SB375 image of gas cell (test.science.10092.fits)
In this image I've inverted the grayscale so that more flux shows as white, less flux shows as black. The position of the image on the array is affected by the (randomly pointed) alignment mirror and isn't relevant to spectral work, but the relative centering of the gas cell on the s/d/f cassette aperture is.
From here we did a lo-med focus run, and settled on 21.5 T for lo-med best focus:
SB375 lo-med image of C2H2 Q-branch (test.science.10098.fits)
FWHM for the column plot shown is about 6 pixels, close to the diffraction-limited size. If we zoom in to look at the individual images of the pinhole we see that they are approximately round...about as good as we've seen.
Zoomed image of C2H2 Q-branch (test.science.10098.fits)
At his point we went on to do a focus run on the echelon side. We picked 25T for the echelon hyperboloid (narrow slit):
SB375 hi-med image of C2H2 Q-branch (test.science.10108.fits)
FWHM looks like about 20 pixels here and the signal looks smaller, though it would take some work to make a useful comparison. The log book says 'narrow slit', but not 'narrowest', though the fits header indicate the 108µm slit for this file. This is not as sharp an image as we have seen in the lab, where we got to ~12 pixels FWHM. This might be equivalent to 13-14 pixels on the smaller pixels of the SB375, but probably not as many as 20. Maybe pressure broadening?? The lab book isn't clear on the condition of the gas cell. We settled on a best focus sum of 46.5T, echelon + echelle.
Next day (12 January), we ran along the 46.5T diagonal looking for optimum focus, imaging the Q-branch with the slit-wheel pinhole. Here is an image at 23.5T echelle, 23T echelon:
SB375 hi-med image of C2H2 Q-branch, 23.5/23T (B_11_01_12/b001.fits)
And here is a somewhat sharper image, taken at 33.5/14T (sum is 47.5 turns, curiously):
SB375 hi-med image of C2H2 Q-branch, 33.5/14T (B_11_01_12/b005.fits)
In this last image, the spots look slightly de-focussed along the spatial dimension, but are pretty sharp spectrally. If we look at the order marked with the yellow line, we see a spectral FWHM of about seven pixels (see inset plot). The 23.5/23T image just above it looks more like about 15 pixels FWHM spectrally. Surprisingly there isn't a definite note in the book where the mirrors were set for our first night on the telescope.
The image above was acquired at about noon on January 12 in the prep room. Next notes in the log book refer to more detector linear-range testing in the prep room, but there are some files saved starting at 1:47am on January 13 with much sharper slit-gas cell images:
SB375 hi-med image of C2H2 Q-branch, (11JAN13/c2h2science10065.fits)
We get a spectral-dimension FWHM of about 6 pixels here along the yellow cut line (see inset). This shows that the focus inside the instrument was pretty good at least spectrally when we went on the telescope on the morning of January 13 for our first engineering night. We also see the '20-row' noise in this first-minus-fourth-scan difference image.
4 comments:
I have trouble believing the gas cell in hi-med mode was correct. By eye, that looks awful. We wouldn't have accepted that as the best focus.
I wonder if imaging the gas cell can be used for lining up the slit/decker/filter wheel. We should have a good mechanism for this. Maybe we want some sort of detent? I don't think Hall sensor will be accurate enough. Or maybe there is an optical way to align the open positions? Something to think about.
I agree about the hi-med slit/decker image, and I think maybe we had some pressure broadening....there is a note in the book alluding to sucking down the gas cell. The last images (pinhole rather than slit/decker) look a bit sharper, but it doesn't indicate where we were set for the first night on the telescope.
We left it at the final position in the prep room, I remember. I think it was decided that we'd reached "good enough" and needed to worry about other things.
I agree that the best focus went off our diagonal, but that could be we just weren't really on the right diagonal or that we don't understand the best spectral images too well. I distinctly remember, though, that while I was doing the turning and you were looking at the screen, you mentioned the intermediate position looked better than the final one. When we went back to it, everyone agreed.
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