Re: ATM Ready to Jump ! Focal length ???

["Bill Prewitt" <bprewitt@neo.rr.com>]

Try it again with the flashlight held up to the side of your head..... even with
your eye.

Here's a handy online calculator for the sag/fl/roc math.
http://www.atmsite.org/software/sagcalc/sagitta.html
It agrees with your f6.2 if the sagitta was correct.

Grind TOT to lengthen it.
          Bill Prewitt

My webpage of telescope making and caving
http://home.neo.rr.com/imaging

----- Original Message -----
From: "G Hooper" <hooperg@optonline.net>
To: "ATM" <atm@shore.net>
Sent: Thursday, December 28, 2000 10:00 PM
Subject: ATM Ready to Jump ! Focal length ???


>
>
> I wanted to do a quick test to see what my focal length
> was before proceeding with fine grinding, this is a good thing
> right ? Well I know it is, but wonder now if I should have picked up
> the game of golf instead !
>
> Test one
> 1) prop the 6" mirror up on the back edge of a countertop
> 2) Place a bright flashlight 10' or so away shining into the mirror at a
slight
> angle
> 3) stretch a tape measure from the mirror, back ten or so feet...
> 4) catch the reflection and start bobbing my head and moving away from mirror.
> 5) reflection stops moving the same way I do at 60"
> 6) More then 60" the reflection goes the opposite way.
> 7) close my eyes, take a break do it again, 60" not 61, not 59" but 60".
> 8) Go back upstairs watch some TV, while the wife says, "What's wrong Hun? "
> 9) Test again and get 60"
>
> Mirror actually measures 5.95"
> 60" = 2F( or R)  so 30"= F (Am I correct this far ?)
> 30/5.95=  f5 !
>
> My problem is that I had an f8 not to long ago ! (exactly what I wanted...)
>
>
> Sweating now I measure the sagitta multiple times many ways and
> consistently get .060 thou removed , this means a F 6.2 ??
> to be an f5, like the first test above the sagitta would need to be .074"
> a .014 thou  difference, FYI I worked in a machine shop, on a bridgeport for 3
> years
> straight, I can spot a 14 thou error in a flash
>
> Now just What the !&%# is going on ???
>
> Any Ideas before I loose my mind completely ???
>
> What is the Most sure fire way of checking my mirrors focal length...
>
> Thanks,
> Glenn
>
>
>
>