The Focal Length
Making a 4.25 Inch Dobsonian
Reflector Telescope
Some
Background, the F Value
The focal length of a mirror is the
distance from the mirror to where an object at infinity will focus. This is usually measured in inches or
centimeters. You will also run into the F value a lot. Typically F values are F/6, F/8 or
F/10. This value has the same meaning
as the F stop values on your expensive camera. The F value is measured by dividing the focal length by the
diameter of the lens. So a 10 inch
F/8 mirror has a focal length of 10 x 8 = 80 inches. This is almost 7 feet.
While a Newtonian reflector will bend this path sideways, you can still
expect the telescope tube to be this long.
Spherical
and Paraboloid Surfaces
You may recall that when grinding an
polishing a mirror, we are trying to mate two surfaces. This creates a spherical surface. But the type of surface we need for a
telescope is a paraboloid. This is
just the three dimensional surface you get by spinning a parabola around its
axis. It's only when you get to the
final stages of figuring the mirror that you change the surface to a
paraboloid. This works because the
difference in the two curves on the surface of the mirror is very small, just
thousandths of an inch. There is no
such thing as a mathematically perfect mirror, but a mirror that delivers light
to where its going to an accuracy less than the wavelength of light is
effectively perfect.
And that's where the F value comes
in. For a 4.25 F/10 inch mirror the
difference between a sphere and a paraboloid is not enough to keep the mirror
from being optically perfect. You
don't have to figure the mirror at all, just polish it into a sphere. For larger mirrors and smaller F values,
the difference increases, and the figuring step becomes crucial.
Magnification
If someone every tries to sell you a
telescope based on its magnification, walk away. Any telescope can have just about any magnification you
want. That doesn't mean you will want
to look through it. The magnification
you get from a telescope is approximately the ratio of the main objective's
focal length, that's the mirror, and the focal length of the eyepiece. Larger focal length of the eyepiece, lower
magnification. Smaller focal length of
the eyepiece, higher magnification.
Unfortunately there are many other things to consider. To begin with, higher magnification is not
always a good thing. With high
magnification it becomes increasingly hard to find a stellar object. Also, objects will move quickly across the field of view, forcing
you to re-adjust the position frequently.
That is unless you have a clock drive tracking the object. In addition, increasing the magnification
on a small fuzzy object will generally only allow you to see a very large and
extremely fuzzy object. So the bigger your mirror, the longer the
focal length will be, and therefore the higher the magnification you will
get. But if you want lower
magnification you will need an increasingly larger focal length eyepiece. And large focal length eyepieces are more
generally more expensive. The lens
itself will have a larger diameter.
This is a good thing, as it makes it easier to look into.
The
Goal Here
The point of all this is how do you
choosing the focal length of your mirror.
It is the second most important choice you will have to make after
deciding how big a mirror to use. In
fact both choices should be made together.
There are two issues to consider.
First, what do you want to see with your scope and second, how much
agony are you willing to deal with when trying to figure the curve. I'll suggest a few options.
The
Beginner
Maybe you just want to look at the
moon, the prominent planets; Jupiter, Saturn, Mars and Venus, as well as a few
prominent and easy to find objects, eg. the great cluster in Hercules. In that case a 4 or 6 inch telescope will
serve you well. An F/8 value is
considered a good compromise. A
larger F value will give you greater magnification, which if you read the
section above, is not what you really want.
A smaller F value and you will spend many many hours trying to get your
mirror curve to be a paraboloid.
The
Planet Watcher
The nice thing about planets is that
they are bright. The compensation is
that they are small. On a blurry
night, you will see blurry planets, no matter how nice your telescope optics
are. While you are not all that
concerned about light gathering power, this is a catch with using a small
telescope. What you want to see is
details on the surface of the planet.
That would be the great red spot and the bands around Jupiter.
Saturn also has bands, as well as separately rings. You might also want to see canals on Mars,
who knows. So to get both the
resolving power and the magnification, you might want a medium size mirror with
a somewhat longer focal length, say an F/10 8" mirror. That catch here is that this scope will
have a fairly long tube. You will
probably end up needed a step stool and maybe even a ladder. The good news is that figuring the mirror
for this hard to transport scope will be straight forward.
Deep
Sky Objects
To see many deep sky objects you want
three things, light, light and light.
To see nebulae, star clusters and galaxies, you want as much light as
you can get. Ok, so like me you think,
a 12.5 inch mirror would be nice. But
to move around the F/10 version of this, you need a pickup truck. On top of that, having a long focal length
increases the magnification and thereby narrowing the field of view. And for many deep sky objects you want the
opposite, as wide a field as you can get.
So you have to think about a fairly small F number, maybe F/6 or F/5
even. Any smaller and you will have
difficult with the field flatness.
That is, when you focus on the center of the eyepiece, the outer edges
will be out of focus. The catch here
is that figuring an F/5 12.5 inch scope could take more time than grinding and
polishing it. It just has to be done
carefully