Expert: Tom Whiting - 10/27/2008

Question
I know how to point out saturn.but the telescope information i can get here in my country is impossible.i want deep sky objects
so why is the focal ratio you going to tell ne is better for this?
thanks alot

Answer
Hi Hady,
Focal ratio determines the degree of curvature of the primary
mirror. f3 to f6 are considered fast ratio's that is, low magnification but relatively wide field of views.
Where f8, f10 and upwards are called slow ratio's and with the
same eyepiece give rather high magnifications but a corresponding
very small field of view. Several formulas which I'm sure you
already know, or should...
Focal length (FL) (where focus forms) = diameter of mirror x focal ratio.  And Magnification = FL mirror divided by FL eyepiece.

Now a larger focal ratio gives your mirror a higher FL, and a higher
FL means higher magnification for a given eyepiece.
But you can't have it all....anytime you go up in magnification,
you lose 3 things (Sorry, just the laws of optics wether its a
telescope or microscope)
1. light, the image is dimmer
2. detail..it's not as sharp
3. Field of view, the area of viewing shrinks down dramatically.

So lunar/planetary people want to magnify a very small part of the
sky, namely a planet or crater, so they want an f8,f10 maybe even
an f15 or f20 scope. But us people who are into extended deep
sky objects want the reverse, a large field of view and a large
mirror to bring in lots of light, so typically a DSO viewer wants
a fast focal ratio, f4 or f5.

Sorry to hear that in your country (whichever one it is) you can't
get much information on Deep Sky Objects, but the nice thing is,
it appears that you have a computer, so you can at least research
all the deep sky objects and learn their locations in the night
sky, using your computer.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA