Friday, June 13, 2008

SolarC, OFIS, and more Apogee

Today we all woke up early and got going on SolarC. The weather cooperated for most of the day so the REU's got to calibrate, test and all that good learning stuff. The OFIS instrument (the spectropolarimeter on SolarC) was working ok and we got to characterize the chip a bit.

OFIS data on the sun... Each row is a spectrum from a fiber at the primary image. Each spectrum would correspond to some point on the solar disk. I think the wavelengths are someting like 1040-1080nm (infrared). The polarimetry happens when two liquid crystals up near the image in the telescope modulate the incoming polarization state of the light. Basically, you take 4 exposures and do some differencing to get the polarized spectra.

A 'dark' frame - an exposure taken with no light so that all you see is the structure and defects on the chip. You use dark frames to remove these ccd properties so you have a properly calibrated image.

I spent the afternoon over at HiVIS making the wavelength calibration mosaic. I took a bunch of exposures with a Thorium-Argon lamp (think neon sign tubes). The spectrum of Th-Ar is well known so I now know what colors fall where on my chip.

There are some funny things that happen as you move the ccd around - ghosts, vignetting and small misalignments. Got to deal with those.... - below is one ThAr minus another. Ideally they'd be blank.
The bad thing about this new CCD is that it's not quite as efficient. The old Pixis had a QE of 85% at H-alpha (650nm). If you throw 100 photons at the pixis, you count 85 (lose 15). The Apogee is more like 50%. Bummer.

This image is for Jesse: QE curves are just funky. My pixis is the red curve - IR optimized. We're going to be buying the UV one for about 30k next week.... For OFIS.


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