two degrees, it may look like a color cast that can be simply processed out. It won't have a shape, just a pretty uniform cast over the whole field. 

 

And, unless there are lots more Aurora than I had thought, and they are a lot stronger than I thought, they should affect your imaging less than cloud cover and other such ugly things. 

 

Alex



#14 Greensky

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Posted 02 December 2020 - 10:07 AM

Exelent point on the different colors.
While we mostly see them as green from the ground, they often, if not always, have different color
further up which you can often see where the "curtains" breaks up, as a blue, red, purple-ish layers over the green.
Good example here: https://imbo.vgc.no/...88bdda3f866c38c

so while I may only see green from the ground, I still need to pass all these layers and
thus I would need to perhaps take out to much with filters.
Anyway, I got a lot of useful information here, and I'm hopeful I can get decent results.
I have seen a handful of great deep sky pics close from here, so I know it's certainly possible.


Edited by Greensky, 02 December 2020 - 10:09 AM.


#15 DRK73

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Posted 02 December 2020 - 10:29 AM

 Btw, if you envy my location for the NL, just remember that it also comes with another, perhaps even bigger "problem" and price to pay in this hobby: Midnight sun from May to late July. fingertap.gif

 

Would I trade disgustingly hot and humid summers, dense overpopulation/congestion, light polluted skies, etc. for northern lights in the winter and midnight sun in the summer? Umm...YES!   



#16 endless-sky

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Posted 02 December 2020 - 12:28 PM

PS, I am aware some may think it's inane to call the beautiful NL a "problem"

And I can assure anyone that I love to observe NL, and also take pictures of it, but now I just want to try out something new smile.gif

I agree: too much of anything can become tiresome / take away from the beauty of something. Just imagine if we had a total solar eclipse every month, instead of every few years. It would become boring, just as much as a Full Moon is boring because you've seen it so many times.

 

I live close Venice. People come from all over the world to see it. I have seen it so many times that I am just about sick of it. I would only go back to it if I am taking some distant relative to visit it. For sure, though, I wouldn't go there just to spend the day, if I can find myself something better to do.

 

As for your filtering problem, I really can't help you, though. Maybe figuring out the wavelengths corresponding to the Northern Lights and get a filter that suppress them. Or use a monochrome camera and do narrowband (hoping the NL wavelengths do not match the OIII or SII).




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