[ATM] need confirmation on sagitta depth

[Roman Toledo <rtoledo2002@yahoo.com>]

         Hello gents.

       I know what my instructor told me about the sagitta depth is
right, and one of my test told me so, still let me tell you what's
going on.

       I'm working on a mirror 13" diameter, and using what I could
find that was "easy" I used this site
http://www.stellafane.com/atm/atm_select_scope/atm_scope_calc.htm  to
get the sagitta, maybe I'm not understanding what this page is for BUT.

     I put in 13" diameter and a  focal ratio of F/6 which is 78", and
it came back saying  Sagitta (s) = r2 / 2R    = to 0.135"  so I ground
the mirror to that depth, but in the sun the reflection was like 30
feet into my garage and still was 4" round, so I was confused

    My instructor told me I needed about  .40 of the top of his
head(not bad!) or like a penny's thickness deep for a rough number, I
was a bit skeptic, but knew he was close from my garge wall test.

    I just got home and did a search and found Bob May's site as one of
the first hits, and my math is VERY rusty, I tried the first formula
and had trouble , so I went with the second formula

S = (D/2)2 / 4*F

so I get  6.5 squared 13 / 312 = .041

so I guess my question is  IS the stellafane web site talking about a
different sagitta ?   or am I just not getting it?  what does  0.135 
mean???


thank you all,  guess tommorow I'll be back grinding till I get close
to .04 on those feeler gauges
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