Re: [ATM] Improving my focault tester/testing skills

["Bob May" <rmay@nethere.com>]

I'm a strong proponenbt of using a camera behind the KE to see
what is happening on the test.  Any camera will work as it is
basically the same as the eye in its action.  The difficulty is
in getting a camera lens close enough to the KE so that you can
see all of the light coming off of the KE.  What is happening
that is bad is tht the light cone is expanding (at the same angle
as the light was coming together from the mirror surface) and the
camera really expects that the light be a parallel bundle so you
have to put the lens very close to the KE, just like you have to
with your eyeball.  Thus simple lenses work a lot better than
some fancy lens will.  I am presently using a video camera of the
hceap single board sugar cube sized survelience type and putting
a different lens in front of it rather than the little lens that
comes with them.  The stock lens just has too wide of a FOV for
best usage so I grab a lens of about an inch of focal length and
set that one up for focusing the light to the camera imager.
Right now tho, the lens is a CS sized zoom lens that just happens
to not havve anything of a sunshade on it so the front o9f the
lens glass can be put right on the KE without any real
difficulty.  Even then, if the light coming back isn't right on
the money, I will see the iris of the lens in the image which can
be a bit nasty with the shorter FL mirrors.  The simple lens is a
lot better for this but it is nice to have the zoom function when
testing both fast and slow optics in rapid succession.
The last video camera I got was from the drug store and sold for
survelience work at home and cost me only thirty five dollars and
was ready to use with all of the cables and so forth that were
needed to use it.Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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