Re: [ATM] Open Question Concerning Pitch Laps

[Mike Lockwood <melockwo@uiuc.edu>]

Jonathan,

Jonathan Bishop wrote:
> So I think we agree that to get back a spherical figure a hard lap will 
> work better.

Yes, but we want a parabola.  A sphere is NOT a necessary step in 
gettting to a parabola.  Its importance is overstated in the 
literature (okay, that's my opinion after making 20 mirrors).  Once 
the mirror is polished out and has a good edge and no serious zones, 
parabolization can (and should) begin, and the mirror maker should be 
comfortable with testing at this point.

> 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.

Here's what I'm getting at - not getting a sphere with medium softness 
pitch is a symptom of a technique problem.  If that problem isn't 
isolated, the lap may NEVER act like it should, no matter what the 
hardness or technique.  There will always be unanswered questions.

> 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.

I'm not sure we (list members) asked enough questions before Bryan 
went back to grinding.  (We tend to jump to our own conclusions.) 
That's why I came up with that giant list of questions.  I even missed 
a few - I didn't ask about polishing speed.  Others made good points 
about the laps getting "crusty" after use.

Seems like a standardized list of questions would save time.  The 
problem is, everyone's results can be unique.

> 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.

In large mirrors, subdiameter laps are often used to add correction, 
rather than just tweak zones.  They are necessary for mirrors of 
faster focal ratios.  The rules get much more complex when you're 
working strongly non-spherical surfaces.

> 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!

Go for a 16" - the 8" lap will come in handy.  Hopefully you won't 
need the 4.25" lap.  I made a 16" F/4 with just an 8" lap for 
figuring.  I learned tremendous amounts on that mirror.

Bryan, if you're still listening to all of this, you're hearing some 
differing viewpoints.  We'll help you through the figuring process no 
matter what route you take.  I still recommend sticking (no pun 
intended) with the pitch that you already have.

	Mike Lockwood


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