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
Help With Test.

Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response To
Posted by Robert May on November 16, 2001 00:50:37 UTC

You may be seeing a reflection from the backside of the mirror! It may be a bright spot but that won't affect the reading of the test as it won't be in focus at the knife edge. I see a reflection on some of the mirrors that I test, the 12.5" porthole being the worst at the moment as the back surface is a nicely polished surface. The cast pyrex blanks tend not to produce a good reflection as the backside is usually fairly rough.
Make sure that you are at the ROC as best you can and then get the light from the frontside of the mirror to the knife edge. See if the light is going the same way as the KE moves (inside of ROC or the other direction (outside of ROC) and move the tester until it's tending to blank off the whole mirror at one time. At that point, you can start using the distance control on your tester to find the shape of the mirror. I might note that the reflection from the backside can be as bright as the light from the frontside of the mirror if the back is nicely polished.
Please note that I like to use a bright light to find out where the mirror is reflecting to and that also allows me to look at the reflection of the light and get the ROC to an approximate point as then the light will be in focus where it returns from the mirror.

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