Re: ATM ronchi question

["AstroJack" <AstroJack@AVATM.com>]

Hal,

A sphere eh! Yes that does change things a bit.  For a parabola the inside out and outside in thing is the key.  For a sphere you just want nice straight lines.  And as smooth a surface as you can manage for flexing.  One thing to keep in mind, as long as you are EVENLY undercorrected from a sphere and have a smooth surface you can flex.  It will just take a little more pull is all. On the other hand if you end up a little over corrected compared to a sphere then it still will flex but you will need a little less tension.  Also my tester is the same as yours except that I have an erecting prism to correct the view. However, This will not change the orientation of the curves of the lines.  Even upside down and backwards curved in lines will still curve in and out curved lines still curve out.  I think you should print out the page I mentioned last night and use that to help you diagnose your mirror.  Once you know what the shape is that will really help you to figure out what to do next.  Also the ronchi image doesn't really suggest a hill, only that the center and edge are high compared to a sphere. Of course without actually seeing the ronchi test direcly it really is impossible to know if I am in the right ballpark or not.  Once you get those nice straight lines then the knife edge for me works the best to see the overall smoothness of the mirror.
best of luck!

Jack Day


---------- Original Message ----------------------------------
From: "Harold" <adams@rmci.net>
Reply-To: "Harold" <adams@rmci.net>
Date:  Tue, 29 Jan 2002 09:56:40 -0700

>
>
>> NOT...     A sphere will have ZERO thousands of difference and you will
>not
>> be able to tell where the edge differs from the center because it doesn't
>
>> You are going in the right direction, > just how much of a central
>depression you'll need  (if you are going to parabolize).
>
>  Thanks Ken. I didn't mention that im building a flex design mirror and i
>need the sphere. Also i am a bit confused still because Jack pointed out
>that the lines curved in at inside roc indicate a hill not a depression. I
>will need to re examine some things before i continue.
>
> Hal
>
>
>
>