ls13 - UCF AST 3722C summary of lecture for tuesday april 8...

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UCF AST 3722C summary of lecture for tuesday april 8 ---- We're in chapter 18. -- detectors. continue with CCDs are in 18.15. last time we started talking about CCDs. - QE, function of wavelengths -- handouts #26 and #27. -- More Subtleties About CCDs: Readout time: can take 15-20 sec or as much as a couple minutes for some big old CCDs. This is time that you can't do any exposing -- just reading out all those pixels. So you'd like to minimize this amount of time. Non-linearity/Saturation: A CCD doesn't have this QE for any number of photons. If too many photons hit, then you've got too many atoms with lost electrons already -- and if another photon hits it, then you're not getting another e- out of that atom. tought luck. so at some point the CCD pixel will have a non-linear response. generally this happens when you get up to approx. 100,000-500,000 photoelectrons in a single pixel. When observing you don't want to build up that many photoelectrons. so you generally observe with sufficiently small expo times so that you don't saturate. CTE: The bucket-brigade process is not 100% perfect. It might be as high as 99.9995% efficient, or even 99.99995%. this seems really good, but think about how many transfers you've got to do in a single row of say 1000 pixels. In pixel #1, you've got to do 1000 transfers. So how many electrons wind up in the transfer register. OK, start with 1 million electrons say in pixel #1. transferring to pixel #2 leaves you with just 10^6 * 0.999995 = 999,995 to pixel #3 leaves you with 10^6*0.999995*0.999995 = 999,990. Keep going. .. how many e- do you wind up with? at the end? 1e6 * (0.999995)^100 = 995,012 e-. so you've lost 4987 elections -- that's 0.5% of them! CCDs generally have really high CTE nowadays, so that this problem is not too harmful. sensitivity to wavelength: First note that a CCD has a broad sensitivity, as I mentioned last time -- to get specific wavelengths, you need a filter -- which is usally a piece of glass or some efficient material that will only transmit certain wavelengths but be opaque to others. why are CCDs sensitive to visible light? what limits the wavelength capability of the CCD? turns out that silicon doesn't interact with photons with less energy (infrared). this has to do with the lattice structure
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of silicon and the covalent bonds between Si atoms. the IR photons aren't energetic enough to break bonds, so the photons wind up simply being transmitted through the lattice. You need 1.1 eV to liberate an electron -- that corresponds to light with wavelength 1.1 microns. So CCDs can't be used as infrared cameras beyond this 1.1 micron wavelength. Temperature of the CCD: you want to operate the CCD as cold as possible. In pro observatories, this means using LN2 -- liquid nitrogen. LN2 is at 77 K, and a common operating temp for the CCD is about 175 K. why do you want the CCD cold? the
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This note was uploaded on 11/09/2009 for the course AST 4700 taught by Professor Fernandez during the Spring '09 term at University of Central Florida.

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ls13 - UCF AST 3722C summary of lecture for tuesday april 8...

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