Here is the best image obtained so far:
Image of laser fiber focus to focus using the foreoptics ellipsoid alone
The detector here has 5.4µm pixels, so the FWHM across the widest part of the image (yellow cut line) is about 190µm...not sure how this would scale to the detector, maybe someone can comment. This was done with an f/35-ish beam size incident on the ellipsoid.
To get this image we intentionally decentered the ellipsoid by 10mm in the dimension perpendicular to the plane formed by the centers of the foreoptics mirrors and the laser fiber. Dana calls this the Y-axis. With this intentional de-center, running the camera through best focus looks like this:
Decentered focus run (click to see animation)
Focus run w/beam nominally centered on ellipsoid (click to see animation)
There are only three frames is the 'centered' .gif because we did not use as fine a resolution for focus checks, but the plate scale is the same for the two animations, and we get to a smaller spot size when we decenter. Is this an indication of a clocking error? Inquiring minds wish to know.
We can also look at the image as we translate the mirror, though it's harder to make a movie out of since the image moves around on the camera:
Image variation with 'Y-axis' translation of ellipsoid (click to see animation)
Translating in the perpendicular direction (in the plane of the back surface of the ellipsoid, if that makes any sense, we don't see much sensitivity to decenter:
Image variation with 'X-axis' translation of ellipsoid (click to see animation)
We'll need to cogitate over what this may mean, if anything. The foreoptics is now at the position which gives the image at the top of this post. Dana has talked about shifting the clocking and repeating the decentering tests, but it might be well to compare these images to Zemax predictions to guide the next moves.
6 comments:
I think 190 microns ends up being 4.75 pixels on the IR array.
Being off by 10 mm is more than I'd like to think we should have been off at the IRTF. What do you all think?
It seems very improbable that you were off as much as 10mm at IRTF.
Nevertheless, I'd like to see where this takes us - the next step in my mind would be to try clocking the ellipse.
I agree that we were likely better aligned than 10mm. I was wondering whether there are any known aberrations in the ellipsoid that we would expect to be canceled by the hyperboloid. If so we'll need to keep those in mind as we look at the ellipsoid alone.
Whoops! My earlier comment regarding the spot size in pixels is inaccurate since I missed the detail you were looking at the ellipsoid alone. My bad.
In the interest of exposing my ignorance of optics to the widest possible audience: Isn't the simple translation of the ellipsoid in the direction perpendicular to the plane of incidence analogous to imaging a point source at a different location in the telescope's focal plane? In other words, if we move the ellipsoid 'down' by 10mm, isn't the 'point source' created by the laser fiber/iris arrangement just a different star in the field of view? The central ray of the light cone is still parallel to the table....
If that's right, does anyone know the physical size of the IRTF focal plane? The 10mm shift in the position of the 'star' moved the image well off Dana's camera, which is about the same size as the hyperboloid.
Sorry if this question amounts to a drop in the SNR of the discussion....just want to get a better idea what we're doing...
The central ray isn't exactly parallel to the table, but it is close.
At the intermediate focus, the plate scale for the 3000 mm IRTF, which is f/35, is 2.06e5/35/3000 ~2"/mm. This says something at an angular distance on the sky 10 arcseconds away from the center is 5mm away.
Since the intermediate focus is about 1.25 meters away from the ellipsoid, the central ray from our offset star hits the ellipsoid, 10/2.06e5 * 1250 mm or 60 microns different from its position at the intermediate focus. Each spot on the ellipse is about 35 mm in diameter
Post a Comment