Back to Home

ATM Forum Message

Forums: Atm · Astrophotography · Blackholes · Blackholes2 · CCD · Celestron · Domes · Education
Eyepieces · Meade · Misc. · God and Science · SETI · Software · UFO · XEphem
RSS Button

Home | Discussion Forums | Amateur Telescope Making | Post

Be the first pioneers to continue the Astronomy Discussions at our new Astronomy meeting place...
The Space and Astronomy Agora

Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response To
Posted by Robert May on November 17, 2001 22:25:39 UTC

If it's a 4.6" lens and it's F8 then the formula is Focal Ratio = Focal Length / Diameter of Lens.
Thus, moving things around so that you have the two known values on one side of the equation you get 4.6 * 8 or 36.8".
This value is going to be fairly close to that value but may vary by about 1/8" or so from the real value. You can find the exact value by putting the objective in a tube and not attaching the back end of the tube to the tube, check the focus of the scope. Start with the tube long and cutoff the tube by 1/2" or so at a time until you have the focus where you want it.
I might note that the EPs in the 15-32mm range have a nice field stop (the ring that doesn't seem to be doing anything in the EP's barrel) and that is where the focus actually is of the objective when you have the scope focused.

Follow Ups:

Login to Post
Additional Information
About Astronomy Net | Advertise on Astronomy Net | Contact & Comments | Privacy Policy
Unless otherwise specified, web site content Copyright 1994-2022 John Huggins All Rights Reserved
Forum posts are Copyright their authors as specified in the heading above the post.
"dbHTML," "AstroGuide," "ASTRONOMY.NET" & "VA.NET"
are trademarks of John Huggins