Re: [ATM] Help my 17.5 inch coulter mirror is astigmatic

[Charles Broward <broward32666@yahoo.com>]

Our Club built a truss type scope around one of these mirrors....we have astig problems that seem temp related.  The mirror is silicone glued to a 18 pt cell, and sometimes the scope behaves well.  When it works, it is great,  when it's not, it will drive you nuts.  Thin mirrors take some care in mounting.

Chuck in N. Florida
 
C.S.Broward
GhastlySky Observatory
Gainesville, Florida 
WWW.Floridastars.org
ATM Coord, Alcor

----- Original Message ----
From: "astroguy@nas.com" <astroguy@nas.com>
To: carlveh@wxs.nl; ATM@atmlist.net
Sent: Tuesday, February 26, 2008 9:42:57 AM
Subject: Re: [ATM] Help my 17.5 inch coulter mirror is astigmatic



I 
own 
a 
Coulter 
17.5 
and 
the 
mirror 
has 
an 
excellent 
surface.  
Because 
this
is 
a 
"thin" 
mirror 
it 
does 
require 
proper 
support. 
My 
mirror 
is 
in 
an 
18
point 
classical 
cell.  

Problems 
from 
astigmatism 
could 
have 
been 
from 
a 
"lesser" 
cell, 
pinched
optics, 
or 
possibly 
a 
poor 
finished 
surface.  
Your 
refiguring 
with
resulting 
astigmatism 
could 
possibly 
be 
to 
inadequate 
support 
when 
you
refigured. 
You 
might 
also 
have 
a 
bad 
piece 
of 
glass.  
Did 
you 
check 
the
glass 
for 
internal 
strain?

Kreig



Original 
Message:
-----------------
From: 
Carl 
Vehmeyer 
carlveh@wxs.nl
Date: 
Tue, 
26 
Feb 
2008 
13:51:32 
+0100
To: 
ATM@atmlist.net
Subject: 
[ATM] 
Help 
my 
17.5 
inch 
coulter 
mirror 
is 
astigmatic


Hello 
everybody,

 

7 
years 
ago 
I 
bought 
a 
old 
coulter 
17.5 
inch 
mirror 
to 
build 
my 
first 
big
telescope. 
Unfortunately 
I 
wasn't 
able 
to 
test 
the 
mirror 
at 
that 
time 
and
could 
only 
startest 
the 
mirror 
after 
I 
had 
finished 
the 
telescope 
a 
year
ago. 
It 
turned 
out 
to 
be 
undercorrected 
and 
had 
severe 
astigmatism. 
The
mirror 
looked 
somewhat 
like 
this 
mirror
http://www.astro-foren.de/showthread.php?p=34674 
. 
With 
help 
from 
some
fellow 
amateurs 
I 
tried 
to 
correct 
the 
problem 
by 
refiguring 
the 
mirror. 
We
started 
out 
with 
grit 
400 
and 
worked 
our 
way 
up. 
All 
seemed 
to 
go 
well, 
but
after 
polishing 
and 
figuring 
I 
ended 
up 
with 
well 
corrected 
mirror 
which
still 
has 
astigmatism. 
We're 
sort 
of 
stuck, 
any 
help 
or 
advise 
is 
welcome.
Does 
anybody 
have 
experience 
with 
astigmatism 
of 
this 
kind, 
Coulter 
mirrors
or 
with 
refiguring.

 

Thankful 
for 
any 
advise,

 

Carl 
Vehmeyer

The 
Netherlands

 

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