From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "James Lerch" 
>
>I can't offer any pointers to documentation, but I can recount a recent
>experience with a 6" F/2 sphere I tried testing.
>
>Using a slitless tester, the problem I ran into was I could not get the
>entire mirror to be illuminated.  I could only get the center 50%
>illuminated, which showed an obvious null at ROC, but only for the center
>50% of the mirror surface!  It was almost like my light source wasn't big
>enough (anyone have any suggestions? I'm using a 5mm LED behind the knife)
>
>I'm currently thinking I'm going to need a point source light, like a
>LED/Laser pointer reflected of a Ball bearing (Or something) to test such a
>fast mirror surface.
>
>Of course this brings up an interesting question in my mind.  The cause of
>the  inability to fully illuminate the mirror at F/2, does this translate
>over to any possible error for slower mirrors?
>
>As for testing the hyperbolic surface, if you can get the mirror
illuminated
>and get some zonal readings, you can use either Figure45 or Sixtests to
>reduce the data.  Just enter the appropriate conic constant in either
>program.  (BTW, if you enter the zonal measurements into Sixtests, the
>program will spit out a best fit conic constant as well, which I found
>rather handy for figuring an Oblate concave mirror I was making with a b of
>+2.3)
>
>Best of Luck!
>James Lerch
>http://lerch.no-ip.com/atm (My Telescope Construction Page)

Hi James
 Two things. One is that you need to have the light
source close to the knife edge. You don't want it so close that your
imaging the surface of the light, instead of the edge.
 It is like matching F-ratios. The ratio of the diameter
of the mirror to the distance of the edge should be smaller than the ratio
of the size of the light source to the distance to the edge. I use on of
the jumbo green LED's. I don't modify the lens or diffuser as many do. I
find it is reasonable as it is. It gives a somewhat collimated beam with a
reasonably uniform brightness across the width.
 The other problem is eye placement. You need to get your
eye close enough that the diameter of your pupil to the distance of the
edge is greater than the mirror to edge. You can help this a little by
using the opposite of a telescope. Take a Barlow lens out of the Barlow an
put it right close to the edge ( on the eye's side. We don't want to
measure the Barlows figure ). This will reduce the effective diameter of
the mirror as seen by the eye. There are tradeoffs here but at least you'll
see it. Dwight

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