Re: [ATM] Stubborn Oblate Spheroid.. update

[Jason Hissong <jhissong@gmail.com>]

Mike,

Excellent information.  Alternating TOT and MOT with short W strokes 
seem to be doing the trick.  I cannot visible see any pits (it is hard 
to get focused on the surface with my 8x slide magnifier I use).  The 
laser test still does not pass (and the layers just really get me 
confused) but I don't know if what I am seeing is true grayness from 
pits or residue/second BVC layer..  It is difficult to get all of the 
junk off of the surface of the mirror so that I am not seeing left over 
polishing compound.  I am heading off to the store tonight to pick up 
some more distilled water and ivory dish soap.  Maybe finger nail polish 
remover?

My latest ronchi test:
http://www.undermidnight.com/images/atm/sphere_1.jpg

I find OROC is more revealing of the edge.. and it is going away:
http://www.undermidnight.com/images/atm/sphere_1_oroc.jpg
For comparison (before the current effort using your advice):
http://www.undermidnight.com/images/atm/oblate_spheroid_5_oroc.jpg

I think I am settling into the right technique for this lap (and for 
myself) which it appears is doing something.  I still have a slight 
raised outer zone, but I am not too worried anymore (polishing more... 
worry less as someone mentioned earlier).  I just ordered some medium 
Acculap and will use that on my 14".

Right now I will keep polishing until I see nothing on the surface when 
I do the laser test (or do you always see something?)

Clear skies...
Jason

Mike Lockwood wrote:

> Hi,
>
> Nice work Jason, and excellent Ronchi images.  Keep going.
>
> If it is indeed TDE, short strokes will work (probably TOT).
>
> If you are not polished out, simply keep polishing with normal length 
> strokes, working both MOT and TOT and it is likely you will simply 
> polish it away.  It is left over from the oblate sphere figure.  Keep 
> an eye on it and make sure it keeps going away.
>
> Many think TDE is usually caused by soft pitch.  While this is 
> sometimes the case, though, it is my theory that the TDE due to an 
> oblate sphere (as shown on my web page) is the most common.  This TDE 
> is often leftover after the oblate is long gone!  At least, that's the 
> TDE I see most often.
>
> As far as checking for pits, I like a 10x loupe.  Set the focus for 
> the surface of the mirror (using another flat surface if necessary), 
> shine a light on the surface, and inspect.  This way you can't mistake 
> the next layer of glass (in BVC) for pits because you're not focused 
> on it.
>
>
> Jason, when you get ready to start figuring, read my short article on 
> figuring with a full-size lap that is posted at:
> http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/full_lap/full_lap.html 
>
>
> I know that I say F/5-F/8 is the range that I'm talking about, but 
> that is conservative and you can likely get there with and F/4.3.  If 
> your pitch is fairly soft the techniques in the article will work.
>
> Beware - at first during figuring it may not look like much is 
> happening, but really it is.  Just keep going, and test carefully.
>
>     Mike Lockwood
>
>

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