How and Why to use Color Filters
by Jeff Medkeff
images provided by Maurizio Di Sciullo
Colored filters have a long history of use among planetary observers. For the better part of a century, they were considered essential equipment, and publications routinely specified their use. Large, professionally funded observing projects were even completed in order to find the best color bandpasses for various targets. The use of broad bandpass colored filters became a standard technique in astronomy. Unfortunately, as the 20th century's planetary science techniques have diverged farther and farther from the 19th century's visual astronomy methods, the use and application of colored filters is not always well understood in modern times. So we'll have a look here at the question of what color filters do, why they work, and, most importantly, how to use them.
Color filters work on a simple principle: they exclude some wavelengths of light from the telescopic image, while allowing other wavelengths through. The areas in the image that are bright in the excluded wavelengths get darker, while the areas that are of a color that is let through stays roughly as bright as without a filter. This increases the contrast - the difference in brightness - between the two areas of different color.
The canonical example used in photography courses is that of a red apple laying in green grass, photographed with black and white film. If the photographer uses no filter, the apple will be quite difficult to distinguish from the grass by its brightness - only its texture and shape will make it discernable. If a red filter is used, the apple will look almost white and the grass almost black. If a blue filter is used instead, the grass will be almost white and the apple almost black. By intelligently selecting the proper color filter, the photographer can control the contrast of the image that is produced.
Astronomers using color filters visually attempt to do something similar. The color contrasts on the planets are not as dramatic as the color contrast between a red apple and green grass. While the eye is a very good detector of contrast, it is not always very good at detecting subtle differences in color, especially on bright and glaring objects. The goal of using a color filter is usually to be able to see something that would be otherwise completely invisible, or to be able to see something which is already visible even more clearly than without the filter.
In this discussion, we are therefore talking about a contrast of intensity. For the numerically inclined, contrast is computed by:
b - b' / b
Where b is the brightness of one area and b' the brightness of the other. One common way of quickly measuring contrast is by taking the average value of several closely-spaced pixels in a CCD image as the brightness of the first feature, and the average for several more pixels showing the second feature, and plugging the values in to the above. (By doing this, we are measuring contrast in its original sense - we are not measuring how sharp an image is or how detailed it is.) We'll do this with an image of Mars below, and, in a later version of this page, Jupiter will be added to the analysis.
A Planetary Case Study
To offer an example of how colored filters work on the planets, Maurizio Di Sciullo has kindly given permission to use images of Mars he took during the 1999 apparition, and provided the original FITs format images for our use. I've taken the images and processed them to fit our needs, as you will see below. Some of the processing is pretty severe, and that's my doing, not Maurizio's. (To see his original work, click on his name.) None of the raw images that Maurizio supplied were processed to increase their contrast or resolution (i.e., I did no unsharp masking or anything of that nature), but as you will see a few images were processed to degrade the quality.
The 1999 apparition was not a very favorable one, but Maurizio's shot looks pretty good to me:
If you had very excellent seeing, and an extremely high quality telescope of sufficient aperture, you might be able to discern something approaching this view of Mars visually. But in reality, it is rare to get anything approaching this kind of view of the planet even in an entire lifetime. Mars is simply a tough object to observe. Most of the time, with most telescopes, you will get to see something like this - complete with veiling glare from atmospheric and instrumental scattering (please walk halfway across the room and view this from a distance of several feet or a few meters to really get the effect):
Pretty ugly, eh? But we'll assume you can get the better view of the planet at least some of the time - and given this assumption, we might well ask why we would want to observe the planet through color filters.
The first reason is that the view of Mars shown above is never possible unless Mars is at, or very near, the zenith. This is because atmospheric refraction tends to smear images of the planets out into short, stubby spectra. This happens even when the seeing is perfect. The result looks roughly like this:
It's pretty clear that this view is not very good! (And imagine how bad it would look if we started with the more "typical" view just above.) Not only are there color fringes at the top and bottom of the planet (a blue crescent at the bottom and a red one at the top in this case), but the fringes even affect some of higher contrast boundaries between surface markings on the globe. And the contrast of some of the most subtle surface features is entirely wiped out! The view here was processed to simulate a Martian disk of moderate size while Mars was pretty high in the sky - 45 degrees, or halfway from the horizon to the zenith. When Mars is really large, and therefore at southern declinations, observers in the temperate latitudes of the northern hemisphere will encounter a good deal more refraction than this, since refraction increases as altitude decreases. The basic problem with refraction is that along the line of color smearing, somewhere between one and six or seven arcseconds of resolution can be totally lost - just thrown away. Since that's up to a fifth of the size of the Martian disk when it is at its largest, most people won't want to put up with that.
Observing with a properly selected color filter wipes out the problem that refraction poses, since it blocks a lot of wavelengths from the eyepiece and thus attenuates this smearing. This is perhaps the "first order" use for color filters - to defeat blurring caused by atmospheric refraction. Another beneficial effect is to also eliminate or attenuate such refractive effects caused by seeing. As the air boils over the planet, these refractive effects take place at small scales and can obliterate fine details. A filter won't completely eliminate the effects of seeing, of course, but it can be a modest help.
The other uses of color filters almost all relate to increasing the contrast between features on the planet's surface. In Mars' case, two colors of filter are particularly useful at bringing out the contrast between very common features. On the one hand, there are the dark surface markings of the planet, which appear gray or greenish against the reddish backgrounds; and on the other, there are the thin clouds that the Martian atmosphere produces consistently in certain areas and at certain times. The surface markings tend to be brought out best by the use of a red or orange filter, while a blue filter makes the polar caps, frosts, and atmospheric clouds pop out more clearly.
Since Maurizio took three images through color filters to arrive at his full-color view above, we can use the individual frames to demonstrate the effects of filtering the image. Below is the full-color view, and next to it the view through the red filter, followed by an image pointing out two specific areas:
Look carefully at the differences between these images. Remember that the red filtered image is in the middle. On the red image, the contrast between Sinus Meridiani (the club-shaped feature in the lower left part of the disk) and the deserts of Edom (the red deserts right in the middle of the disk) is about three times that in the integrated image. We get this by measuring pixel values (in the original FITs files) and plugging them into the formula above. A good deal of subtle detail is visible more clearly in the red image than in the color image, especially around the edges of small features at the top, left, and right of the disk. Even the fork in Sinus Meridiani is more clearly seen in red than in the color image. At the two areas indicated in the right-hand image, there is subtle detail in the red image that is lost in the color view.
In real-life observing conditions, the real view of the unfiltered red planet will not be as contrasty as depicted here. Compare this more typical representation:
In practice, the red-filtered view depicted on the right won't really be quite that sharp and the dark areas won't be that strongly etched - so you can mentally add a bit of blurring and washing out. But the additional details seen in the right hand image that are not present in the left hand image is pretty typical of what a red filter does in practice to reveal new detail on this hemisphere of the planet. The difference is due to two effects - increasing the intrinsic intensity contrast between the dark areas and the bright areas, and knocking down blurring caused by atmospheric refraction.
Moving on to the blue image, we can see that there are some substantial changes in it compared to both the color image and the red image:
Here the effects are pretty evident. There are clouds on the left hand limb. There is a bright bit of polar cap at the top. On the lower left limb is the frost-filled Hellas basin. And a small white spot near the right hand limb marks a cloud over Syrtis Major. These details are suspected in the colored view (except perhaps for the Syrtis Major cloud), but really come barreling out through the blue filter. But note that the surface markings are much less visible. Again, a comparison with our typical telescopic view is somewhat more instructive:
On the left-hand image it is hard to even see the clouds and frost features in some cases, and where they can be seen, they are far from clear.
How It Is Done
The examples given above are grounded in physics. The image formed by the telescope with the filter is the same, whether the eye looks at the image through an eyepiece, or a CCD camera captures the view electronically.
But there is one substantial difference between a CCD camera and an eyeball - the eyeball sees in color! Indeed, when you put red and blue filters in the telescope to look at Mars, you will see something roughly like this:
Here is where certain observing skills come into play - and where you have to do a little work to get the benefits of the filtered view. The human eye is used to looking for slight differences of color - and on apparently large objects, such as paintings, the eye is very good at it. It is not quite so good at it on small objects, especially bright glaring ones in the middle of a dark background. So the first thing that typically happens when you put in a color filter is that your eye and brain see a big red ball - or a big blue ball, as the case may be.
For some people, the effect can at first be distracting enough that it wipes out any benefit received by the filter. Even when severe atmospheric refraction is being eliminated by a weak yellow filter, a few observers can't get past the weak yellow color of the image to enjoy the increased resolution. And when the red filter is put in, the "rose colored glasses" syndrome is certainly quite common.
The solution to the problem is to have some patience, and very consciously do two things. The first is to acclimatize your eye to the view. The average human being takes about ten to fifteen minutes to adjust to wearing colored glasses, and in my experience the average observer takes about that long to get used to the colored view of the planet. So the first step is simply to give it time - keep on looking at the red (or blue) dot. The more you practice, the quicker this acclimatization will occur later on. After some twenty years of doing this, I can get used to any filtered view (except strong violet) in less than a minute, and most planetary observers share this experience.
The second thing to do is to begin to train your eye to see differences in brightness, and completely ignore differences in color. This is not always easy to do, to start. In effect, it is like training yourself to see the world as though your eye were a black & white television camera. They key to it is to think monochromatically. The whole image is going to be red (or blue) when filtered - so look for bright red and dark red, but don't go in search of orange and ochre, because you won't see it. When you turn your attention to a particular part of the planet, ask yourself consciously whether that part is brighter than or darker than the surrounding areas. After a few sessions, doing this will become second nature.
By the time you are done with your first session, you will have spent a half hour or so looking at the planet through a colored filter. (Or perhaps an hour or more looking through two or three different filters.) When you are done, take the filter out and look at the "natural" view. The people who spend time on it and cultivate their "black and white eye" will notice the washed-out appearance of the unfiltered image immediately. Areas where there were new details in the filtered view should be pretty evident, and sometimes - armed with the knowledge gained by the filtered view - the observer will barely make out those details in the unfiltered view.
But it is worth emphasizing that these benefits will not be enjoyed unless significant time is spent on the project. Half an hour per color is about right for those just starting out. Those who give the color filter two minutes to work their magic will almost always lose out.
By the time you have begun to cultivate these skills, it will be evident that using color filters is not really an aesthetic enterprise. It really doesn't do much of anything to make the view more romantic or more artistic. The technique is strictly a tool, and the tool is designed to increase the amount of detail that can be seen on a planet, or else to increase the ease of seeing such detail as is already visible without a filter. Speaking for myself, I like the aesthetics, and I also like getting the details - so filters, and the special skills needed to use them to their potential - are parts of my kit.