# Why do we use filters in telescopes for astronomical imaging?

I have read that if we image without a filter we get no information about the color or SED of objects. Can anyone elaborate the reasons for using filters for imaging/photometry? What happens if we image without a filter?

In general, the CCDs used to capture images do not register the energy (therefore, colour) of the incident photons on them - they just count the number of photons observed by each pixel (or a value proportional to the number of photons, as the are not 100% efficient). So, they essentially just show overall brightness variations across the image.

If you want to capture colour information you therefore have to use filters. I.e., if you want to get information on the number of red photons (e.g., the intensity of red light incident on each pixel of your image), you use a filter to block out all other light. You can do this with multiple different filters to build up a full colour image.

In most commercial digital colour cameras the CCDs have a filter mask over four pixel patches: two filtering for green light, one for blue light, and one for red light. The outputs from these pixel is used to build the full colour image.

As WDC pointed out in his comment, without filters, you simply get a recording of the received irradiance as a function of the sensor's spectral response function. In other words, a normal CCD that detects the light in a camera isn't capable of picking up every wavelength of light perfectly and the response function tells you how good that CCD is at picking up every wavelength of light.

Sometimes though, you don't want to take a picture and record every possible photon the CCD is capable of recording. Sometimes you want to record specific wavelengths. You do this by applying a filter, before the CCD, that let's in only specific wavelengths.

This has all sorts of uses. A simple example would be taking three pictures, one with a red filter to primarily let in red light, another with a green filter to primarily let in green light, and a third with a blue filter to primarily let in blue light. When you look at the individual images on your screen, the computer doesn't know what colors (i.e., wavelengths) of light the CCD saw, it just knows how many photons were observed so it can only show you a greyscale. However, you can then, in post-processing, tint your image with the red filter red, tint your green image green, etc. and then combine you red, green, and blue images into a single, colored picture to get a close-to-true, color image of your object. In fact, that's actually how digital cameras actually work to take colored pictures!

Besides using filters to get colored pictures, Astronomers use filters for a wide variety of science goals. It is very possible to create a special filter that only lets in a single wavelength (or as near to a single wavelength as one can get). Often single wavelengths of light are tied to specific physical processes. By that I mean, only specific physical processes can create that exact wavelength. So by looking at something with a filter for a specific wavelength, you're looking at the components of that object that created that wavelength of light.

A common single-wavelength filter people like to use is the H-alpha filter and a common target for observation is the Sun. Shown below, and taken from APOD, is a picture of the Sun using the H-alpha filter.

Or simlarly, the Solar Dynamics Observatory (SDO) spacecraft is constantly observing the sun in all sorts of filters. Note how different the sun looks at the different wavelengths!

Note: these are false color images for effect!

A group of students are browsing the internet before class.

One of the students asks, &ldquoDo you guys ever look at the Astronomy Picture of the Day website?&rdquo

• Cyle: &ldquoOf course. I love APoD, especially those colorful nebulae and supernova remnants. It&rsquos too bad there aren&rsquot any of those close by that you can see just with your eyes.&rdquo
• Donna: &ldquoThose aren&rsquot the real colors. They have to do things to those images, like color in the clouds and gasses.&rdquo
• Eric: &ldquoI thought they had to take different images of the same object and merge them together.&rdquo
• Fiona: &ldquoThen how do they know what colors x-rays are?&rdquo

Taking color pictures with optical telescopes such as Hubble, or any ground-based telescope with CCD detectors, is very different and much more complex than using film in a traditional camera. Electronic detectors do not read out information in color&mdashrather, the energies of the photons must be assigned colors in a computer process known as image processing. For visible light images, the color choices are sometimes assigned to try to faithfully reproduce what our eyes could see (if they could stare at the object for a long time without blinking or otherwise taking &ldquosnapshots&rdquo). Many full-color images are combinations of data taken in separate exposures of red, green, and blue visible light. When mixed together, these three colors of light can simulate almost any color of light that is visible to human eyes. That is how televisions, computer monitors, and video cameras recreate colors.

The standard colors that are mixed together on a television screen are called R, G, and B for red, green, and blue. This is done with a set of filters that pass light of wavelengths centered around 650, 520, and 450 nm, respectively. They block all other colors. Each filter is typically about 100 nm wide. While RGB filters are adequate for creating color images on a screen, these are not the filters that astronomers typically use.

Astronomical filters were not developed to create color images, though they can be used for that purpose. Rather, they were designed to study the physics of stars and other astrophysical objects. For instance, by comparing the brightness of a star in two filters, it is possible to determine its temperature. This is because the separate filters sample different points on the star&rsquos Planck spectrum. Because Planck curves with different temperatures are unique, these two points are sufficient to uniquely determine the shape of the curve, and thus, its temperature. But stars are not perfect Planck emitters. They have absorption lines. (Sometimes, they even have emission lines.) Filters can be designed to be especially sensitive to these absorption lines, and thus, provide the ability to distinguish one type of star from another by means of the absorption features. Filters allow this determination to be made by simple imaging techniques rather than more complicated spectral techniques, usually saving a lot of time at the telescope.

Standard photometric filter sets have been developed over the past 50 years. The most common set is called the Johnson/Cousin system. It was developed in the 1960s and uses U, B, V, R, and I filters, for ultraviolet, blue, visible, red, and infrared. These filters typically have widths of about 100 nm, give or take, and they are centered at 365, 445, 551, 658, and 806 nm, respectively. Additional filters have been developed that push farther into the near and mid-infrared, going out into the region between 1,000 and 5,000 nm (1 and 5 microns). Other filter sets have been developed as well, usually with some specific use in mind. For instance, both Hubble and the Sloan Digital Sky Survey developed special filter sets based on their instrumentation and science goals.

In addition to these broadband filters, there are narrowband ones that only pass light close to a particular wavelength. These narrowband filters typically have widths less than 10 nanometers and are centered on an emission line from hydrogen, oxygen, sulfur, etc. Many of the beautiful pictures we see of nebulae (gas clouds) are taken using several of these narrowband filters to highlight emission from different atomic species.

All astronomical observations are done using these (or other) standard filter sets. This standardization allows one set of observations to be easily compared to another, a very important ability to have when doing science. Whenever a new filter standard is created, a lot of effort goes into understanding how it is related to others so that the new observations can be compared to older ones. Of course, it would be much easier to always use the same sets of filters for all observations, but sometimes, the science goals make that impractical or impossible.

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### Seeing Planets in a Different Light Reveals More Detail

Earth's atmosphere is in constant fluctuation turbulent air currents blur fine surface detail on solar system objects viewed through a telescope. Faint contrasting areas blend together due to "irradiation" &mdash a distortion of the boundaries between light and dark surfaces.

When you employ a color filter to zero in on a narrow region of the spectrum, the scattering of interfering wavelengths is enormously reduced. Suddenly, the smeared, pale bands of Jupiter resolve into loops and festoons. Delicate markings appear on Saturn's globe, and the Cassini ring division darkens and solidifies. Mars' polar caps stand out like tiny pearls, and vague lunar rilles acquire greater depth and contrast. Bad "seeing" becomes acceptable good seeing becomes superb!

Because many planets have a characteristic color (e.g., Mars is reddish), you will dramatically increase detail by reducing the predominant hues, uncovering hidden contrast and surface markings. That's why "the Red Planet" is most effectively enhanced with a green filter.

Below is a guide to using color filters to best view the planets in our solar system.

Mercury
#25 Red will make the planet's disk stand out against a blue sky, permitting daytime or twilight viewing. Mercury is usually best observed just after sunset, when the sky is awash in orange light, so employ #21 Orange with high magnifications to see the planet's phases.

Venus
No matter what telescope aperture you use, Venus's excessive brightness usually causes a very "overexposed," roiling image. With a #47 Violet filter, or stacked #58 Green and #80A Medium Blue filters, you'll reduce the severe twinkling for a better view of the fascinating changing phases.

Mars
#25 Red passes the predominant reflections of surface plains and maria, and #21 Orange is good for reducing the intense glare to enhance detail and mottling. The polar caps stand out with #15 Deep Yellow and #80A Medium Blue examine the melt lines with #58 Green.

Jupiter
This great planet reveals its cloud bands, loops, festoons, ovals, and Red Spot with #11 Yellow-Green, #80A, #58, and #21. Go from seeing only two bands without a filter to seven or more with a filter! Try stacking filters to reduce heavy glare.

Saturn
Many subtle globular details are improved by #15 Deep Yellow. See the difference in brightness of the extremes of the rings with #25, #11, #58, or #80A. #15 helps sharpen Saturn's image in photographs, improving the resolutions of Cassini's division.

Moon
Reduce the Moon's glare with #80A Medium Blue, and enhance the contrast of lunar rilles and strata with #15 Deep Yellow.

Other Uses
You will improve black-and-white photographs by blocking UV light with #15 Deep Yellow. Refractor chromatic distortion is also reduced by #15, and by #80A. The #82A Pale Blue is great for stacking with other colors, and can adjust film color balance by absorbing excess yellow and red. #58 Green will block street light while passing much of the wavelength of doubly ionized oxygen in emission nebulae. Try #25 Red for long black-and-white exposures of the Omega or Rosette Nebulas.

## The Lake County Astronomical Society

Newbies to observational astronomy quickly learn that the full moon is a daunting adversary that blots out contrast and detail in the faint fuzzy objects we pursue. If you're getting your feet wet in astrophotography, you've probably noticed that the bright moon does an even nastier job on your photos. The light pollution in your sky is amplified by the sensitivity of the camera. Moisture in the air reflects the moonlight and gives a smoky look to the sky through the camera. And nasty gradients start harmlessly at one end of your image and increase until they fog the details of your image.

All in all, the moon is a much bigger enemy to the imager than the observer. But what if there was a way to neutralize the moon and even neutralize the local light pollution we're constantly bemoaning? Wouldn't you like to be able to image even during the 13 brightest days of the lunar month from the 9-day old moon to the 22-day old moon? Well of course there's something you can do and involves doing something that is totally the opposite of what we normally do in astronomy: You want to throw away light. Huh? Yes, throw away light. Whah? Yep, get rid of a LOT of light. Let me explain.

Conventional progress in astronomy is all about collecting lots of light: We get big lenses and mirrors - more surface area means more light collected. In photography, we use those big optical surfaces with cameras that can stare at the sky for a long time to pile up the incoming light. When we're collecting light from a nebula or stars or dust the goal for better pictures is to use bigger optics, more sensitive cameras, and longer exposures. That's the philosophy from light pollution-free areas and (relatively) moonless nights.

When there's light pollution and/or a bright moon, the rules change. Bigger optical systems amplify light pollution - man- and moon-made. Long exposures on sensitive cameras do the same thing. And what's worse is that the sky mess isn't uniform. The area of the sky closest to the terrestrial or lunar light source is brighter than the areas that are farther away. The camera can catch this gradual brightening and dimming - a "gradient" - much better than your eye can detect. So collecting a big bunch of "bad" light makes your photos much poorer instead of richer.

The solution many astrophotographers use is to be selective - VERY selective - in our light "shopping". Discard all the bad light, and keep only the very good light. We'll skip the physics lesson, but the most popular lights to collect are Sulphur II (SII), Oxygen III (OIII), Hydrogen beta (Hbeta) and the most popular of all, Hydrogen alpha (H-alpha). These specific wavelengths of light correspond to different atomic states of different elements. To collect only specific light requires a filter and a small detour to explain terminology.

Normal astronomical filters 1 , like an infrared blocker or primary color filters will allow a fairly large part of the optical spectrum to pass through the filter. The part (or "band") of the spectrum is wide, so these are called "broadband filters". On the other hand, when we want to be snobbish about the light we want to collect we choose a filter that only allows a thin part of the spectrum onto the camera. So these specialized filters are called "narrowband filters". Narrowband imaging is quite popular these days because it allows astrophotographers to take photos regardless of the phase of the moon and it can substantially eliminate the effects of your local light pollution. No more hiding out during bright phases of the moon and no more chagrin at your local light pollution.

In this article, I want to concentrate on H-alpha imaging, first because it's so popular (in the universe and among astrophotographers) and second because it's important to understand the limits of H-alpha imaging before you get too excited about it.

Picking your targets: So what objects benefit from H-alpha filtering? The H-alpha light comes from excited hydrogen. Certainly stars have a lot of that so stars show well through an H-alpha filter. And other types of energy can excite clouds of hydrogen gas, so clouds surrounding star creation and star destruction activity are also candidates. What we see in objects like the Eagle Nebula, the Orion Nebula and the Lagoon Nebula are clouds of gas surrounding locations that represent forming stars. The energy of these stars excites the gas that then emits light. So these nebulae are a called "emission nebulae". Emission nebulae are VERY GOOD candidates for H-alpha filtering. The light they send out is broadcast in one or a few very narrow parts of the spectrum.

On the other hand, objects that are merely reflecting light usually have light that is broadcast along much larger parts of the spectrum. Reflecting nebulae are poor candidates for H-alpha imaging. The Running Man Nebula (NGC1977) and The Pleiades (M45) are famous reflection nebulae.

There are composite objects like the parts of the Veil Nebula (including NGC6992 and NGC6940), and the Trifid Nebula (M20) that have an emission nebula that illuminates a reflection nebula. You can use H-alpha filtering to capture part of the object, but the reflection part of the nebula will need other techniques to be captured.

Some galaxies have strong areas of H-alpha emissions (like M82 and M33), but overall, galaxy images don't benefit from H-alpha photography. Like galaxies, there are some planetary nebulae with portions of H-alpha light, but again, the reflection part of the nebula will not be seen through the filter.

The moon and other solar system objects don't benefit from H-alpha imaging. H-alpha light is significant in solar photography, but YOU MUST USE SPECIALIZED SOLAR FILTERS OR YOU CAN DAMAGE YOUR EQUIPMENT OR CAUSE SERIOUS PERSONAL INJURY. In this article we're discussing nighttime H-alpha images (otherwise the phase of the moon wouldn't matter!).

So, the primary targets for H-alpha imaging are the emission nebulae and the open clusters that include an H-alpha gas cloud (for example, M16 - the Eagle Nebula).

Camera matters. Not all cameras are good at collecting H-alpha light. Many dedicated astrophotography cameras have good or excellent sensitivity to the H-alpha wavelength (6463 angstroms). Consumer digital cameras sometimes contain a chip that is sensitive to H-alpha (as well as infrared) light, but the manufacturers mount a blocking filter to help get a good color balance for everyday photos. So the normal pocket digital camera won't do the job in H-alpha.

The popular DSLR's (e.g., Canon 350XT and Nikon D-70) have the same problem, but there are home modifications that can be made to the camera to remove the manufacturer's filter and allow the H-alpha light onto the chip. Sometimes, this renders the camera unusable for routine photography and sometimes you can correct the colors using external filters or special settings and processing. One company, Hutech Astronomical Products, even sells fully modified Canon and Fuji cameras to permit H-alpha photography.

By far, the best candidate for H-alpha photography is a dedicated astronomical camera. Cameras from SBIG, Starlight Xpress, FLI, Meade, Orion will almost always have decent H-alpha sensitivity. The easiest way to find out if your camera has good H-alpha sensitivity is to find the spectral response specifications or just use Google to search for examples of imaging in H-alpha. The exact sensitivity varies with the camera models. For example, my SX MX716 has great H-alpha sensitivity while its cousin the MX916 has fairly poor sensitivity.

Mount requirements. So you've decided your camera will gladly accept H-alpha light, you like the choice of imaging objects and you think you're ready to go. But hold on - the mount you will use needs to do a little more work than you are accustomed to. Here's why: In normal, multi-color light imaging (for example with a DSLR), you collect light from a wide spectral band - about 400nm wide. H-alpha filters come in bandpass widths of 3nm to 13nm. So you are only collecting 1/30th to 1/130th the amount of light. This means you need to take longer exposures (or many, many short exposures) to compensate. That image you captured with 10 exposures of 30 seconds (5 minutes total exposure) will need 30 to 130 TIMES more exposure - 150 minutes to almost 11 hours exposure. There are processing techniques so that narrowband imagers don't necessarily endure these marathon exposures, but the bottom line is be prepared for many and long exposures.

This requirement puts a lot of pressure on your mount. Getting smooth tracking in a short exposure is (somewhat) easy. But in a 4, 5, 8, 10 minute exposure? Only the best mounts can do unguided images that long. If you have an autoguider setup you will get a lot of use out of it during H-alpha and other narrowband imaging. 2

Another mount-related aspect is the quality of your alignment. For sequences of short exposures any errors in your alignment won't cause too much field rotation. However when your exposure length get longer - 4 to 10 minutes - you might notice that the stars on the edge of the image seem to be more oblong compared to the round stars in the center. This is the effect of field rotation. A poor polar alignment (or a good alt/azimuth alignment) will show arcs of stars - small arcs in the middle and larger arcs as you get farther from the center.) If the polar alignment is good (but not perfect) you don't see the effect much in the middle of the image, but nearer the edge, the short arcs make the stars a little squat. The solution?? Put more effort into refining your polar alignment if you want the long exposures needed for good H-alpha (and other narrowband) images.

Filters. Narrowband filters, including H-alpha, are like typical colored filters. They come in 1.25" and 2" sizes (priced accordingly). The filters have standard threads to attach to most cameras. Like normal filters, narrowband filters can be used in color filter wheels and strips.

Each filter has a specific bandwidth and bandpass center. The H-alpha filters are centered on the 6563 3 angstroms or 656.3 nm. The bandwidth of the filters varies from 3nm to 13nm. The 3nm filters have a very, very narrow bandwidth and gets only the purest H-alpha wavelength and normally have a higher price. This bandwidth results in really small, sharp stars. On the other hand, the 13nm filter has a more "open" bandwidth and gives slightly larger (but still small!) stars and fractionally less contrast compared with the narrower filters.

The masters of narrowband imaging prefer the narrowest bandwidth forms. They feel that the precise exclusion of all other light makes for better photos. Of course, their images take about 4 times as long as images with the wider filter because they need to compensate for the narrower slice of the spectrum. Most of these experts have excellent mounts and automated controllers, so some extra hours of exposure aren't a burden to them.

One other consideration when selecting a bandwidth is "drift". The coatings used for narrowband imaging presume that your optical system operates within a specific range of f/-ratios. Often, the comfort zone is about f/4 to f/11. As you go outside that range, the filter's "center" will shift red-ward or blue-ward. If your filter has a narrow bandwidth and you use optics outside the comfort zone, the filter can totally miss the targeted light. The wider bandwidth filters have the same shift, but being wider they won't miss the goal light.

So if you plan to use optics outside the comfort zone (for example, very long focal length telescopes or very fast camera lenses), you should stick to the wider bandwidth filters.

Other filters, other applications - We only touched on the other kinds of filters for narrowband imaging. Each has a use for a specific kind of light that corresponds to specific astronomical environments and events such as star formation, star destruction, etc. You are welcome to explore the use of other filters via on-line resources, but I recommend starting your imaging with H-alpha to get the best choice of targets and ease of imaging.

Conclusion and Resources: So, do you want to get out and take images regardless of the phase of the moon or even if your sky isn't perfectly dark? Can you mount handle longer exposures? Can your camera record the fun parts of the spectrum, like H-alpha?

If so, buy a nice H-alpha filter and give narrowband imaging a try.

You can find great examples of narrowband imaging at Richard Crisp's website -
http://www.rdcrisp.darkhorizons.org/
The Starizona website has a good discussion of narrowband imaging located at -
There is a Yahoo! group dedicated to narrowband imaging located at -
http://groups.yahoo.com/group/narrowbandimaging/

You should also check with astrophotography groups for your specific camera to see what tips for narrowband imaging apply to your equipment.

1 Filters come in two forms: "pass" and "block". A pass filter allows specific light to pass. So a red filter allows red (but no other) light to pass. A block filter allows all light except a specific light to pass through the filter. An "IRB" blocks the infrared light, but allows all other wavelengths. Unfortunately, the common terminology for filters is not specific, so a reference to an "infrared filter" could mean a filter that blocks infrared light or a filter that passes only infrared light. For this article, the term "H-alpha filter" means a pass filter that only allows light near the H-alpha wavelength.

2 There is an upside to the laughably long exposure requirements for H-alpha imaging. When you kick off a 90-minute sequence of images (under computer control), you can use that time effective for other tasks. Some people do observing with a separate set of optics. Me? I take a very nice, short nap!

3 In New Mexico, most highways have a 3-digit designation, but there is one 4-digit highway: 6563 also known as the "Sunspot Highway". It leads to the National Solar Observatory at Sacramento Peak.

One of the simplest ways to boost the quality of views offered by your telescope is the addition of filters to your setup. Whether scrutinizing the ice caps of Mars or surfing the clouds of distant nebulae, looking through a telescope filtered for its target can radically improve the viewing experience. However, choosing the proper filter for the job can be a daunting task. Never fear! After reading this article, you will be prepared to outfit your gear no matter what your favorite celestial target is.

### Night Skies and Our Eyes

The quality of views offered by your telescope depends primarily upon three factors: resolving power, contrast, and sharpness. While resolving power depends largely upon the aperture of your scope, sharpness and contrast can be boosted by the application of filters. Both attributes are affected by the imperfections of human vision and what astronomer’s call seeing, the effects of light scattering in the atmosphere. Proper filtration can do wonders for both issues.

If you have ever forgotten your sunglasses on a trip to the beach, you know the benefits of filtering light before it reaches your eyes. Your retinas are loaded with light-sensitive cells and when they become exposed to large quantities of light it can be difficult, if not painful, to see. Viewing the sky at night poses the opposite problem—our eyes are under-stimulated by the light in their surroundings. It might seem counterintuitive at first that the solution to this problem involves the use of filters which, by nature, reduce the amount of light reaching your eyes. Without getting lost in the complexities of human vision, it is worth knowing a little about how our eyes work to understand why filters are an astronomer’s best friend.

You might remember from biology class that your eyes contain two kinds of photoreceptors: rods and cones. Three types of cone cells, each responding to a different wavelength of light (red, green, and blue) are responsible for our perception of color. They are fewer in number and require much brighter conditions than our rods to relay sensory information to our brains. Our cones take the reins during photopic (daylight) conditions. Scotopic (night) vision relies entirely upon our rods. Although you have about twenty times as many rods as cones, there is only one type of receptor at work, and it responds ideally to a wavelength of light situated between the blue and green spectra. If you have ever wondered why the world seems a little bluer at night, it is because your rods are working solo. Scientists call this the Purkinje Effect, named after the Czech neuroscientist, Jan Evangelista Purkinje. So, what does all of this mean for astronomers? Because scotopic vision is monochromatic, the determining factor for perception in the dark is contrast. This is where filters come into play.

Peak response curves for rods and cones

Telescope filters screw into the barrel of your eyepiece and are sized accordingly. All filters operate by reflecting some light and transmitting the rest. Their value to astronomers resides in their ability to let you pick and choose which wavelengths of light reach your eye. By controlling the type and quantity of light that your eyes perceive, you will be better able to distinguish differences in low-light conditions. By reflecting certain wavelengths, transmitted light appears much brighter, increasing contrast and improving your view. Since different celestial bodies reflect or emit different wavelengths of light, filters allow you to fine-tune your instruments, depending upon where you are looking in the sky.

### Turn off the Bright Lights

Unless you are planning to use your telescope thousands of miles from civilization, your overall viewing experience will be improved by the application of a light pollution reduction (LPR) filter. If you are looking for one filter to enhance your views across the board, add one of these to your bag. LPR filters reflect the wavelengths of light associated with sodium-vapor and mercury-vapor lamps, the most common types used for street illumination. If you live near or in a city, an LPR is as necessary as a tripod. While the elimination of all artificial light is impossible, LPRs will give you a noticeably darker sky to work with.

While most of this article is aimed at providing solutions for boosting our perception of light associated with distant subjects, one beloved night-time target is so bright that it requires dimming: the moon. The best options for teasing detail out of the moon are the use of polarizing and/or neutral density filters. If you have ever experimented with filters on your camera, you are probably familiar with these two photography staples. Neutral density filters (often marketed as “moon filters” for the astronomy crowd) reduce glare while leaving the colors transmitted to your eyes unaltered. Because neutral density filters uniformly reduce light across the spectrum, they will not increase contrast, but they will cut back the intensity of light reaching your eyes, allowing you to see otherwise invisible details. Polarizing filters cut back on reflection and have the added benefit of allowing you to manually adjust the filter strength. With the turn of a thumbscrew, you can choose the precise amount of filtration for optimal viewing.

### Neighborhood Watch

As we move further away from Earth, the variables that affect our view of the sky begin piling up. The atmospheric conditions and physical composition of each planet present unique challenges, depending upon where you are looking and what you are looking for. Consequently, there are countless ways to enhance your views for each planet. This is where color telescope filters come in handy. Several manufacturers offer “planetary sets” that consist of varying grades of red, yellow, and blue filters. As a bonus, most sets include a neutral density filter in their lineup. Color telescope filters are described using Wratten numbers, the same industry standard used to categorize camera lens filters. A comprehensive list of filter-planet combinations would triple the length of this article. However, a few general tips should get you started. Red filters help with daytime viewing of Mercury and Venus. Yellow filters boost contrast in Neptune and Uranus while teasing out detail in the belts of Jupiter and the surface of Mars. Blue filters are the most versatile of the group, revealing dust storms on Mars, the belts of Jupiter, and the rings of Saturn. Finally, if you are interested in the stormy skies of Venus, try a violet filter. It is never a bad idea to try a couple of different filters for each subject—you may be surprised by what you see!

### In a Galaxy Far, Far Away

It comes as no surprise that the most difficult (and often beautiful) celestial phenomena to observe are the ones furthest away. Narrowband and line filters add clarity to this otherwise cloudy subject. Narrowband filters block out all light except for small ranges of wavelengths associated with specific phenomena. Line filters are even more precise, blocking out all but one or two wavelengths of light. The most popular of these groups is the Oxygen III (OIII) filters, which reflect all but 496 and 501nm lines, associated with planetary and emission nebulae. Such extreme filtration provides a clean, black background for observation.

No matter what your favorite celestial subject may be, there is a filter out there that will help you get to know it better. Experimenting with the many available options is the best way to determine what works best for you. So, grab some filters and have a look!

*Note: The photographs in this article serve as simulations. Cameras register greater detail and more colors than the human eye is capable of perceiving when looking through a telescope.

## Moon filter

A neutral, grey or moon filter is used to lessen the intensity of bright moonlight and to slightly increase contrast. Anyone who has ever been to an observatory and looked at the Moon through a large telescope without a filter will vividly remember the experience and know why this filter is so important. Observing the moon without a filter will not cause any damage, but it is so bright that it really dazzles you. If you then turn away from the telescope and look into the darkness you will often still have a ghostly afterimage of the moon in the eye you observed with. Although this afterimage will gradually fade, it is still very irritating.
Of course these filters are available in different light reduction levels. They range from a light transmittance of about 8% up to 50%. The filters with a high transmittance are suitable for the smaller telescopes and those with a low transmittance are suitable for larger telescopes.

Adjustable polarizing filters are the luxury version of Moon filters. This is not just one, but two filter elements, which are connected to each other. Rotating one filter element relative to the other continuously adjusts the amount of darkening. Most polarizing filters allow light transmission levels from 1% to 40%. They can be used to set the optimal balance between light level and contrast for the size of telescope you are using.

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## Final Words

All in all, telescope filters are a must-have accessory for all telescope users. It is obvious that without a filter, the universe viewing experience is incomplete. You can begin by getting a moon filter because it is the most basic one. Every astronomer imagines what it must be like to go to the moon and look at the craters by yourself. That is a little impossible but you can get a taste of that experience by viewing the moon through a good telescope filter.

If you are more inclined towards viewing the planets, get the colored filter that works best to enhance the details of your favorite planet and get lost in the beauty of it. Or if there's a solar eclipse coming up, it is absolutely necessary and important to get a solar filter to be able to observe different phases of the eclipse. In other words, get filters for your telescope and look at the universe with a new perspective.

You can choose anyone from the above list according to your requirements and you will not be disappointed surely.

## Why do we use filters in telescopes for astronomical imaging? - Astronomy

The single biggest problem facing any observer wishing to undertake a programme of high resolution photography is the atmosphere. When a good quality, well collimated telescope is used the atmosphere is responsible for nearly all deterioration of the image quality delivered at focus. Astronomical seeing is a very well-documented phenomenon, but with the increasing number of observers employing large aperture telescopes for high resolution imaging, another not so well-known process can affect image quality far more than observers realise. Indeed until recently I had rather underestimated the effect of this phenomenon. This effect is atmospheric dispersion.

Atmospheric dispersion and its effects

The atmosphere imparts many deleterious effects on the light that passes through it. Astronomical seeing (the mixing of air of different temperatures) is undoubtedly the most destructive property when it comes to obtaining high resolution images, however atmospheric dispersion also imparts serious effects, especially when employing large aperture telescopes with the object of interest located well away from the zenith.

Dispersion is the ‘smearing out’ of light of different colours due to differential refraction as it passes through our atmosphere. The level of dispersion present is related to the wavelength of light and the filter passband. Shorter wavelengths / wider filters are more seriously affected than longer wavelengths / narrower filters. Effectively our atmosphere behaves as a prism, splitting white light into its spectrum of colours. Dispersion is worse the lower in the sky you observe, as the light is passing through more air. For example when observing an object at about 30° altitude you are looking through around twice as much air as you would be at the zenith – a considerable difference.

Pressure, temperature and humidity all affect the amount of dispersion that will occur for a given altitude but, for the typical amateur observer, these secondary effects are very small. The main culprit is the altitude of the object above the horizon, as shown in Figure 1.

Larger aperture telescopes are affected more than smaller ones because of their better resolving power, so the effect of dispersion becomes significant at a higher object altitude. For example, as a 16" (40cm) aperture delivers four times better theoretical resolution than a 4" (10cm) aperture it becomes clear that for this larger telescope to deliver performance to its maximum resolving potential, the object must be located very high above the horizon. Even at an altitude of 60°, around 0.7 arcseconds of dispersion is present from 400-650nm. Since many observers live at latitudes where the planets do not pass close to the zenith it soon becomes apparent that we are often imaging objects well away from the zenith where dispersion has serious potential to degrade image quality.

A typical 6" (15cm) telescope should achieve a performance not hindered by dispersion down to an altitude of around 40° in white light, although when we consider the wide spectral response of CCDs and the resolution possible under excellent seeing it becomes clear we should look at ways to try and overcome dispersion in order to maximise the potential of our telescopes.

Overcoming dispersion

There are ways in which we can overcome the effects of dispersion. The general consensus among observers is that the use of filters eliminates these effects, meaning typical RGB colour imaging ‘bypasses’ the effect as the filters are passing only a narrow band of wavelengths. However, in reality this is not the case.

As discussed above the amount of dispersion is dependent on the bandwidth used. Typical RGB filters cover around 100-150nm in bandwidth and a red filter of 100nm bandwidth will be less affected by dispersion than a blue filter of the same bandwidth. Dispersion does have less effect for filtered light compared to unfiltered light as shown in Figure 1. This figure also shows that white light is quite seriously affected by dispersion. For example, if we say the typical highest resolution attained on a planetary target by a 36cm telescope is around 0.25 arcseconds (which in practice is about right from my own imagery), then we can conclude that for a 36cm aperture, to maintain 0.25" resolution unaffected by dispersion, with different filters the altitude of the object above the horizon must be greater than the following:

 UV/IR blocked white light: 77° Astronomik blue filter: 72° Astronomik green filter: 52° Astronomik red filter: 42°

It is therefore apparent that dispersion can play a major role in the attempt to obtain high resolution imagery of the planets even when using filters. From typical northern European latitudes the planets only rarely attain an altitude of 60° and, for much of the time, we must work at altitudes much lower than this. Therefore while filters can provide some relief from the effects of dispersion they certainly do not cure the problem. We must turn to another device for this purpose.

Dispersion correctors

Basic correctors that reduce the smearing effects of dispersion have been employed by visual planetary observers for many years. 2 The 19th century astronomer George Airy employed a set of wedge prisms to correct for the effects of dispersion during his observations. In use a prism was orientated so that its dispersion was opposite to that produced by Earth’s atmosphere.

Depending upon the altitude of the object more than one prism would be required to exactly nullify dispersion, but it is possible to use two wedge prisms that rotate with respect to one other to provide an adjustable corrector for almost any altitude. This is known as a Risley prism. This type of system is ideal for the observer as it offers an easily adjustable system without the need for multiple single prisms.

Single wedge prism correctors are typically specified as 2° or 4° prisms which will nullify dispersion in unfiltered light for a given altitude. For example a 2° prism will nullify dispersion across the visible spectrum at 65° altitude, while a 4° prism will work at 35° altitude. Adirondack Astronomy in the USA manufactured a set of such prisms which they marketed as Prismatic Atmospheric Dispersion Correctors 4 (PADCs) which could either be used alone or as a pair for adjustable correction. Sadly these have since been discontinued.

Fortunately, fully adjustable dispersion correctors are now available with a prism pair incorporated into a single convenient unit. Astro Systems Holland (ASH) manufactures such a device which is available to amateurs. 3 This unit is ideally suited to the task of high resolution imaging as it offers easily adjustable correction via a pair of prisms with levers extending out of the device barrel for quick and easy adjustment.

Typical prices for such correctors are not especially cheap coming in at around the £250 mark for the adjustable ASH corrector. Single prism correctors are less expensive, however I know of no current source for them.

Dispersion correctors in practice – are they worth it?

In theory a corrector sounds as if it should be an essential piece of equipment for the serious planetary observer, but what about in practice under the night sky? My own experience so far is an extremely positive one – so much so it has prompted me to compile this article. I began with an Adirondack 2° single prism which I still have. During the 2011 apparition of Saturn this device enabled me to obtain a notably higher level of image quality despite the mediocre altitude of the planet at just 37° at maximum. It enabled me to use unfiltered light to obtain sharp images, something which would have been impossible without the corrector in place at such an altitude. Even red light images showed a notable increase in sharpness. These positive results prompted me to obtain a fully adjustable dispersion corrector identical to the one detailed earlier.

One tricky problem faced by users of such a device is keeping the corrector aligned properly rotationally with regard to the direction of the dispersion. For example a planet’s position angle relative to the local horizon changes as it rises, culminates and sets. This means we must slowly adjust the corrector over time to keep it correctly aligned to counteract the direction of dispersion. This sounds complex but in practice is easily achieved if we know an object’s position angle relative to the horizon in our field of view. In practice the corrector needs to be adjusted every 30-60 minutes to keep the orientation of the device optimal for dispersion correction.

In truth there is no simple answer to the question ‘Are dispersion correctors worth acquiring?’ It depends upon a number of factors. Those using smaller telescopes would not really see much benefit apart from times when the planets are very low in the sky. For those using large apertures a dispersion corrector would appear to be essential equipment when seeking to obtain the best possible image quality.

For those fortunate enough to be located within the tropics it is likely that only a small benefit would be realised since for most of the time the planets are high enough in the sky to be well away from the worst effects of dispersion.

Many observers employ colour cameras for a single shot colour image. These are especially vulnerable to the effects of dispersion, and the figures quoted for white light apply for the amount of dispersion for a given altitude. I would consider a corrector essential for anyone using a colour camera for planetary imaging purposes. Simply re-aligning the colour channels back into line to remove colour fringing does not remove all of the dispersion affecting the image.

For those located in the northern hemisphere the years ahead, while very favourable for Jupiter, are not so good for Mars and Saturn, both of which are sinking lower in our skies. Obtaining good quality images of these planets will become increasingly difficult. A dispersion corrector such as those discussed in this article would help greatly to improve both image quality for CCD users and the view in the eyepiece for those observing visually.

For the casual observer the expense of a dispersion corrector may seem rather steep, however for more serious observers it is a very worthwhile investment, especially those employing large aperture telescopes for high resolution imaging or using colour CCD cameras.

Dispersion has been a largely forgotten issue from an amateur standpoint in recent years, however the use of dispersion correctors is on the increase, and in the age of very high resolution imaging many now consider these devices an essential piece of equipment to help coax the best out of their telescopes.

1. Prost J. P., ‘Atmospheric dispersion’, http://www.astrosurf.com/prostjp/Dispersion_en.html
2. Dall H. E., ‘Atmospheric dispersion’, J. Brit. Astron. Assoc.,71, 75-78 (1960 April)
3. Van Kranenburg A., ‘The atmospheric dispersion corrector’, http://www.astrosystems.nl/
4. Dobbins T. A., ‘AVA’s Dispersion Corrector’, Sky&Tel, 2005 June, 88-91

Article originally published in the JBAA 122, 4, 2012

[To search for planetary observations uploaded by BAA members, following this link to search the BAA Member Pages]

## XRISM telescope filter wheel, calibration system sent to Japan for assembly

SRON engineers wrap up the filter wheel for transport to the Japanese space agency JAXA. Credit: SRON

On June 9, SRON Netherlands Institute for Space Research sends its contributions to the XRISM X-ray telescope to Japan, where space agency JAXA will mount it on the satellite. SRON has been working on a filter wheel plus calibration system for the past few years. In 2023, XRISM will be launched into space, where it will observe phenomena such as black holes and supernovae.

The Earth's atmosphere blocks X-rays from space, much to the relief of people and animals, because it can be harmful to every living species. But because of this protective layer, astronomers miss out on a lot of information about, for example, black holes, the thin matter between clusters of galaxies, supernovae and cosmic particles. Space telescopes offer a solution. In 2023, the Japanese space agency will launch the X-ray satellite XRISM into orbit. Together with the University of Geneva, SRON contributes to XRISM with a filter wheel and the accessory calibration system.

On June 9, SRON sends the filter wheel plus a backup copy to Japan, where all XRISM components will be assembled. In September, an SRON team will fly over to carry out a number of tests on the filter wheel, which will be mounted in the telescope next year. "Everything has been delayed for a year and a half because of corona," says engineer Martin Grim, a member of the team that is traveling to Japan. "We actually wanted to carry out the instrument tests in Japan in May 2020 and XRISM was initially scheduled for launch in 2022."

The filter wheel puts several filters in front of XRISM's X-ray camera, allowing astronomers to filter out the brightness and wavelength of the cosmic rays as desired. For example, they will use the molybdenum neutral-density filter if a star or black hole emits too much X-ray radiation and they will select the beryllium or polyimide aluminum filter to block certain wavelengths. A low-radioactive iron-55 filter is part of the filter wheel to calibrate the camera. Iron-55 continuously emits a known X-ray spectrum serving as a reference point. The calibration system also includes Modulated X-ray Source (MXS) that provide a reference spectrum. The Dutch company Photonis has supplied these MXS units to SRON.