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Q : Do anit-reflection coatings improve light transmission?
A : Yes. Conventional thought seems to be that all the light not reflected is transmited through to the next medium. This is critial to the performance of high-element wide angle designs with many refractive surfaces.
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Q : How important it is to get a parfocal series of eyepiece?
A : Parfocal eyepiece sets reduce the amount of refocusing when changing powers, but it is rare when no refocusing is required. Parfocallizing of eyepiece sets is a non-performance factor when choosing oculars.
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Q : How many eyepieces should I have?
A : Eyepieces are the most critical factor concerning the performance of your telescope, excepting a dark sky. Eyepieces create the image your eye will see, and the right ones will give you the experience that makes amateur astronomy so rewarding. Even the best instrument will never perform to it's potential visually with poor oculars. Since most manufacturers sell their telescopes with inexpensive ones, and since most people selling a telescope keep their good eyepieces, the aftermarket is your best source. Borrow as many as you can and try them out; for every object there will be an eyepiece that works best with your particular telescope. You'll probably be satisfied with 5-8 good eyepieces; and you'll use your telescope much more often with good ones.
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Q : How much magnification can I use with my telescope?
A : Every telescope is different, but a rough rule of thumb is 30-50X per inch diameter of the objective. A good refractor may, however, use 100X/inch on bright objects, so this is not a hard rule. You can always increase the magnification above these limits, but it is pointless if you're not seeing more. This rule breaks down for larger instruments, as the distortion of the atmosphere limits practical magnification to 300X. See Usable Magnifications.
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Q : Is high magnification better?
A : Only for some objects, although undermagnification is often a problem, even for experienced observers. The penalty for increased magnification is reduced field of view and brightness; faint objects grow fainter as the magnification is increased This is why larger aperture telescopes are so effective on faint objects; they provide enough light to stimulate the eye at high magnifications.
For example, a 4-inch telescope will only view a globular cluster effectively at 80X, and it will appear as a blob. A 6-inch will resolve the outer stars at 130X, an 8-inch will resolve further in at 200X. 10 and 12.5-inch telescopes will make them glitter to the core at 300 and 400X.
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Q : The image with my low power eyepiece is clear, but my high power is fuzzy. What's wrong with it?
A : There's probably nothing wrong with the eyepiece: you have probably exceeded the resolving power of your telescope. A television set looks clear 10 metres away, but up close you can see the imperfections.
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Q : What is a Barlow Lens
A : A barlow lens has a negative focal length which increases the effective focal length (E.F.L) of the objective lens or mirror of the telescope. It is always placed between the objective and the eyepiece and results in increased magnification and decreased field of view.
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Q : What is an eyepiece?
A : An eyepiece is a magnifier, much like a high power magnifying glass. When placed at the real image made by the lens or mirror of a telescope, the eyepiece projects a virtual image into your eye, enabling you to see the target.
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Q : What is Apparent Field?
A : Apparent Field (A.F.) is the angle viewed by the eye when looking into the eyepiece. An eye by itself has an A.F. of about 100 degrees, so any well corrected design up to this value would be a benefit.
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Q : What is Exit Pupil?
A : Exit Pupil is the size of the light beam the eyepiece projects into your eye. Exit pupil can be calculated as follows:
Most night adapted eyes open to 5-7mm, so it's not a good idea to use eyepieces which give an exit pupil much larger than this, as the beam won't fit into entrance pupil of your eye.
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Q : What is Eye Relief?
A : Eye relief is maximum distance between the eye and the eye lens of the eyepiece to see the eyepiece's field stop. (The field stop is the baffle at the image plane that produces the field edge.) Adequate eye relief is a very important factor for comfortable viewing.
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Q : What is the "Kidney Bean Effect"?
A : The "Kidney Bean Effect" is not the same phenomenon as the before mentioned "black spot". In some long f.l or wide angle eyepieces, it is sometimes necessary to move the eye closer to the eyepiece in order to see the edge of the field. Sometimes, when this occurs, parts of the field between the centre and the edge are cut off, as part of the quickly converging beam misses the eye's pupil. This appears to the observer as a giant kidney bean shaped dark region that meandors around the field as head moves.
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Q : What is the black spot I see in a low powered eyepiece in my reflector during daylight?
A : A low powered eyepiece in a reflector produces a large exit pupil with a large image of the secondary mirror obstruction. During the day, when the pupil of the eye is small, if the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the center of the field. At night, when the pupil of the eye is large, the darkened region is not noticed.
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Q : When I'm observing a bright object like a planet, I see an opposing ghost image. What causes that?
A : The ghost image, and it's evil twin, the out-of-focus ghost is caused by internal reflections inside the eyepiece. The only way to eliminate these is to eliminate air-spaces in the eyepieces, as the ghost is caused by a double bounce between two lenses in close proximity. While the ghost is an annoyance, the out of focus ghost is more of an enemy, as it reduces overall contrast of the image, which determines how much detail you'll be able to see. The treatment, if not eh cure, is di-electric multicoating of the lens-facing surfaces inside the eyepiece.
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Q : Which eyepiece design is best?
A : This often asked question is quite irrelevant, as different design's performance varies with different telescopes. Different eyepiece designs have various characteristics. For example, and expensive widefield design is not required for planetary viewing, where the only important thing is maximum contrast. A Plossl or Orthoscopic would probably be best, but almost all design s are good performers on-axis for any f/ratio. Telescopes with F/ratios>10 are quite tolerant of simple low element eyepieces up to 55 deg. A.F., but telescopes <6 are a different matter. Off-axis performance requires powerful correction to properly image the highly convergent beam. Each eyepiece and telescope performs as a system, and their image can only be evaluated as much.
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Q : Which eyepiece should I choose?
A : If brightness is not a factor, choose the eyepiece that will encompass the object, then allow for a suitable backdrop. If you want to know the actual field on view the eyepiece will give (True Field), this can be calculated as:
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Q : Which works better? An eyepiece or a Barlow+eyepiece giving the same magnification?
A : The only time the eyepiece alone may perform as well, is on-axis, in a high-contrast application, as the extra optics of the barlow may cause a slight depreciation. Optically, for all other sues, the eyepice+barlow outperforms the eyepiece working alone. The reason? Most of the aberrations caused by positive spherical lenses (Coma, Astigmatism, Curvature of Field and Spherical Aberration) can be reduced and sometimes almost eliminated by introducing a negative system (barlow) which has the same aberrations in negative quantities! Spherical aberration of the system is reduced as the positive spherical aberration of the eyepiece is cancelled by the negative spherical aberration of the barlow. The other aberrations cancel in a similar way!
This is one of the eyepiece designer's most powerful weapons, and it is used in most of the shorter focal length ultra-wide designs. Another great benefit of this idea is that the longer eye relief of the longer f.l. eyepiece used with the barlow is retained.
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Q : Why are some eyepiece more expensive than others?
A : When you pay more for an eyepiece you are usually paying for: Field of view: Eyepieces that have many lenses to correct for the five major aberration (these aberrations give increasingly worse, the lower the focal ratio of the telescope) have obviously higher costs in lenses and coatings.
Eye relief: Using larger, more expensive elements in eyepieces allows for a greater distance between the eyes and eyepiece.
Coatings: 2-layer multicoatings on both faces of all lenses will typically add 25% to the cost of an eyepiece, but this is absolutely necessary to preserve the contrast of the image when the light has to go through 7-9 lenses.
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Q : Why is the image better at the center of the field?
A : All commercial eyepieces are made with spherical elements, as these are the only type that are easily mass produced. These naturally produce aberrations, which become much worse in highly convergent light beams. There is no way to avoid all aberrations when using spherical elements. Clever eyepiece designer can, however, minimize the objectionable ones and cause others to manifest themselves in an acceptable form.
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Q : Will a telescope work without an eyepiece?
A : Not for visual purposes, as the eye cannot process the real image made by the objective. The telescope may be used without an eyepiece for camera and other instruments.
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