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Re: [ATM] need confirmation on sagitta depth

Roman,

Some coincidence: I was just completing a little Excel program which
calculates the Saggita. Then I read your email.
In the test run of the program I entered the dimensions of my own mirror:
the program gave a correct saggita. 
The second test run was your mirror which gave a saggita of: 0,1354 inch.

Good luck
Harro Treur

> -----Oorspronkelijk bericht-----
> Van: Roman Toledo [mailto:rtoledo2002@yahoo.com] 
> Verzonden: donderdag 27 juli 2006 8:32
> Aan: atm@atmlist.net
> Onderwerp: [ATM] need confirmation on sagitta depth
> 
> 
>          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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