From: "Vladimir Galogaza" 
To: "ATM shore" 
Reply-To: "Vladimir Galogaza" 


David,

If it is true that  mirror has so wide "edge" than you might
consider that it was never really finished . You said that it takes you six
months to regain your composure. It seems to me that   you are bound to
another disappointment if you will not modify your approach.
 This is my reaction to the sentence:"Last summer I took a telescope making
class
from John Dodbson. A not entirely pleasurable experience. "

If you do not understand what you are doing, no amount of authority can
save you from yourself. You may read good books and may work with good
people and falsely think that this suffice to protect you from mistakes.

If you left  your instructor to measure focal length  FOR you , or INSTEAD
of you,  than even if he made mistake you made it also, and much more
important one. You did not check on his measurements which you should, not
in order to question his authority but to see if you understood what he
learned you by duplicating (or not) his measurement.

Richard Schwartz, "authority on antiauthority"   wrote a lot on this
subject for our list. So the problem is not uncommon.

Some are in need for advices from gurus to be led by hand comfortably but blindly,
 and some to confirm their own conclusions. It is up to you to choose.

From your letter I see that now you are inclined to follow the second path
which is optimistic development and kind of warranty for success and
satisfaction.

>I will need to remove the pitch and begin grinding anew.

Make new tile tool and regrind the mirror to sphere to the very edge before
starting polishing.
Save your polishing tool for later polishing of a reground mirror. It can
be  modified ,  to the new  f/6 curve.

>My suspicion is that the curve never made its way out there in
>rough grinding.

You should be able to see this immediately by simple Sharpie test, and
later at polishing by KE and Ronchi test. You did not mention testing. Have
you done it and how?
I will suggest, before doing anything at all  make a good KE and Ronchi
test of the present mirror and find out what kind of surface you have got.

Measure the ROC while grinding, with spherometer, template, or other method
while keeping sphericity until you are at required f-number but do not
insist on 6.000 because Richard will get you.

From your present sagita of 2.5 mm you have to dig additional
  0.8 mm for f/6 ( approximately). You may do that with finer than 80 grit.
There is no law on this. Its compromise between spending time to remove deep pits
and  making smaller pits. I think there is no consensus about this.

>I really don't think that fine grinding could have made such an impact on
>the curve...particularly because I was sooooo careful in fine grinding of
>not going over the edge of the mirror with the tool.

Going over the edge with a tool is not uncarefulness, it is part of the
usual procedure. Because you did not,  it may caused  this edge problem.
Regarding possible impact of the fine grinding on the curve, it suffice to
say that  figuring usually starts from polished sphere and is done  by
POLISHING.
 So grinding with any size grit can have considerable impact on curve.
 Do not fool yourself. We are speaking in terms of wavelengths.

>This is based on the tremendously sucky reading I got off the star test.

How you did your collimation? Is your mirror coated?

>. I took the advice of someone who either has diminished visual acuity or
>diminished faculties.That is over ...............

How can you tell that it is over? What makes you so sure about it?

Starting with 12.5" mirror is not easy task. That you have made some
mistakes could be expected, but you have made it to the complete scope
which is great
achievement. Your decision to make it even better is also great. I wish you
all success in this.
Have a happy New Year.


Regards
Vladimir.

--- BBBS/NT v4.00 MP