Re: [ATM] first light on a Lurie-Houghton

["Bob May" <rmay@nethere.com>]

>From the calcualting that I've done for Dale Horn's 12.5" F4.6
design, I've found that those two surfaces do affect the focal
point but that almost seems all that they do.  The much more
difficult surfaces are the other two which Dale wants to have
within two waves of each other and those tend to move the color
correcction around a lot.
If you can, build a large focuser so that you can take a small
eyepiece that you put in a plate and let it move about on the end
of the focuser so that you can investigate the edges of the FOV
with the EP.  A Jap standardd four mm EP would probably be a good
choice fort the EP.  Sliding it about on the back of the focuser
will allow you to see if there is any focus shit over the field
and let you see the edges of the field for when astigmatism
starts to get in the way.  The focus shouldn't shift at all if
things are square and the astigmatism should be very tiny until
you get to the edge of the field where it will suddenly blow up.
Bob May
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