Barlow Lenses No matter how many eyepieces a CAT user accumulates, a Barlow lens will increase the number. What is a Barlow? In its simplest form, it is a singleelement negative lens mounted at one end of a barrel. An eyepiece is inserted into the other end, and the whole thing is placed in the CAT's star diagonal. What good is that? When an eyepiece is combined with the Barlow's negative lens, its magnification is doubled (usually). A Barlow is an eyepiece multiplier. For example, adding a Barlow to an eyepiece collection that consists of 15-, 10-, and 6-mm oculars adds "virtual" 12.5-, 5-, and 3-mm eyepieces. How? The Barlow lens makes the light cone coming from the telescope's optical system longer and skinnier. This "stretching" has the practical effect of making the telescope's focal length longer, and longer focal length telescopes produce higher magnifications than shorter focal length ones with any eyepiece.
Some new amateurs are skeptical about Barlows. Getting additional eyepieces just by adding a relatively inexpensive item to the accessory collection seems like a violation of the time-honored "there ain't no such thing as a free lunch" rule. For once, there is no catch. If well made, a Barlow not only can add focal lengths to an eyepiece collection, it can actually improve the images in these eyepieces. Most oculars perform better at longer telescope focal lengths, and a Barlow increases a telescope's focal length. Barlows can also increase viewing comfort. A 12-mm eyepiece, for example, is usually more comfortable to use than a 6-mm. It will likely have more eye relief and a larger eye lens. A Barlow will make this comfortable 12-mm eyepiece into a comfortable 6-mm eyepiece.
What should a buyer look for in a Barlow? Single-lens-element models may work okay, but it is best to choose a multielement achromatic or apochromatic Barlow to keep from adding spurious color to the eyepiece. The Barlow should be well built with a well-blackened barrel interior to prevent stray reflections. One that holds eyepieces in place by means of a compression ring instead of a setscrew is also desirable since a compression ring will hold heavy eyepieces more securely and will not mar their barrels like a tightly cranked-down setscrew. Finally, choose a Barlow in an appropriate "power." The 2x Barlows, which double an eyepiece's magnification, are most common, but 1.8x and 3x ones are also commonly available. An SCT owner should probably stick with the lower-power models since a 3x will not be very usable with most eyepieces except on nights of the best atmospheric seeing.
As for which specific Barlow, this is a golden age for this device, and all the models tested recently, even very inexpensive imported models such as Orion's $40 Shorty Barlow, performed well optically. The main difference between cheap and expensive Barlows is in their mechanics—things like barrel baffling, setscrews, and compression rings. Do you want the best? TeleVue makes Barlow lenses in both 1.25- and 2-inch models that are as renowned as their eyepieces. The 2-inch Big Barlow is not cheap at just over $200, but it is about as good as a conventional Barlow gets. The company also makes a superpremium model, the TeleVue Powermate. It is a top-of-the-line two-element model that also features a two-element corrector lens assembly for a total of four lens elements. It is even more expensive than the Big Barlow, at about $300 for the x2 2-inch version (1.25-inch versions with powers up to x5 are also sold), but it is famous for excellent images, especially in high-power planetary imaging.
Focal Reducers and Reducer/Correctors If only there were such a thing as a reverse Barlow. For years SCT users, who sometimes felt saddled by the CAT's long focal length, dreamed of a magic lens that would decrease a scope's effective focal length instead of increasing it. It is easy enough to get high power out of an SCT by adding short focal length oculars and high-power Barlows. It is harder to get low-power and wide-angle views from f/10 telescopes. Low-power eyepieces are expensive when they are really good, and these long focal length oculars can be uncomfortable to use because of their long eye relief. Simple lenses called focal reducers have been around years and do decrease the SCT's focal length. Unfortunately, these lenses do not work very well. What good is increasing the field of view if the stars at the edge of that field look like comets, even in expensive eyepieces?
Nothing much changed until the late 1980s when Celestron enlisted the efforts of master telescope and optics maker Jim Riffle to design a reducer that would not just be a reducer; it would be a reducer/corrector (r/c). The Celestron r/c (Plate 39) (Meade also sells one) is a two-element lens in a special housing that screws directly
Plate 39. (Assorted Accessories) A few of the accessories amateurs find themselves constantly buying, (l - r) Lumicon UHC LPR filter, Celestron f/6.3 reducer/corrector, Thousand Oaks OIII LPR filter, Meade 12mm illuminated reticle eyepiece, Lenspen, Celestron LED astronomer's flashlight. Credit: Author.
onto the rear port of an SCT. The threads on the other end of the r/c duplicate the SCT's rear-port threads, so anything that can be attached to a normal SCT port can be screwed onto the r/c. What is the magic of the r/c? It takes an f/10 SCT and turns it into an f/6.3 scope. With the r/c in place, a 25-mm eyepiece yields 50x (rather than 80x as it would at f/10) and delivers a concomitantly wider field. That is not all. The r/c also flattens the SCT's naturally curved field. Amazingly, stars at the edge of the field look better with an r/c in place than without it—in any eyepiece.
The r/c is a remarkable device, but it does have a few minor drawbacks. First, while the Celestron and Meade r/cs will work in any SCT, they seem to work best in 8-inchers. Field-edge correction in the larger telescopes does not seem quite as good. Also, r/cs, while usable for imaging, can cause vignetting with larger CCD (charge-coupled device) chips (those that approach 35-mm film frame size, like the sensors of digital single-lens reflex cameras). A photo taken with a large-chip camera through an r/c may be slightly darker at the edges and corners than it should be. Visually, an eyepiece with a longer focal length than about 32-mm may also show this vignetting. Despite its few faults, the r/c is a remarkable and remarkably useful accessory, especially given its reasonable price, about $130 for either the Celestron or the (apparently identical) Meade version.
Dew Shields Lucky astronomers do not just live where the sky is dark; they live where the atmosphere is dry. For CAT owners who observe where humidity is high and the dew point is low, dew is a huge roadblock on the path to productive observing. What happens to that big glass lens on the end of the scope when it cools below the dew point? It fogs up. Soon thereafter, it will be dripping wet, and the observing run will be over. In some parts of the world, like the southeastern United States, an SCT that is unprotected from dew will become useless in little more than an hour on many nights. How do southerners—or anybody else—keep dew from "falling" on their CATs? A simple dew shield is the first line of defense.
"Simple" is right. A dew shield is nothing more than a plastic or metal extension to the telescope tube that fits over the corrector end and shields the big lens from some of the heat-sucking sky. The less of the sky the corrector can "see," the longer before it dews up. A dew shield also has the added benefit of protecting the optical system from stray light (just like a lens shield on a 35-mm camera). A dew shield is a common item and is probably the first accessory most CAT users should buy. Meade and Celestron have sold them in the past, but today most U.S. amateurs are buying the nicely crafted Astrozap dewshields (Appendix 1), which are available either in metal painted to match the telescope's tube or as "flexible shields," flat plastic sheets that can be formed into a tube, fastened in place with Velcro, and slipped over the corrector end of the tube. In the United States, Orion also sells flexible dew shields, and a Google search will turn up a host of other makers of these simple accessories.
Dew Zapper Guns Outside the lowest-humidity areas, a dew shield alone will not be enough to allow all-night observing runs. The second line of defense is the dew "zapper" ( ). What is a zapper? That depends on where it is bought. At an astronomy store, it will be sold as a "dew removal gun." Anywhere else, it will be known as an automobile window defogger or 12-volt hair dryer. And that is actually the best description: a little 12-volt hair dryer. These devices put out amazingly little heat, but that is perfect for our purposes. High heat is not needed and can cause a corrector to deform slightly and produce poor images until it cools again. When dew begins to creep onto the corrector (halos will begin to appear around bright stars), fire up the zapper. Just a minute or two of use is enough to dry the corrector unless things have gone too far.
What if you cannot find a zapper for sale from an astronomy dealer or do not want to pay what one costs at the scope store? Check boating and outdoors suppliers where, as mentioned, these devices are sold as window defrosters and hair dryers. What if you cannot locate one there either? In a pinch, a plain old hair dryer can work if there is a source of 117-volt AC available. Just do not set the blow dryer to "high" and pump 2,000 watts onto the corrector. Using the lowest setting, hold the dryer a couple of feet from the corrector plate and keep moving it continuously. That will get the job done without ruining "local seeing."
Dew Heaters In the most humid areas, it may be necessary to go to the third line of defense in the war against soggy corrector plates: dew heaters. A dew zapper will work at the worst locales, but most observers soon tire of zapping the corrector every 10 minutes or so when dew is heavy. Dew heaters are the ultimate fix (Plate 40). They are narrow cloth bands that can be wrapped around the corrector end of the tube and fastened in place with Velcro (Velcro really is the astronomer's best friend). Heating elements made from resistors or resistive "heat rope" are sewn into these bands. Each heater strip has a cable that connects to a control box, usually via a phono-style (RCA) plug. In the past, some dew heaters were powered with AC current, but these were unreliable and even dangerous. All dew heaters sold today are DC powered.
For years, the answer to, "Where do you get a dew heater system?" was, "From Jim Kendrick." Kendrick, a Canadian amateur, did not invent dew heaters, but he was the first to integrate them with a control box that allowed users to adjust the heat applied to the corrector. His Kendrick System (Appendix 1) is still popular with
Plate 40. (Dew Fighting Tools) Dew fighting tools. Left to right: Dew-Buster heater controller, Kendrick 8-inch heater strip, 12 volt dew zapper gun." Credit: Author.
amateurs, and in addition to his original controller, he now offers a digital model that senses ambient temperature and applies just the right amount of heat to the lens, saving battery power and preventing seeing effects.
Actually, Kendrick was not there first with a temperature-regulated dew controller. A creative amateur, Ron Keating in Louisiana, did that with his DewBuster. The DewBuster uses a probe to provide temperature feedback to the control box. The user sets the system to a desired temperature above ambient, and dew is a thing of the past. The top-of-the-line Kendrick temperature-regulated Premier controller costs $350, while the DewBuster is $160. It should be noted, however, that the Kendrick can be interfaced to a computer; the DewBuster cannot. The Premier also features a much more sophisticated control panel and display—if that is important to you. A basic non-temperature-controlled Kendrick can still be had for less than $100.
The heater strips themselves can be purchased from Kendrick, with an 8-inch corrector heater costing about $65. DewBuster does not make heaters at this time, but Kendrick heaters work fine with it. Also available are the cheaper but still effective 8-inch heater strips sold by Dew-not for about $40. Both Kendrick and Dew-not sell heaters in sizes for any aperture CAT. Eyepieces, finders, and other accessories can also dew up, and both companies also make heater strips for these devices. Ken-drick's and DewBuster's controllers have multiple outlets that allow the box to run multiple heater strips.
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