From: "Nathan D. Spaulding" 
To: 
Cc: "ATM" 
Reply-To: "Nathan D. Spaulding" 


> Be careful, or the math may screw you back.  ;-)

Truer than you know!!!!!  The reason I said what I did is that I seriously
screwed up on my first mirror checking saggita with a dial indicator.  I
have always had a hard time with math,and will aviod the use of it if at
all possible.  I'll let a computer do it for me, and only in an emergency
resort to paper.

> As an example to highlight my concern, the sagitta of a 12 inch f/6 mirror
> differs from the sagitta of a 12 inch f/8 mirror only by (please check my
> math ;-)...about 0.030 inch...which is about 1/32 of an inch.

When a line, in my experience is scribed with a beam compass, it is
suprisingly accurate.  I've used this method several times with good
success.  I've even used it to very carefuly measure the saggita in fine
grinding and make the nessecary adjustments.

> Can our mirror maker scribe a line that accurately?  Can he cut/file that
> accurately?  Actually he probably needs to be accurate to about 1/64th of
> an inch...or more.  Does our ATM have steady hands?  Good eyes?  Patience?

In my case a resounding YES!!!!!  I do nat feel that this is a difficult
thing to construct at all.  simply cut outside the line and CAREFULLY file
to the line.

> We need to ask the telescope maker in question the $64 question:  What
> f/ratio is your target?...and more importantly...what is your tolerance
> plus and minus?

Excellent point.  This is a case where the TN wants to achieve an exact focal length.

> Depending on his answer...the scribed template method may not meet his
> needs.  Especially for a slower mirror that he is comtemplating.

No the template may not meet his needs, but I will contest that a template
will work for shoter focal lengths just fin, though mabye not in this case.

> Hmmmmm.  Our angst ridden ATM may not know how well it works until after
> many hours of grinding and polishing to the point he can measure focal
> length more accurately by other means.  (Assuming he does not make a
> spherometer or use the sun/wet reflection test during fine grinding.)
> Perhaps your advice will give him lots of frustruation instead?  ;-)

Point conceded.

> Leon Trotsky supposedly said "you may not be interested in revolution, but
> revolution is interested in you."

Again conceded.

> Let's put that in math terms.  "You may not be interested in math, but
> math is interested in you."  ;-)

True, true!!

> If you make a spherometer with a 6 or
> 8 inch spacing of supports (for this 12 inch mirror example)...you can
> center it on the mirror and take readings...then off-center...and if your
> mirror is spherical you should get the same readings.

I agree, though I do not have the expertise to make a spherometer.  You are
100% correct!


Ciao,
    Nutso Nate

--- BBBS/NT v4.00 MP