Pirich's Full Review: Celestron Light Pollution Reduction Filter 1.25'' ...
The Celestron Light Pollution Reduction (LPR) Filter is a device designed to cut out the effects of urban light pollution so dim astronomical objects such as nebulas and galaxies can be seen from the city. Light pollution is the pinkish glowing sky caused by poorly designed street, building, and parking lot lights which waste energy by directing light upwards to the sky instead of where it is needed on the ground. This filter is designed to give the evening sky back to you by blocking the worst effects of light pollution, and is surprisingly effective at doing this. This review is set up in the following sections:
Background
Observations
Usage
Conclusion
Background
Up to the 1970s, amateur astronomers could see quite a bit from the back yard. People living in large cities near downtown had a problem with light pollution blotting out the stars, but you didn't have to go far to get away to dark skies. Then the light cult came to America, and in the 1980s, absurd amounts of lighting became the norm for night. The usual cited reason was crime prevention, which flies in the face of much of the lights added being for advertising purposes and decoration, as well as the fact crime rates have not responded to lighting. In my own observation, the only cars I have seen broken into (including my own) were parked under streetlights. Most of these lights spill a large portion of their output upwards, where it causes the sky to glow the color of the light hitting it. If you have ever flown over a city at night, all of the lighting you see pointed at you from below is the source of light pollution as millions of watts of electricity are wasted. Today, the skies are so badly lit by upward facing lights, all but the brightest stars have disappeared and the Milky Way has been forgotten for most of the population in the US, Europe, and Japan.
There is an anecdote from the California Northridge earthquake in 1994; the evening after the earthquake, when the city lights were knocked out, people started calling the police saying something was wrong because the stars looked unusually bright; they had never seen a sky without light pollution. I lived near a 20 screen cineplex for several years, and only the very brightest stars, planets, and the moon are visible in the orange-pink sky light pollution causes.
If you want to see a really clear view of the stars today, the city and the suburbs are too bright. But more importantly, the light haloes of even small towns have become so bright you really have to get about 25 miles away from any place with a Wal-Mart before you will have a chance to see the full night sky. And, that means really getting out in the middle of nowhere. For most of the population of the US, getting to a truly dark sky involves a trip of over 50 miles.
When looking through a telescope, the effect of light pollution is sort of like looking through a window with a thin white curtain over it. When looking at something on the other side, the brightness of the curtain masks any detail dimmer than it is, so only the outside features brighter than it are visible. For example, if you were looking at a car, the areas on top where it is reflecting more light are visible, but areas in shadow disappear and the curtain is visible.
Now, to transfer this to the case of light pollution, the light is from streetlights, so it is in the colors of sodium yellow, mercury blue, and a mix of the secondary peaks from other minor lighting types. The result is anywhere from a peach orange to a pink colored glow competing with anything else in the sky.
The basic countermeasures you can use in the city to see dimmer objects washed out by light pollution are all based on drawing a contrast in the image between the object and the sky behind it. Some reduction is possible simply by increasing magnification. This technique uses the nonlinear light response of your eyes to get around light pollution. For example, if you look at a bright nebula, such as the Orion Nebula, in the city at low magnification in a telescope, the nebula will appear to be a dim fuzzy patch in the sky you likely would skip over if you didn't know there is a nebula there to see. This is because the background sky is lit almost as brightly as the nebula, which is an enormous and beautiful object at low magnifications in the countryside. But the human eye is a complex and subtle sensor and includes some interesting abilities to distinguish items from a background. When you increase magnification, the image gets dimmer. So, for example, a 5" scope operating at 40X produces an image about 8 times brighter than what the unaided eye would see. At 80X, the same telescope will produce an image about 4 times brighter than what the unaided eye would have seen with half the field of view.
Strangely enough, the eye is actually able to discern features more easily in the dimmer image, and the structure of the nebula becomes visible as the background drops out. This is a quick, simple, and fairly effective technique, but there are some problems with it: First, this works on your eye, but is totally ineffective for cameras, which have linear response sensors, and will continue to be blinded by the background. Second, large scale objects can't be seen in their entirety at higher magnification. Third, larger telescopes have to go to very high magnifications and narrow fields of view for this to work. Forth, though it isn't obvious when looking, the magnification technique is mostly showing green ionized oxygen to you since the eye is more sensitive to its color, while the red of hydrogen is lost.
As a result, conventional wisdom for urban observers has been to use smaller telescopes since the light pollution problem is a little more manageable. Nebulas, galaxies, and star clusters for the urban observer have largely been reduced to a short list of the few brightest ones in the sky.
A light pollution reduction filter is designed to reduce the brightness of the light polluted sky more than it dims just the colors of the light pollution and allows everything else through. In practice, the LPR filter blots out a large amount of incoming light and has less interference with some key colors for the hydrogen red, oxygen green, and reflection blue of deep space objects.
This is done with a relatively new technology in coatings. By putting many thin layers on an optical window with a sequence and precise thicknesses, it is possible to cause the wavelengths of some colors of light to bounce back while others go through. The idea is sort of like one of those double-mirror frames with the lights inside which give an impression of looking into a huge distance. Here, that effect causes destructive interference in wavelengths the color of common streetlights and lets the major colors in nebulas through.
Observations
The Celestron LPR filter is one of the stranger looking pieces of glass I've ever happened across. From straight on, it looks a bit like a mirror and from an oblique angle it appears magenta and clear, yet makes green reflections. The housing is well made with a knurled front edge to make it easier to thread in the dark.
I was curious about how it would appear to work in daylight to have some feel for what I was seeing when using it at night. When held to the eye, the experience can only be described as psychedelic. The center of the filter looks to be a sort of dirty green color which fades to magenta at the edge. Everything through it gives the impression of being dimmer, yet objects with blue or red in their color appear to be fluorescent. An object like a computer display will appear to be bright blue-green, while regular white paper looks dirty green. If you tilt the filter, it quickly brightens to magenta, red, orange, and at the sharpest angle where you can still see through it, it will have almost turned clear (since you are looking through the coatings at an angle where their thicknesses are greater).
When installed in a telescope eyepiece or in the front of a diagonal, the light hitting the filter falls in a narrow angle, so the wide-angle magenta effects do not occur. Instead, there is an image where blue-green colors appear very bright, while yellow and violet nearly vanish completely.
Usage
Using an LPR filter is complicated. I first started encountering LPR filters at astronomy club observing meetings. Incidentally, I highly recommend these- amateur astronomy is interesting because people from all sorts of occupations from firemen to nurses to engineers all show up. The use I first saw was in skies where there was little light pollution to reduce, and in this mode, the filters seem to slightly help bring out some detail on objects, but clearly at the price of dimming everything in the field by at least one magnitude. Or, in other words, the shape of nebula clouds may be a little sharper, but everything is dimmer and many dimmer stars vanish. As a result, using one in relatively dark skies has felt a like wearing an inch thick rubber suit on a warm summer night to keep mosquitos from biting.
After seeing this, I was not sure what to expect in night time use deep inside the city. I put together three telescopes to try it out with- an 80 mm achromatic refractor, a C5 5" Schmidt Cassegrain, and a NexStar 8 GPS 8" Schmidt Cassegrain. I expected to be able to get a determination of how effective the filter was, and how useful it would prove depending on the type of telescope and its size.
Use with the 80 mm Achromatic Refractor:
This scope was the small 80 mm F/5 scope I have reviewed previously (and I have come to think everyone thinking of giving a child a telescope should see this as the minimum). These telescopes do have some chromatic aberration, which means the far blue and red don't come to the same focus, so images have some color in their fringes. At the same time, the size of the telescope has modest light gathering ability.
I put the telescope on Orion and tried out the image with no filter, the LPR filter, and the Baader Contrast Booster I had looked at previously. The image of the center of Orion comes through fairly well at modest power and four stars of the trapezium at the center come through very well. With the Baader Contrast Booster filter, some of the violet fringe disappears and the contrast does look a bit better. However, the results with the LPR filter were nothing short of disastrous. The image dropped in brightness so far the trapezium stars vanished. At the same time, the image turned to a ghost of an impression with an apparent double-image. I tried the Pleads star cluster only to see many of the stars which look so nice in this telescope otherwise simply drop out. In short, the prescription of this filter is a vast overmatch for a telescope this small, and appears to be inappropriate for short focal length achromatic refractors of any type.
Use with the 127 mm dia. C5 5" SCT:
This scope has become my general purpose grab-and-go telescope due to its combination of light weight and easy portability. These telescopes are reflectors and therefore have perfect color focus across the spectrum, so their images do not have the violet fringes achromatic refractors do. This telescope has approximately 252% of the light gathering ability of the 80 mm scope.
In this instrument, the LPR filter made a marked improvement to the Orion Nebula. The background light dropped out and the structure of the clouds became sharper. However, the trapezium stars, which otherwise shine like diamonds, were dimmed to the point where some of them would disappear when looking at them directly. I did a few test images with a camera, though, and found the results are indeed far superior to taking an image with no filter then attempting to subtract the light pollution. The conclusion I drew from this is this is the smallest telescope diameter a visual observer should consider using this filter with. A less intrusive filter, such as the Baader Contrast Booster is more appropriate for smaller scopes. In an experiment looking at Saturn, Jupiter, and Mars, the image was a clear disappointment. The colors through the filter do nothing to enhance planet detail, and the reduction in brightness makes some fine details difficult to resolve.
Use with the 203.2 mm dia. NexStar8GPS 8" SCT:
This telescope is currently the big Kahuna at my place- I was fortunate enough to get an 8" tube with superb optics, so this one is a keeper, though its size and weight mean it only gets used for serious observing. Like the C5, these telescopes are reflectors and therefore have perfect color focus across the spectrum, so their images do not have the violet fringes achromatic refractors do. This telescope has approximately 254% of the light gathering ability of the C5 and 645% of the 80 mm scope's capability.
In this telescope, the LPR filter finally seemed to come into its own. The telescope's 8" diameter allows it to overcome the filter's dimming effects to produce an image of both nebulas and star clusters which is clearly superior on cloud detail, and still retains most stars. In photographic tests, this is a very clear winner at this point. I recommend folks who have a scope this big or something really large like 9.25", 10", 11", 12" and larger will see obvious gains when observing deep space objects. A trial on Jupiter, Saturn, and Mars again showed the filter does nothing for these images, and actually degrades them, so this behavior is consistent.
Conclusion
The Celestron LPR filter is a fairly effective solution to an extremely difficult problem. Light pollution has done so much damage to the visible sky, a filter capable of doing much about it is an extreme optical problem. Even so, this filter does improve the image noticeably, and is clearly more effective in larger telescopes.
I do not recommend the Celestron LPR filter for anyone using a telescope smaller than 5" in diameter for visual use. It may be useful in small diameters for photography, but be prepared for long exposure requirements. Even at 5" diameter, be prepared for a large hit in the visual image brightness.
I do not recommend the Celestron LPR filter for achromatic refractors. The filter cuts the center of the spectrum where sodium yellow is, and leaves red and green-blue, thus leaving only the image fringe. Other filters such as minus violet or the Baader Contrast Booster are effective in these scopes, and do have some ability to combat light pollution.
To some extent, I find myself wishing there was a small-scope version of this filter which damped the same colors, but was just a bit less aggressive a filter. In small telescopes, just some reduction of light pollution would probably be enough to greatly improve their performance without reducing the image brightness as much. As it is, the filter works so well there is a certain "All or nothing" character which means I have found myself balancing what I really want to see every time I consider using it. But, given the choice of 2 hours of driving to go observe and return, it is still very useful.
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