Re: [ATM] need confirmation on sagitta depth

["Mel Bartels" <mbartels@bbastrodesigns.com>]

>        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


>S = (D/2)2 / 4*F
> so I get  6.5 squared 13 / 312 = .041

I'm not sure how you got the 2nd line, but the correct expansion of the
formula is:  6.5*6.5/(4*13*6) = 0.135

Here are all the sags for a wide range of FRs for the 13 inch:

Mirror Dia (inches) =	13	
Focal Ratio	Focal Length      Mirror Sagitta
3.00 		39.00 		0.271 
3.25 		42.25 		0.250 
3.50 		45.50 		0.232 
3.75 		48.75 		0.217 
4.00 		52.00 		0.203 
4.25 		55.25 		0.191 
4.50 		58.50 		0.181 
4.75 		61.75 		0.171 
5.00 		65.00 		0.163 
5.25 		68.25 		0.155 
5.50 		71.50 		0.148 
5.75 		74.75 		0.141 
6.00 		78.00 		0.135 
6.25 		81.25 		0.130 
6.50 		84.50 		0.125 
6.75 		87.75 		0.120 
7.00 		91.00 		0.116 
7.25 		94.25 		0.112 
7.50 		97.50 		0.108 
7.75 		100.75 		0.105 
8.00 		104.00 		0.102 
8.50 		110.50 		0.096 
9.00 		117.00 		0.090 
9.50 		123.50 		0.086 
10.00 		130.00 		0.081


Mel Bartels


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