[ATM] Polishing to a Sphere Question

[Michael Coslo]

On Jan 29, 2009, at 10:18 PM, michael dodge wrote:

>
> I'm working on my first mirror (8" F6), polishing is going well (I  
> think), had some problems with a "dog biscuit" surface which is  
> getting alot better with some longer presses and slower strokes.

Let me tell you about my dog story.......

Just kidding.



> I have a zone at about 90% of the dia. which has a shorter ROC than  
> the rest of the mirror. The center zones null out at the same time  
> but the 90% zone nulls about .075" closer to the mirror using my  
> knife edge tester. Have two questions for the experts:
> How can I knock this zone down closer to the sphere?

Have a seat Michael - I think you have a rolled edge. Not so good, but  
every cloud has a silver lining. You need to take the rest of the  
mirror down, not the last 10 percent. But by the time you are  
finished, you'll have an awesome polish.

Long strokes, and you might want to undersize the lap a little bit. It  
would be tempting to make a 90 percent lap, but I'm not so sure that  
one just a little undersized wouldn't do the trick for you. How is  
your lap doing? On my first mirror, I wore the lap down to where it  
was acting a little strange, it was thin at the edges, and acting very  
hard and got some dog biscuit. Don't know if that is the case here,  
but I remade my lap for my big push after messing the mirror up.


> and
> How "close" to a sphere should I be before I start to think about  
> figuring?
> Thanks for any advice!

For a first mirror, I recommend going the whole way to the sphere  
first,  then parabolize.  Folks with more experience can often hit a  
paraboloid without going to the sphere, but for a first mirror, it  
would be incredibly lucky.


Another reason is that first timer's often don't polish enough, so  
that extra time will be well spent. But when you have the needed  
correction for your mirror, you'll have a great polish as mentioned  
above.


-73 de Mike N3LI -