Re: [ATM] A Null Test

[Ric Rokosz <radicell2@yahoo.com>]

--- "Richard F.L.R. Snashall" <rflrs@rcn.com> wrote:

>  From what I've read, the Ross Null Test is good at
> f/4 or slower.  


>From my experience that isn't true.Have done a 24
incher f/3.3 and a 18 inch f/2.8 with a 6 inch custom
made lens I did and a 13 inch lens where I redid the
convex side.

For the faster mirrors you do need a larger lens which
explains why people were having problems getting good
wave rating with smaller 3 to 4 inch lenses.You also
have to factor in the focal lenght of the lens which
is very important.A 4 inch lens of 5 inch focal lenght
will cover the whole mirror but only give a wave
rating of say 1 or 2,whereas a 4 inch lens of 15 inch
focal length will give a much better wave rating but
not cover the entire face of the mirror.
> 
> The spacing does have to be adjusted to change goal
> f/# and conic constant.

There was long thread on the Ross many years ago
now,and is in the list archives.

I sort of wondered why you went to three lenses
instead of the easier Offner two.Making a spherical
convex surface ought to be very easy for any atm.The
spacing for the Offner elements is more critical and
you do need a venier caliper to set things up.

Ric


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