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Assistance understanding nm for filters

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#1 joeytroy

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Posted 25 January 2021 - 10:36 AM

All,

 

I am a complete n00b with filters and figured I would ask away in the experienced section. I am looking at two different type of filters to create the following types of images with my ASI533

 

https://www.cloudyni...ith-osc-images/

 

Based on the post James recommend the following filters

 

L-eXtreme Dual Band 7nm HA/OIII Filter $239.99

Astronomik SII 6 nm CCD Filter $199.95

 

Being forever frugal with my money (not sure it's possible with AP) I wanted to understand the difference in nanometers and how it would effect my images. I was looking at the following filters

 

ZWO 1.25" Duo-Band Filter which has HA at 15nm and OIII at 35nm $99.00

Astronomik SII 12 nm $129.95

 

Can some explain to me in layman's terms how these filters are different in terms their capability? I realize the nanometers are different and the only thing I can guess based on that, is the lower the nm number maybe the better quality the filter is in terms of isolating a specific band? Just a guess but sounds reasonable in my head lol.gif

 

Any assistance is always appreciated!


Edited by joeytroy, 25 January 2021 - 10:37 AM.


#2 kathyastro

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Posted 25 January 2021 - 10:48 AM

The nanometre measurement is the bandwidth or bandpass of the filter: what range of wavelengths on both sides of the desired wavelength it will pass.  Hydrogen alpha, for example, has a wavelength of 656.28 nm.  A 3nm Ha filter will pass wavelengths of 654.78 to 657.78, a total range of 3 nm.

 

The smaller the number, the more "pure" the wavelength selection will be.  On the other hand, a smaller bandpass will let fewer photons through, requiring longer exposures.  Smaller bandpass filters also cost more to make.

 

You would want a larger bandpass for reasons of cost, for visual use (because of the small number of photons and the lack of sensitivity of the human eye), to catch red- or blue-shifted wavelengths from radial motion, or because a "fast" scope causes light to hit the filter at more of an angle.  The filter will pass the specified wavelength only for light that hits the filter at 90 degrees.  At other angles, the wavelength will be shifted.  A broader bandpass will allow the desired wavelength through in spite of the angle.


Edited by kathyastro, 25 January 2021 - 10:49 AM.

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#3 Cfreerksen

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Posted 25 January 2021 - 10:59 AM

Being on a budget is fine. The ZWO filter does a fair job. I use both the ZWO and the L-enhance (l-Extremes older brother)

 

The number is the width in nano-meters of the specific wavelength of light that is let through for that particular filter (bandpass). The tighter/lower the number it is, the more specific the wavelength of light that it lets through. This generally indicates the quality of the filter but not always. With a smaller number you get more specific light and generally less halos around bright stars.

 

If you have a controlling budget and light polluted skies or want to shoot during bright moon, the ZWO is an OK choice. It will work, just not as well as other filters. You don't have to spend top dollar for equipment and get good results. Better equipment can be helpful but it is often not used to its full potential anyway due to user skill level.

 

And I would not worry about SII at this point especially if you are using a color camera. SII is generally way less prevalent than Ha or OIII. 

 

What type of scope and camera are you using?

 

Chris


Edited by Cfreerksen, 25 January 2021 - 11:03 AM.

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#4 JamesTX

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Posted 25 January 2021 - 11:00 AM

As Kathy mentioned, its how large the bandpass is.  Smaller nm means a tighter window. 

 

Generally the narrower the filter, the better the contrast.  That said I only have experience with the above mentioned filters in a bortle 4/5.

 

There are actually a number of threads in cloudy nights comparing filters of different band pass and a nice review of the zwo narrowband filters.

 

Here's a great thread comparing 3nm, 6nm and 12nm:

 

https://www.cloudyni...-advice-needed/


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#5 JamesTX

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Posted 25 January 2021 - 11:07 AM

Being on a budget is fine. The ZWO filter does a fair job. I use both the ZWO and the L-enhance (l-Extremes older brother)

 

The number is the width in nano-meters of the specific wavelength of light that is let through for that particular filter (bandpass). The tighter/lower the number it is, the more specific the wavelength of light that it lets through. This generally indicates the quality of the filter but not always. With a smaller number you get more specific light and generally less halos around bright stars.

 

If you have a controlling budget and light polluted skies or want to shoot during bright moon, the ZWO is an OK choice. It will work, just not as well as other filters. You don't have to spend top dollar for equipment and get good results. Better equipment can be helpful but it is often not used to its full potential anyway due to user skill level.

 

And I would not worry about SII at this point especially if you are using a color camera. SII is generally way less prevalent than Ha or OIII. 

 

What type of scope and camera are you using?

 

Chris

Actually, s2 shows up pretty good.. at least with brighter targets and works well if someone wants to do SHO with a OSC.  :)


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#6 joeytroy

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Posted 25 January 2021 - 11:09 AM

The nanometre measurement is the bandwidth or bandpass of the filter: what range of wavelengths on both sides of the desired wavelength it will pass.  Hydrogen alpha, for example, has a wavelength of 656.28 nm.  A 3nm Ha filter will pass wavelengths of 654.78 to 657.78, a total range of 3 nm.

 

The smaller the number, the more "pure" the wavelength selection will be.  On the other hand, a smaller bandpass will let fewer photons through, requiring longer exposures.  Smaller bandpass filters also cost more to make.

 

You would want a larger bandpass for reasons of cost, for visual use (because of the small number of photons and the lack of sensitivity of the human eye), to catch red- or blue-shifted wavelengths from radial motion, or because a "fast" scope causes light to hit the filter at more of an angle.  The filter will pass the specified wavelength only for light that hits the filter at 90 degrees.  At other angles, the wavelength will be shifted.  A broader bandpass will allow the desired wavelength through in spite of the angle.

Kathy,

 

I think that makes sense if I understand you I got the following information

 

1. smaller the nm the more "pure" the bandpass but also the more exposure I need to capture said bandpass

2. because I have a fast scope RedCat F/4.9 the bandpass will cause more of an angle, due to that I am assuming the broader (cheaper) filters maybe the way to go?

 

Being on a budget is fine. The ZWO filter does a fair job. I use both the ZWO and the L-enhance (l-Extremes older brother)

 

The number is the width in nano-meters of the specific wavelength of light that is let through for that particular filter (bandpass). The tighter/lower the number it is, the more specific the wavelength of light that it lets through. This generally indicates the quality of the filter but not always. With a smaller number you get more specific light and generally less halos around stars.

 

If you have a controlling budget and light polluted skies or want to shoot during bright moon, the ZWO is an OK choice. It will work, just not as well as other filters. You don't have to spend top dollar for equipment and get good results. Better equipment can be helpful but it is often not used to its full potential anyway due to user skill level.

 

And I would not worry about SII at this point especially if you are using a color camera. SII is generally way less prevalent than Ha or OIII. 

 

What type of scope and camera are you using?

 

Chris

Chris,

 

All my gear is in my signature. In terms of bortles class your guess is as good as mine. https://www.lightpollutionmap.info/ has me at Bortles 4 based on the World Atlas of 2015, and https://clearoutside.com/ has me at Bortles 2-3 based on the size of the moon. Last week it was 2, tonight it is 3.



#7 happylimpet

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Posted 25 January 2021 - 11:48 AM

1. smaller the nm the more "pure" the bandpass but also the more exposure I need to capture said bandpass

2. because I have a fast scope RedCat F/4.9 the bandpass will cause more of an angle, due to that I am assuming the broader (cheaper) filters maybe the way to go?

 

1 isnt correct (Kathy was slightly wrong about this). The narrower filter will let less broadband light through - ie starlight spread throughout the spectrum. But it should let ALL the light through at the desired wavelength, so exposures neednt be any longer. For other reasons, they CAN be longer, but thats not something to think about now.

2 f4.9 isnt fast for the purposes of this issue, so dont worry about it.


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#8 kathyastro

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Posted 25 January 2021 - 11:58 AM

1. smaller the nm the more "pure" the bandpass but also the more exposure I need to capture said bandpass

2. because I have a fast scope RedCat F/4.9 the bandpass will cause more of an angle, due to that I am assuming the broader (cheaper) filters maybe the way to go?

Happylimpet is correct that the bandpass doesn't affect how many photons of the desired wavelength are passed.  But it does affect the total number of photons (i.e. desired + unwanted), which will in turn affect exposure.

 

The bandpass doesn't cause the angle.  The scope causes the angle, and the bandpass determines how much off-angle light gets through.  For my f/4 scope, I selected 5 nm filters rather than 3nm because of the off-angle light (and because of cost).  At f/5, I wouldn't worry about it.


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#9 joeytroy

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Posted 25 January 2021 - 12:25 PM

Perfect, thank everyone for your assistance. Based on all this I am going to try and get the lowest nm I can afford. I am trying to be thoughtful about this in the event I ever upgrade my scope or camera in the future.



#10 happylimpet

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Posted 25 January 2021 - 01:04 PM

Happylimpet is correct that the bandpass doesn't affect how many photons of the desired wavelength are passed.  But it does affect the total number of photons (i.e. desired + unwanted), which will in turn affect exposure.

This is true but it doesnt mean longer exposures are REQUIRED to get a certain result, it just means that the option of better results with using longer exposures becomes available....I dont want the OP to think he'll need to use longer exposures.


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#11 ryanha

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Posted 25 January 2021 - 01:10 PM

Here are a couple good presentations from FarPoint Astro's website about narrowband in general. It does talk about bandpasses as well.

 

- https://farpointastr...rowbandTalk.pdf

- https://farpointastr...rowband_FAQ.pdf

 

--Ryan


Edited by ryanha, 25 January 2021 - 01:11 PM.

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#12 joeytroy

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Posted 25 January 2021 - 01:28 PM

Here are a couple good presentations from FarPoint Astro's website about narrowband in general. It does talk about bandpasses as well.

 

- https://farpointastr...rowbandTalk.pdf

- https://farpointastr...rowband_FAQ.pdf

 

--Ryan

Ryan,

 

Thanks that very helpful!




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