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Dear Hepping,

I think that one needs to remember that the source of "non-descanned" 
signal light is not a point. It is sort of a 3D blob caused by light 
that emerged from a point but then scattered off refractive features 
in the specimen. As a result, strictly speaking, it cannot be 
"focused" onto any remote detector.

What seems to work best is locating the sensitive element of the 
detector as close as possible to the back of the objective. It would 
seem to me that if you are planning to use this 2-photon system to 
look at thick specimens (brain, embryoes, etc) then mounting the tube 
lens between the dichroic and the objective might mean you losing 
some signal you could otherwise have recorded.

Cheers,

Jim Pawley



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>
>  Short answer to (1) is, astigmatism.
>
>  Transmission through a flat, tilted piece of glass gives only a lateral
>deflection to a collimated beam, but it gives astigmatism to a focusing
>beam. You could imagine using a dichroic that reflects the excitation
>rather than transmitting it, but be careful about flatness (most dichroics
>are quite curved, sadly) which can also give astigmatism.
>
>  Because of some bad decisions, I ended up using a dichroic in the same
>manner you describe. You can cancel a lot of the resulting astigmatism with
>a second piece of glass tilted the same amount but rotated 90 degrees about
>the optic axis. On the other hand, why bother solving a problem you don't
>have to have?
>
>On Sat, Nov 29, 2014 at 11:23 AM, Heping Yuan <[log in to unmask]> wrote:
>
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>>
>>  Hi everyone, I was looking to modify an old confocal system into a
>>  two-photon system. I
>>  had two questions and would appreciate any help.
>>
>>  1) A section of the schematic of typical two-photon is as follows
>>  (http://www.thorlabs.com/tutorials.cfm?tabID=32729):
>>
>>  Galvos -> Scan Lens -> Tube Lens -> Dichroic (690 LP) -> Objective
>>
>>  where fluorescence emission is reflected off Dichroic towards a collection
>>  lens and PMT's
>>
>>  I'm wondering if there are some unforeseen problems in changing the order
>>  as follows:
>>
>>  Galvos -> Scan Lens -> Dichroic (690 LP)  -> Tube Lens -> Objective
>>
>>  where the tube lens can focus the fluorescence emission back to the
>>  Dichroic and directly
>>  into the PMT's (without a collection lens).
>>
>>  2) What is a common procedure to image the back aperture of the objective
>>  to the input
>>  window of the PMT? My first thought is to create a collimated source with
>>  size > than
>>  back aperture and shine directly into the back aperture with objective off
>>  (to see spot size
>>  at PMT input). Would this be correct? If so, practically speaking what
>>  type of source is
>>  typically used?
>>
>>  Thanks,
>>  Heping
>>


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