# Thread: Question: How To Find Where To Put The Focuser Using the F#, and How To Find The F#

1. ## Question: How To Find Where To Put The Focuser Using the F#, and How To Find The F#

i understand how the F# tells you how long your tube must be, but how does that same number tell you where to put your focuser? and does that number supposed to represent inches? millimeters? is there some equation to find out where the focuser go's?

also how do you determine what the F# is?

2. The focal ratio ("F#") tells you neither the OTA length, or where to put the focuser.

Focal ratio is a unit-less metric, obtained by:

focal ratio = (focal length / aperture)

Focal length and aperture can be expressed in inches or millimeters (or any other unit of length, as long as the same unit of measure is used for both).

The focal ratio is a measure of light gathering capability or "optical speed".

To compute placement of a focuser, one must know the type of telescope (i.e. Newtonian reflector, refractor, SCT, Mak, etc), along with the total focal length of the telescope, the back focal length, the size of the imaging plane, along with ray-tracing of the optical path along the normal and other vectors. It takes mechanical ray tracing on a full-scale or precision scale drawing at a minimum, and in a contemporary design one might use an optical design tool like MODAS, MOADS, Oslo, or similar tool in conjunction with the ray-trace diagram.

3. Additional help: the focal length is relative to the telescope design and primary/secondary optics.

In a Newtonian reflector, the primary mirror curvature determines where the secondary mirror must be placed, and consequently where the spider resides in the tube, along with the opening for the focuser assembly.

In a refractor, the primary objective lens (almost always a lens assembly or compound lens) along with any secondary optics (correctors, etc.) determine the focal length and consequently focuser placement.

In SCT, RC, Mak-Cas and other designs, the combined primary optic, mirror(s) and correctors determine the instrument's focal length.

5. sorry guys. yes i am referring to a newt. i will look at that download now. thanks! and please if there are any other places or downloads you or anyone else think would be useful please let me know.

ok, downloaded the file. but its a PC file. is there one for the mac? i have a powermac.
Last edited by Looking_Glass; 10-15-2009 at 02:00 AM.

6. Originally Posted by Looking_Glass
is there one for the mac? i have a powermac.
Poor boy. The advantage of having a PC is that you are forced to learn somewhat how they work in order to keep them running.

7. Originally Posted by Looking_Glass
sorry guys. yes i am referring to a newt. i will look at that download now. thanks! and please if there are any other places or downloads you or anyone else think would be useful please let me know.

ok, downloaded the file. but its a PC file. is there one for the mac? i have a powermac.
I don't know of any for mac but here is a link to an on line calculator,
JIM FLY'S NEWTONIAN TELESCOPE DESIGN PLANNER
Also check out The ATM Site: Resources and Techniques for Amateur Telescope Makers a huge resource for building scopes.