Re: ATM what did I do wrong????

[George Anderson <georgea@cam.org>]

Hi Charles

It sounds like your second reading is giving you the ROC, much like a
foucault or ronchi test. If the flashlight and paper are beside each
other and you moved back and forth until you got a focused beam image on
your paper you just measured the ROC. Which would be the focal point if
you were trying to look at something that close. But if you were to use
the same mirror to look at stars or planets then your focal length would
then be roughly half of what you just measured.
Relax, wait for the sun tomorrow, and then do the test again and you
will probably find a focal length of 49 inches.

George Anderson
Montreal Canada

Clear skies and good health

c f wrote:
> 
> Hello to all,
> I am working my first mirror, a 6" f8,  sounds simple enough,  got through
> the 80, measured and I was right on, .0468 for a depth in the center.  I did
> all my grinding TOT.  I then went to 120, did a focal check, used the sun,
> bingo, + - 48 ".   On to 220, 30 Micron, 15 micron. all with TOT.
> everything looks awsome, did a sharpie on the 15 micron, and the grid came
> out in 2 wets, and it came out evenly,  checked under a magnifying glass,
> not a pit in site that isnt of the same size as the others. that tells me
> good things.
> 
> But............ I wet the mirror, put up a peice of paper, flashlight, and
> took another focal reading. and SOB, it is wayyyyyy off.  in the other
> direction to boot!!!!!!!!!!!!!!!!!!!  instead of a 48" +- focal,  I have a
> freeking 98" focal!!!!!!,  I went from .04687 to .03125!!!!!!!!  this was
> done with the TOT, using a "W" stroke, 1" over hang during the stroke.  Now
> with all the reading I have been doing, it says that if you keep the TOT,
> then you will dig a deeper hole so it is suggested to swap them.  well, I
> decieded to not do this, and keep the TOT to maybe bring it closer to a F/7.
> 
> can anybody tell me what I did wrong??  because at this time, it looks like
> I am heading back to 120 maybe 220 to get this back to the correct focal.
> but it would be nice to know what I did so I dont repeat this again.
> 
> thanks to all for helping out the newbie
> 
> Charles in wilmington ma
> 
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