Narrowband filters for fast optics

RASA telescopes, Hyperstar systems and other fast optics is growing quickly in popularity among astro photographers. However popular, these systems requires costly, special narrowband filters with shifted band-pass, and finding good such filters is not straightforward. Thus, I want to learn from the experiences in the AP community.

To explain why this is filter issue is a challenge, let me start by defining the term "fast". When used in this context, fast optics in AP refers to lenses / telescopes with large apertures with a rather short focal lengths, giving rise to a low f-number, typically in the range between f/3.5 and f/1.8. With such fast lenses, the light cone has a wider edge angle compared to a lens with a long focal length where most of the incoming light is close to perpendicular to the camera.

If you are interested in the math here, this angle is calculated through the relationship: tan(alpha) = (Aperture/2) / (Focal length). To exemplify, a Celestron 8" SCT telescope has an incoming light beam that is maximum 2.8 deg tilted away from the central, perpendicular beam, while this angle in a fast system like a Celestron RASA becomes 14 degrees.

This light angle becomes is an issue for narrowband filters in the optical train, as the light passes through the optics in a way that shifts the bandpass region for the filter proportionally to the increase of the light angle. For a very narrowband filter, like a 3nm Hydrogen alpha, this becomes a serious issue, and the incoming light from Ha close to the edge of the telescope will not pass the filter, effectively stopping down the lens / telescope to a slower system. The user may still be able to capture great images, but at the cost of a slower speed. Jim Thompson has written an excellent article for those who are really interested in the details: https://www.cloudynights.com/topic/738068-narrowband-filters-and-fast-optics-good-or-bad/

Currently, there are filter sets from Astronomik, called MaxFR that comes in 6nm and 12nm versions. Baader had for a long time a "highspeed" filter set that I have heard had some issues, but has now announced the release of CMOS optimized filter sets for fast optics in two versions: 6.5nm and 4nm.

Does anyone have any real world experience to bring forward regarding narrowband filter sets for fast optics and if the narrower version is worth the extra $ when used in a light polluted backyard?

Regards,
Martin, aka @urban.astronomer

The math is way over my head here but I’s shooting OSC so I’m following this thread.

Dale

Anecdotally, I just switched to 6nm zwo filters in my f/4 newtonian (previously used 12nm Astronomik) and they don't perform as expected.. Not sure if the bandpass shift is the problem, Planning a head-to-head comparison of the 12nm vs 6nm Ha filter here soon. I guess it could also be the quality of the zwo filters vs the astronomik too.

First, your calculation only accounts for the Marginal ray but the angles get steeper for part of the ray bundles if you also include the Chief ray to account for off-axis ray bundles. Either way, the net effect of the filter is to decrease the effective speed of the system.

With faster systems, the most common approach is to use a NB filter with a wider bandpass. Some companies bias the filter peak for larger angle rays to accommodate faster beams and I believe that's the way that the Baader filters for F/2 work (but I'm not 100% sure.) I've spoken with the designer at Chroma and he tells me that their standard 3 nm filters minimize center shift out to a F/4 beam, which is pretty good. Chroma is working on a new version of their NB filters that are specifically designed to work down to F/2. Unfortunately, I don't believe that they are widely available yet–even though I've seen them listed by some sellers.

John

Hi Martin,
three years ago, i performed some transmission measurements on a VASE (variable angle spectral ellipsometer) on my BAADER filterset.
I found, they are quite good for down to f/2.8, which is more than i expected. However, this is not a transmission spectrograph, but an ellipsometer, R & T is not too accurate, since the baseline is measured first, and due to mechanical design features.
I will do the same for my BAADER f/2 filters somewhere in future, but am too busy at the moment.

Since i missed these data, but recently stumbled across it when i again ran out of disk space, i'll upload some graphs here:

Transmission of BAADER H-alpha 7nm, for different angles of incidence

Effective transmittance of this H-alpha filter as a function of f-number, calculated for an unobstructed lens

Same stuff for the BAADER O-III 8.5 nm

Similar stuff for the BAADER 8nm S-II filter. However, on the left there are the integrated transmission curves for the different f-numbers, individually.

Edit:
For O-III and S-II, where two emission lines are present, if i remember right i found typical line intensities, and used those for calculating the integrated transmission.

Hi John et al., I wonder if someone might comment about the wider passbands (35nm) used by the Altair quadband filter. Not a perfect solution, but a seemingly reasonable approach for Hyperstar and RASA users that don't want to give up their speed advantage. Looking at the charts (following the links from the OP), it seems this approach could work well (especially for OSC) and has an additional benefit that it's reasonably priced. Any comments would be appreciated. Thanks & CS Doug

https://www.landseaskyco.com/altair-2-quadband-filter.html

As a newbie that is just starting to play with some of the Optolong filters l-enhance and l-pro I’m considering getting an OIII for my setup to try along with an Ha filter. I’m using several newts that are at f5 shooting with a OSC (ZWO ASI294MC pro) what NM would be best for me then?
You guys mention that filter sets are good down to say f2.8 does that mean that they will work with any system down to the f2.8 or does it mean that the the slower f ratio (say f5 in my situation) the less effective they become?

Sorry if this is somewhat off topic.

Dale

Maybe a topic which should be considered as well if one thinks about choosing a ultra-narrowband filter…..what about the wavelengths shift of Ha due to the speed of the desired nebula or whatever?
This is quite large, in the range of more than 1 or 2nm for many cases, right? If one uses a 3nm filter and the nebula is "running away", what then…..less transmission, less photons…..more money spent for less performance?

What is your opinion in this matter?

Thanks and CS,

Georg

Georg N. Nyman:
Maybe a topic which should be considered as well if one thinks about choosing a ultra-narrowband filter.....what about the wavelengths shift of Ha due to the speed of the desired nebula or whatever?
This is quite large, in the range of more than 1 or 2nm for many cases, right? If one uses a 3nm filter and the nebula is "running away", what then.....less transmission, less photons.....more money spent for less performance?

What is your opinion in this matter?

Thanks and CS,

Georg

Ya thats why I commented about it being off topic. Probably should be a separate post. If others feel that way I’ll go ahead and do a new topic with that question as I’m wanting to get an Ha & OIII filters but don’t want to buy the wrong ones as these are expensive filters. At least to me they are.

Dale

Dale Penkala:
You guys mention that filter sets are good down to say f2.8 does that mean that they will work with any system down to the f2.8 or does it mean that the the slower f ratio (say f5 in my situation) the less effective they become?

Dale

Hi Dale,
usually, a filter that works for f/2.8, will also work at f/5.
It might be slighly less transparent, since the central wavelength usually is a bit further in the red than the emission line. This is done to lead the blue shift due to tilted transmission. You can see this effect in my measurements above.

Georg N. Nyman:
Maybe a topic which should be considered as well if one thinks about choosing a ultra-narrowband filter.....what about the wavelengths shift of Ha due to the speed of the desired nebula or whatever?
This is quite large, in the range of more than 1 or 2nm for many cases, right?
Georg

Hi Georg,
red shift from an object's relative speed can be an issue, but this only applies to galaxies - i didn't consider that for my imagin, yet, but i'm not very into galaxies.

CS Horst

Hi Martin, AS the thread poster, is your interest in this subject theoretical, OR are you planning on moving to Mono Hyperstar or RASA?

I started with Celestron Fastar in 2005 and moved to Starizona Hyperstar 2, then upgraded to Hyperstar 3 on a C11. I abandoned Hyperstar several years ago for fast refractors. I can discuss my experience of the pros/cons of using a Celestron F2 system. But, I wont go into all that if your question is theoretical. That seems to have been answered very well. I only used Astronomik 12nm narrow band filters, and did primarily because of the issue mentioned above. I will say it is fast, but not easy. Note: I had to use 2" filters to not vignette. Even with a moderate size chip due to the steep angle of the light cone.

I will speak more to the issue of application rather than optical theory.

I am returning to a fast reflector, but at F3 with a Riccardi Honders Veloce RH200. However I have not had a chance to use it. My savings of buying a used scope is being eaten up as it is in Italy for repairs and re-coating. I too have to be concerned about the narrow band filter at F3. They have worked with my Tak FSQ106ED at F3.64. I will have to go to 2" filters with the camera I want to use. That increases the cost form the 1.25" I have been able to get away with. Chroma is out, because as John mentioned they are good to F4 and cost. I the Astrodons claim they are good to F3 and will work, but no 2" mounted, only 49.7 round un-mounted ( $870 for the 5nmHa /SII & $1438 for the 3nm OIII = $3178 ).

I am considering the Astronomiks MaxFR as a cheaper option.
Cheaper filters will not be well centered on the narrow wave length and wave length transmission is not guaranteed or at poor levels. Farfocal is not an issue, because at F2 you will be re-focusing anyway using a filter drawer.. A 8" Hyperstar has a focus zone of 8.37 microns as compared to 55 microns of a F5 106 refractor.

I would like to mention that my reason for using narrow band and fast optics is primarily to combat light pollution and limited field of view. I have no other choice.

Lynn K

Georg N. Nyman:
Maybe a topic which should be considered as well if one thinks about choosing a ultra-narrowband filter.....what about the wavelengths shift of Ha due to the speed of the desired nebula or whatever?
This is quite large, in the range of more than 1 or 2nm for many cases, right? If one uses a 3nm filter and the nebula is "running away", what then.....less transmission, less photons.....more money spent for less performance?

What is your opinion in this matter?

Thanks and CS,

Georg

Most of the time you use NB imaging on emission nebula within the Milky Way but sometimes it's desirable to try to image Ha sources within distant galaxies. So it's relevant to ask how wide of a filter do you need for distant galaxies? It turns out to be hard to find the constant between distance and redshift in nm so I computed it myself. Since I can't type equations here you'll have to put up with pseudo text to denote the the relationships:

1) Recession velocity = (delta lambda/lamda)c, where c = speed of light
2) Recession velocity due to distance = H*d, where H = Hubble Constant and d = distance
3) Therefore: d = (c/(lamda*H))*delta Lamda

Now for the constants:
H = 2.11e-18 (1/sec)
c = 3e8 m/sec
lamda (Ha) = 656.3 nm
1 Ly = 9.461e15 m

Therefore: d =(2.17e23 m/nm) * delta lamda = 2.29e7 Ly/nm = 22.9MLy/nm--call it 23 MLy. That means that Ha light from a galaxy at a distance of 23 MLy will be red-shifted by 1 nm. That's probably as much shift as you want if you are using a 3 nm filter, but that works for a lot of galaxies in the local group. The main thing to realize is that the red-shift from "local" Ha nebula in the Milky Way is nothing by comparison so using a 3 nm filter is totally acceptable for those sources. The main reason to use such a narrow bandwidth is to isolate Ha emissions while increasing the SNR when the sky is bright. If you are in a dark, clear location, a 5 nm bandwidth is just fine and I wouldn't hesitate to put a 7 nm or even a 10 nm filter on a F/2 Hyperstar system. My C14 system has 5 nm filters and I've gotten good images of Ha in galaxies out to around 40-50 MLy so it works fine.

John

John Hayes:
That means that Ha light from a galaxy at a distance of 23 MLy will be red-shifted by 1 nm.

I would like to add on this. The calculation John did was based on the redshift caused by the expansion of the universe. In this case, one also knows the direction of the shift (towards longer wavelengths, hence "red-shift" ). However, one might need to consider also the motion within the universe as I did some number crunching and you even don't need to consider relativistic effects.

Let me present this: Let's assume like above that we can allow a red-shift of 1nm, this means the radial velocity of the object is about +/-0.15% of the speed of light. Let's take c as above, this gives 457km/s. (equation 1 from above: delta lambda/lambda = v/c )
I checked the numbers how fast the Andromeda galaxy approaches the Milky Way (I looked it up in Wikipedia). It's redshift is -0.10% or 300km/s so about 2/3 of the calculated limit above (with the neg. sign, it becomes a blue-shift). Therefore, the effect may appear with objects even closer to us than with the red-shift from the expansion of the universe.

Cheers,
Björn

John Hayes:
First, your calculation only accounts for the Marginal ray but the angles get steeper for part of the ray bundles if you also include the Chief ray to account for off-axis ray bundles. Either way, the net effect of the filter is to decrease the effective speed of the system.

Thanks, John

Can you explain the term "Chief ray" and how the angles get steeper? In my simple logic, the sum of light entering the lense must be an integration of all incoming light from edge to edge. And with systems like RASA or Hyperstar, the central ray is also obstructed by the camera, so this needs to be subtracted.

-Martin

three years ago, i performed some transmission measurements on a VASE (variable angle spectral ellipsometer) on my BAADER filterset.
I found, they are quite good for down to f/2.8, which is more than i expected. However, this is not a transmission spectrograph, but an ellipsometer, R & T is not too accurate, since the baseline is measured first, and due to mechanical design features.
I will do the same for my BAADER f/2 filters somewhere in future, but am too busy at the moment.

Thanks, HR_Maurer. I believe you are right, as the standard Baader narrowband filters are relatively broad (7-8.5nm). A shift in the central wavelength for Ha, O-III and S-II will still be contained within the bad pass of the filter. I have used these filters with success with a Mitakon Lens stopped to f/2.8.

-Martin

Doug Summers:
Hi John et al., I wonder if someone might comment about the wider passbands (35nm) used by the Altair quadband filter. Not a perfect solution, but a seemingly reasonable approach for Hyperstar and RASA users that don't want to give up their speed advantage. Looking at the charts (following the links from the OP), it seems this approach could work well (especially for OSC) and has an additional benefit that it's reasonably priced. Any comments would be appreciated. Thanks & CS Doug

https://www.landseaskyco.com/altair-2-quadband-filter.html

Doug,

I am not familiar with this particular Altair filter, but with such a broad pass band as 35nm, the small shift in wavelength is negligible. Thus, this filter should be feasable for fast optics and still have the advantage of speed.

In my light polluted area (Bortle 8-9), having a tight band pass is crucial, so 35nm is not optimal for me. For OSC, I have been using an IDAS NBZ Nebula boost filter from Hutech for fast optics (https://www.sciencecenter.net/hutech/idas/nb-filters/nbz.htm) with my Hyperstar f/1.9 with some success. This is a dual band filter, and I believe it has some ~10nm band pass around Ha and O-III. Here is an example: https://www.astrobin.com/ptf466/

-Martin

Lynn K:
Hi Martin, AS the thread poster, is your interest in this subject theoretical, OR are you planning on moving to Mono Hyperstar or RASA?

I started with Celestron Fastar in 2005 and moved to Starizona Hyperstar 2, then upgraded to Hyperstar 3 on a C11. I abandoned Hyperstar several years ago for fast refractors. I can discuss my experience of the pros/cons of using a Celestron F2 system. But, I wont go into all that if your question is theoretical. That seems to have been answered very well. I only used Astronomik 12nm narrow band filters, and did primarily because of the issue mentioned above. I will say it is fast, but not easy. Note: I had to use 2" filters to not vignette. Even with a moderate size chip due to the steep angle of the light cone.

Lynn,

Thank you for asking. My story goes like this: As a long time user of a Celestron Edge telescope with Antlia (3-4)nm narrowband filters, I finally made the last upgrade and bought a Hyperstar v4 system for Christmas (using the company Christmas bonus!). I can also mention that the Antlia filters made a great improvement from my Baader filter set (7-8.5nm) in my Bortle 8-9 environment. Especially the O-III needs a tight band-pass.

In my ignorance, I believed Antlias statements, claiming that their filters could also be used with fast optics, like f/2. In my first run with the Hyperstar system and Antlia filters, I collected 2 hours of Ha-data from the Jellyfish nebula with the rather poor result shown below. I now know that the 8" Celestron telescope had only 1-2" effective aperture due to the tight band pass combined with the spectral shift. So the Antlia statement is not incorrect, they did just not say that my f/1.9 scope effectively became something like f/8 instead.

So, I am looking to buy a new set of narrowband filters especially for fast systems, and there suddenly seems to be several options in the making out there (New Baader filters, Chroma and Astronomik). Also, they seem to come in two versions 6 or 12nm (like Astronomik MaxFR: https://www.astronomik.com/en/fotografische-emissionslinienfilter/maxfr-emission-line-filters.html). Is it worth going for the 6nm version? Will it improve on light pollution at the cost of stopping down the telescope, or is it simply just better? Any shared user experience is appreciated.

Also, as mentioned in an earlier post, for OSC I have found the IDAS NBZ filter from Hutech useful for the Hyperstar (see my image: https://www.astrobin.com/ptf466/)

-Martin

Hi Dale,
usually, a filter that works for f/2.8, will also work at f/5.
It might be slighly less transparent, since the central wavelength usually is a bit further in the red than the emission line. This is done to lead the blue shift due to tilted transmission. You can see this effect in my measurements above.

Thank you HR, so if I’m looking at say an OIII filter I see 2 options a 12nm & 6.5nm. Which one would be the best to get to use with my OSC camera? I’m guessing the 12nm because its more “broad” then the 6.5nm?

Sorry for my dumb questions, just trying to understand what I should purchase. I’ve been using a 2” Lumicon OIII that I used 15-17yrs ago for visual work and my guess is its not really geared for photographic work. This is why I’m looking at getting one that is more suited for what I’m trying to do. I was looking at the Optolong 12nm or 6.5nm filters.

Thanks!
Dale

Thanks, John

Can you explain the term "Chief ray" and how the angles get steeper? In my simple logic, the sum of light entering the lense must be an integration of all incoming light from edge to edge. And with systems like RASA or Hyperstar, the central ray is also obstructed by the camera, so this needs to be subtracted.

-Martin

Martin,
The Chief Ray defines the maximum field angle—typically it is the angle of the ray going to the corner of your sensor. So the maximum ray angle that has to pass through the filter occurs at the maximum field angle. The maximum angle will be given by the angle of the Marginal Ray (which is the maximum angle of the on-axis ray bundle—what you computed) plus the angle of the Chief Ray. That maximum angle is what sets the limit on the performance of the filter. Another way to think about it is that if the wavelength shift at that maximum ray angle works, it will work for ALL of the rays passing through the filter.

John

Dale Penkala:
Hi Dale,
usually, a filter that works for f/2.8, will also work at f/5.
It might be slighly less transparent, since the central wavelength usually is a bit further in the red than the emission line. This is done to lead the blue shift due to tilted transmission. You can see this effect in my measurements above.

Thank you HR, so if I’m looking at say an OIII filter I see 2 options a 12nm & 6.5nm. Which one would be the best to get to use with my OSC camera? I’m guessing the 12nm because its more “broad” then the 6.5nm?

Sorry for my dumb questions, just trying to understand what I should purchase. I’ve been using a 2” Lumicon OIII that I used 15-17yrs ago for visual work and my guess is its not really geared for photographic work. This is why I’m looking at getting one that is more suited for what I’m trying to do. I was looking at the Optolong 12nm or 6.5nm filters.

Thanks!
Dale

Hi, I got the Optolong filters with 12nm and I am very happy with them.....Lacerta Newton F4/1250Q.....
CS, Georg

Georg N. Nyman:
Dale Penkala:
Hi Dale,
usually, a filter that works for f/2.8, will also work at f/5.
It might be slighly less transparent, since the central wavelength usually is a bit further in the red than the emission line. This is done to lead the blue shift due to tilted transmission. You can see this effect in my measurements above.

Thank you HR, so if I’m looking at say an OIII filter I see 2 options a 12nm & 6.5nm. Which one would be the best to get to use with my OSC camera? I’m guessing the 12nm because its more “broad” then the 6.5nm?

Sorry for my dumb questions, just trying to understand what I should purchase. I’ve been using a 2” Lumicon OIII that I used 15-17yrs ago for visual work and my guess is its not really geared for photographic work. This is why I’m looking at getting one that is more suited for what I’m trying to do. I was looking at the Optolong 12nm or 6.5nm filters.

Thanks!
Dale

Hi, I got the Optolong filters with 12nm and I am very happy with them.....Lacerta Newton F4/1250Q.....
CS, Georg

Thanks Georg! That is the one that I was thinking of for myself.

This might be a dumb question but then a filter states 12nm & 6.5nm does that mean that the 12nm allows more wavelength of light? Or put another way is “more broadband” within its OIII range?

Dale

John Hayes:
Thanks, John

Can you explain the term "Chief ray" and how the angles get steeper? In my simple logic, the sum of light entering the lense must be an integration of all incoming light from edge to edge. And with systems like RASA or Hyperstar, the central ray is also obstructed by the camera, so this needs to be subtracted.

-Martin

Martin,
The Chief Ray defines the maximum field angle—typically it is the angle of the ray going to the corner of your sensor. So the maximum ray angle that has to pass through the filter occurs at the maximum field angle. The maximum angle will be given by the angle of the Marginal Ray (which is the maximum angle of the on-axis ray bundle—what you computed) plus the angle of the Chief Ray. That maximum angle is what sets the limit on the performance of the filter. Another way to think about it is that if the wavelength shift at that maximum ray angle works, it will work for ALL of the rays passing through the filter.

John

Thanks, John.

This is interesting. I feel the need to start studying optics in more detail now;-) Does this mean that the size of the sensor plays a role in the wavelength shift as well, as a smaller sensor will lead to a smaller Chief ray angle and vice versa?

Martin

Thanks, John.

This is interesting. I feel the need to start studying optics in more detail now;-) Does this mean that the size of the sensor plays a role in the wavelength shift as well, as a smaller sensor will lead to a smaller Chief ray angle and vice versa?

Martin

Most narrow band filters are designed to work at normal incidence. When a ray passes through the filter at an angle, it shifts the the transmission characteristics of the filter. It may change the shape of the transmission curve, the wavelength of the peak, and the amount of light that gets through. For small angles, tilting the beam won't make much of a difference. Obviously what we are talking about here is at what point do the ray angles get so big that the filter characteristics change enough to matter. At the center field position, the maximum ray angle that pass through the filter is determined by the marginal ray. When you move off-axis, some of the rays will be at an even larger angle, which means that the transmission characteristics vary a little bit depending on where you look in the field. In general, it's not a huge effect, but if you have a very fast system using a large sensor, it's something to keep in mind. With a big sensor used with very narrow bandpass filters, it's probably a good idea to have a little "head-room" on the focal ratio spec for the filters that you select. If the filters limit the transmission, the main effect will be to see a bit more signal fall off into the corners, which should be easily corrected using flats.

John

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