how does my scope work

I have a 130mm Newtoniam with a 1000mm focal length. The tube of the scope
is short and the eyepieces still fits into the side of the scope. I can see
an extra reflection if I look into the scope without an eye peice. Does
anyone know how the 1000mm focal length is achieved?
This is the scope I have http://www.opticalvision.co.uk/15.html

Robert

You will find a lens at the bottom of the focuser tube. This lens acts as a
Barlow, increasing the effective focal length of the scope.

Adam


"Robert Summerfield" wrote in message
.. .
I have a 130mm Newtoniam with a 1000mm focal length. The tube of the scope
is short and the eyepieces still fits into the side of the scope. I can
see
an extra reflection if I look into the scope without an eye peice. Does
anyone know how the 1000mm focal length is achieved?
This is the scope I have http://www.opticalvision.co.uk/15.html

"Adam" wrote in message
...
Robert

You will find a lens at the bottom of the focuser tube. This lens acts as
a
Barlow, increasing the effective focal length of the scope.


If you find an A-level physics textbook, you should be able to find a ray
diagram in the optics section described as a 'telephoto' lens.

If thought of geometrically, the convergence caused by a concave mirror (or
a lens) has a particular angle which is defined by the focal length. In a
short tube reflector, the mirror may have a focal length of 500mm, but
adding a barlow lens (a weak diverging lens), the angle of convergence is
reduced for the last few centimeters to the same angle that a 1000mm mirror
would have had.

This allows a tube not much more than 500mm to have the same effective focal
length as a 1000mm scope.

The downsides are that
- figuring a 130mm mirror to a 500mm focal length (f/3.8) is much harder
than for 1000mm. (f/7.7)
- collimation of a f/3.8 telescope is much less forgiving than a f/7.7
scope
- the barlow may add chromatic aberation
- the barlow may reduce the light getting through by a few percent

The first two of these are more likely to be important for most users.

Hope this helps