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I have a custom coupled camera/spectrograph imaging system that I need to focus at infinity for a microscope setup. I can't move the device outside, where I would be able to use some distant object to set the focus, because it is too fragile and bulky.

Are there other tricks that will allow me to find infinity focus for my setup?

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  • $\begingroup$ Can you illuminate the output of your imaging system with a laser for example? $\endgroup$ – eranreches Dec 11 '18 at 19:19
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If I understand your question correctly, you need to point your setup at a point source located at infinity, and adjust the setup until the point source is focused on your camera's image sensor.

Light from the point source is precisely equivalent to a collimated beam. Among the ways to obtain a collimated beam to use an expanded beam from a laser pointer. You can expand a laser beam by first running it through a microscope objective followed by a convex lens; but that leaves the problem of placing the convex lens in the right position to precisely collimate the beam. A very straightforward way to do that is to place a pinhole at the focus of the microscope objective, and reflect the expanded beam back to the collimated lens using a flat mirror. Adjust the angle of the flat mirror so that the beam passes back through the pinhole. (That is, both the outgoing beam and the return beam pass through the pinhole). A beamsplitter between the laser and the microscope objective will allow you to monitor the brightness of the return beam. Adjust the position of the convex lens until the amount of light returning through the pinhole is maximized, and the beam will be collimated.

Once you've got a collimated beam, you just send that beam into your setup and focus the camera. NOTE: the beam will be very bright, and might damage your camera's image sensor unless you attenuate the beam. To attenuate it, you can put a neutral density filter in front of your setup, in front of the beam collimator, or between the beamsplitter and the laser.

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well, to point the obvious, if the microscope is infinity corrected, just focus the microscope using the regular eyepiece, then replace the eyepiece and the tube lens with your camera system and try to get the image in focus without touching the microscope knobs.

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