Archive for February, 2011

SkyView Fermi LAT Surveys

Monday, February 28th, 2011

We have just released two gamma-ray all-sky surveys based upon the Fermi LAT weekly data. Fermi images go much deeper and have somewhat higher resolution than the old EGRET surveys. Fermi is also sensitive to gamma-rays at significantly higher energies, up to 300 GeV.

The two surveys cover the bands 0.1-5.4 GeV and 5.4-300 GeV. There are many more counts in the lower energy survey and many more sources are apparent there. The surveys are generated from the weekly exposure maps and photon lists supplied by the Fermi LAT team. The exposures are summed together to create an overall exposure map and the photon lists are combined and resampled in 1/8 degree pixels in a Cartesian projection. The combined exposure at the center of each of these pixels is then sampled and weighted by the size of the pixel to get the normalized exposure. The ratio of the counts to the weighted is used to get the intensity of the pixel. Currently we are only providing the intensity maps but we can also include the counts and exposure maps if there is interest. We also hope to provide a 3-D Fermi survey with perhaps 10 energy bands.

Fermi’s new hard band extends SkyView‘s frequency coverage a full decade above the previous upper limit.

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SkyView updates GALEX survey

Wednesday, February 16th, 2011

Galaxy Evolution Explorer (GALEX) surveys have been updated to use the GALEX GR6 release. As mentioned in a previous post GR6 improvements include more sky coverage and a bit better calibration. Image generation for GALEX may be slow for a period of time as the data file caches get rebuilt. More GALEX mission information can be found at the Space Telescope Science Institute GALEX page.

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Clip resampling of intensive quantities in SkyView

Friday, February 11th, 2011

The SkyView web page now has two different options for using the Clip resampler. The clip resampler computes the overlaps between the user image’s output pixels and the original survey pixels and adds up the contributions from each overlap. The default behavior sets the output pixel to the sum of all of the inputs. This is the only option that used to be available in the Web interface. Using this option if you could integrate over exactly the same region of the sky in both the input and output images, you’d get exactly the same number: flux is exactly conserved.

This makes sense when what is being measured is an extensive quantity like counts or flux. If the output pixels are bigger than the input pixels, then the values in those pixels should typically be bigger too.

Often however, the original data are an intensive measurement: the size of the pixels should not directly affect the values we place in them. E.g., the map might be of temperature or intensity. The temperature at some point should not be proportional to the size of the pixel. If you select the new Clip (Intensive) sampler, the output pixels will the averages over the overlapped pixels in the input. So the output values will not depend directly upon the size of the pixels.

If you are using SkyView-in-a-Jar, this option has always been available with the combination of settings
sampler=Clip ClipIntensive
and we’re now making the choice available on the web as well.

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New Fermi and GALEX surveys soon in SkyView

Wednesday, February 9th, 2011

Over the next week or two we’ll be releasing a new and an updated survey set. The Fermi all-sky data will give us a major new image of the gamma-ray sky. Fermi has higher sensitivity and resolution than the EGRET all-sky map — which was one of SkyView‘s original datasets. Once we get Fermi on-board we anticipate updating this survey every few months until we get to the end of the mission, gradually increasing the sensitivity of the survey. If there is interest we can also provide Fermi data as a time resolved image, e.g., with two month resolution.

We’ll also be updating the GALEX surveys to use the GALEX GR6 release. GR6 has more sky coverage and a bit better calibration than the GR4 data we’ve been serving for the past couple of years. We retrieve GALEX data from STScI caching each image locally the first time it is used. We’ll need to flush our caches when we do the upgrade, so getting GALEX images may slow down a little for a while as the caches get rebuilt.

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WMAP 7 Year Data Available in SkyView

Wednesday, February 2nd, 2011

The WMAP survey has been updated to use the 7 year integrated datasets from the HEASARC/Lambda archive. Previously we had been using the 5 year data. While there are small changes in the pixel values the underlying organization of the surveys is unchanged. All six WMAP surveys (5 colors and the galaxy subtracted image) have been updated.

If you would like to use the older 5 year datasets within SkyView please let us know.

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