Wednesday, July 26, 2006

Nerd Post - Point Spread Functions

Ok, as the title suggests, this will be a nerd post. Since my blog is named with an optics term, I figure I should probably write some science in here too.

Definition of Terms

  • Point Spread Function: Measuring the spot created in the focal plane when you focus a beam of collimated light down to a point.

  • Collimated: All travelling in the same direction. Laser light is collimated light.
  • Grism: a combination of a grating and a diffraction grating. Splits up light into its component colors (aka wavelengths).

  • Full Width Half Maximum (FWHM): A measurement of how steep a peak on a graph is. Imagine a mountain. You take the peak elevation, divide it by two, and go find two towns on opposite sides of the mountains at this elevation. Then, you measure the horizontal distance between the two towns. The smaller the distance, the steeper the slope.

  • CCD: Charge-Coupled Device. In laymans' terms, a digital camera, just more expensive. The one I'm using has a cooler on it which can cool the chip down to -20 C. It's also $5,000.

Doug and I got the optics bench pretty well aligned and focused. Since our optics aren't achromatic, we have two focii for our two laser beams (one green, one red). We've got our camera lens on a linear adjustable stage so we can repeatably focus the camera on the two laser spots.

Anyway, once we focused the system, we inserted the grism that Doug's been working on. It's still got some plastic on it from the time when he accidentally melted some zip-ties in 180 C phosphoric acid. After two further rinses with the hot phosphoric acid, we've managed to get most of the plastic off, but there's still a little bit. Anyway, we were able to measure the FWHM of the PSF of the grism, and found it to be approximately 2 pixels, which is no worse than the FWHM of a flat mirror. This is pretty good, implying that our grism is not any worse than a flat mirror, which is good.

Tomorrow, I get to take it all apart and put a faster lens in so we can get more orders on the CCD chip. Then we get to measure the efficiency of the grating, i.e. how much light is reflected into the different orders versus how much light we shine onto the surface of the grating.

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