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


>From: "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.

Hi
 In the class I have a spherometer. Most measure with it but
it isn't as accurate as using a small pin light, a tape measure and some
water spray. With the spherometer, most focal lengths are around .018 to
.023 someplace on the scale. The dial gage is +/- .001 ( typical cheapy ).
The placement of the corner ball bearings is also a little in question. For
what the class needs, it does a reasonable job.
 Still, when they bring the mirror to me, I bring out one
of those small pocket keychain lights. I watch the reflection of the wetted
mirror and can locate the ROC within about 1/4 inch. I rock my head from
side to side and watch how the reflection acts. Since I can also see the
entire mirror, I can see when they haven't fully ground out to the edge. I
get a more accurate reading and more subjective information from the
mirror. Still, many in the class feel they are getting more from the
spherometer.
 I gave up trying to convince them that what I was doing was
really more accurate. They can see the needle on the dial gage. I have to
admit, it does take a little practice with the light but it is simpler and
much more accurate ( about a factor of 4 or 5 for a f/6 to f/8 ). It does
take more than one person. One to take the sitings and one to measure the
location.
 It is low tech but works well.
Dwight

>
>> 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