From: "Dwight K. Elvey" 
To: atm{at}shore.net
cc: DAVE-OESTREICH{at}ATTBI.COM
Reply-To: "Dwight K. Elvey" 


>From: "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!

Hi
 What makes you sure he miss read it. It is
quite possible to extend the mirrors focal length that much by the wrong
stroke, during fine grinding.
 Why were you not measuring all along? I watch the curve as often
as I can to make sure I'm still on track.

>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.

 I don't see how you could have a balance problem if you built
the scope by John's method. He builds from the tube outward. You'd have
placed the trunion bearings at the right place if done this way.

>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.

 I'd make another tool from tile and leave the pitch covered
one alone. I'd make a full sized tile tool and then use the 10 inch for
polishing. Working with a sub-diameter tool is a lot harder than a full
sized.

>The question is where do I want to start?
>Do I want to go all the way down to 60?

#60 or #80.

>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.

 This is almost garenteed to have caused TDE. This is the most
common cause that I've seen. You need to move the tool so that it extends
to 1/6 of the tool width over the edge. You just need to make sure that the
tool doesn't go so far that it teaters over the edge.
 The reason you end up with turned edge is no that you've ground
the edge down, it is that you over worked the center and made it too small
a focal length, compared to the edge.

>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.

 It may be that John is getting to old to do classes, I've not
seen him in a couple of years. Still, it is real hard to keep a number of
students doing the right thing. I've been sharing the duties of running a
class for SJAA. I find that the students don't listen. I see them doing
something wrong. I show them the right way. I watch them to make sure they
are doing it correctly. When I come back a few minutes later, they are
right back doing it wrong again. This is with a group that is only about 6
at a time. I need breaks more than the students do.

>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'd recommend rough grinding with either 60 or 80.

>I do want to dig my curve deeper and make a f6. That seems to be the proper
>course but I need some input.

 You are in the SF area. I would recommend either Chabot in Oakland
or come to San Jose. I think you are one of the types that need closer watching.

>Am I on the proper course?

 At this point, I wouldn't attack it on your own unless you
are the type that can't handle instruction well. You also need to know that
the instructor can't tell you how long a particular action will take. A lot
depends on how much preasure you use, the exact stroke and the amounts of
grit. Since there are often more than one student, learn how to evaluate
your own work. Dwight

>Thank you for your patience and attention.
>Input would be greatly appreciated.
>-Karlo
>
>
>

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