Re: [ATM] Spherometers and sphericity

["kreig" <astroguy@nas.com>]

Ken hunter is perfect in his description of the Sharpie test.  If you need 
to
determine if you have a sphere this is the way to go.

A sphereometer is used to determine the radius of curvature, only.  Dragging
a sphereometer around on ground glass is a sin and will quickly wear the
contact points making the sphereometer inaccurate.

Use the correct tool and proceedure for each step and you will have great
success.

Kreig




----- Original Message ----- 
From: "Ken Hunter" <atm_ken_hunter@yahoo.com>
To: "Mike Lockwood" <melockwo@uiuc.edu>; <atm@atmlist.net>
Sent: Tuesday, November 27, 2007 3:50 PM
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)
>
> Mike Lockwood <melockwo@uiuc.edu> wrote:
>
> The pencil test is another option. Put lines on the mirror with a
> pencil, then gently rub it against the tool WITHOUT any abrasive. The
> two are in contact where the pencil lines go away or are disturbed.
> Again, this does not guarantee sphericity, but it's a fairly good
> indicator of it.
>
> Mike Lockwood
>
>
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