Telescope Is 127mm/5 "???

Question

I was looking at a telescope that says there is a "large" 127mm aperture. Does that mean it is only 5"???

Answer 1

That's right, (25.4mm = 1 ").

Do be aware that 5" is a pretty large refractor as amateur optics go.

Answer 2

It sounds about right.  Aperture refers to the diameter of the objective lens or mirror.  

Answer 3

Welcome to the world of telescopes.  If you start to move around in the world of refractors, where apertures start at 60mm and proceed up to, say, a TEC 180mm for $18,000, then people will seriously debate the merits of "moving up" from say an 80mm to a "large aperture" like 102mm.  A five inch aperture in this world begins to look pretty big.  It also can be very expensive.   A chinese 5 inch refractor will run in the multiple hundreds, say $500 to $800, an apochromatic will run $5 to $6 thousand dollars, roughly $1,000 per inch up to the first five inches, and then the price sky rockets after that.  So for refractor addicts this is a big deal and a big price point.  Refractor owners like to quote aperture in mm because it makes them feel larger.  

For Newtonian reflector dob guys like Geoff this is pipsqueak stuff.  In fact just to get a secondary mirror the right size to illuminate a good eyepiece in a newtonian you'd want *at least* a six inch (150mm) telescope which among Newtonian owners is considered a small scope.  For $10,000 in the world of Newtonian dobs you can get 25 to 30 inch (635mm plus) scopes which require a truck to move around and are quite heavy.  

The Schmidt Cassegrain owners (think c8) run in between 200mm or eight inches (there are some smaller ones running down to about 90mm, these include Celestron and Meade Maks as well as the very expensive Questar) to about 14 to 16 inches.  So the aperture point there is roughly mid-size by Newtonian standards and "very large" by refractor standards.   You can make a kickass Newtonian with premium optics which sometimes get nicknamed "apo eaters" for much less than an apochromatic refractor, but these Newtonians really don't deliver the 3, 4, and 5 degree fields of view that you can get out of various refractor designs.    The refractors will also deliver full illumination of the eyepiece so there is no light fall off near the edge; to deal with that problem in  Newtonian you need to push up to eight inches and beyond.  At that point your focal length is increasing and your field of view is getting smaller.  So refractors have their place.

A five inch scope is not to be sneezed at.   You can do a lifetime of observing with it.  Some people prefer the larger apertures.  I'm neutral owning examples of several.  

If you are getting into amateur astronomy you should just remember that available apertures run anywhere from two inches -- the typical finder-- to about thirty inches and that "large" depends on whom you're talking to.  

There are some people who just have to own refractors and never want anything else.  There are others who like to own all kinds.  There are still others who see refractors as a waste of money given the capabilities of well made Newtonians and even, for some, SCTs.  Refractors are good lookin instruments and can be good to look through, too.    

Hope that helps,

GN

Answer 4

Yes. A telescope's aperture is the diameter of its main lens or mirror, the area which actually gathers the light. 127 mm aperture is quite large compared to the diameter of your eye (perhaps 7 mm maximum), and the light gathered depends on the _area_ of the aperture: 127 squared as compared to 7 squared, or 329 times. The 8" and 10" scopes recommended in my answer to your earlier question gather three or four times that that.