Re: [ATM] Spherometers and sphericity

[Ken Hunter <atm_ken_hunter@yahoo.com>]

All you need to do to answer this question is to imagine the shape put on the glass during manual hogging. Deep hole in the center tapering to the edge.
   
  Now apply the SHARPIE TEST, you will see that contact is only at the edge and there is NO CONTACT in the center. As mentioned earlier, the SHARPIE TEST will tell you if you are:
   
  FLAT-SPHERICAL
  SOME OTHER SHAPE
   
  That's all it's good for but that is a lot.
   
  If you apply longer strokes, you will eventually find a displacement where the flat surface near the edge of the tool fits down into the hole in the center of the glass and the marks are disturbed... but the glass is not spherical. In your example the edge rubs the edge but the center DOES NOT RUB THE CENTER unless the glass and mating tool are spherical.
   
  Ken Hunter
   
  

Scott Ewart <scotte@frontiernet.net> wrote:
  Ken,
I agree that it is frustrating hearing about people grinding during
a Sharpie test, knowing they may be led astray. But please explain why
short strokes are necessary.
For example, it seems to me that if you have a slightly deeper
center than edge, then with a very short stroke, the edge only rubs the edge
and the center only rubs the center. How can you see that they are
different? Wouldn't a longer, more normal stroke reveal more?

Scott


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Ken Hunter
Sent: Tuesday, November 27, 2007 6:50 PM
To: Mike Lockwood; atm@atmlist.net
Subject: Re: [ATM] Spherometers and sphericity

The SHARPIE test is misunderstood by many. 

I originally coined the term back in the 70's while working at Kitt Peak
National Observatory. One day we (Lynn Hepburn, several other opticians and
myself) were discussing the subject *sphericity and how to determine* when
it was noted that the pencil test sometimes left scratches from impurities
in the lead. I suggested using a SHARPIE marker to put the lines on the
glass and the SHARPIE TEST was born. It is the same exact test as the pencil
test described in the old ATM series of books using a less abrasive marking
medium.

As most understand and describe the SHARPIE test, they put the marks on
the glass and continue grinding until the lines are gone. This is NOT A
SHARPIE TEST. Is is different by one very important distinction. If you are
using the typical grinding strokes, YES, the marks will eventually disappear
but... you do not necessarily have a sphere. You have merely managed to
grind away all the marks.

The real SHARPIE test is done with a very few, very short, strokes in
several directions to determine where the glass is in contact with the tool.
If you are fairly clean in your habits, it can be done dry with little
regard to scratching except in the very finest grades of abrasives. 

I usually will do the SHARPIE test at the intermediate grades by cleaning
the disks, putting on the lines, add a drop of 5micron slurry and give some
very short strokes for a few seconds. Longer strokes or longer times will
reduce the effectiveness of the test.

Ken Hunter
Inventer of the SHARPIE TEST 
(my one claim to fame)


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