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DATA Processing

DATA Processing. Data volumes. STSI generate images of entire sky 1500 each having 14000x14000 16 bit pixels New cameras with mosaic of large CCDs generate images 2048x2048 or larger Data rates exceed 100 Mbytes every 5 minutes for many years. Compression for transmission and archival.

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DATA Processing

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  1. DATA Processing

  2. Data volumes • STSI generate images of entire sky • 1500 each having 14000x14000 16 bit pixels • New cameras with mosaic of large CCDs generate images 2048x2048 or larger • Data rates exceed 100 Mbytes every 5 minutes for many years

  3. Compression for transmission and archival • Astronomical images consist largely of empty sky • Compression can reduce data for effective archiving and transmission • Important consideration for large scale digital sky surveys

  4. Compression Issues • Images are subject to stringent quantitative analysis • Astrometric: positional measurement • Photometric: brightness measurement • Do not discard useful information when compressing the images. • Lossless compression • But discard the noise • It is essentially random, and so cannot be completely removed from the image

  5. Preprocessing: Calibration • Calibration of an astronomical CCD deep-sky image consists of removing the bias and thermal contribution (DARK FRAME) and dividing the resultant image by the FLAT-FIELD in order to standardize the response of each image pixel. Calibrated = (Raw-Bias-Thermal)/Flat=(Raw-Dark)/Flat

  6. Calibration DARK FRAME • It records the BIAS noise and THERMAL noise for a specific CCD temperature and integration time DARK FRAME = BIAS FRAME + THERMAL FRAME • To take a dark frame, cap the telescope and integrate using the same time used for acquiring the raw image.

  7. Calibration BIAS FRAME • It contains the amplifier zero-point offset, the random readout noise from the amplifier and the noise from camera electronics. FLAT-FIELD • is a photosite-by-photosite map of a CCD's sensitivity to light. • it is an image of a uniform object such as twilight sky or a sheet of opal glass attached to the inside of the observatory dome. Chip sensitivity, dust all appear as variations in the sensitivity of the CCD itself: division by FLAT-FIELD will remove these defects.

  8. Aberration • in optical systems (lenses, prisms, mirrors) generally leads to blurring of the image. • It occurs when light from one point of an object after transmission through the system arrives in different points.

  9. Restoration • is used to increase the definition of a CCD image. • Optical aberrations, seeing, and tracking efficiency affect the images obtained with a CCD detector reducing its sharpness. • The blurred image of a star, planet or galaxy can be significantly improved by deconvolving its Point Spread Function (PSF) in such a way that the end result is a sharper and more detailed image.

  10. Convolution • The integration of the product of two functions in time. • Convolution in the time domain is equivalent to multiplication in the frequency domain.

  11. PSF • the kernel in the superposition integral (convolution product) that expresses the effect of a linear optical system in the formation of an image of an object. • A point spread function in astronomy takes a point of light, and returns an image of how the point appears to our instrument. • Since many astronomical objects are points, the same point spread function should apply to all point sources in a given image. • To sum up, a point spread function is the observed non-pointlike shape of a real point source.

  12. PSF Can be also stellar, elliptical or irregular Depends on what we are subtracting

  13. Flux • If flux is the energy / s through a surface (e.g. the pupil of your eye), then the total energy / s from the star must be the flux through a sphere of radius r, where r is the distance to the star. • This total energy / s, or power, is called the luminosity (L).  Since such a sphere has an area of 4pr2, we have the relationship:   F  =   L /4pr2

  14. Restoration: interactive techniques The best are interactive techniques: • The PSF of the image has to be determined before using any image restoration algorithm. • This usually consists in isolating a non saturated star in the image to be treated and using this information as its PSF. • The software works in an iterative way calculating several approximations of the deconvolved image. • Best examples • Maximum Entropy Deconvolution (MEM) • Lucy-Richardson Deconvolution (LR) • Van-Cittert Deconvolution (VC).

  15. Direct algorithms • can also be used with good results, such as the Wiener algorithm. • There are however several drawbacks associated with the application of these algorithms • deconvolved images are usually noisy and they can not be used for photometry.

  16. Image subtraction: Example The stellar shape of PSF, multiplied by the flux of each star is subtracted out of the image, resulting in a relatively clean image.

  17. Subtraction: Example • There are noticeable residuals around several stars, indicating a sub-optimal fit. • What kind of error does this residual represent? • High residuals are generally found on very bright stars, where the CCD is saturated, or non-stars such as galaxies, where the PSF is not fit correctly.

  18. Community uses Standard DAOPHOT package for photometric extraction DAOPHOT is a stellar photometry package designed by Peter Stetson to deal with crowded fields. The package performs various tasks including finding objects, obtaining the point-spread function, and profile-fitting photometry.

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