The key to purchasing a good telescope is understand what a telescope really does - not what  it does not do.  Telescopes do not use magnifying lenses to make distant stars and planets visible.  Magnifying lenses do nothing to help you see the sky.  A telescope does one thing only - it gathers light!   The sun is so bright even distant planets get a good dose of light and they reflect it back. So much reflected light,  it makes them appear as stars. The larger the opening of a telescope the more light it gathers and the better the image a person can see when the light is concentrated into one spot.  A telescope concentrates this light in one of two ways. Light is gathered either through a lense in the front of it (Refractor type), or via a mirror in the end of it which is curved in such away it reflects it back onto a concentrated spot on a smaller second mirror which then sends it out through the eye piece (Reflector type). 

           REFRACTOR                                                                 DOBSONIAN                                                                     LIGHT BRIDGE

Magnification  is created by  the focal length of the telescope,  or the length of the tube necessary to create a clear image and the size of the hole in the eyepiece. Both are measured in millimeters.  The length of the tube is divided by the size of the eye piece which creates the magnification power.  A tube of 300 mm in length being viewed through an eyepiece with a 10mm hole will increase the size of  space objects 30 times or in laymen's terms 30 power (300/10 = 30). 

All telescopes work well, with some limitations.  The most expensive types are the Refractors as they require multiple lenses to intensify the gathered light  to artificially create focal length.  This is why telescopes with short fat tubes can have a high focal length. A good example of a refractor type telescope would be the type seen in the movies aboard wooden sailing ships in which the captain or pirate uses it to spy on distant vessels to determine if they are friend or foe. These are usually some type of tube with a large lense on one end and a very small lense on the opposite end.  These are the earliest examples and work well for most near earth viewing.   The least expensive types are the reflector types that use reflected light to create the image.  These are simple and use  two mirrors.  They are often larger and bulkier, have no fancy computer controls for the most part, but they are also unbeatable for seeing large, clear images of everything in space. In fact nearly all the super sized telescopes seen in observatories are this type of telescope.  But in order to appreciate the heavens all telescopes  require filters to see the planets in full color.  Without colored filters the planets and nebula will look silver or white.  The minimum number of filters one will need is 6.  Two grey filters to cut down glare when looking at the moon, and 4 colored ones for viewing planets. The most common colors are Red, Green, Blue, and Yellow.  These lenses are fairly inexpensive, usually screw onto the bottom of the eyepiece, but will help filter out the white light and allow the true colors of deep space and the planets to be seen with the naked eye through the scope. 

The minium power a telescope needs to see near earth planets is 100 power.  Nearly all telescopes easily achieve this and will let you see the planets.  The difference  is do you want to see a pea sized planets, or in the case of Saturn a BB in a small washer, or do you want to look at a realistic sized planet that can provide you detail?  The only way to get the bigger image is through bigger telescopes.  Bigger is Better in the telescope world.  To see the planets well will require a minimum of a 5 inch opening with a 1000mm focal.  This will let you also see some nebula, but they will be fuzzy and small.  The ideal size for near planet viewing is closer to 6 inches.  Five inches and below you can view the moon great and other terrestrial objects like the neighbor's back yard a couple of miles away.  To view the more distant things like nebula will require a minimum 8 inch telescope.  The planets will be fantastic and the distant galaxies will begin to appear with better light and color.  A good size for all levels of star gazing is a 10 inch opening.  This is large enough  to provide  breath-taking views of distant galaxies and nebula as well as stunning planet detail. The 10 inch size telescope has been said to be the perfect size and a size virtually all astronomer's will never outgrow.   Telescopes in  12 inch and above size become extremely expensive.  The views are better, but when the cost is added in,  it becomes questionable if it is worth the added expense. They also become bulky in these larger sizes.   In the case of reflector type scopes - sometimes called Dobsonian or Light Bridge telescopes - at 12 inches and above they become very hard to use and to move around so I wouldn't recommend them.Many of them require the use of a small step ladder just to be able to use the eyepiece. In my opinion at 12 and above inches the telescope becomes more of a fashion statement of the owner than a tool of the sky.  Instead of looking to see, the monster socpes becomes more of, "EVERYONE LOOK AT ME!" for the owner.

The biggest bang for the buck is to avoid buying the "Go-To-Computer-Driven Telescopes" and acquire a good reflector such as the Dobsonian or Light Bridge type.  They aren't complex, and they are the real work horses of the telescope world, but they are large and require storage space. For those in Apartments, these might not be the best, but many models are designed to either be broken down for storage or the tube can be raised nearly up and down to save space.  A 10 inch light bridge will cost you less than a 5 inch computer model and do many times the work and provide you far better views of the sky.  Computer controlled stuff is nice, and I have one I use for camping, but they can be difficult to use as the owner has to be able to decipher true north from magnetic north and set the telescope up accordingly.  Usually, I skip the set up of these which almost always should be done before dark in order to get everything level as it is nearly impossible to see in the dark without getting night blinded by a flash-light, and just use it to star hop by pointing the telescope in the area I want to view.  Once you use a telescope a few times, it becomes obvious, you don't need a computer to tell you where to go.  If a person can find the constellation Orion - or Orion's belt - which appear as three bright stars in a near straight line in the sky about the width of a person's hand from one end star to the other they can just about find everything else with a simple sky chart or star map.  These can be had for FREE on many different websites and be tailored to the area you will be viewing from.  They aren't as complex to use as first glance might make one think.  Once Orion's belt is found (and there are lots of deep space nebula in this area alone) the rest of the sky aligns perfectly to the map. 

The great thing about the universe is although it is limitless and as large as the sky, it is no bigger than the size of your own  back yard! My 3 year helps me view the stars. He likes seeing all the "pretty balls" in space - especially the big one with a large red eye (Jupiter) and the one with rings around it (Saturn).  On any given night he can point to any one of several stars and tell you that is Jupiter, that is Saturn, that one is Mars, and that one over there all by itself is Venus.  And that is worth all the telescopes in the world!