- #1
Chen
- 976
- 1
Hi,
I'm performing an experiment whose goal is to analyze the splitting of the green Mercury line under Zeeman's effect. To do this we use a Fabry-Perot etalon, which first has to be calibrated. This means both making sure that the two reflective mirrors are parallel, and then measuring the distance between them by watching the specturm of some known elements.
My question comes down to this:
When I see, in a specific angle, clear and sharp fringers coming out of the etalon, is this an indication that the plates are parallel (at least locally, i.e around the line of sight)? Or is there another set of circumstances under which I would get the same results with the plates not parallel at all?
If you feel like it, you can continue reading more about my situation and set up.
We are watching the fringes through a telescope, and in order to measure them the etalon is mounted on a rotating disk, and we have digital means to record its rotation angle. So the procedure would be rotating until the telescope crosshair hits a fringe, writing down the angle, rotating again until we hit the next fringe, and so on.
Now, I am busy with making the plates parallel. What bothers me most is that the plates themselves are not completely planar; I can clearly see that they are in fact concave (or perhaps convex - I don't remember the English word - either way it doesn't matter).
Those are the facts of the matter. Now here is are my observations and analysis:
Since the plates are, say, concave, you can never say they are actually parallel. What you can say, is that they are parallel for a specific incidence angle, in which the concavity of the plates is such that they are locally parallel.
I have three screws through which I control the alignment of the plates relatively to each other. I can clearly see in the lab, that it's possible for me to get very very thin and clear fringes for a specific angle, however when I rotate the etalon the fringes become blurry and start expanding/contracting (depending on the direction in which I rotate the etalon).
Which brings up back to the question I asked above.
Many thanks,
Chen
I'm performing an experiment whose goal is to analyze the splitting of the green Mercury line under Zeeman's effect. To do this we use a Fabry-Perot etalon, which first has to be calibrated. This means both making sure that the two reflective mirrors are parallel, and then measuring the distance between them by watching the specturm of some known elements.
My question comes down to this:
When I see, in a specific angle, clear and sharp fringers coming out of the etalon, is this an indication that the plates are parallel (at least locally, i.e around the line of sight)? Or is there another set of circumstances under which I would get the same results with the plates not parallel at all?
If you feel like it, you can continue reading more about my situation and set up.
We are watching the fringes through a telescope, and in order to measure them the etalon is mounted on a rotating disk, and we have digital means to record its rotation angle. So the procedure would be rotating until the telescope crosshair hits a fringe, writing down the angle, rotating again until we hit the next fringe, and so on.
Now, I am busy with making the plates parallel. What bothers me most is that the plates themselves are not completely planar; I can clearly see that they are in fact concave (or perhaps convex - I don't remember the English word - either way it doesn't matter).
Those are the facts of the matter. Now here is are my observations and analysis:
Since the plates are, say, concave, you can never say they are actually parallel. What you can say, is that they are parallel for a specific incidence angle, in which the concavity of the plates is such that they are locally parallel.
I have three screws through which I control the alignment of the plates relatively to each other. I can clearly see in the lab, that it's possible for me to get very very thin and clear fringes for a specific angle, however when I rotate the etalon the fringes become blurry and start expanding/contracting (depending on the direction in which I rotate the etalon).
Which brings up back to the question I asked above.
Many thanks,
Chen
