Optimal Compression of Floating-point Astronomical Images Without Significant Loss of Information

We describe a compression method for floating-point astronomical images that gives compression ratios of 6 -- 10 while still preserving the scientifically important information in the image. The pixel values are first preprocessed by quantizing them into scaled integer intensity levels, which removes some of the uncompressible noise in the image. The integers are then losslessly compressed using the fast and efficient Rice algorithm and stored in a portable FITS format file. Quantizing an image more coarsely gives greater image compression, but it also increases the noise and degrades the precision of the photometric and astrometric measurements in the quantized image. Dithering the pixel values during the quantization process can greatly improve the precision of measurements in the images. This is especially important if the analysis algorithm relies on the mode or the median which would be similarly quantized if the pixel values are not dithered. We perform a series of experiments on both synthetic and real astronomical CCD images to quantitatively demonstrate that the magnitudes and positions of stars in the quantized images can be measured with the predicted amount of precision. In order to encourage wider use of these image compression methods, we have made available a pair of general-purpose image compression programs, called fpack and funpack, which can be used to compress any FITS format image.Comment: Accepted PAS

Lossless Astronomical Image Compression and the Effects of Noise

We compare a variety of lossless image compression methods on a large sample of astronomical images and show how the compression ratios and speeds of the algorithms are affected by the amount of noise in the images. In the ideal case where the image pixel values have a random Gaussian distribution, the equivalent number of uncompressible noise bits per pixel is given by Nbits =log2(sigma * sqrt(12)) and the lossless compression ratio is given by R = BITPIX / Nbits + K where BITPIX is the bit length of the pixel values and K is a measure of the efficiency of the compression algorithm. We perform image compression tests on a large sample of integer astronomical CCD images using the GZIP compression program and using a newer FITS tiled-image compression method that currently supports 4 compression algorithms: Rice, Hcompress, PLIO, and GZIP. Overall, the Rice compression algorithm strikes the best balance of compression and computational efficiency; it is 2--3 times faster and produces about 1.4 times greater compression than GZIP. The Rice algorithm produces 75%--90% (depending on the amount of noise in the image) as much compression as an ideal algorithm with K = 0. The image compression and uncompression utility programs used in this study (called fpack and funpack) are publicly available from the HEASARC web site. A simple command-line interface may be used to compress or uncompress any FITS image file.Comment: 20 pages, 9 figures, to be published in PAS

Results of the 1980 NASA/JPL balloon flight solar cell calibration program

Thirty-eight modules were carried to an altitude of about 36 kilometers. In addition to the cell calibration program, an experiment to evaluate the calibration error versus altitude was performed. The calibrated cells can be used as reference standards in simulator testing of cells and arrays

Results of the 1979 NASA/JPL balloon flight solar cell calibration program

Calibration of solar cells to be used as reference standards in simulator testing of cells and arrays was accomplished. Thirty-eight modules were carried to an altitude of about 36 kilometers during the solar cell calibration balloon flight

Results of the 1981 NASA/JPL balloon flight solar cell calibration program

The calibration of the direct conversion of solar energy through use of solar cells at high altitudes by balloon flight is reported. Twenty seven modules were carried to an altitude of 35.4 kilometers. Silicon cells are stable for long periods of time and can be used as standards. It is demonstrated that the cell mounting cavity may be either black or white with equal validity in setting solar simulators. The calibrated cells can be used as reference standards in simulator testing of cells and arrays

A randomized comparison of branded sodium stibogluconate and generic sodium stibogluconate for the treatment of visceral leishmaniasis under field conditions in Sudan.

OBJECTIVE: To compare the outcome of treatment of Sudanese kala-azar patients treated under field conditions with either branded sodium stibogluconate (SSG) (Pentostam GlaxoWellcome) or generic SSG (Albert David Ltd, Calcutta, supplied by International Dispensary Association, Amsterdam). METHOD: Randomised comparison. 271 patients were treated with Pentostam and 245 with generic SSG. RESULTS: No statistically significant differences in cure rate or mortality were detected between Pentostam and generic SSG. No differences in side-effects between the two drugs were noted. The initial cure rate at the time of discharge was 93.7 and 97.6%, respectively; the death rate during treatment 5.9 and 2.4%. Six months follow up was achieved in 88.5% of the discharged patients. Two patients had died in the Pentostam group and two had died in the generic SSG group, giving a final death rate of 7.5 and 3.7%. The number of relapses in the Pentostam and generic SSG groups were 3 and 1, respectively. The final cure rates, calculated at 6 months after discharge, were 91.3% and 95.9%. CONCLUSION: No difference was observed in the performance of generic SSG compared to Pentostam for the treatment of visceral leishmaniasis in Sudan. Generic SSG can be routinely and safely used for the treatment of kala-azar. Generic SSG costs only 1/14 of the price of Pentostam. The use of generic SSG may make treatment of kala-azar affordable for national governments in Africa

Managing an archive of weather satellite images

The author's experiences of building and maintaining an archive of hourly weather satellite pictures at NOAO are described. This archive has proven very popular with visiting and staff astronomers - especially on windy days and cloudy nights. Given access to a source of such pictures, a suite of simple shell and IRAF CL scripts can provide a great deal of robust functionality with little effort. These pictures and associated data products such as surface analysis (radar) maps and National Weather Service forecasts are updated hourly at anonymous ftp sites on the Internet, although your local Atsmospheric Sciences Department may prove to be a more reliable source. The raw image formats are unfamiliar to most astronomers, but reading them into IRAF is straightforward. Techniques for performing this format conversion at the host computer level are described which may prove useful for other chores. Pointers are given to sources of data and of software, including a package of example tools. These tools include shell and Perl scripts for downloading pictures, maps, and forecasts, as well as IRAF scripts and host level programs for translating the images into IRAF and GIF formats and for slicing & dicing the resulting images. Hints for displaying the images and for making hardcopies are given