Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/~fruchter/ERO/A2218/README Дата изменения: Tue Feb 8 04:26:21 2000 Дата индексирования: Sat Dec 22 06:24:23 2007 Кодировка: Поисковые слова: http www.badastronomy.com phpbb index.php

README: SM3A ERO observation of Abell 2218

This directory contains the drizzled images created for the SM3A ERO
obervation of Abell 2218. The images are:

Image File Drizzle Weight File Filter Exposure Time (s)
F814W_mos.fits F814W_mosw.fits F814W 12000
F606W_mos.fits F606W_mosw.fits F606W 10000
F450W_mos.fits F450W_mosw.fits F450W 12000

All individual exposures were 1000s long, with two exposures
taken per orbit.

The mosaic images contained in this directory were created
using onlychips 2 and 3 of the WFPC2 images. The purpose of
the EROs was to rapidly demonstrate the success of the HST
servicing mission; as these two chips contained most of the
objects of visual and scientific interest in these datasets,
only they were included in the mosaic.

The observations were dithered using a combination of the
standard two and four-point WFPC2 dither patterns on top
of larger throws, for a total displacement of the telescope
by more than 5" in both the X and Y axes of the chips.

The dark created for the NGC 2392 observations was also used to
process this data; however, in addition, a further bad pixel mask
was created by combining the F450W data in place without removing
the dithers so that only features intrinsic to chip remained.
Cosmic rays and other image defects were located and
masked using the techniqe described in the Fruchter and Hook
drizzling paper (see link in parent directory).
The pixel mask containing both chip defects and cosmic rays
was applied to weight files which reflect the sensitivity of the chips
in each filter as a function of position. The final
resulting weights reflect the relative sensitivity across the
images fairly accurately, but cannot be used to predict the absolute
variance of the final image.

The drizzled images have a pixel scale one-half that of the
original WFPC2 images, or about 0."05. The full set of Drizzle
parameters used is recorded in the image headers. The user of these
images should remember that because the noise in drizzled images is
correlated, the true image noise over an area of N (>>1) pixels is
greater than root(N) times the image r.m.s. In particular, with the
drizzle parameters used here, scale=0.5 and pixfrac=0.6, the final
the true image noise is about 1.66 times greater than that predicted by
root(N).

Note for users wishing to work on individual images:

The dark calibration image used to process this data has been placed
in the archive. It is now the "recommended" darkfile for these datasets,
and will be used in calibration-on-the-fly processing by the archive
of these datasets.