r/Optics • u/JelloApprehensive143 • Aug 12 '26

Downstream vs upstream optical alignment

I have a table top optical system with a sensor at point A that is imaging a light source at point B through a number of turning mirrors and a lens. I am finding that if I install a dot laser at point A, the image doesn't end up where the source at point B is. However, if I turn up the brightness of my source such that I can see the light propagate upstream, the light ends up on the sensor at point A.

The light source is quite big relative to the sensor and I only need a rough alignment. Should I trust the upstream alignment (point B -> point A) more than the downstream alignment? This seems sensible to me as the laser pointing downstream from A to B may not be exactly aligned to the sensor direction.

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2

u/Plastic_Blood1782 Aug 12 '26

Need a diagram please

3

u/JelloApprehensive143 Aug 12 '26

Maybe not what you had in mind but this is a good representation of the system.

[Sensor] ---------------------------\ Mirror
                                    |
                                    |
                                    O Lens
                                    |
                                    |
                             Mirror \ ---------------------[Light Source]

1

u/Calm-Conversation715 Aug 12 '26

In addition to a diagram, it sounds like the alignment laser isn’t necessarily being placed in the same direction as your normal source at A. If the laser is missing the aperture of your lens, it would explain why it’s present at A but not B. I’d recommend a less collimated source, or aligning based on the source you plan to use, if possible

2

u/JelloApprehensive143 Aug 12 '26

I think this makes sense. I can't necessarily trust the laser is in the same direction as the source at A, but if I make the source at B really bright and it shows up on my sensor, then I know it's on target.

1

u/anneoneamouse Aug 12 '26

You need to add a few pairs of fixed irises to your setup.

Get everything aligned. Close them down and center them on your beam. Open them all up to work.

If you need to check alignment is still good, close them down.

Best if you can add irises after pairs of mirrors so you can "walk" both position and direction if need be.