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: Foucault Testing 8" F/4.5

Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response To
Posted by steve c/">steve c on February 4, 2000 07:45:13 UTC

: I'm testing my first mirror (8 in. f/4.5) and can't get the edge to shadow test. The right edge goes into the heavy shadow but the left side stays lit (Knife enters from the right) Do I need to re-adjust the lateral as I test the outer zones or leave it alone? A Ronchi test looks picture book from inside to outside and the surface looks smooth. I'm also having a problem with the edges of the silt showing up in the mirror during testing. If I widen the slit the problem goes away but then what's the fuss about narrow slits and tint pinholes in Texereau. Out to the 70% zone things are as expected. The problem may be your foucault testers light source or slit,your light should have a piece of fine ground glass in front of the bulb to diffuse the light, and this should be parallel to the knife edge, The slit width varies with the distance from the mirror, the greater the distance from the mirror the wider the slit needs to be. Use your ronchi screen to determine the approx imate shape,move the knife away from the mirror until you see 3 or 4 lines,the lines should curve slightly outward from the center to edge,if you have t.d.e the lines will curve abruptly outwards at the edge,(at the radius of curvature)and inwards if turned up-edge is present,if you determine you have t.d.e you should take the mirror partially back to a sphere ,with the mirror on top of the lap,press for about 10-minutes then use a short center over center stroke extending the mirror about 1-inch over the edge of lap,do this for 2-minutes then press for 2-minutes with no weight on top of mirror,repeat this process 4 or 5 times then test again(use very slow strokes with light pressure). good luck.

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