The Celestron Contrast Booster Filter made by Baader Planetarium in Germany is an extremely effective device for overcoming chromatic abberation in achromatic refractors. This filter even moderates light pollution effects when a light pollution filter would use so much light as to make the small refractor useless. This review is set up in several sections:

Background
Observations
Usage
Conclusion

Background

Short tube achromatic refractors- telescopes which focus light through lenses, have become very popular in the last few years since they can offer fairly sharp wide angle images of planets, stars, and nebulas at modest cost compared to apochromatic refractors with perfect color correction. The down side of these telescopes is images of planets and stars have blue colored haloes around them. This is because these telescopes have fairly strong chromatic abberation, the effect of different colors of light coming to focus at different locations.

Refractors all have to deal with an inherent problem of physics. Different colors of light bend differently when they hit a material like glass, which bends light based on its wavelength. This is how prisms are able to break light up into its constituent colors- the red, orange, yellow, green, blue, and violet light are all bent differently because of their different wavelengths.

A lens is a lot like a prism in that it is trying to bend light, and they have the same physics- the different colors are bent different amounts. To overcome this, optical designers use compound lenses where multiple lenses of different types of glass with different indicies of refraction are stacked together to produce a hybrid system with better uniformity. The simplest of these is the achromatic lens, which uses plate glass (what windows are made from) for one lens and lead crystal for the other (these are often referred to as "Crown glass" and "Flint glass" for archaic reasons). The acromatic lens is much better than a single piece of glass like a magnifying glass, but the more it tries to bend light, the more severely it will have false color effects, anyway. Any telescope with a reasonable aperture will have to bend light a lot to have a short focal length.

Extremely sophisticated multi-lens groups can do better, and these are called apochromats- however, they are many times the cost of achromatic telescopes. Historically, achromatic telescopes tried to balance the light bending problem by using long focal lengths, which require less bending, and attempted to correct for the full visual spectrum. The results are fairly good, though the telescope necessarily will have a high focal ratio and can only bring in narrow fields of view. The short tube achromatic refractors essentially threw out even trying to compensate fully for the color effects. So, what they did was compensate for red through green light, and the focal point for blue and violet is so far off, this light is spread out so much it is more difficult to see. This works fairly well on groups of stars like the Pleiades, where they don't appear to have individual haloes, but the entire view has a violet background. But on a planet, such as Jupiter, the image will have a bright blue ring around the planet, and on the Moon, the effect is so severe it interferes with seeing details.

Baader and other filter manufacturers have come up with another answer to this probelm. In the last 15 years, multi-coated filter technology has made it possible to selectively filter specific different colors of light. The way this works is the glass has a set of many thin coating layers applied to it. When light hits it, some of it bounces back and forth between the layers and some goes through. By precisely controlling the thickness of layers and tuning the number of layers, it is possible to make a filter allow some colors of light through unimpeded while others bounce back.

The idea in this case is to allow through colors of light where the telescope has the best performance. By selectively letting through the most useful parts of the spectrum, it is possible to make the image you see look sharper since the best focused area of the spectrum your eye can use is the only part coming through.

Observations

The contrast booster filter looks a bit unusual in everyday light. The cell is precisely machined and has the company name and filter type on the side in chrome letters. The glass itself looks yellow, but at an angle will reflect pure blue violet light. If you put it on a dark background, it looks blue-violet.

Holding it up to you eye, the everyday world looks much the same as it does through yellow tinted glasses, except you may notice while red is visible, the color blue looks black. Compared to a yellow filter, the effect is much more extreme in the Contrast Booster. In the same way a yellow filter makes your surroundings look bright and crisp, so does the Contrast Booster.

The filter is made to screw into the base of a 1.25" size telescope eyepiece. The Contrast Booster is a bit different from other telescope filters in that it is optically flat, so you can put in a diagonal with the filter on the front end and the telescope will still focus. This is important because it means you don't have to keep threading it on each eyepiece while changing eyepieces for different magnifications.

Usage

When I first tried out the filter earlier this year, Orion, Saturn, and Jupiter were all high in the sky. The filter is advertised as being the one to have for Jupiter. I set it up on an eyepiece and had a second of similar size so I could swap back and forth. The telescope I used it with is an 80mm short tube. I also had a 5" diameter schmidt cassegrain (mirror) telescope to compare what it did to the image in a telescope without chromatic aberration.

The image on Jupiter was a clear and obvious improvement. Jupiter is white colored overall, so it generates a very strong blue halo when seen without the filter, to the point where the outer edge of the planet is hard to make out against the blue glow. Now, the simple solution with these telescopes is to put the lens cap on with the center cap removed to take it from being an 80mm scope to being a 45mm scope. So, I did this, then compared with the Contrast Booster. The Contrast Booster with the lens cap off virtually eliminates any trace of the blue halo. At the same time, the red and blue features on the planet's cloud bands are indeed much more crisp and easy to see. In this case, the filter is a clear win. I tried the filter out again with the 5" telescope, which has no color correction problem. In this case, although it did make the blue features appear darker, I have to say more detail and color is visible without the filter. In summary, the filter appears to be a major bonus only if the telescope is a short tube achromatic refractor.

The next target was Saturn. Saturn is an interesting object because the planet and its rings all appear to be shades of a light beige color. Saturn doesn't cause as bright a blue halo effect as Jupiter since it has less blue light in its color. The difference in this case was fairly subtle compared to Jupiter's dramatic improvement. However, there is some improvement in the image contrast. On the 5" scope, there was no improvement and I would again conclude this filter is best suited for the achromatic refractor and not other telescope types.

The filter isn't advertised as working as a light pollution filter, but I had noticed the background sky did seem a bit darker with it. This is important since normal light pollution filters, which block reflected light from sodium street lights and mercury vapor lights, end up blocking a large amount of the total light a telescope uses. I am working on a review for one of these, so a more complete explanation is coming. The long story short is blocking that much of the spectrum in a little 80mm scope makes the image so dim you can't make out details, anyway. The Contrast booster was blocking a lot of the blue spectrum, a primary contributor in light pollution.

Target number three was the Orion Nebula, which is very sensitive to urban light pollution. This really is the best of the nebulas, so it is a good test object since a small telescope should be able to show quite a bit on it. Here the Contrast Booster didn't color the view an obvious yellow- the nebula is relatively dim and most people see them as being a green-gray color except in the largest telescopes. Here the filter did provide a subtle reduction in the background sky glow and the filaments of the nebula jumped out. It was a surprising improvement in the image, to say the least. In the 5" scope, there was some improvement, but not as much as a light pollution filter, but I am fairly certain in this case it is because of the telescope's larger size. The 80mm scope has a dimmer image overall, so the background sky glow is less of a problem, anyway.

The next object was the Plieades, which is a bright group of stars near to us. The stars appear to be a brilliant blue-white in the 5" scope. The 80mm scope has the advantage of being able to see all of the Plieades with the surrounding sky, but the blue-white color is lost. As mentioned before, the stars have a blue-violet background in the 80mm scope. The Contrast Booster simply subtracts this background. The stars all have a golden yellow color. In the 5" scope, the image was obviousy preferable without the filter since the stars have their true blue-white color.

Perhaps the most impressive difference seen was on the Moon. The Moon is extremely bright in a telescope, even though it only reflects about 4% of the sunlight hitting it. Here the color is nearly white, and the 80mm scope has serious problems with false color haloes, as well as making the edge of the moon look a sort of lime-yellow color. The Contrast Booster puts the image in sharp relief without the blue edges. Some subtle features, like different lava flow fields on the surface, are also visible. In the 5" scope, the moon is stark black and white, and very bright without a filter. The Contrast Booster did help reduce eye strain here in the larger scope, though I would be hard pressed to say any change in visible detail was apparent. In either case, the Contrast Booster works very well as a Moon filter.

Conclusion

After starting to use a small inexpensive achromatic refractor, false color will very likely be the one feature most users come to dislike about them. The alternatives in terms of other types of telescopes are generally expensive. The Celestron Baader Contrast Booster filter is an extremely effective way to improve the performance of these small telescopes. Even if you get a larger telescope later, these small telescopes are favorites for grab-and-go-look use since they are light, easy to use, and rugged. The Contrast Booster also makes them perform well enough to always be worth the trouble. In fact, I ended up finding this filter so useful, my small short tube refractor now has the Contrast Booster installed on its diagonal at all times.

Recommended: Yes