From: "DAVID K OESTREICH" 
To: 
Reply-To: "DAVID K OESTREICH" 


I am afraid this posting might run rather long. I am hoping there are one
or two patient souls that will muddle through this posting and give me some
much needed advice. Last summet I took a telescope making class from John
Dodbson. A not entrirely pleasurable experience. My goal was to create a
12.5" Dobsonian telescope. Furthermore I wanted to create one along
the lines of the Richard Berry style Truss tube scope. The physical
construction of the scope went fairly uneventful... as much as can be
expected for one's first outing. The mirror on the other hand had its share
of problems. John erroneously read a watercurve of the focal length midway
through fine grinding. While I have heard that a water curve is far from
precise for determining focal length none-the-less John's reading was off
by 2 1/2 feet! Rather more than a slight miscalculation. So instead of a f5
and some change scope I have and f8 plus which translates to a tube length
of 7 feet and some change.
Being greater than average height 6' 3"; I was mildly perturbed to
find that I needed a ladder for the bloody thing. But the fun doesn't end
there. When we finally star tested the polished blank we come to find the
thing has a horribly downturned edge. Yeah I know you were anticpating
that. The thing has this horrid dark prominient dark ring when focused one
direction and looks like a tribble that has touch a van der graf generator
in the other direction.
The dilemma is this. I reallllllllly do not want an f8 scope. I would be
much happier at f6. The shorter focal length would also hopefully resolve
some balancing and counterweight issues that I am having. Currently it
needs 35 lbs on the backend. The OTA and paricularly the Secondary cage
runs a tad heavy (particularly the 2" focuser). This rather horrid
mutation has sat in my basement for five months while I regained my
composure. Now with cloudy skies I find I have the appetite to take on this
project again. So what is one to do. I still have my 10" tool (still
covered in pitch). I purchased a grit kit for a 12". So I have plenty
of grit. I will need to removed the pitch and begin grinding anew. The
question is where do I want to start? Do I want to go all the way down to
60? If I understand the problem correctly the curve did not make it all the
way out to the edge of the mirror.
My suspicion is that the curve never made its way out there in rough grinding.
This is based on the tremendously sucky reading I got off the star test.
Estimates are that the edge is from an inch to an inch and 3/4 wide. Fairly
downturned I'd say.
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. I chalk the whole
experience up to a case of the blind leading the blind. I wasn't skilled or
knowledgable enough to know when I should begin my fine grinding and flip
from mirror on top to bottom. I took the advice of someone who either has
diminished visual accuity or diminished faculties.
That is over so now I am left with a problem that I would like to correct.
So how do I correct it?
Do I follow my gut instinct to return to rough grinding and bring that curve out?
I do want to dig my curve deeper and make a f6. That seems to be the proper
course but I need some input.
Am I on the proper course?
Thank you for your patience and attention. Input would be greatly appreciated.
-Karlo

--- BBBS/NT v4.00 MP