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UW ASTR 480 - Study Notes

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IRAF Tutorial for Exercise III Image Processing: Bias Frames and FlatfieldingLearning GoalsFiles UsedIntroductionFlatfields: Dome Flats vs. Sky FlatsDark Frames versus Bias FramesImage Arithmetic in IRAF - The steps needed for the exercise answer sheet start here. You will be directed as to when to answer questions on the answer sheet.After this practice, answer questions 1, 2, and 3 of the exercise on the answer sheet.SubarraysAnswer question 4. You will need to imstat again using a different section of the chip.Flatfielding in IRAFPATH 1. – Step by Step FlatfieldingUse the worksheet for the following sections; move between the tutorial and worksheet as you work, starting with question 5.PATH 2. – CCDPROCThe CCDPROC task is a very useful way to reduce a large batch of data in an efficient way, but caution must be used.Do not delete these images since they will be used in later exercises!IRAF Tutorial for Exercise IIIImage Processing: Bias Frames and FlatfieldingLearning Goals - List the various sources that contribute to the amount of charge present in each pixel after an exposure.- State which sources of unwanted charge are additive and which are multiplicative.- Demonstrate proficiency in the IRAF tasks used in this tutorial and exercise.o imcopy, imarith, unlearn, imstato disp, implot, colbias, sections, darksubo CCDRED, package, type- Summarize the overall steps needed to process a raw CCD image. Files Used/astro/classes/Astro_480/exercises/exercise2/m92.tar.gzIntroductionAlthough CCDs have revolutionized astronomy, they are not perfect. Each individual pixel in a CCD array can be thought of as a device that linearly converts light to charge, q (electrons), so that for an integration time t, where Ai and Bi are the "gain" and "offset" parameters that characterize the pixel i, Ii(l) is the incident light flux and QEi(l) is the pixel’s efficiency, which is wavelength dependent, integrated over some passband that is usuallydefined by an optical filter. There is also some thermally generated "dark current," Dit, (dark count per second times the number of seconds) that accumulates during this interval. You should think of a CCD as an array of pixels, each with their own individual gain and offset values.The values of Ai and Bi vary from pixel to pixel across the CCD array, and we need to correct for this. As Craig Mackay, one of the grand practitioners of CCD astronomy, once said: "The only uniform CCD is a dead CCD." In addition, the electronics used to read out the detector also introduces a nonzero "bias" value that is added afterthe signal is converted to an analog voltage, so that the measured voltage associated with pixel i is This bias structure can be quite complex. Sometimes the bias level changes steadily during the course of the detector being read out. In other cases the bias structure changes along a row as the pixels are read out, and thereis a constant bias "vector" that needs to be subtracted from each row. These effects arise because of non-idealities in the readout electronics, due to loading effects and temporal and thermal instabilities in the 1/15/19 1electronics. (You should have some sympathy for the challenge of maintaining microvolt stability over the environmental variations that occur in a telescope dome.) If we illuminated the entire CCD with a uniform ("flat") intensity distribution, we could correct for the variationsin pixel sensitivity, for the pixel-to-pixel differences in offset values, as well as for the dark current and the bias structure. In addition, the telescope and instrument that lie between the detector and the light source can also introduce both spatial ("vignetting") and wavelength-dependent variations in overall system sensitivity, which must also be compensated for. The process of manipulating the raw CCD data file to correct for these effects is called "flatfielding." This involves compensating for both additive and multiplicative non-idealities. Flatfielding is something of an art. You might think that the task of overcoming sensitivity variations would be simple: just take a frame of a uniformly illuminated field, use the image to determine the pixel-to-pixel variations, and divide all data frames by the appropriately normalized "sensitivity flat." This is certainly the basic approach we take, but the practical challenges include:- How do you generate a genuinely uniform flux incident upon the system? - Is the spectral energy distribution of the calibration source the same as the science targets?- How do you disentangle the effects of the detector, the instrument, the optics, the filters, the telescope and the atmosphere?- How can you best compensate for changes in sensitivity that depend on telescope orientation?Flatfields: Dome Flats vs. Sky FlatsThere are two common ways that astronomers attempt to generate uniform illumination for characterizing CCD instruments: 1) dome flats and 2) sky flats (twilight or blank night sky). Regardless of the technique used, it is essential that you obtain flats for every filter used. One set of bias frames will work for all filters, though, as that part is wavelength independent.Dome flats use a reflective screen painted on the inside of the telescope dome and illuminated appropriately. The observer points the telescope at it for calibration frames. Dome flats are often taken in the afternoon before an observing run. It is a good time to characterize and understand the detector and instrument for an unfamiliar setup. The dome flats can also be reduced before the sun sets, allowing for near real-time preliminary reductions of frames as they are being acquired. This has, in our experience, often proven very useful in maximizing the useof telescope time. There are some major disadvantages to dome flats, though. For one thing, there is often a lot of stray and scattered light that simply isn’t present during night-time observing conditions. Also, the spectral energy distribution of the light from the flat-field screen is seldom much like the light from astronomical objects,so the integral across the passbands is not a good match to that of the data frames. It is still worth obtaining dome flats if possible, since it provides a consistency check and, as noted above, they are often good enough for mountaintop reductions.Sky flats use the brightness of the sky itself for obtaining flatfield calibration data. There are even two kinds of sky flats: twilight flats and blank sky


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