Filters & Light
Picking out the best photons
On the first page of the FAQ I asserted that Astrophotography is a practice of maximizing Signal to Noise in an image. This is true, but it is accomplished through the capture of photons raining down on us from space, so Astrophotography could also be pared down to the directive of gathering as many photons as possible. The rest, including the equipment covered on this FAQ page, is just the details, and while the details are certainly complicated, everything goes back to that basic goal. But not all photons of light are equally useful, and even the definition of useful can change depending on the nebula in question.
Visible Light
On a cosmic scale only a fraction of light is within what humans consider visible light. Many objects in our sky produce brilliant amounts of Radio, Infrared, Ultraviolet, or even X-Ray light, but we are not capable of observing those wavelengths without specialized equipment. Since most consumer-grade cameras are designed to emulate the limits of human vision, they have similar limitations to visible light as well. So to start, the amount of useful light for (non-professional) astronomers caps out at less than 1% of the total electromagnetic spectrum (before anyone leaves an angry comment on the Contact page, yes, you can build a backyard radio telescope, or just watch the static on your TV). The light which concerns us today can be broken down into two categories: narrowband emission and continuous broadband.
The Visible Light Spectrum
Narrowband
This is light of specific colors associated with excited or ionized atoms. Take a pure element and expose it to a sufficient energy source like an electric current, open flame, or stellar radiation, and it will glow in a set of precise wavelengths in response.
Many of you reading this may have taken a basic chemistry course in school which demonstrates this effect in which you burned copper, phosphorous, or other elements over a Bunsen Burner, noting that the reaction produced a flame of varying colors as a result. Or at least, the energized atoms produced photons at specified wavelengths which our eyes interpret as these varying colors.
Spectral Lines from 3 different elements
Other common examples include fireworks – through combustion, varying elements are prompted to produce brilliant colors for a brief time. The Neon signs in the front window of many businesses and the streetlights (often using Mercury or Sodium vapor) punctuating the highways at night demonstrate this effect as well, though they use a more elegant electric current as opposed to explosive combustion. Beyond Earth, many of the Deep space objects we wish to photograph emit light along narrow wavelengths as well. Most of the nebula we photograph are all made from the same few ingredients (the universe only has one periodic table of elements, so far as we know anyway) and these ingredients are often energized through exposure to stellar radiation - more on that later.
Broadband
The other category of incoming light is called broadband, which is light not associated with any one emission line. Broadband light is emitted primarily through reflection or scattering (and usually both at once).
Our Moon is an excellent example which should be familiar to about every person on the planet. The Moon shines entirely due to reflected sunlight which scatters across the surface regolith. Of course that’s not entirely truthful - the Moon does emit light in an emission line, however this emission line is in Infrared, which is a long-winded way of saying the Moon is cold. Galaxies and some nebula would also fall into this category. Galaxies do have small areas containing emission nebula (which are just far away versions of the nebula we shoot up close in the Milky Way), but much of their emitted light comes from massive amounts of dust scattering and sometimes obscuring starlight. Reflection nebula work in the same manner. These nebulae receive energy from nearby stars but not enough to cause ionization. Instead, they reflect and scatter the light, and due to Rayleigh scattering these nebulae often appear blue in color.
The Moon and the Pleiades are two broadband emitters
As a final example, some nebulae are thick enough to block starlight completely. Imaginatively called ‘Dark Nebula,’ these clouds are notable not for the light they produce, but for the absence of it. Dark Nebulae usually contain larger and heavier particles than your average hydrogen cloud which are better suited for scattering starlight to the point that stars can only be detected in infrared or longer wavelengths. Visually, these looks like empty spots in the sky. Photographically, they are difficult to capture in all but the darkest skies since the nebula itself is barely brighter than the true background of space.
Practically these different types of nebulae are not so easily separated. The Orion Nebula (M42) contains emission, reflection, and dark nebula all in the same vicinity, and galaxies like Andromeda (M31), while dominated by reflected starlight, feature eye-catching spots of emission nebulae and dark nebula which reveal its spiral arm structure.
M31, M42, and NGC 7023 all feature various amounts of Dark Nebula, but M31 and M42 also output emission lines as well
Filters
And now I finally get around to talking about filters. As with the section above, I will separate them into their use on broadband versus narrowband targets. Broadband filters, which are designed for use in light polluted conditions or on objects in space which do not emit narrowband wavelengths, often let in a majority of the visible light spectrum while surgically removing wavelengths which are unhelpful. Narrowband filters are often the opposite, rejecting all photons except those within a small percentage either side of a specified wavelength. Remember, filters never make an image brighter – they may allow better quality images to be captured as a result of their use, but by their nature they can only take away light.
IR/UV Cut
The most common type of cut filter is one you likely already use. While consumer-grade cameras are designed to mimic the limits of human vision, they are not perfect and can be sensitive to some IR and UV light beyond our perception. Photographically this can cause pale red or blue haze and discoloration so most cameras will have small glass filters placed above the sensor which cap off either end of the visible spectrum. The limits of these filters may be slightly different depending on their designed use; a typical IR cut filter in a DSLR will severely reduce the sensitivity to the visible red light emitted from emission nebulae, leading some astrophotographers to have this filter removed.
Light Pollution Filters
As mentioned above, streetlights often produce light through ionizing sodium or mercury vapor, and most LP filters are designed to cut off these emission lines while still allowing wavelengths commonly seen in space. These are not a silver bullet against light pollution but can allow a city-bound astrophotographer to be able to take longer exposures than otherwise possible. At one point I was nearly unable to shoot objects in my southern sky due to city lighting, but with this filter the maximum exposure time while pointed South increased from about 1 minute to 5 minutes.
These filters are indeed helpful, but still have a limited use. As they cut off a large portion of the visible spectrum they should only be used to shoot emission nebulae. Broadband objects like galaxies and reflection nebulae emit light along the entire spectrum and using these filters would unfortunately cut off much of the light coming from these night sky objects. Sometimes the best way to combat light pollution is to drive away from it.
The bandpass of Astronomik’s CLS Filter. The cutoff between 540-650nm rejects much of the emission lines from Sodium and Mercury lamps
LED lighting
LEDs are slowly replacing vapor lamps and making light pollution an increasingly difficult opponent. Unless specifically manufactured otherwise, LEDs glow along the entire light spectrum and that makes LP filters increasingly obsolete. All else being equal LEDs are more power efficient, so I can’t blame any city planners for their use, but legislative options can still be used for reducing LP while still allowing night time lighting. Some cities in New Mexico have been known to vote for use of only monoband LED lighting, among other considerations for reduce the spread of light pollution including light shades, downward facing lamps and wattage limits.
Monochrome LRGB / True Color
Shooting the night sky in LRGB is a special case as far as this category is concerned. LRGB filters are not tasked with reducing the effect of light pollution, instead they are tasked with splitting the visible spectrum into even sections. As mentioned in the Equipment Page, most cameras have a Bayer filters which allow capture of color images. This makes taking color photos as easy as pressing a button, but unfortunately this filter lowers the effective resolution of the photo since it is split into 3 color channels and 25-50% of the photons will be reflected since they hit the wrong part of the color filter.
To circumvent this issue a night time photographer can use a monochrome camera. Lacking a Bayer Matrix, color is captured through use of 3 separate images, each with a different Red, Green, or Blue filters placed in front of the camera (as every color image can be separated into Red, Green, and Blue channels). Once these 3 images are combined the result is a color image of higher effective resolution than what would be taken with an equivalent sensor using a Bayer Matrix. Astrophotography is not the first use of this technique; combining monochrome RGB photos was used over a century ago to photograph Mohammed Alim Khan who was nice enough to sit still for 3 different exposures.
Photo by Sergei Mikhailovich Prokudin-Gorskii (1911), with the individual Blue, Green, and Red channels shown on the right top-to-bottom. Note that the Khan’s predominantly blue robe appears dark in the Red exposure in the bottom right
The final piece of the LRGB filter set is L, which is for Luminance. Luminance filters are often little more than an IR/UV cut filter, though made not to interfere with the narrowband emissions of deep space nebulae. Astrophotographers shooting in LRGB often separate the data into two distinct images for much of the post processing workflow. Luminance images, as they encompass the entire visible broadband spectrum, may receive the bulk of sharpening and contrast work while the combined RGB photo is processed to ensure color accuracy and reduction in color noise. Once the two are combined the resulting “LRGB” image should display the best results from both.
Narrowband Filters
The second type of filter is for isolation of emission lines. These filters only allow a narrow bandwidth of light through and reject the rest. The bandwidth which is permitted corresponds to the wavelength produced by an ionized atom of a specific element, with some room for error on either side. Cheaper filters have a wider bandpass, often in the 12-15nm range, while the most expensive filters will be 5nm or less. The purpose for shooting in these filters is explore more in the section below, but the short version is that these filters can be used to stand in for the color channels of RGB and, as they allow higher contrast photography of some nebulae, they can also stand in for Luminance. Another benefit is that, due to their outright rejection of most of the visible spectrum, they are generally the only option for astrophotographers in heavy light pollution to take long exposure night sky photos, though I will again mention that the increasing use of LED lighting is making this more difficult in some areas. Astrophotographers mainly use 3 narrowband filters today:
Hydrogen-Alpha / Hα (656.28 nm)
3 out of every 4 atoms of normal matter is Hydrogen. Combine enough of it together and you get a new star, and that stellar radiation is great ionizing all the remaining Hydrogen around it, causing them to emit a deep red light called Hα. This self-serving process is often seen in the emission nebula in deep space, making Hα an incredible wavelength for photography. While mostly used for producing clean and higher contrast images of nebulae, Hα filters are also useful for photographing the star formations areas of nearby galaxies.
IC 443 in Hα, 23 hours integration. Note the presence of Hα in both the shock fronts of the supernovae and the background nebula to the right
Double-Ionized / Oxygen O III (500.7 nm)
O III is typically strong in Planetary Nebula, which are nebula created by a White Dwarf Star which has shed most of its former matter into the surrounding space. It is teal in color
IC 443 in O III using 33 hours of exposure. While much dimmer, O III can be seen on the leading edges of both Supernovae
Ionized Sulfur / S II (672.4nm)
Sulfur may seem like an odd choice since it is the 10th most abundant element, but it is usually present in areas of space which have experienced energetic events. Sulfur is created late in the life cycles of stars and during supernovae, so areas of an image strong in S II are often regions of high velocity and/or radiation. Visually, S II is a slightly deeper red than Hydrogen Alpha
IC 443 in 14 hours of S II. While appearing similar to Hα, S II is noticeably dimmer in the supernova area and provides little nebulosity to the lower right
Lesser Used Filters
Astrophotographers may use other filters at times, but many of them will have limited use. Hydrogen-Beta (Hß) is a good example and it is used somewhat frequently on certain nebulae. Peaking around 486nm (close to O III’s ~500nm), Hß is actually emitted from ionized nebula just the same as Hα but it requires more energy and is typically not as ubiquitous as the more common Hα, so its use in providing detail for astrophotographers is limited. All else being equal, an Hα filter is likely to provide more contrast since it is a more common type of photon.
Ionized Carbon filters may be useful in some cases. The ethereal green often found in the glow of comets is due to Carbon, but every Comet has a slightly different chemical makeup. A bandpass of Carbon which excessively isolated it from other bandwidths may also cut off other bright emission lines produced by the Comet, including CO2 and Arsenic.
Helium and Nitrogen filters are another example of seldom-used filters. Helium is the second most common element of normal matter, but its ionized state requires a higher energy than O III or S II. Ionized Nitrogen (N II), while also more common than Sulfur, produces light which is already picked up by all but the tightest Hα bandpass filters (N II has dual emission lines which both peak +/- 2nm off the Hα line of 656 nm).
Bi/Tri-band filters (DSLR Bayer issue)
Shooting in any one of these Hα, O III, or S II lines is often counterproductive when using a color camera. In the LRGB section above I mentioned that shooting with a Bayer Matrix will require longer exposure since a given photon will only hit the right section of the Matrix 25-50% of the time. With narrowband filters this challenge is compounded since all other wavelengths are rejected, meaning the time necessary for a properly exposed image is much higher.
The solution to shooting narrowband with a DSLR could be to take significantly more subexposures and trust the stacking process to increase the signal to noise ratio, but another option is to use a filter which passes the Hα, O III, and S II spectral lines all at once. Various dual, triple, or quad-band filters such as the Radian Triad are quickly gaining popularity for those using color cameras since it allows the entire Bayer Filter to be used while capturing Hα, Hß, O III, and S II all at once.
Color PALETTES
Now that you’ve spent precious and unretrievable minutes of your life reading the fascinating topic on how stuff glows in different colors, you may now be wondering what we can do with narrowband images. The use of Broadband LRGB seems obvious since it forms a true color image corresponding to human vision, but narrowband is much more selective in the photons it accepts. And in some cases a stacked Hα image of an emission nebula is enough on its own. Artistically, a black and white photo can be stark, compelling, and [insert other artistic adjective]. However, one of narrowband imagery can also assist both the color and contrast quality of RGB photos, or even form color images of its own. This is generally split into true color and false color.
Narrowband / Broadband Combination
Hydrogen-assisted Red / HαRGB
Hydrogen-α, itself being a deep red wavelength, is often blended into the broadband red channel to boost visible nebulosity or reveal the star-forming regions of a galaxy. For emission nebula like the Flaming Star (below), Hydrogen can be used in place of a broadband Luminance channel since the narrowband image likely has more detail in the first place.
The addition of Hα to color images will increase nebula color and contrast as well as bring bloated stars back under control
S II could also be used in this regard since it is a red color close to Hα, though it is usually too weak a signal in emission nebulae to be useful in the face of just using Hydrogen instead. A more obscure use of S II is to capture what should be (and technically is) Hα light. Try and photograph a Galaxy which is more than a couple hundred million lightyears distant and you will need to consider red shift. The universe is expanding, and the doppler shift of light waves at that distance will actually move the Hα regions into the S II band by the time it reaches us.
This is not necessarily an accurate representation of color since emission objects are often more pink. The reason for this is that while Hydrogen-alpha is red, the other Hydrogen-beta and -gamma lines emit photons of blue color, leading to an overall pink glow. As such, sometimes Hα data, which is itself honestly red since the filter ignores the photons from the beta and gamma lines, is sometimes blended at reduced intensities into the blue channel to simulate other Hydrogen wavelengths.
The glow of a Hydrogen lamp via nasa.gov
O III-assisted G/B
Just as Hydrogen can boost the color and contrast of the Red channel, O III (naturally teal in color) can do the same for Green and Blue channels. This is sometimes used with Planetary Nebula and Supernova remnants since those objects can be strong in O III signal. While O III is useful for boosting color, is not generally good in for Luminance on its own. Even in objects strong in O III like Planetary Nebulae and Supernovae Remnants, the detail provided by O III is distinctly separate from the Hα, though blending the O III and Hα images together can form a composite image which can be better tasked for Luminance detail.
For many emission nebulae, particularly the Planetary Nebulae and Supernova Remnants, boosting the color using both Hα/R and GB/O III is quite common. The details on how narrowband is combined with broadband is explored more on the Post Processing page and includes further visual examples on the effects of narrowband combination with RGB images
False Color
False color, as its name implies, is not concerned with color accuracy. The strategies used until now take definite artistic license but they are still generally bound by some limits of accuracy. R-filtered images are mapped to Red, G to Green, and B to Blue, and even on the narrowband side Hydrogen is used to assist red (and sometimes blue), Oxygen to blue/green, etc. False color, however, will map various narrowband images to any of the 3 color channels either arbitrarily or in order of their wavelength to show the highest contrast of detail. While in true color about any emission nebula will be dominated by the red color of Hydrogen, in false color these narrowband wavelengths can be any number of contrasting colors.
The Hubble Palette / SHO
One of the most famous color palettes maps the narrowband channels in order of their wavelength, with Sulfur mapped to Red, Hydrogen to Green, and Oxygen to Blue. While Sulfur and Hydrogen are close neighbors as wavelengths go, Hydrogen is still a higher wavelength by a small margin, so it is stuck being green. This order of color mapping is often denoted as SHO, and you have probably seen at least one SHO image before, even if you were at the time unaware.
The Eagle Nebula in SHO from the Hubble Space Telescope
Bicolor HOO
Another color combination which offers some shortcuts and interesting results uses only two narrowband images, which is why some call it Bicolor. As false color goes, HOO is visually close to true color since it maps Hydrogen to Red and Oxygen to both Green and Blue (though some use an average of Hα and O III for the Green channel instead of just O III). And if you recall the section above detailing how Hα and O III can boost the Red and Green/Blue channels respectively, the HOO palette almost sounds like the same thing, just skipping the broadband entirely.
IC 443 in HOO. Note the neutral star colors
The results would agree with this since the nebula looks about the same, just with somewhat bland star coloration. Given that narrowband imagery often has a monopoly on the interesting detail of a nebula in question, some astrophotographers find processing their photos to be easier by starting with an HOO combination and then blending in the RGB stars using careful masking.
Other Combinations
False color offers near endless combinations, and for photo processing programs like PixInsight, which allow images to be combined with more explicitly mathematical means, the combinations truly are endless.
The SHO-AIP Script can be downloaded for PixInsight and allows easy combination of SHORGB photos, and even includes an entry for N II images.
Non-Visible Wavelengths
While not relevant to an article explicitly only covering visible light, I should still mention that non-optical light is still captured by professional astronomers and sometimes mixed in with visible light images. Ultraviolet, Infrared, X-Ray, and even Radio waves are sometimes translated down into the visible spectrum so our limited vision can visually appreciate what the instruments have detected in nonvisible light.
Jupiter’s Northern Lights captured in Ultraviolet but the Hubble Telescope, presented as a bright blue on a separate visible light image of Jupiter
This has been a lengthy yet brief explanation in how light is used (or not used, in some cases) by astrophotographers. However, filters are only a small part on how astrophotography works. If you want to learn more and have not already seen the other FAQ articles, be sure to check out the Equipment, Capture, and Processing pages to learn how these photos are taken, processed, and the equipment required to accomplish this.