Intensifying your viewing experience.
Switching from photons to electrons allows this eyepiece to go where no eyepiece has gone before.
If you've never looked through a modern image intensifier, the first experience is nothing short of profound. The merest hint of illumination is all it takes to transform the night's deepest shadows into well-lit scenes. The glow of a digital clock is enough to brighten the far corners of a large room. The crescent Moon will render a landscape with the brilliance of a noontime Sun.
Because image intensifiers have such incredible power to enhance Earthly scenes, it's reasonable to expect that turning them skyward should provide an equally profound experience. Sweeping the heavens should be like scanning long-exposure photographs with stars everywhere and nebulae glowing like carnival lights. Right?
Well, not exactly. Primed with the terrestrial experience, veteran observers have often been unimpressed with their initial image-intensified look at the heavens. It's the same sort of feeling many novices have after their first telescopic encounter with the Andromeda Galaxy.
This discrepancy between expectation and reality is probably the main reason image intensifiers, which have been around for decades, haven't made a bigger impact on amateur astronomy. But that may well change with the introduction of the [I.sup.3] Piece by Colorado-based Collins Electro Optics.
Why is the [I.sup.3] Piece different? First of all, at its heart is a state-of-the-art, generation-three image tube manufactured by ITT Night Vision. While more efficient than generation-two technology, especially at extremely low light levels, these tubes are light-years ahead of the generation-one devices developed nearly 40 years ago that pervade today's surplus markets and are used in many "inexpensive" night viewers sold through mainstream consumer outlets. Generation-three devices are claimed to amplify light 30,000 to 50,000 times compared to only 1,000 or 2,000 times for some other night-viewing devices.
Another feature enhancing the astronomical friendliness of the [I.sup.3] Piece is that it's made specifically for use with amateur telescopes. There's no need to kludge together couplings. It comes ready to slip into conventional 1 1/4-inch or, with an optional adapter, 2-inch focusers, and the 1 1/4-inch nosepiece is threaded for standard filters. The unit is powered by a small 3-volt lithium battery, which typically delivers 30 to 40 hours of continuous operation. There are a host of video accessories for the 13 Piece, but more about them later.
How It Works
In principle, a generation-three image intensifier is quite simple. Incoming photons strike a photocathode, releasing electrons that bombard a microchannel plate. As electrons flow through microscopic tunnels etched in this plate they ricochet off the specially coated tunnel walls, releasing a cascade of additional electrons, which emerge to strike a phosphor screen. In addition to the screen's image being thousands of times brighter than the light falling on the photocathode, its glowing green hue closely matches the human eye's peak response. Thus, the tube's full range of input sensitivity, which extends from roughly 4500 angstroms in the blue to 9000 in the near infrared (where the eye is completely insensitive), is rendered as a monochromatic color readily visible to the eye.
The overall length of the 13 Piece is only 4 1/4 inches and it weighs 15 1/2 ounces - similar to some of today's premium eyepieces. Its magnification is about the same as a conventional 25-millimeter eyepiece, and it has an apparent field of 35 [degrees]. (I found the view reminiscent of those I had with the low-power Kellner eyepieces offered as standard equipment on early-model Schmidt-Cassegrain telescopes in the 1970s.) The [I.sup.3] Piece has sophisticated "output" optics, which are necessitated by the strongly curved surface of the phosphor viewing screen. The image becomes slightly soft only at the very edge of the field. An extremely generous eye relief of 38 mm makes it easy and comfortable to view through the [I.sup.3] Piece whether or not you wear glasses while observing.
Last fall I spent a number of nights under the stars with an [I.sup.3] Piece borrowed from the manufacturer specifically for testing. All my observations were made in suburban skies, where 5th-magnitude stars mark the typical naked-eye limit on a good night. I also asked Ron Dantowitz at the Boston Museum of Science to make an independent evaluation of the eyepiece at the museum's rooftop Gilliland Observatory in the light-polluted heart of the city. We both had previous experience with image intensifiers and weren't expecting miracles. Despite these different observing conditions, we had similar initial impressions - the [I.sup.3] Piece was fascinating to use. What we both planned as quick first assessments of the device ended up being multihour observing sessions prompted just by the allure of the view.
There are some caveats that go with the [I.sup.3] Piece, however, and I should explain them first. Because you are looking at the image on a phosphor screen, the view does not have the same "depth" and fidelity as with a conventional eyepiece. It is somewhat like watching a baseball game on television rather than in person. In addition to the screen's green hue, there is a very subtle electronic noise, which appears similar to the "snow" seen with a weak television picture. There is also scintillation mixed with the noise - a steady peppering of momentary bright specks on the phosphor screen. Noise and scintillation are most apparent when the overall image is very dark, causing the [I.sup.3] Piece to automatically operate at maximum gain. When the incoming illumination is higher (as with many terrestrial viewing situations), the gain drops and the scintillation is dramatically reduced.
These characteristics of image intensifiers are usually the ones first-time users criticize. Nevertheless, after a few minutes both Ron and I hardly noticed them. "Believe it or not," Ron said the following morning, "I quickly got used to the green screen and scintillation. After a little while the view seemed very natural"' The bright screen can reduce the dark adaptation of your observing eye, which is a hindrance in a darkened observatory.
While it's difficult to pin exact performance numbers on the [I.sup.3] Piece, in my sky conditions I typically found that I could see stars a solid two magnitudes fainter with the device than I could with a conventional eyepiece yielding a similar magnification. This is impressive considering that the same boost without the intensifier would require increasing a telescope's aperture by two and a half times!
In addition to improving the magnitude limit, there was another aspect of the [I.sup.3] Piece I found very appealing. With a conventional view, seeing the faintest stars always requires a great deal of work. You have to use averted-vision techniques, and even then the stars come and go. It can be quite tiring to constantly push a telescopic view to its limit. With the [I.sup.3] Piece, however, I always saw the faintest stars on the phosphor screen with direct vision. For the most part, faint objects were either visible or not. While the very faintest stars sometimes lurked in the intensifier's noise, I didn't need averted vision to see them. This made observing with the [I.sup.3] Piece extremely pleasant.
Ron was quick to point out that the see-it-or-not quality of the [I.sup.3] Piece would be a big advantage for visitors to the museum's observatory. "For the first-time observer, the eyepiece would make all the difference in the world between seeing objects like M13 from the city and not seeing anything. It's great for showing people a star cluster rather than just fuzz. Many times they can't even see the fuzz."
Pushing the visual limit two magnitudes fainter made the [I.sup.3] Piece perfect for viewing star clusters. The first night I used the device at the f/10 focus of a Meade 16-inch LX200 Schmidt-Cassegrain telescope, I spent several hours viewing virtually every star cluster plotted in the Millennium Star Atlas that lies within a few degrees of the galactic plane running from northern Cygnus through Cassiopeia. Scores of these objects were new to me, and all except the most sparse clusters were immediately recognizable in the eyepiece.
Perusing nebulous objects was another story and one not easily summarized because the eyepiece's performance was not as uniform on these objects. The result was also dependent on the brightness of the sky background. In general, it was good for planetaries, with objects like the Ring and Eskimo nebulae offering superb views. Some small planetaries had to be carefully identified since they could be confused with halos that appeared around bright stars in the [I.sup.3] Piece. (In time, these halos became "part of the view" and were not bothersome.)
Some notably blue reflection nebulae like M78 were seen a bit better in conventional eyepieces than in the intensified view. In most cases, however, if a nebulous object was not seen in the [I.sup.3] Piece, then it wasn't visible by any means.
The central issue pertinent to the visibility of diffuse objects is contrast - the difference in brightness between the object and its background. While an image intensifier makes diffuse nebulae appear brighter, it also amplifies the sky's brightness and thus does not usually change the contrast. Red nebulae, are an exception because the intensifier transforms red wavelengths, which the eye sees poorly, into green. The result is a disproportionate increase in the apparent brightness of the nebula relative to the background. The effect is even more pronounced for bright nebulae, which would intrinsically have good contrast were it not for the eye's insensitivity to red light.
A perfect example is the North America Nebula in Cygnus. Visible to the unaided eye under extremely dark and transparent skies, this glowing red cloud of hydrogen is washed out by the tiniest amount of light pollution or moonlight. Nevertheless, from my suburban location I immediately picked it up while wide-angle sweeping with the [I.sup.3] Piece attached to a 35-mm camera lens. (This was accomplished with an adapter I cobbled together.) The view was further enhanced when I added a red filter in front of the camera lens. As an aside, my improvised wide-angle system was great for hunting artificial satellites and faint meteors (not to mention checking out nocturnal critters wandering through the backyard).
I experimented with a variety of lenses ranging from a 16-mm f/2.8 fisheye to a 500-mm f/8 catadioptric. Making a direct comparison of views with and without the image intensifier was difficult, but again I believe the [I.sup.3] Piece gave at least a two-magnitude improvement for the faintest stars visible. With a 200-mm f/3.5 lens I could easily see all the Pleiades plotted on the Millennium Star Atlas.
With the 13 Piece on the 16-inch telescope, I easily saw galaxies such as NGC 6946 in Cepheus. Galaxies that would have been glimpsed by experienced observers but possibly missed by beginners using traditional eyepieces were conspicuous. Again, objects were immediately visible - no observing tricks were needed to see all that was to be seep.
The enhancement of spiral galaxies was occasionally more pronounced in the central cores than in the spiral arms; perhaps this is due to the predominance of red stars in the cores and blue stars in the arms. Elliptical galaxies, on the other hand, are red and seemed uniformly enhanced. Nevertheless, because of lower contrast, none of these extended objects exhibited the two-plus-magnitude gain that occurred with point sources such as stars.
More Than Just Visual
Collins Electro Optics sells an impressive line of video accessories for the 13 Piece. It includes custom adapters, a monochrome CCD camera, and a real-time frame averager that combines 2, 4, 8, or 16 individual video frames as they come from the camera and displays them as a continuous image on the monitor.
The frame averager was extremely effective at reducing (and eliminating) scintillation from the video image when two or four frames were averaged. The view became a bit muted at the 8- or 16-frame setting. While averaging frames helps beat down noise and improves the detection of faint objects, everything that appeared clearly on the image intensifier's phosphor screen was bright enough to be captured by the CCD camera without the need to average frames. Thus, the averager was best for removing scintillation and "smoothing" the view. It also gave a slight improvement in the appearance of star images that were at the intensifier's limit, though averaging 8 or 16 frames would sometimes mask the faintest stars that were only momentarily visible in poor seeing.
The video system revealed stars to about 15th magnitude at the f/10 focus of the 16-inch telescope. Star clusters, especially globulars such as M13 and M15, were nothing short of spectacular on the video monitor. The Ring Nebula was also an easy target for the video system. The eyepiece's video performance was similar to the visual results. The video system would certainly be great for public viewing. It would also be great for supernova hunting, especially when used with a computer-slewed telescope.
I found some of the most interesting potential for the video system when it was connected to camera lenses. The resulting wide-field views of the sky were excellent for viewing meteors.
Overall I was impressed with the [I.sup.3] Piece. It is yet another tool to enhance backyard astronomy. The gain of two-plus magnitudes for stars is substantial, especially when you consider the ease with which the faintest objects are seen in the eyepiece.
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|Title Annotation:||Collins Electro Optics I3 image intensifier|
|Author:||Di Cicco, Dennis|
|Publication:||Sky & Telescope|
|Date:||Feb 1, 1999|
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