Re[6]: ATM 8" f3.8 -- Worthy project?

[Dwight Elvey <elvey@hal.com>]

Hugues_Laroche@ses-astra.com wrote:
> 
> 
>                wire
>                 |
>    Eye          |  <-- blocking part
>                 |
>             ----|           ------> to the mirror
>             |   |
>  screen --> |   |  <-- lit part
>             |   |
> 
> So the testing would be very slightly off-axis. On fast mirrors
> may that be a problem?

Hi Hughes
 I think you are missing the point. When the wire is exactly
at ROC, if you put your eye behind it, the diameter of your
pupil will see around the wire at both the wire in front of your
eye and the wire imaged from the source. It would look like
holding the wire up in front of your eye and looking at a
white screen.
 The idea of both the wire tester, slit-less and regular
Foucault tester is to have both the source and the resolving
device block light that doesn't go exactly through the
center of focus.
 When using a slit or pin hole source with a wire tester,
the idea is that the wire will block any light that goes
through the focal point from the source, while letting
light that isn't focused go by. If you use the setup
with a screen and wire, you'd need to use a pin hole or
slit for the resolving device, a wire wouldn't work.
 Another way of looking at it is that what your eye sees
on the face of the mirror is a completely out of focus
source. That means that every point on the screen will be visible
from every point on the mirror surface. Picking out the
missing light from the sliver of the wire would be impossible.
 Does this make any sense?
Dwight