From: Guy Brandenburg 
To: DAVID K OESTREICH 
CC:  atm{at}shore.net
Reply-To: Guy Brandenburg 


David,

Am I to understand from this that John Dobson doesn't use spherometers or
Foucault testers or Ronchi testers, and that he only had you measure the
focal length ONCE during the entire grinding/polishing process? Good grief.
That sounds rather irresponsible to me.

In any case, if it really is f/7 or f/8, and you want an f/6, then, yes, I
think you will have to go back to rough grinding, with either 60 grit or 80
grit, take your pick. Why not use a full-size tool? You can save yourself a
lot of $$ if you get some dental stone and cast yourself a full-size tool
and glue either washer dumps or hard ceramic tools to the top, if you can't
find a full-size glass tool.

Guy Brnadenburg

DAVID K OESTREICH wrote:
> 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