Filter Research

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Using Deep-Sky (Band Pass) Filters

Before you can decide what deep-sky filter is best, and how best to use it, you have to ask yourself a number of questions:

  1. What kinds of objects do you want to look at?

  2. What kind of telescope do you use?

  3. Where do you do your observing?

  4. How much are you willing to pay?

The answer to these four questions will help you decide what filter or filters are best for you.  I had to answer the same questions before I purchased my deep-sky filters.  I'll share my answers and the filter choices I made with you a little later.  Right now lets look at some of the additional information that I used to help me make my decision.

Type of Objects:

The type of objects you want to observe will have an impact on what filter is best for the task.  The wavelengths at which an object emits light has an obvious link to how the view of that object is affected by the different filters.  The nature of the light that we see through our telescope is that it has either been emitted or reflected from the object we are viewing.  The most obvious source of light that we see is star light.  Star light, and the light from objects that are visible due to reflected star light (eg. moons, planets, asteriods, reflection nebulae, star clusters, galaxies) tends to have a broad spectrum with no obviously dominant wavelength.   Light can also be emitted directly from clouds of ionized gas such as from emission nebulae, planetary nebulae, and supernova remnants.  Light emitted from these sources is at discrete and very specific wavelengths due to the way the light is generated (Google "quantum theory of light" to learn how this works).  This is perhaps a good opportunity to finally explain the vertical dashed lines on my spectral response graphs.  Table 6 below summarises the list of emission wavelengths that are desirable and undesirable from the astronomer's perspective.  The vertical lines in my graphs correspond to these desirable and undesirable wavelengths. 

Desirable Undesirable
Moniker Origin Wavelength Moniker Origin Wavelength
H-beta Hydrogen in nebulae 486.1 Hg Mercury in man-made lighting 404.7
O-III Oxygen in nebulae 495.9 Hg Mercury in man-made lighting 435.8
O-III Oxygen in nebulae 500.7 Hg Mercury in man-made lighting 546.1
C2 Acetyl (a carbon based molecule) in comet tails 511.0 nightglow natural glow from Oxygen in upper atmosphere 557.7
C2 Acetyl (a carbon based molecule) in comet tails 514.0 hp Na Sodium in high-pressure man-made lighting 568.8
H-alpha Hydrogen in nebulae or our sun 656.3 Hg Mercury in man-made lighting 578.2
N-II Nitrogen in nebulae 658.4 lp Na Sodium in low-pressure man-made lighting 589.3
S-II Sulphur in nebulae 672.4 hp Na Sodium in high-pressure man-made lighting 616.0
      nightglow natural glow from Oxygen in upper atmosphere 630.0
      nightglow natural glow from Oxygen in upper atmosphere 636.4

Table 6   Summary of Desirable & Undesirable Wavelengths for Astronomy

Deep-sky filters take advantage of the fact that the desirable and undesirable wavelengths do not coincide with each other, allowing for a filter to be made that allows all of the desirable but none of the undesirable wavelengths through to your eye.  The result is darkening of everything in your eyepiece view except the desirable emission wavelengths.  This works great for emission nebulae, planetary nebulae, and supernova remnants, but what about everything else?  Some manufacturers and users have stated that certain types of deep-sky filter can enhance the visibility of deep-sky objects like galaxies and clusters too.  I have no experience myself to say whether or not this is true.  Based on my research plus my own limited experience, I have prepared a table that lists the recommended applications for each of my 11 filter categories.



O-III Group A

Viewing of emission nebulae, planetary nebulae, & supernova remnants under heavy light pollution

O-III Group B

Imaging of emission nebulae, planetary nebulae, & supernova remnants

H-beta Group A

Viewing of faint emission nebulae, with or without light pollution

H-beta Group B

Imaging of faint emission nebulae

Narrow Band

Viewing & imaging of emission nebulae, planetary nebulae, & supernova remnants under moderate-to-no light pollution

Medium Band

Viewing & imaging of emission nebulae, planetary nebulae, & supernova remnants under moderate-to-no light pollution

Wide Band

Viewing of emission nebulae, planetary nebulae, & supernova remnants under mild-to-no light pollution; imaging of all deep-sky objects

Extra Wide Band

Viewing or imaging of all objects under mild-to-no light pollution

Multi Band

Viewing or imaging of all objects under mild-to-no light pollution

Special A

Lumicon Comet for comets; Orion Mars for Mars; all others for contrast improvement while viewing moon or planets

Special B Contrast improvement while viewing moon or planets

Table 7   Deep-Sky Filter Application By Filter Category

You will note in Table 7 that I don't recommend the use of O-III Group B and H-beta Group B for visual use.  The bandwidths and %transmissivities for these filters are very small, making them best suited to CCD imaging applications.  Clearly the most common application of deep-sky filters is the observation of emission-type objects.  David Knisely did a pretty exhaustive visual survey of different nebula types using a variety of Lumicon brand filters.  The end result was a list of filter recommendations for different nebulae. His results are summarised below in Table 8, but with my filter categories substituted.

Nebula Name Best Filter Nebula Name Best Filter
M1 Crab nebula Narrow / Medium NGC7000 North America Nebula O-III Group A
M8 Lagoon Nebula Narrow / Medium NGC7008   O-III Group A
M16 Eagle Nebula Narrow / Medium NGC7009 Saturn Nebula none
M17 Swan Nebula O-III Group A NGC7023   Wide / Extra Wide
M20 Trifid Nebula Narrow / Medium NGC7026   O-III Group A
M27 Dumbbell Nebula Narrow / Medium NGC7027   O-III Group A
M42 Orion Nebula Narrow / Medium NGC7048   O-III Group A
M43 Orion Nebula H-Beta Group A NGC7129-7133   Narrow / Medium
M57 Ring Nebula Narrow / Medium NGC7139   O-III Group A
M76 Little Dumbbell Nebula Narrow / Medium NGC7293 Helix Nebula O-III Group A
M97 Owl Nebula O-III Group A NGC7538   Narrow / Medium
NGC40   Narrow / Medium NGC7635 Bubble nebula O-III Group A
NGC246 Skull Nebula O-III Group A NGC7662 Blue Snowball none
NGC281 Pac-Man Nebula Narrow / Medium NGC7822   Narrow / Medium
NGC604 in M33 O-III Group A IC405 Flaming Star Nebula Wide / Extra Wide
NGC896/IC1795   Narrow / Medium IC410   O-III Group A
NGC1360   O-III Group A IC417   H-Beta Group A
NGC1491   Narrow / Medium IC434/B33 Horsehead Nebula H-Beta Group A
NGC1499 California Nebula H-Beta Group A IC1318   H-Beta Group A
NGC1514 Crystal Ball Nebula O-III Group A IC1396   Narrow / Medium
NGC1999   none IC1848   Narrow / Medium
NGC2022   O-III Group A IC2177 Seagull Nebula H-Beta Group A
NGC2024 Flame Nebula Narrow / Medium IC4628   Narrow / Medium
NGC2174   Narrow / Medium IC5067-70 Pelican nebula Narrow / Medium
NGC2327   H-Beta Group A IC5076 inNGC6991 H-Beta Group A
NGC2237-2239 Rosette Nebula Narrow / Medium IC5146 Cocoon Nebula H-Beta Group A
NGC2264 Cone Nebula Narrow / Medium PK64+5.1 Campbell's Hydrogen Star H-Beta Group A
NGC2359 Thor's Helmet O-III Group A PK164+31.1 Headphone Nebula Narrow / Medium
NGC2371-2   O-III Group A PK205+14.1 Medusa Nebula O-III Group A
NGC2392 Eskimo Nebula O-III Group A Sh2-13   Narrow / Medium
NGC2436   Narrow / Medium Sh2-54   Narrow / Medium
NGC2438 in M46 O-III Group A Sh2-84   Narrow / Medium
NGC2440   Narrow / Medium Sh2-101   Narrow / Medium
NGC3242 Ghost of Jupiter Narrow / Medium Sh2-112   O-III Group A
NGC4361   Narrow / Medium Sh2-132   O-III Group A
NGC6210   O-III Group A Sh2-142   O-III Group A
NGC6302 The Bug Nebula O-III Group A Sh2-155   Wide / Extra Wide
NGC6334   Narrow / Medium Sh2-157   Narrow / Medium
NGC6357   O-III Group A Sh2-170   Narrow / Medium
NGC6445   Narrow / Medium Sh2-171   Narrow / Medium
NGC6543 CatsEye Nebula O-III Group A Sh2-235   H-Beta Group A
NGC6559   Narrow / Medium Sh2-254-5-6-7-8 inIC2162 H-Beta Group A
NGC6781   O-III Group A Sh2-261   Narrow / Medium
NGC6804   O-III Group A Sh2-276 Barnard's Loop H-Beta Group A
NGC6888 Crescent Nebula O-III Group A Sh2-311 in NGC2467 Narrow / Medium
NGC6905 Blue Flash Nebula Narrow / Medium vdB93(Gum-1) near IC2177 H-Beta Group A
NGC6960-6995 The Veil Nebula O-III Group A      

Table 8   Recommended Filter For Use With Particular Nebulae

Type of Telescope:

The type of telescope you use can also affect how useful the different deep-sky filters are to you.  The primary parameter of concern is aperture.  Deep-sky filters can attenuate a large percentage of the light coming through your telescope, making some limited to use only on larger aperture telescopes.  Table 9 below summarizes the recommended minimum telescope apertures for the 11 filter categories.  I used the aperture recommendations from various filter manufacturers (primarily Astronomik) to come up with my own relationship between scotopic %transmissivity and telescope aperture.


Scotopic % Transmissivity

Minimum Aperture

O-III Group A



O-III Group B


10" (254mm)

H-beta Group A


8" (203mm)

H-beta Group B


11.5" (292mm)

Narrow Band


4.5" (114mm)

Medium Band


3.5" (89mm)

Wide Band


2.5" (64mm)

Extra Wide Band


no limit

Multi Band


no limit

Special A


Lumicon Comet 5.5", all others no limit

Special B 49-72 no limit

Table 9   Minimum Recommended Aperture By Filter Category

Now don't assume if you use a filter on an aperture smaller than recommended above that you won't see any improvement in your view, because you will.  The recommended apertures are just what is needed to get the best performance out of your filter.  Another way to look at the issue of pairing a filter to a telescope is to consider the exit pupil size.  The exit pupil size is calculated by dividing your eyepiece focal length by your telescope's focal ratio.  For example:  a 26mm eyepiece on an f/10 telescope would have an exit pupil diameter of 2.6mm.  Lumicon has a table on their filter webpage recommending exit pupil size ranges for their various filters.  I have reproduced that table below.

Filter Type Deep Sky (wide band) UHC (narrow band) O-III (Group A) H-Beta (Group A)
Bandpass 90nm 22-26nm 10-12nm 8-10nm
Optimum Exit Pupil (Light-polluted sky)  0.5-2mm 1-4mm 2-5mm 3-7mm
Optimum Exit Pupil (Dark sky)  1-4mm 2-6mm 3-7mm 4-7mm

Table 10   Recommended Exit Pupil Size For Lumicon Filters

Other than having sufficient aperture, and picking the right focal length eyepiece, there really is no limit that I can find on what type of telescope you can use deep-sky filters on.  Well, actually I guess there is one thing:  band pass filters can have poorer performance on low focal ratio telescopes.  I believe that this is generally true of all telescope accessories, and has something to do with the fact that light from the outer edge of the objective is passing through the filter at a significantly different angle than from the the center of the objective.  Testing by others (see the work done by Christian Buil) shows that a filter's pass band wavelengths shift with the angle of the filter relative to the incident light.  This effect is used constructively in the fancy (ie. expensive) tunable H-alpha filters used for Sun observing.  The result for deep-sky filters on low focal ratio telescopes is a non-uniform view over the filter area.  Some web reviews I've found suggest that this effect is worsened when using lower quality filters.

Observing Location:

Ahh if only we had a time machine, that we could jump into and go back 100 years or so for our evening's observing session.  Back then you'd only have to go a short wagon ride to get to where the light pollution was basically zero.  The sad reality is that the majority of amateur astronomers are forced to do their observing from urban or sub-urban environs.  Sure we may get to go on the odd road trip to a dark sky sight, but if any of us plan on observing with any regularity we will have to learn to accept our light polluted backyards for what they are.  To give you an idea of we astronomers are having to deal with, I have put together a composite map of Southeastern Ontario region where I live and do my observing.  The map overlays satelite light pollution data over the Google map for the region so you can see clearly what cities are generating the light pollution.  I live in the white blob second from the right.

Figure 14   Light Pollution Map for Eastern Ontario, Canada

With this reality in mind it is easy to understand why there is such a large variety of deep-sky filters available to choose from.  So if you are like me, and you live in a large city where light pollution is high, you probably should consider an appropriate deep-sky filter if you want to have any hope of seeing nebulae from your backyard.  If you live further outside a large city, the light pollution rejection capability of the filter you choose maybe doesn't need to be so much.  If you are a lucky bum, and you live or can observe regularly from a dark sky sight, then you may not need a deepsky filter at all.  For you dark-sky bums, you may want to at least consider that deep-sky filters can also improve your view of faint nebulae by reducing the brightness of other neigbouring objects like stars.


The one thing I did not include in my research is price.  There are simply too many different suppliers out there for me to compile a list of price along with each filter's performance.  That work you'll have to do for yourself.  One piece of advice though is that in general, it seems that you get what you pay for; expensive filters tend to be the better performers.  So what is a good performer?  It is any filter with the following attributes:

  • has a transmittance as close to 100% as possible at all the desired wavelengths;

  • has a transmittance as close to 0% as possible at all the undesired wavelengths; and

  • transition from undesired to desired wavelengths is very steep, essentially a step change from 0% to 100%.

There are a few other signs of good quality filters to look for such as:  optically polished glass, anti-reflection coatings on both sides, and hard protective coatings for anti-scratch.  Also, companies that provide you with a print-out of YOUR filter's factory measured spectral response is always a good sign of a quality manufacturer.

My Choice:

The answers to the initial four questions for me are:

  1. I am just getting back into astronomy, and not knowing yet what I can and can't see from my backyard, I want to explore everything that I can.  Therefore I want to have filters that can enhance all the different types of deep-sky objects.

  2. I have an f/10 8" SCT and an f/5 80mm achromat.  That way I can view small dim objects or planets as well as large nebulae or star fields.

  3. I do my observing primarily from my backyard, in the middle of a fairly large urban center.  I do on a few occasions observe at my in-laws out in the country.

  4. I am willing to pay a premium in order to get a high quality, high performance filter.  I don't have a giant telescope nor a dark sky observing site, so I want to maximise the performance of the filters I choose.

It is my nature to explore, experiment, optimise...the engineer's curse!  The choice that best meets my needs is a selection of high performance filters, from wide to narrow.  This provides me with the largest flexibility, and the freedom to experiment and learn for myself what filters work the best on different objects for my circumstances.  Figure 15 is an image of the filters I selected:  Astro Hutech IDAS LPS-P2 (Multi Band, 72.7% trans), Baader Planetarium UHC-S (Wide Band, 54.7% trans), Astronomik UHC (Medium Band, 33.6% trans), Astronomik O-III (O-III A, 20.5% trans), and Astronomik H-beta (H-beta A, 12.6% trans).

Figure 15   My Deep-Sky Filter Choices

Well, that's it.  I've said all I wanted to say, researched all I wanted to research, and tabularised all I wanted to tabularise.  It is now time to get outside and do some observing!  Thank you for listening.  If you have any questions about my background material, please check out my References page, or contact me at  Cheers!


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Last updated: 07-Mar-12

Copyrights to all content from the webpages hosted here belongs to Jim Thompson. Nov. 2009.