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
Login

Be the first pioneers to continue the Astronomy Discussions at our new Astronomy meeting place...
The Space and Astronomy Agora
Re: Negative Focal Length 3-Simple Ways

Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response To
Posted by Rick Crockett/">Rick Crockett on March 2, 2000 23:10:57 UTC

Hi Guys: Steve that works but not for refractive optics of course. Here are 3 simply ways. They are not the only ways but they are a fair start.

Negative refractive element: Place (1/2") aperture stop against lens surface and pass sunlight through lens, measure backfocus to point that spot is 1" in diameter. This is going to be pretty close to negative Fl.. Take another measurement where spot is 2" in diameter and divide by 3 for refinement on the answer.

Negative convex reflector element: Do the same except reflect sunlight through the mask and back onto a scaled target. A square mask may be easier to use.

Negative concave refector. Using any light source at any distance greater than Rc pass light through hole in paper and allow returning light to come to focus the image of the hole on the paper. This is the Radius of Curvature which is twice the focal length.

Even though the sun is not a point source the spread is less than the error in reading saggita is likely to be.

Cheers, -Rick

Follow Ups:

    Login to Post
    Additional Information
    Google
     
    Web www.astronomy.net
    DayNightLine
    About Astronomy Net | Advertise on Astronomy Net | Contact & Comments | Privacy Policy
    Unless otherwise specified, web site content Copyright 1994-2020 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