To: atm{at}shore.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


>From: "John Murray" 

>...I found the following details

>Diameter       230mm
>ROC            4200mm!!

>       This makes it approximately  a nine inch F-10 mirror!! After
>allowing the mirror to acclimatise (24 hrs)before testing  we can see
>that the surface is very smooth with very gentle ronchi lines. Getting
>readings with the ronchi and foucault proved extremely difficult. The
>shadows moved at the slight thought! of movement. Any suggestions for
>this?

That's the nice thing about long focus mirrors.  You can see all the
surface to evaluate smoothness when you null the 70% zone.  When I made my
4 1/4 inch and 6 inch f/9 or f/10 mirrors...I only took three zonal
readings with the knife edge:  Center, 70% zone, and edge.  If the figure
is as smooth as you say it is...and you get decent readings for the three
zones...then you can be fairly certain that it's a pretty good mirror.  (I
will not make claims of specific accuracy here.)

>       Mick and a mate who is a boilermaker are considering making an
>Alt/Az based on ideas from Berrys book "How to make a Telescope". Being
>a boilermaker though they are both looking at making the entire mount
>from tubing and welding it. Any ideas on whether this would work or
>suffer any effects due to being made from metal? I've thought of
>vibration and weight so far but any thing else? They believe they can
>make it quite light.

My scope is in the design stage, and not quite as long as yours, but check
out: http://overton.tamu.edu/aset/krajci/
specifically:
http://overton.tamu.edu/aset/krajci/scope-design.htm

There may be some useful ideas for making the scope not-too-heavy.

Hope this helps,
Tom Krajci
Tashkent, Uzbekistan

--- BBBS/NT v4.00 MP