While a number of interesting celestial objects are readily identified by the naked eye, sometimes with the aid of a star chart, many others are so faint or inconspicuous that technical means are necessary to locate them. Many methods are used in amateur astronomy, but most are variations of a few specific techniques.

Star hopping is a method often used by amateur astronomers with low-tech equipment such as binoculars or a manually driven telescope focuser . It involves the use of maps (or memory) to locate known landmark stars, and "hopping" between them, often with the aid of a finderscope. Because of its simplicity, star hopping is a very common method for finding objects that are close to naked-eye stars.

More advanced methods of locating objects in the sky include telescope mounts with setting circles, which assist with pointing telescopes to positions in the sky that are known to contain objects of interest, and GOTO telescopes, which are fully automated telescopes that are capable of locating objects on demand (having first been calibrated).
Setting circles are angular measurement scales that can be placed on the two main rotation axes of some Achromatic Lenses . Since the widespread adoption of digital setting circles, any classical engraved setting circle is now specifically identified as an "analog setting circle" (ASC).

By knowing the coordinates of an object (usually given in equatorial coordinates), the telescope user can use the setting circle to align the telescope in the appropriate direction before looking through its eyepiece. A computerized setting circle is called a "digital setting circle" (DSC). Although digital setting circles can be used to display a telescope's RA and Dec coordinates, they are not simply a digital read-out of what can be seen on the telescope's analog setting circles. As with go-to telescopes, digital setting circle computers (commercial names include Argo Navis, Sky Commander, and NGC Max) contain databases of tens of thousands of celestial objects and projections of planet positions.

To find an object, such as globular cluster NGC 6712, one does not need to look up the RA and Dec coordinates in a book, and then move the telescope to those numerical readings. Rather, the object is chosen from the database and arrow markers appear in the display which indicate the direction to move the Apochromatic refractor . The telescope is moved until the distance value reaches zero. When both the RA and Dec axes are thus "zeroed out", the object should be in the eyepiece. The user therefore does not have to go back and forth from some other database (such as a book or laptop) to match the desired object's listed coordinates to the coordinates on the telescope. However, many DSCs, and also go-to systems, can work in conjunction with laptop sky programs.

Computerized systems provide the further advantage of computing coordinate precession. Traditional printed sources are subtitled by the epoch year, which refers to the positions of celestial objects at a given time to the nearest year (e.g., J2005, J2007). Most such printed sources have been updated for intervals of only about every fifty years (e.g., J1900, J1950, J2000). Computerized sources, on the other hand, are able to calculate the right ascension and declination of the "epoch of date" to the exact instant of observation.

Aapochromatic telescopes have become more popular since the 1980s as technology has improved and prices have been reduced. With these computer-driven telescopes, the user typically enters the name of the item of interest and the mechanics of the telescope point the telescope towards that item automatically. They have several notable advantages for amateur astronomers intent on research. For example, GOTO telescopes tend to be faster for locating items of interest than star hopping, allowing more time for studying of the object. GOTO also allows manufacturers to add equatorial tracking to mechanically simpler alt-azimuth telescope mounts, allowing them to produce an over all less expensive product.

Amateur astronomers engage in many imaging techniques including film and CCD astrophotography. Because CCD imagers are linear, image processing may be used to subtract away the effects of light pollution, which has increased the popularity of astrophotography in urban areas.

from:wiki

Amateur astronomy, also called backyard astronomy, is a hobby whose participants enjoy watching the night sky (and the day sky too, for sunspots, eclipses, etc.), and the plethora of objects found in it, mainly with portable telescopes and binoculars. Even though scientific research is not their main goal, many amateur astronomers make a contribution to astronomy by monitoring variable stars, tracking asteroids and discovering transient objects, such as comets. Such efforts are one of the relatively few ways interested amateurs can still make useful contributions to scientific knowledge.

The typical amateur astronomer is one who does not depend on the field of astronomy as a primary source of income or support, and does not have a professional degree or advanced academic training in the subject. Many amateurs are beginners, while others have a high degree of experience in astronomy and often assist and work alongside professional astronomers.

Amateur astronomy is usually associated with viewing the night sky when most celestial objects and events are visible, but sometimes amateur astronomers also operate during the day for events such as sunspots and solar eclipses. Amateur astronomers often look at the sky using nothing more than their eyes, but common tools for amateur astronomy include portable apochromatic telescopes and binoculars.

People have studied the sky throughout history in an amateur framework, without any formal method of funding. It is only within about the past century, however, that amateur astronomy has become an activity clearly distinguished from professional astronomy, and other related activities.

Collectively, amateur astronomers observe a variety of celestial objects and phenomena. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep sky objects such as star clusters, galaxies, and nebulae. Many amateurs like to specialise in observing particular objects, types of objects, or types of events which interest them. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Astrophotography has become more popular for amateurs in recent times, as relatively sophisticated equipment, such as high quality CCD cameras, has become more affordable.

Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. The pioneer of amateur radio astronomy was Karl Jansky who started observing the sky at radio wavelengths in the 1930s, and interest has increased over time. Non-visual amateur astronomy includes the use of infrared filters on conventional telescope focuser , and also the use of radio telescopes. Some amateur astronomers use home-made radio telescopes, while others use radio telescopes that were originally built for astronomy research but have since been made available for use by amateurs. The One-Mile Telescope is one such example.

Amateur astronomers use a range of instruments to study the sky, depending on a combination of their interests and resources. Methods include simply looking at the night sky with the naked eye, using binoculars, and using a variety of optical telescopes of varying power and quality, as well as additional sophisticated equipment, such as cameras, to study light from the sky in both the visual and non-visual parts of the spectrum. Apochromatic refractors are available new and used, but in some places it is also common for amateur astronomers to build (or commission the building of) their own custom telescope. Some people even focus on amateur telescope making as their primary interest within the hobby of amateur astronomy.

Although specialized and experienced amateur astronomers tend to acquire more specialized and more powerful equipment over time, relatively simple equipment is often preferred for certain tasks. Binoculars, for instance, although generally of lower power than the majority of telescopes, also tend to provide a wider field of view, which is preferable for looking at some objects in the night sky.

Amateur astronomers also use star charts that, depending on experience and intentions, may range from simple planispheres through to detailed charts of very specific areas of the night sky. A range of astronomy software is available and used by amateur astronomers, including software that generates maps of the sky, software to assist with astrophotography, observation scheduling software, and software to perform various calculations pertaining to Achromatic Lens .

Amateur astronomers often like to keep records of their observations, which usually takes the form of an observing log. Observing logs typically record details about which objects were observed and when, as well as describing the details that were seen. Sketching is sometimes used within logs, and photographic records of observations have also been used in recent times.

The Internet is an essential tool of amateur astronomers. Almost all astronomy clubs, even those with very few members, have a web site. The popularity of CCD imaging among amateurs has led to large numbers of web sites being written by individuals about their images and equipment. Much of the social interaction of amateur astronomy occurs on mailing lists or discussion groups. Discussion group servers host numerous astronomy lists. A great deal of the commerce of amateur astronomy, the buying and selling of equipment, occurs online. Many amateurs use online tools to plan their nightly observing sessions using tools such as the Clear Sky Chart.

from:wiki

The achromatic telescope is a refracting telescope that uses an achromatic lens to correct for chromatic aberration.
When an image passes through a lens, the light is refracted at different angles for different wavelengths. This produces focal lengths that are dependent on the color of the light. So, for example, at the focal plane an image may be focused at the red end of the spectrum, but blurred at the blue end. This effect is particularly noticeable the further an object lies from the central axis of the telescope. The image of a star can appear blue on one side and orange on the other.

Early refracting telescopes with non-achromatic objectives were constructed with very long focal lengths to mask the chromatic aberration. An Achromatic telescopes uses an achromatic lens to correct for this. An achromatic lens is a compound lenses made with two types of glass with different dispersion. One element, a concave lens made out of Flint glass, has relatively high dispersion, while the other, a convex element made of Crown glass, has a lower dispersion. The crown lens is usually placed at the front due to the higher susceptibility of flint glass to atmospheric attack. The lens elements are mounted next to each other and shaped so that the chromatic aberration of one is counter-balanced by the chromatic aberration of the other, while the positive power of the crown lens element is not quite equaled by the negative power of the flint lens element. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus.

Uses an equiconvex crown with R1=R2, and a flint with R3=-R2 and a flat back. Can produce a ghost image between R2 and R3 because they have the same radii. May also produce a ghost image between the flat R4 and rear of the telescope tube.
R1 is set greater than R2, and R2 is set close to, but not equal, R3. R4 is usually greater than R3.

Uses an equiconvex crown with R1=R2, and a flint with R3~R2 and R4>>R3. R3 is set slightly shorter than R2 to create a focus mismatch between R2 and R3, thereby reducing ghosting between the crown and flint.
The use of oil between the crown and flint eliminates the effect of ghosting, particularly where R2=R3. It can also increase light transmission slightly and reduce the impact of errors in R2 and R3.

• The Intane Starviewer 60 APO Refractor is 360mm focal length at a focal ratio of f/6 that measures only 300mm long and weighs only 1.4kg.
• The Intane Starviewer 60 APO Refractor has extreme quality optics that make it a superior choice as a telephoto lens, spotting scope or small achromatic telescope .
• 1:10 dual speed crayford focuser,360-degree rotatable, 2" to 1.25" adapter inclued.
• All-metal construction is precision machined for the perfect fit and finish.

Specifications         

Standard Equipment  

• Main tube assembly (Cemented triplet system) x1
• 2" 360 deg. rotatable Crayford focuser x1
• 2" to 1.25" adapter x1
• Mounting Ring  x2
• Dovetail x1
• 2" extension tube x1
• Interferometer Report for objective x1
• Bag with custom fitted foam x1

An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.

The most common type of achromat is the achromatic doublet , which is composed of two individual lenses made from glasses with different amounts of dispersion. Usually one element is a concave lens made out of flint glass, which has relatively high dispersion, while the other, convex, element is made of crown glass, which has lower dispersion. The lens elements are mounted next to each other, typically cemented together, and shaped so that the chromatic aberration of one is counterbalanced by that of the other.

In the most common type (illustrated above), the positive power of the crown lens element is not quite equalled by the negative power of the flint lens element. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus. Negative doublets , in which the negative-power element predominates, are also made.

Theoretical considerations of the feasibility of correcting chromatic aberration were debated in the 18th century following Newton's statement that such a correction was impossible (see History of the telescope). Credit for the invention of the first achromatic doublet is often given to an English barrister and amateur optician named Chester Moore Hall. Hall wished to keep his work on the achromatic lenses a secret and contracted the manufacture of the crown and flint lenses to two different opticians, Edward Scarlett and James Mann. They in turn sub-contracted the work to the same person, George Bass. He realized the two components were for the same client and, after fitting the two parts together, noted the achromatic properties. Hall failed to appreciate the importance of his invention, and it remained known to only a few opticians.

In the late 1750s, Bass mentioned Hall's lenses to John Dollond, who understood their potential and was able to reproduce their design. Dolland applied for and was granted a patent on the technology in 1758, which led to bitter fights with other opticians over the right to make and sell achromatic doublets.

Dollond's son Peter invented the apochromat, an improvement on the achromat, in 1763.

Hyperbolic mirrors are intended to form image of object in focal plane, and normally are used in astronomy telescopes, as well as other optical systems demanding broadband reflection and excellent photographic effect.
   For example, R-C telescopes utilize two hyperbolic mirrors as the primary and secondary mirror, which correct for coma, which also results in a smaller spot size on and off axis; Cassegrain telescopes have a primary parabolic mirrors focusing its rays onto a convex hyperbolic secondary mirror, which yields excellent correction over moderate fields both for visual and photographic astronomy.

   Like ellipsoidal mirrors and parabolic mirror, hyperbolic mirror also can be on-axis or off-axis and can be supplied in a variety of materials, including BK7 glass, copper, fused silica, UV grade fused silica, nickel, and other optical glasses. Common coatings include none (uncoated), enhanced aluminum, protected aluminum, dielectric, protected gold, and silver. 

Intane can provide both on-axis and off-axis hyperbolic mirrors with the shape of either convex or concave, and especially specialize in production of medium to large-size custom hyperboloid with exacting tolerances, high grade surface finish and difficult coatings. Surface accuracy can be verified by the Foucault test or on the Zygo interferometer upon the request.
           Optical designers of Intane are always available to assist with your special needs and production in China is ready to meet your quantity requirements. 

from:Intane|Hyperbolic mirrors

    * The Intane Starviewer 60 APO Refractor is 360mm focal length at a focal ratio of f/6 that measures only 300mm long and weighs only 1.4kg.

    * The Intane Starviewer 60 APO Refractor has extreme quality optics that make it a superior choice as a telephoto lens, spotting scope or small astronomical telescope.

    * 1:10 dual speed crayford focuser ,360-degree rotatable, 2″ to 1.25″ adapter inclued.

    * All-metal construction is precision machined for the perfect fit and finish.

from:Intane Blog|Telescope

   Achromatic lenses are corrected for chromatic aberration with respect to three wavelengths (blue, green and red). Chromatic aberration is produced by dispersion, or the variation of refractive index with wavelength, and causes different wavelengths to have different focal points.

     Using separate materials like crown glass and flint glass for the converging and diverging lens elements, the dispersion of each can be compensated for by the other thereby minimizing the total effect.

     Achromatic doublet is the simplest form of achromats including two types: cemented doublets and air-spaced doublets. Surely achromats could have multi-elements as well, which is up to its application. Usually achromatic lenses with large aperture are air-spaced. All the air-spaced achromats show exceptional performance and provided with mounts. The air gap acts as a third lens element and provides a greater degree of correction than found in a cemented lens. In this way it is possible to balance the aberrations over a fairly wide range of wavelengths so that the lenses perform well as broadband imaging devices.

     All achromats are computer optimized to minimize aberrations and coma, and they are fully coated with either multiplayer broadband or with a single layer anti-reflection coating.A wide range of focal lengths and diameters are available.

Flat mirrors are smooth, highly polished, flat surfaces, for reflecting light. The actual reflecting surface is usually a thin coating of silver, aluminum, or other material. They may be fashioned from a number of materials, including BK7 glass, copper, fused silica, UV grade fused silica, nickel, and optical lens crown. Flat mirrors can either come round, square, rectangular or ellipse. 

Interferometer Report for Optical Flat

Dia:200mm

Dia:300mm

from:Intane|Telescope lens

A mirror is an object with at least one polished and therefore specularly reflective surface. The most familiar type of mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to produce magnified or diminished images or focus light or simply distort the reflected image.

Mirrors are commonly used for personal grooming (in which case the old-fashioned term "looking-glass" can be used), decoration, and architecture. Mirrors are also used in scientific apparatus such as telescope mirror and lasers, cameras, and industrial machinery. Most mirrors are designed for visible light; however, mirrors designed for other types of waves or other wavelengths of electromagnetic radiation are also used, especially in non-optical instruments. In the modern day people adore looking at the reflection of themselves, which makes the sale of mirrors quite high. They are an necessity in most modern households.

Telescopes and other precision instruments use front silvered or first surface mirrors, where the reflecting surface is placed on the front (or first) surface of the glass (this eliminates reflection from glass surface ordinary back mirrors have). Some of them use silver, but most are aluminum, which is more reflective at short wavelengths than silver. All of these coatings are easily damaged and require special handling. They reflect 90% to 95% of the incident light when new. The coatings are typically applied by vacuum deposition. A protective overcoat is usually applied before the mirror is removed from the vacuum, because the coating otherwise begins to corrode as soon as it is exposed to oxygen and humidity in the air. Front silvered mirrors have to be resurfaced occasionally to keep their quality. There are optical mirrors such as mangin mirrors that are second surface mirrors (reflective coating on the rear surface) as part of their optical designs, usually to correct optical lens .

The reflectivity of the mirror coating can be measured using a reflectometer and for a particular metal it will be different for different wavelengths of light. This is exploited in some optical work to make cold mirrors and hot mirrors. A cold mirror is made by using a transparent substrate and choosing a coating material that is more reflective to visible light and more transmissive to infrared light. A hot mirror is the opposite, the coating preferentially reflects infrared. Mirror surfaces are sometimes given thin film overcoatings both to retard degradation of the surface and to increase their reflectivity in parts of the spectrum where they will be used. For instance, aluminum mirrors are commonly coated with silicon dioxide or magnesium fluoride. The reflectivity as a function of wavelength depends on both the thickness of the coating and on how it is applied.
A dielectric coated mirror used in a dye laser. The mirror is over 99% reflective at 550 nanometers, (yellow), but will allow most other colors to pass through.

For scientific optical work, dielectric mirrors are often used. These are glass (or sometimes other material) substrates on which one or more layers of dielectric material are deposited, to form an optical coating. By careful choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from the mirror can be specified. The best mirrors of this type can reflect >99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror. Such mirrors are often used in lasers.

In astronomy, adaptive optics is a technique to measure variable image distortions and adapt a deformable mirror accordingly on a timescale of milliseconds, to compensate for the distortions.

Although the most of mirrors are designed to reflect visible light, surfaces reflecting other forms of electromagnetic radiation are also called "mirrors". The mirrors for other ranges of electromagnetic waves are used in optics and astronomy. Mirrors for radio waves are important elements of radio telescopes.

from: wiki

Time: November 21,2008

Location:Yinna Mountain,Meizhou, GuangDong, China

Photographer: Wei Huang

Refractor:Intane Triplet 80mm F7 APO Refractor with Vixen 0.67x field reducer

Mount: Vixen Sphinx SXW

Camera: Canon 450D Baader IR Filter Modified, Cooling Modified

Guide: Mizar GT68(D=68mm,f=600mm)+ Vixen GA4+ Mizar Or6

Exposure: ISO800 composite of 16X8min ,4X2min,4X12sw

Processing: Deepskystacker, Photoshop CS

Weather : Clearness, Transparency fine.

Naked eye limiting magnitude: 6

The Crayford focuser design makes very fine adjustments possible, with tolerances up to 100 times better than conventional rack-and-pinion focusers. Its zero image shift and zero backlash makes it outstanding for visual and photographic work and a must for CCD imaging.

JMI recognized the benefits of this design and was the first company to bring it to the amateur market in a commercial product. Many telescope accessory companies have used the design for add-on focusers since JMI first adopted it and it is now being incorporated into many telescopes as standard equipment, as well.

The illustration shows one of Jack Wall’s original design

  Nanjing Intane Optical Engineering Co., Ltd.(a certified ISO9001:2000 company), is a leading manufacturer of customized high-quality precision optics in China. Founded by Professor Bifang Zhou, this esteemed manager is the former director of the Nanjing Astronomical Instrument Research & Development Center under the Chinese Academy of Sciences.

Employing a team of scientists and highly qualified fine optic engineers, Intane specializes in client-specific, high precision optical components, as well as complex optical and optoelectronic systems. Especially,  have competitive advantages at manufacturing medium to large-size precision aspherical optical components.   hot products are craydord focuser,  Apochromatic Lens , Optical Components

from:News|Intane

•  The mainframe components locating the probe are made of massive stainless steel, hardened and fine ground on machine-tools to the highest achievable accuracy. Concentricity of mainframe components to the probe is less than 1 micron.
•  The spherometer rings represent one of the key components of the Super-Spherotronic including an incredible amount of work to eliminate any possible error source. Made of heat-treated high quality stainless steel, the rings are machined on high accuracy grinding machine-tools and finally fine-lapped of Apochromatic refractor. An ingenious design combined with a special electro-erosion procedure allows the ball and the ball location site to be matched with highest accuracy. Each ring is delivered with a calibration certificate. The measuring accuracy is directly traceable to NIST standard.
•  Super Spherotronic
•  The supporting balls are also concentrically positioned with an extreme accuracy of less than 1 micron. This highly accurate positioning of the ring balls is an essential and distinctive feature of the Super-Spherotronic, ensuring excellent repeatability. To avoid any thermal and mechanical deformation, the supporting balls are made of wolfram carbide (an extremely hard metal), while the probe center is an ultra-precision ruby ball. This completely eliminates any errors introduces by metallic probe centers used in other spherometers.
•   The probe itself is a high precision Heidenhain encoder, specifically selected for this use, having a total systematic and random error of less than 0.2 microns.
•   The test plates used for calibration of the instrument are manufactured to high quality standards to an accuracy better than 1/10 wave for sphericity. The test plates are certified by NIST of USA, so that the accuracy of Super-Spherotronic is directly traceable to international standards.

Different Scopes for Different Folks :

Now that we understand these basic points of telescope performance and mounting, we can discuss the three basic optical designs of telescopes: the refractor, the reflector, and the compound (or catadioptric) telescope.

A Apochromatic Refractor is what most non-astronomers think of when they hear the word "telescope." Its tube is most often long and skinny, mounted on a tripod, with a lens at one end and the eyepiece at the other. Refractors were the first type of telescope invented, and the finest refractors still provide the best images of any design for a given aperture. They are often chosen by observers with a dominant interest in the planets and Moon, because they can provide sharp, high-contrast views at high magnification and are less bothered by atmospheric "seeing" than the other designs. They also require less maintenance than reflectors or compound scopes, and are therefore popular with beginners. The refractor’s good performance at high power and relative insensitivity to light pollution makes it a good choice for a city-based observer, as the design performs best on the objects that are most easily seen from urban or suburban locations.

These advantages do not come without a price — literally: refractors are the most expensive telescopes per inch of aperture. Big refractors can cost several thousand dollars, and still are considered too small in aperture for serious deep-sky observing. The long focal length of most refractors restricts the field of view, making it difficult to take in large extended objects like some clusters of stars. And the long tube, with the eyepiece located at the back end, requires a tall tripod, which, if poorly made, can allow the scope to shake and shimmy in the breeze, rendering high-powered observing difficult.

The reflector uses a mirror, rather than a lens, to gather and focus light. By far the most common design is the Newtonian reflector, which places a concave (dish-shaped) primary mirror at the bottom end of the telescope tube. A small secondary mirror at the other end directs the focused light out the side of the tube and into the eyepiece. Newtonians offer the largest aperture available at given price, and when well made, they can provide sharp, contrasty views that rival all but the finest refractors. A Newtonian’s low center of gravity and eyepiece location at the top of the tube allow for comfortable viewing with a more compact mounting, which can be made stable with much less bulk and cost than the tall mounting required by a refractor of similar aperture.

Big Apochromatic Refractor of 10" aperture and larger on Dobsonian mountings are the most popular telescopes for astronomers who seek to gather "buckets of light" for deep-sky observing. These giant scopes perform best at remote dark sky sites, away from the glare of city lights. The value and versatility of the smaller 4.5" to 8" Newtonians, mounted either equatorially or as Dobsonians, makes them a fine choice for the beginner with general interests.

Newtonian reflectors require occasional maintenance. Unlike the lenses in a refractor, the mirrors in a reflector need periodic alignment, or collimation, for best performance. While many beginners seem intimidated by collimation, it’s really not difficult, and takes only a few minutes once you get the hang of it. A reflector’s tube is also more open to air and humidity than that of a refractor, and if left uncovered the mirrors can accumulate dust and grime, which necessitates occasional cleaning. While these maintenance concerns are often overstated, a Newtonian may not be the right choice for someone who finds the prospect of occasional tinkering with the telescope unappealing.

The most modern of the three common designs for telescope focuser is the compound, or catadioptric type, which uses a combination of lenses and mirrors to gather and focus light. The greatest advantage of this design is its compactness: the lenses and mirrors "fold up" the light path inside the telescope, reducing large-aperture scopes to a manageable size. If an equatorial mounting is desired, the smaller tube can be carried on lighter and more economical mounts than that required by a Newtonian of the same size. Compound telescopes are most popular with observers who desire both generous aperture and an equatorial mounting in a transportable package.

The names Schmidt-Cassegrain and Maksutov-Cassegrain refer to specific designs of compound telescope focuser, which use differently shaped lenses and mirrors to achieve a similar result. The Maksutov is often cited as offering better image quality, though there is little in the way of optical theory to support this opinion. Most probably the Maksutov has developed its reputation as the superior catadioptric design because its spherical optical surfaces are easier to make to very high precision than the more complex shapes demanded by the Schmidt. As a result, if a telescope maker practices anything less than the strictest quality control, their "average" Maksutov will outperform their "average" Schmidt. In top-quality telescopes from careful manufacturers, both designs can yield excellent images.

There are a few drawbacks to all compound designs. As in any telescope that employs mirrors, occasional alignment is required for peak performance. The cost of a compound is higher than that of a Newtonian of the same aperture, though still lower than the cost of a comparably sized refractor. Most significantly for the planetary observer, the secondary mirror in a compound is much larger than that in a Newtonian, and its presence in the light path of the scope reduces contrast somewhat for high-powered viewing. In general, astronomers who desire a highly capable, easily transportable telescope find these worthwhile compromises, and have made the compound scopes very popular.

Price is a Consideration :

Budget is a factor in almost every telescope purchase decision, but there are at least three major price-related pitfalls to be avoided.

Don’t buy a flimsy, el cheapo scope with the intention of getting a taste of the sky and upgrading later. Many of those scopes are so poor-quality and frustrating that they can turn budding stargazers off of astronomy for good!

On the other hand, don’t give up on astronomy if the scope of your dreams is financially out of reach at this moment. There are many reasonably priced, high-quality beginner’s scopes that can reveal incredible wonders, while helping a novice define his or her particular observing interest.

Finally, if you are one of the fortunate few for whom price represents little obstacle, think twice before buying the biggest, most expensive telescope in stock. Many of the large, fully featured scopes favored by experienced observers are also the most complicated, and are too much to grasp for someone still trying to find the Big Dipper!

What About Astrophotography?

Before concluding, here’s a quick word for the beginner who wants to jump right into astrophotography through their new telescope focuser: Don’t! At least, not until you have taken some time to learn the sky and become familiar with operating your scope. Photography of the heavens can be a wonderfully rewarding pastime, but is a combination of art and science with a steep learning curve that can discourage beginners who try to take on too much at once. Of course, if astrophotography is a primary interest there is nothing wrong with selecting a first scope based on its easy adaptability to camera work in the future. While most telescopes can be used for picture-taking (with varying prospects for success), the most important qualifications for a photographic instrument are a rock-solid equatorial mounting, and ease of attaching a camera so that it can be focused. For a variety of technical and economic reasons, compound telescopes of 8" aperture and larger are most popular for photography. They also make fine instruments for general observing.

The Bottom Line

Which, then, is the right telescope? That’s a decision that must be made individually, but the three best pieces of advice are:

The best telescope(Apochromatic Refractor) for you is the one you will use most often. A huge, optically wonderful scope will bring no joy if it is consigned to the closet!

All else being equal, a larger-aperture (diameter) telescope will reveal more in the night sky than a smaller one ("I know, already!" you may be thinking.)

Buy from a company that’s knowledgeable about telescopes and astronomy, and who will support you even after your purchase (since you will likely have questions).

Given the bewildering array of telescopes on the market, how does an enthusiastic but inexperienced consumer choose the right one? To answer this question I will explain the differences between specific telescope(Apochromatic Refractor) types, but for that discussion to be meaningful it is important first to understand some very basic points about astronomical telescopes in general.

Aperture is the Most Important Factor :

The single most important specification for any astronomical telescope is its aperture. This term refers to the diameter of the telescope’s main optical element, be it a lens or a mirror. A telescope’s aperture relates directly to the two vital aspects of the scope’s performance: its light-gathering power (which determines how bright objects viewed in the scope will appear), and its maximum resolving power (how much fine detail it can reveal). There are other criteria to be considered in selecting a telescope focuser , but if you learn only one thing from this article, let it be this: the larger a telescope’s aperture (i.e., the fatter it is), the more you will see.

Don’t Get Hung Up on Power :

Unfortunately, the first question most beginners ask is not "What is this telescope’s aperture?" but "What is its magnifying power?" The truth is, any telescope can be made to provide almost any magnification, depending on what eyepiece is used. The factor that limits the highest power that can be used effectively on a given scope is, you may have guessed, its aperture. As magnification is increased, and the image in the scope grows larger, the light gathered by the telescope is spread over a larger area, so the image is dimmed. There is also an absolute limit, determined by the physical properties of light, to the resolution that is possible with any given aperture. As the magnification is pushed beyond that limit the image fails to reveal any additional detail and gradually breaks down into a dim, fuzzy blob.

The maximum useful magnification for any telescope is about 50 times the aperture in inches, or two times the aperture in millimeters. This equates to about 100x to 120x with the smallest telescopes, which is enough to see such wonders as the rings of Saturn and cloud bands on Jupiter. The 2x per millimeter figure is a rule of thumb, and can vary up or down somewhat depending on the optical quality of the scope in question and the vision of the individual observer. Experienced observers usually use much less power; 0.5x to 1x per millimeter is more appropriate for most objects. Any manufacturer claiming that their 60mm scope can provide good views at 450x (7.5 times the aperture in millimeters) is trying either to pull your leg or pick your pocket!

Bigger is Better, But...

While aperture is the most important specification of any telescope, there are exceptions to the rule that "bigger is better." One is obvious: the need for portability. The largest amateur telescopes are very big indeed, and demand either housing in a permanent observatory or possession of a strong back, a truck, and a gang of muscular and motivated observing buddies! There is a line to be drawn between performance and portability, and where it will be drawn varies with the individual and his or her capacity for storage and portage. Beginners are encouraged to start out with a scope of sufficient aperture to feed their interest, but of a size that they can manage easily. Avoid succumbing to "aperture fever." Those infected with this psychological malady choose the largest telescope they can afford without regard to portability. Their monster scopes soon gather dust in the garage, exiled for the crime of being too heavy and bulky, while the once enthusiastic would-be stargazers wind up frustrated or in traction.

The Sky IS the Limit...

The second limitation on very large telescopes is less obvious, but becomes apparent after the first couple of viewing sessions: the Earth’s atmosphere limits how much we can see. Stars and planets viewed through a telescope focuser appear to shimmer or wiggle, as their light passes through the air and is distorted. This effect is known to astronomers as seeing, and becomes more noticeable and bothersome as telescope aperture increases. It especially affects observations of the Moon and planets, where high power applied to reveal fine details also magnifies the air turbulence.

The amount of distortion due to seeing varies, depending upon the behavior of air currents in the upper atmosphere, and to a lesser extent upon the altitude and topography of the observing site. But on an average night, at an average site, air turbulence will limit useful magnification to 250x or 300x, and prevent telescopes larger than about 8" or 10" aperture from achieving their full potential for high-powered viewing. Telescopes larger than 10" are most often chosen by observers who want to gather as much light as possible for viewing dim galaxies, nebulas, and star clusters. These "deep sky" objects, affectionately called "faint fuzzies," are most often viewed at much lower power than the planets, so seeing is less of a problem.

Telescope Mounts :

The last important topic to cover before delving into optical designs is that of mounts. Telescopes are offered on either altitude-azimuth (or altaz) mounts, which move up-down (altitude), left-right (azimuth), or equatorial mounts, which are tilted to align with the rotational (polar) axis of the Earth.

Altaz mounts are generally lighter and simpler to use, and are preferred if the telescope is to be used both for both astronomy and daytime observing (or for daytime observing only). The better ones offer slow-motion controls to aid in moving the scope by small increments, and are useful for powers up to about 150x. The Dobsonian mount is a variation on the altaz mount. It employs unconventional (for telescopes) materials like plywood and Teflon in a compact mounting that moves easily, is extremely stable, and can adequately support large telescopes at a very low cost. Though there are no mechanical slow-motions or electric drives on a Dobsonian, a well-made example glides so smoothly on the Teflon bearings that with a little practice it is quite easy to track objects manually at 200x or more!

Equatorial mounts are designed specially for astronomy, and are not recommended for terrestrial viewing. Their advantage is that they allow easier tracking of the stars across the sky. This motion can be achieved with either a single manual slow-motion control or an electric motor drive (or clock drive). The easier viewing they provide at high power makes equatorials preferred by observers who are most interested in the Moon and planets. Also, you’ll need an equatorial mount if you want to do astrophotography.

from:intane

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical lens . A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Lenses are typically made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called optical lenses : for instance, a microwave lens can be made from paraffin wax.

The variant spelling lense is sometimes seen. While it is listed as an alternative spelling in some dictionaries, most mainstream dictionaries do not list it as acceptable.

This kind of lens has a negative focal length, diverges from collimated incident light and forms only virtual images that are seen through the lens. It is often used to expand light beams or to increase focal length in existing systems. Normally it is used in combination with other lenses. Usually it is symmetrical, but can customize it upon the request.the optical designers are always available to assist with your special needs and our production in China is ready to meet your quantity requirements.

Specifications Diameter φ4-φ300mm E.F.L. At the request of customers Wavelength(λ) At the request of customers Material BK7(K9) or as requested Clear Aperture >90% Bevel (0.25-1.0)mm× 45°(depends on the diameter) Diameter Tolerance +0.0/-(0.1-0.2)mm (depends on the diameter) Focal Length Tolerance ±2 % Centering Tolerance 4’ or as requested Surface Figure λ/4 or as requested Surface Quality 60-40 s&d or as requested C.T. Tolerance ±(0.1-0.25)mm (depends on the diameter) E.T. Tolerance Reference Coating At the request of customers

The Crayford focuser is a common focusing mechanism in amateur astronomical telescopes. It is named after the Crayford Manor House Astronomical Society, Crayford, England where it was invented by John Wall, a member of the astronomical society which meets there. The original Crayford Focuser is on display there.

The Crayford focuser was initially demonstrated to the Crayford Manor House Astronomical Society, and then descriptions were published in The Journal of the British Astronomical Association (February 1971), Model Engineer magazine (May 1972) and Sky & Telescope magazine (September 1972). Jack Wall decided not to patent the idea, effectively donating it to the amateur astronomical community.

Crayfords are popular among amateur telescope focuser makers as they are easy to make without any high precision machining, yet provide precise focusing with no gear slop or backlash. They are also made in a variety of designs by companies specializing in amateur telescope supplies.

 Intane 2” Crayford focuser, constructed of strong aluminum with a black anodized finish. a built-in tension knob for adjusting the "feel" to match your preference to give zero image shift and zero backlash, with a second fine adjustment knob with a 10:1 ratio for ultra smooth and fine adjustment. The friction can be adjusted with a large thumbscrew. A second thumbscrew can lock the focuser tightly in position. Features 2 inch brass non-marring compression ring, an adaptor from 2" to 1.25" with brass non-marring compression ring and 2" filter thread.

The features

4 inch racked in and 7 1/6 inch racked out
Ultra smooth focus motion with 10:1 Microfocus knob
Constant movement
2 inch non-marring eyepiece holding compression ring
2" to 1.25" adaptor with non-marring eyepiece holding compression ring and 2" filter thread
Made from strong aluminum

from:intane