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Home  Understanding Equipment
Equipment
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Written by TAAS
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Tuesday, 07 July 2009 23:45 |
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First, let’s start with Newtonian reflectors, which are comprised of an open tube, a parabolic mirror at the bottom of that tube, and a smaller, flat, 45 degree mirror at the top. Mirrors are cheaper to produce than lenses, because only one side has to be within the engineering tolerances required for precision viewing. Newtonian primary mirrors also use a concave design that is much simpler to grind than many other telescope lenses. Thus, if you want the most bang for the buck, this is the scope type for you. Further more, larger "Newts" often use a Dobsonian mount, which is a simple pan and tilt type box that the tube rests in. This design was named after John Dobson, who brought “big” astronomy to amateurs with this design. Dobsonian mounts are very simple and can handle massive structures while maintaining a high degree of strength. If you've ever seen a giant scope with trusses sitting on a hand crafted wooden base, this is it. Unfortunately the optical design can cause blurring of stars in the outer portion of the field of view at the eyepiece. This is the biggest drawback. Hence, if you want clean star images in the full window of your eyepiece, plan to buy more expensive eyepieces, which are designed to handle the optical demands of Newtonian design. Also, since many who buy a Dobsonian mounted Newt (Dob for short) like to go big, the scopes get large and heavy quickly. My own 12" weighs about 90 lbs assembled, and is 5 ½’ tall. An average 6" Dob goes for $250-300 new. If that’s not large enough, you can buy 'em pre built up to 30" or more, for the price of a new family sedan.
Next, there are catadioptric designs (cats), such as Schmidt Cassegrains and Maksutov Cassegrains. These designs are popular for two reasons. First, they are relatively compact because of a multi mirror and lens system which folds the light path into a smaller tube. Second they have a long usable light path (focal ratio), which means less expensive eyepieces and cleaner star fields in your entire field of view. Unfortunately, most of these scopes have larger secondary mirrors than Newts, which slightly reduces the amount of contrast and resolution that is created by the primary mirror in the first place. This is because the secondary mirror (as in Newts) sits right within the light path, compromising the detail in the final image. Still, unless you have perfect atmosphere, or take to detailed planet and lunar sketching or photography, you may never really appreciate such relatively small optical differences. All in all these are fine scopes capable of a myriad of observing purposes, and are typically light enough to easily mount on a tripod with an equatorial mount, while providing enough aperture to keep the image bright and pleasing to the eye.
Finally you have refractors, the gems of the telescope world. Because they rely solely on lenses, they are the most expensive breed per inch of aperture. Aside from price, the other main drawback to many refractors is something called violet fringing. Bright objects, such as the Moon, planets, and bright stars, will give off a small, yet obtrusive purple halo. This is because of the physics involved in the way light is brought to focus with different lens designs and glass types. Fortunately, this can be avoided with apochromatic (apo) designs, which often use exotic glass, additional elements, or both to fix the problem. Unfortunately, this means the cost goes up exponentially. So why buy an Apo? Refractors have no obstruction in the light path, which means better contrast and image sharpness than any other design (fantastic for planets). Apochromatic design also adds perfect color rendition (no purple halo). Put these together and you have optical perfection. A 6" Apo will put you on a waiting list, and plan to spend mucho dinero ($10,000+). That's just the tube. Accessories optional.
If you are staring out, and I assume you are since you are reading such an article, my recommendations are simple. Get a scope with an aperture of 5-6”. Get the strongest mount you can afford. Purchase at least two eyepieces that will give you approximately 50x and 150x. A setup of this design could potentially keep you happy for the rest of your life. |
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What Telescope Should I Get? |
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Written by TAAS
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Tuesday, 07 July 2009 23:36 |
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Please understand that what works for one person will not always work for another. Also know that recommendations here are made from my opinions, but they are based on several years of telescope ownership and many shared experiences with fellow astronomers both novice and veteran alike. That said, initially I recommend starting with a pair of binoculars, since they are easy to navigate with. However, assuming you have made it past this point and you lust for greater reach, you should ask yourself a few questions. First, what do you think you will want to look at? There are dozens of different object types to view in the sky (planets, comets, galaxies, the Moon, etc), and most people will find fascination in certain ones over others. Some will love them all equally. These preferences can effect which scope makes a best choice for observing. Will transportation be required to use the scope, thus how large and heavy of a piece of equipment can you handle? Finally, and often the deciding factor, what is your budget? |
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Last Updated ( Tuesday, 07 July 2009 23:45 )
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Written by TAAS
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Tuesday, 07 July 2009 23:34 |
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First off, let’s discuss the basics behind a telescope. To view anything you will need a scope, an eyepiece, and a mount. The scope serves one major purpose, to gather light. This is determined by the size of the objective optic (either a lens or a mirror), and is measured in millimeters or inches. Just last evening I was doing a “peek at the moon” session at a local campground, using my 12” telescope. A gentleman came up to me and asked, “How powerful is a scope like that?” I’m sure he wanted an answer like, “Six hundred power!” But in reality what I should have said was, “One thousand eight hundred times more light than your unaided eye.” You see, although maximum magnification is partly determined by the size of the objective size of a scope, it is mostly determined by the stability of the atmosphere. So, in actuality, the scope magnifies light more than anything.
Now that you have all that light gathered and flying down to the viewing end of your instrument at 186,000 miles/second, you will need an eyepiece. This is a small object, containing some variety of lenses, which takes the “raw” image from your scope and brings it to a point of focus for your eye. You simply insert the eyepiece into your telescope’s focuser, and turn the focusing knob until the image is sharp. Eyepieces come in various focal lengths which determine the image size, or magnification.
Finally you will need a mount to hold and stabilize your scope. Mounts come in a variety of designs, but the two most common are the equatorial and the alt-azimuth. An equatorial mount uses a two-axis system that holds your scope at the angle of your location of latitude. This enables you to follow objects easily as they slowly move across the night sky. This design also lends itself to easy integration of a motor, which will slowly move the mount for you, counteracting the earth’s rotation. The alt-azimuth is a simpler two-axis design which moves up/down-left/right similar to the movement of canon. This design tends to be more intuitive for the beginner. A word to the wise, a mount can NEVER be too big and strong, in my opinion. You can skimp on many aspects of a telescope, but skimping on the mount means shakes and jitters every time you touch the focuser, bump the eyepiece with your brow, or a slight wind passes by. If an image is rock solid, only then can you truly focus on the detail in your picture window of space. |
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Last Updated ( Tuesday, 07 July 2009 23:45 )
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Written by TAAS
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Saturday, 04 July 2009 01:30 |
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Every time I set my astronomical equipment up in a public group setting someone will inevitably approach me, “My so-and-so is interested in the night sky. What’s a good telescope to get for them?” A fair question. But a few factors certainly need to be considered before answering a question like this.
Ironically, despite the fact that most enthusiastic amateur astronomers end up with a scope at some point, I often don’t recommend a telescope for beginners…at least not initially. It is easy to fall victim to the “department store telescope”, a route that many beginners have taken, or have been offered by way of gift. If I had a dollar for each time a friend or acquaintance has told me of a frustrating experience with a small, wobbly, hard to focus, store bought telescope (which inevitably leads to total abandonment of the hobby), I could afford those 150mm Fujinon binoculars that I lust after. Well, that may be a slight exaggeration. The point is clear nonetheless. Not only does effective use of a telescope require some knowledge of the sky, the additional problem is that a flood of cheap plastic scopes have been sold to the public for decades. If you are in your local hobby store or wholesale club, and you come across a small telescope that promises “views of the planets at 600x or more”, hide your wallet and run. Usable magnification is directly related to size (yup, the bigger the better) And even experienced astronomers will tell you that, thanks to an intermittently turbulent atmosphere, 600x almost never happens…even with the best equipment. Also know that the least expensive good quality scopes will cost you upward of $250, once the minimum accessories (a mount/tripod and eyepieces) are factored in.
So where do you begin? For starters, you can pick up a small constellation guide and a red flashlight, and relax under the stars in a comfortable lounge chair. No kidding. You could spend a year doing this in your spare time and remain fascinated by what you see. Learning the constellations and their major stars will benefit your future endeavors ten fold by reducing your frustration with stellar navigation. Not to mention that using your naked eyes will enable you to catch satellites and meteors by the boatload.
OK, you’ve done that. Now what? Get yourself a pair of binoculars with at least 40mm lenses. A small pair with less than 10x magnification can be hand held. A pair larger than that is probably best on a tripod. Either way you can greatly magnify dense clusters of stars, larger galaxies, and the surface of the Moon. You can also appreciate the four largest moons of Jupiter, or the phases of Venus. A great advantage of binoculars, as opposed to telescopes, is that you view objects in the orientation that your eyes already perceive them – up is up, down is down, left is left, and right is right. With unmodified telescopes, images are always reversed left to right and flipped upside down. But best of all is that you use both eyes with binoculars, so details are easier for your brain to resolve, and objects appear brighter than with one eye alone. Just try stepping outside on a dark night, and after staring at the stars for a few minutes try closing one eye. You’ll see how large of a difference it makes.
Maybe you already have a pair of binoculars, but don’t use them much. Great! Pull them out of the drawer, carefully dust off the lenses, and take a tour of the Orion constellation or the summer Milky Way. You’ll be impressed, I promise.
Jason Scherff |
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Last Updated ( Wednesday, 16 September 2009 04:36 )
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