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


David,

If it is true that the mirror has so wide "edge" than you might
consider that it was never finished .
 If that is so you are only continuing on the project.
You said that it takes you six months to regain your composure. As it seems
to me, and from your questionnaire, you are bound to another disappointment
if you will not radically change your approach. This is reaction to the
sentence:"Last summet I took a telescope making class from John
Dodbson. A not
entrirely pleasurable experience. "

If you do not understand what you are doing, no amount of authority can
save you from yourself. You read good books and work with good people and
falsely think that this suffice to protect you from
mistakes.  You may not blame neither the books or person alone.

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

Richard Schwartz, "authority on antiauthority"   wrote a lot on this
subject for our list.


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

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

I consider what I said up to now is the most important advice in this case
and hope that you will appreciate it. But there are some other:

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

Spare your polishing tool for polishing reground mirror. It can be later
modified ,  to the new  f/6 curve.

Make new tile tool and regrind the mirror to sphere to the very edge before
starting polishing.

>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 do not mention testing. Have
you done it and how?
I will sugest before doing anything at all to make a good KE and Ronchi
test 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.

>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 cause  this edge problem.
Regarding possible impact of the fine grinding on the curve, it suffice to
say that  figuring usualy 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