Telescope Advice Page
The purchase of a telescope is a personal one. It is determined by cost, portability, ease of set up, your desire for gadgets, and by the types of objects that you like to observe. The best way to decide what type of ‘scope is best for you is to look through and use as many as you can. You can observe through many types of scopes at our public stargazes and at regional star parties. What follow are some links that we feel may be of use to a beginner:
Why don’t you come to an OVAS public star gaze and look through our members’ scopes. This way you can get an idea of what type of ‘scope you like and what you can see through a ‘scope. I recommend that you at least read Don Kemper’s and Ed Ting’s advice. Don’t forget the words of an OVAS member who said, "I use my [80mm] binoculars more then my [8" or 200mm Dobsonian] telescope". Why does he feel this way? Two of our more "senior" members have traded in Celestron C8 SCT’s for 4" (101 and 102mm) TeleVue refractors. Do you wonder why? The C8 gathers 4 times the amount of light that the 4 inch scope does. Yet they feel that they see objects better in the 4" because of improved contrast. Can you see the difference? I moved up from a 90mm ETX to a 10" Dob and have almost never used the ETX since. Why? Come and see. Try before you buy. What meets your wants and needs? Everybody is different and likes to look at different things. Different scopes will show you different things. A good quality refractor offers both excellent low power wide field views and narrow field high power views. A large reflector makes objects much brighter but is limited to about a degree field of view at the widest. What is right for you? Our public star gazes are a great way to look through different ‘scopes and decide what (if anything) is right for you! Most of us started out before we joined the club. Most of us fell for the slick color ads that you see in magazines. Most of us have gone through 2-3 scopes before we ended up with what we like. Perhaps we can save you a step and some expense. Perhaps the mass produced scope will meet your needs, perhaps not. Your observing style will guide your choice of scope. Do you prefer the convenience of a Dob? Or is collimation too much of a hassle? What is collimation? Do you desire a gadget laden techno scope? Must you have the best optics? Would you buy a car without a test drive?
Ed Ting’s Scope and Astronomy Equipment Reviews - Ed has reviewed many scopes and accessories. Todd Gross’s Scope and Astronomy Equipment Reviews - Todd, a weatherman, has reviewed many scopes and accessories Cloudy Nights Scope and Astronomy Equipment Reviews - These are reviews submitted by various amateur astronomers. An Observing TutorialTom Campbell’s Observing Tutorial Just getting started? Take Tom’s tutorial. This introduces basic concepts and can teach you about different types of objects in just a few short web pages. Check out his other advice on this nice page. I like his Limericks page. Humor is good. Also What can you see in a scope? Compare eye piece sketches to astro-photographs to learn what you can really see through a telescope. Types of Scopes There are three types of scopes that are commonly used by amateurs. They are the REFRACTOR - the modern version of Galileo's telescope, The REFLECTOR - a descendent of Issaic Newton's telescope, and The CATADIOPTRIC or compound or CASSEGRAIN telescope. The Refractor comes in various quality levels. The achromatic, apochromatic, and the semiapochromatic. The Achromatic is the older and simpler design. It usually has two lenses at the front or objective end. The glass in these lenses will bend light of different frequencies (colors) differently. This creates a ring of "false color" (usually violet) around bright objects like the moon. This problem is reduced by making the instrument have a long focal length. But this biases the scope towards magnifying objects and limits the field of view. As a scope is limits to a useful magnification of about 50x per inch of aperture (width), these scopes have a limited range of available magnification over which they can be used. Short focal length scopes of this type will have more false color and tend not to produce sharp high power views. Semiapochromats and apochromats used more complex lens designs and more exotic (i.e. expensive) types of glass. These have less false color and better high power performance. A good apochomat (or semiapo) can offer pleasing views at low, medium or high power. A refractor has no 'central obstructio' and offers the best possible image sharpness. The best scopes have excellent contrast, they block stray light and allow you to see objects not seen in other scopes with less contrast (though the object may be "dim"). The Reflector uses mirrors not lenses so it has no false color. But its image is inverted and mirrored. This does not matter for looking at objects in space but makes it hard to use to look at objects on earth. This typr of scope gets big as it gets wider so the mount it is used with gets heavy as the scope gets larger. This limits the size of the scope that most people will used on an equatorial mount to about 8". With a Dobsonian (lazy Susan) mount, one can use scopes of up to 20" in size without difficulty. And with a helper, this type of scope can grow to more then 36". A reflector on a dobsonian mount is the cheapest way to get a large scope. Reflectors are the cheapest type of scope. They have a cantral obstruction caused by their secondary mirror. however if the central obstruction is kept to 20% or less of the aperture (diameter) of ther scope, a reflector can produce a sharp image of the type needed to observe the moon, planets, or double stars. The Cassegrain or compound scope uses 3 mirrors and a corrector plate to make an image. All of these optical surfaces dim the image some what. So for scopes of equal aperture (width), this design puts out the dimmest image. Like a Newtonian, this must be collimated (have its mirrors aligned) to perform its best (SCT only, Mak's are usually factory aligned). But these scopes are short and thus easier to move and mount. Placed on a fork mount, this scope is easy to turn into a computer driven scope. By using focal reducers as well as eyepieces one can get both a wide field of view and high magnification. The SCT version of this design (Schmidt Cassegran), tends to have a large central obstruction that makes the image less sharp then other designs. This is a very flexible design that can be used for imaging as well as viewing. But as it is a "Jack of all Trades"; it is "the Master of None". The "Mak" (Maksutov Cassegrain) can be made as a short or long focal ratio device. However the commercially produced "Maks" are long focal ratio instruments. So they are biased towards magnification and tend to produce dimmer but sharper images. These "Maks" make nice small light weigh scopes for observing the moon, planets, sun (with proper filters), and double stars but are less suited for nebulae and large objects. I like to look at Deep Space Objects – DSO’s – here aperture wins. To see David Kriege’s reasoning on why you really want a large scope, take this link. Just remember that the large scope is heavy and setup may become more complex. With a "big Dob" you can actually SEE things! All kidding aside, this does graphically show you the difference in the eyepiece appearance of the same object with several different size scopes. Though sometimes I miss the easy set up of my old 10" tube Dob. ConceptsForget what you think, the purpose of a telescope is to gather light This is why I like to observe with a light bucket. The maximum magnification is dependant on the telescope’s diameter or aperture Forget the claims of cheap telescopes. Useful magnification is 50 power per inch of aperture (diameter). So that’s 100x at 2", 150x at 3" and 400x at 8". Often the atmosphere around here limits you to 200-250x regardless of how big your scope is. In my experience the best atmospheric conditions around here occur in September. Field of view Think about it. The average telescope with the average medium power eyepiece has a field of view of about one degree. There are 360 degrees in a circle and 180 degrees in a hemisphere. From horizon to horizon is about 180 degrees not accounting for trees, buildings and hills. So your average telescope sees only 1/180th of the sky! This is why you need a "finder scope" or ‘reflex sight’ on your telescope. The focal length of the telescope determines your field of view. The larger your focal length the smaller your field of view. The smaller your focal length the larger your field of view (i.e. you can see more of the sky with a "short focal length scope"). The focal length is the diameter of the telescope’s mirror or lens (it’s aperture) times it’s focal ratio. Thus as a telescope gets bigger (wider) it’s focal length increases. So. the telescopes with the widest fields of view are generally smaller (less aperture) with a smaller focal ratio. What about focal ratios?
The Shorter focal ratio scopes have wider fields of view while the longer focal ratio scopes have a smaller field of view. It is hard to make a good short focal ratio scope either as a refractor or as a reflector. You will need to spend more $ if you want to get high power views out of a short focal ratio system. Cheap short focal ratio scopes are for low to medium power viewing. It is easier to make long focal ratio optics. So they are less expensive. Refractors with focal ratios of 12 or more will have less problems at higher magnifications, but are limited in their aperture because of that focal ratio times the diameter of the scope thing. A 4" wide f 12 scope has a tube that is 48" long! That’s why all of the old telescopes in the old observatory domes look long and slender! And these scopes have smaller fields of view which may make it harder to find things once the scope grows over 4" in diameter. Object selection may determine your choice of telescope.
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