From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


There are two requirements for the lens to be used in the imaging of the
test.  First is that the lens needs to have a focal length such that it
will see the whole mirror in the image.  This doesn't mean that the lens
needs to be a physically large lens, just that the camera sees the whole
mirror.  The second is that the lens needs to be big enough to see the
whole cone of light coming past the KE.  If you don't have a big enough
aperture, the edges of the mirror will not be filled with the light of the
source. I use a simple doublet for my lens and it is almost touching the KE
(maybe a 1/32" from the KE) and it has no problems seeing the light
from a mirror. When I draw it back from the KE, the point is soon attained
where the edges of the mirror start losing the light and that isn't a good
place to do the testing.
Those of you that are using complex zoom lenses are actually probably
making things difficult for you as those lenses usually have builtin hoods
and so forth and make it difficult to actually get the lens close enough to
the KE to make the lightpath right for the test. One guy that made a system
for his home used a simple lens imbedded in a sliding box structure (bottom
and back plus sides with the camera and the top, front and sides with the
lens) and this was after trying a 35mm camera lens and having many problems
with it.  He is now getting nice images of the stuff that he's doing at
home.  The simple lens is nicer as it allows the lens to be put right
behind the KE without problems.  Interestingly, he leaves the camera
stationary and moves the KE in front of it and has to move the camera only
to insure that the KE at the rearmost position doesn't hit the KE.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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