Re: [ATM] Open Question Concerning Pitch Laps

[Jonathan Bishop <jonathan_bishop@sbcglobal.net>]

Mike,

So I think we agree that to get back a spherical figure a hard lap will 
work better.

Many beginners, like myself and Bryan, get in trouble during 
parabolization (usually turning an edge or digging a hole) and simple 
would like to get back to a nice clean sphere again. I know that most 
advanced ATMers will find that a little tedious, but for the beginner it 
is probably a good approach. In Bryan's case he actually went back to 
fine grinding to get to a sphere as he could not get there with his 
current setup, no matter what he tried. I was often tempted to do that 
myself until I switch to harder pitch.

I am sure many other beginners went the same route as Bryan. I blame 
this on the soft pitch, but probably the more advanced polishers could 
have dealt with that in some other way than hardenning the pitch. From 
what I saw all the advice that he was given did not help him though.

The reason I like subdiameter laps is that I remember seeing a picture 
of the 200 inch Palomar mirror being figured that way. So while I am 
polishing I can dream of the time I am polishing my own huge mirror. Of 
course this will require a sub diameter lap :-).

On the more realistic side one nice thing about subdiameter laps is that 
the figure changes really slowly provided of course that you use light 
smooth strokes and maintain good contact. I know a lot of people have 
steered away from this approach, but it worked for me. I find it easy to 
understand. Simply polish where you want to remove glass.

BTW, are you saying that the sub diameter approach will get easier for 
large mirrors? That's good to know. So first I made a 4.25 and then an 
8, for wich I used the 4.25 lap. Next I think should be 12 or 16, for 
which I can use both my 4.25 and 8 laps. Then for 32 I use... Seems like 
a nice progression!

Jon

Mike Lockwood wrote:

> Jonathan,
>
> Jonathan Bishop wrote:
>
>> You are going to hear a lot of contradicting advice on figuring a 
>> mirror. So I expect many other people will have opinions that sound 
>> much different than mine...
>
>
> When you boil it all down (not the pitch, the advice!), the list 
> advice is fairly consistent - there are many ways to make a mirror, 
> some easier than others.
>
> Bryan, the most important advice that I saw multiple times was that 
> you should practice and try to perfect your Focault testing.
>
> Jonathan, I was impressed with how well you used a subdiameter lap to 
> figure your mirror a while back.  It's not easy, especially for a 
> mirror of that size.  (I think it was an 8"?)  It was also not the 
> typical path to the parabola.
>
>> I think your pitch is much too soft. I also used soft burgundy from 
>> WB and had similar frustrations. I remember boiling it for a couple 
>> of hours to harden it. After I did that I was able to quickly get 
>> back to a sphere using short W strokes. In fact what actually 
>> happened was that I raised a hill in the middle which gradually got 
>> bigger and bigger until it covered the whole mirror, at which point 
>> it was actually more spherical than I had ever seen before. So I 
>> really am convinced we should stay away from this soft pitch for this 
>> reason.
>
>
> This recommendation is based on experiences in making how many mirrors?
>
> The goal of making a Newtonian primary mirror is not to make a sphere, 
> it is to make a smooth parabola.  For hand work, hard pitch may more 
> readily form spheres, but soft pitch more readily forms smooth 
> parabolas.  The (approximate) sphere is just a stage that we pass 
> through on the way to a parabola.  The necessities before parabolizing 
> are a good edge and lack of serious defects (deep holes or rings, poor 
> polish, prominent zones).  I'm not saying you CAN'T make a smooth 
> parabola with hard pitch, but I am saying it is a lot easier with soft 
> pitch.
>
> At the risk of starting a "pitch war", to me Bryan's pitch sounded 
> like it was in the range to make a nice smooth parabola.  He just 
> needs to get close enough to a sphere so that he can start figuring.
>
>     Mike Lockwood
>
>

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