From: Dale Eason 
To: James Lerch , ATM List 
Cc: atm_free{at}yahoogroups.com
Reply-To: Dale Eason 


Hi James,

I just thought of another contrast reducer.  We are both using the Vesta
Pro for testing.  We have both taken its lens off.  Its lens has an IR
blocking filter in it.  With out it the CCD is very sensitive to IR.  IR
does not focus at the same place maybe we are seeing poor contrast because
of that.  If the majority of the signal is caused by IR light then we are
testing the mirror at IR and not at 550.

I don't have an IR blocking filter to try.  Maybe you do.  I thought that a
green LED would not have much IR but when I look at it with a hand held
spectroscope I see light down into the red end.  I have a dark green
plastic filter when I put it in front of the spectroscope the red end
disappears.  Putting my hand a few inches away from a 100 watt light bulb I
feel heat.  Putting the green filter between seems to block it.  So I tried
the green filter in front of my lens. The image gets dimmer but the
contrast improves.  HMM.

I'm also going to make threads inside my Lens to camera adapter and then
repaint it black. That should keep the low incedent reflections down
better.


-+- James Lerch  wrote:
> The loss of contrast could be a result of lens
> flare, or internal reflections.
>
> However, keep in mind my "lens system" is just a
> single dual convex lens with a
> 35mm focal length and fabricated from a single piece
> of unknown glass.
>
> The lens is 1 of three lenses included with a
> Surplus Shed eyepiece lens set I
> purchased some time ago.
>
> I had 'assumed' the loss of contrast was a result
> of:
>
> #1  Using a broad band white light source (compact
> fluorescent bulb)
> #2  A non-acromat lens
> #3  A color CCD sensor that is also sensitive to
> near IR
>
> I figured the three items combined would make for a
> terribly low contrast image
> as the lens is by no means "color corrected"!
>
> I've also been thinking about how reflections or
> 'flare' might interfere in the
> Foucault test, and here's my thoughts:
>
> #1 the light from the Foucault test passes by the
> knife edge and thru the lens.
> #2 the lens focuses the light onto the CCD
> #3 some of this light would reflect off the CCD
> glass cover back to the lens
> #4 some of this light would reflect off the backside
> of the lens (which is
> convex)
> #5 the light reflected off the backside of the
> convex lens would make it back to
> the CCD
>
> The question is, how much light would make it back
> to the CCD after 2 internal
> reflections, and how would it appear on the CCD?
>
> James
>


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