From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


Hi
 I don't think your calculations are correct? For an
8" F8, my calculations say that a 3.5" diameter spherometer
should read a little shy of .012". Here is my calculations:

 8" f8 has a focal length of 8" * 8 = 64". This
is a ROC of 2 * 64" = 128"

Using right triangles and 3.5" I get the equation:

(128 - x)^2 + (3.5/2)^2 = 128^2

This expands to:

x^2 - 256*x + 3.0625 = 0

I used one one of the net quadratic equation calculators to get:

 x = 0.01196

See:

http://www.math.com/students/calculators/source/quadratic.htm

or use:

 x= ( -b +- sqroot(b^2-4ac) ) / 2a

 on your own calculator.

Later
Dwight


>From: "James P Crombie" 
>
>The .006" was with the 3.5" spherometer, if you calculate it out it
>translate to about .048" across the 8" of the mirror.  So I
have another
>.002" to go for an f8
>
>Frank Q wrote:
>
>>Hi All
>>
>>An excellent effort.
>>
>>I have a question -- My recollections of grinding an 8" f/5 is that
>>I could excavate much more than 0.006" in a few hours - does
>>anybody else find this or is my memory rapidly failing?
>>
>>(I used 80 grit and a full sized plate glass tool)
>>
>>Cheers
>>Frank Q
>>
>>----- Original Message -----
>>From: "James P Crombie" 
>>To: "Atm List" 
>>Sent: Wednesday, July 23, 2003 3:38 PM
>>Subject: ATM Mirror Grinder pictures
>>
>>
>>
>>
>>>Hello all
>>>I did another couple of hours with the mirror grinding machine and got
>>>the sagita to .006"(this is with a 3.5" spherometer
not overall ) or a
>>>ROC of  166" .   I thought I would start with something
easy like an 8"/f8
>>>
>>>http://www.jamescrombie.com/temp/closeupmirror1.jpg
>>>http://www.jamescrombie.com/temp/wideview1.jpg
>>>http://www.jamescrombie.com/temp/mirrorgrinder1.avi    (note 1.7mg)
>>>http://www.jamescrombie.com/temp/mirrorgrinder2.gif     (same as above
>>>but will open in browser window)
>>>
>>>--
>>>--------------------------------------------------------------------------
>>>
>>>James P Crombie
>>>
>>>Summerside      Machinist
>>>Prince Edward Island      Amateur Astronomer
>>>Canada      3D Designer
>>>
>>>E-mail      jpcrombie{at}pei.eastlink.ca
>>>Astronomy webpage      http://www.jamescrombie.com
>>>Rhinoceros 3D webpage      http://www.jamescrombie.com/rhino/
>>>Mirror Grinder page          http://www.jamescrombie.com/pics/
>>>--------------------------------------------------------------------------
>>>
>>>
>>>
>>>
>>
>>
>>
>>
>
>--
>--------------------------------------------------------------------------
>
>James P Crombie
>
>Summerside                  Machinist
>Prince Edward Island        Amateur Astronomer
>Canada                      3D Designer
>
>E-mail                      jpcrombie{at}pei.eastlink.ca
>Astronomy webpage           http://www.jamescrombie.com
>Rhinoceros 3D webpage       http://www.jamescrombie.com/rhino/
>Mirror Grinder page          http://www.jamescrombie.com/pics/
>--------------------------------------------------------------------------
>
>
>
>

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