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Hubble Legacy Archive

Image of the Month for June 2011

ACS Spectral GRISM Extraction: Galaxy Cluster Abell 2218

Interactive Display for 10159_01 ACS/WFC: F814W and G800L

Highlighted HLA Features:

Advanced Camera for Surveys (ACS) GRISM data are slitless spectra acquired with the ACS grism mode. Rather than obtaining one spectrum at a time, this observing mode makes it possible to obtain hundreds of specta in a single observation. The Space Telescope - European Coordinating Facility (ST-ECF) has created data products for the HLA by extracting and calibrating the spectra of individual objects in the ACS and Near Infrared Camera and Multi-Object Spectrometer (NICMOS) grism observations.

This display can be reproduced in the HLA by using the following steps or clicking on the image above.

  1. Type into your browser. Click on the "Enter Site here" button.
  2. Type "Abell 2218" in the search box and then press the "return/enter" key.
  3. Click the "Footprints" tab to view where HST observations of Abell 2218 have been taken for all the instruments.

  4. Abell 2218 footprint
  5. Scroll down the page to find the inventory at the bottom. Under the "Detector" column enter "*ACS/WFC*" (i.e., the ACS / Wide Field Camera) and under the "Level" column enter "4" (i.e., a color image). Press the "return/enter" key.
  6. Click on the "Images" tab. Locate the image "10159_01" (first image), which is the Abell 2218 color image which uses the F814W (red = continuum image) and the G800L (blue = spectra) filters. Click on "Interactive Display". Zoom out by clicking the box with the minus sign in the upper left three times to make the image look like the image displayed at the beginning of this Image of the Month .
  7. Go back to the "Footprints" page and scroll down to the inventory again. Remove the "*ACS/WFC*" and the "4" from the boxes under the heading. Enter "G800L" under the "Spectral_Elt" column and press the "return/enter" key. Scroll back up to view the "Footprints" display to show the locations of the spectral extractions (the "X's").

  8. Abell 2218 footprint for G800L observations
  9. Click the "Images" tab and examine the previews from each of the spectral extractions. These show the continuum image, the 2-dimensional spectral extraction and box used for the extraction, a plot of the spectrum, and AB-magnitude estimates of the source. The example shown below is the 9th extraction.

  10. Image Preview of Abell 2218 Grism extractions
Additional Information:
To access the ST-ECF website directly, and learn more details about how the extractions where made, click on the "Grism Spectra (ST-ECF)" tab.