SkyView Blog

One of astronomy’s primary links to the past are the constellations. We’ve just added a set of overlay files that you can use with SkyView’s standalone package to draw constellation boundaries over an image. You can download individual constellation files, or download a tar file containing all of the constellation files. A constellation overlay can be drawn over all an image using settings like

java -jar skyview.jar position=0.,0. drawfile=all.bound survey=408mhz size=100 pixels=800

The image this produces is

The constellation boundaries used are derived from the official IAU constellation boundaries. However there are a couple of caveats. The constellation boundaries are defined in terms of corners in 1875 coordinates, i.e., only the 1875 declination or 1875 right ascension varies between two consecutive corners. SkyView transforms these corner positions to the user’s requested projection and then draws straight lines between the points. Unless the user has requested B1875 coordinates in a Cartesian projection, it’s unlikely that SkyView will match the precise boundaries. However if you are looking at an image at a scale large enough to see constellations, then discrepancies are likely smaller than the width of the borders.

Another problem can arise in projections that can tile (e.g., the Cartesian projection). E.g., if we are making an image centered at 0,0 then constellations spanning RA=180 degrees will have lines that are supposed to leave the left edge of the image and return on the right. Howver SkyView’s overlay drawer simply joins the two corners with a line the runs through the middle. Using non-tiling projections (like Tan on Sin) or only including constellations within 90 degrees of the center of the image will usually take care of this.

Due to a file system configuration problem some 2MASS images have been unavailable since December 17, 2008.  We are working to fix the situation and should have the 2MASS data back on line soon.  We apologize for the inconvenience.

A new version of SkyView has been installed to accommodate the issues that off center GALEX GR4 images presented. GALEX now uses a special GalexExposure Image finder. This uses knowledge of the GALEX survey to find the center of the actual field of view for every observation. Specifically it looks for the AVASPRA and AVASPDEC keywords in the header and uses the pixel location corresponding to these coordinates as the center of the field of view. The center locations in the survey description file have also been updated to use the FOV centers rather than the nominal image centers.

The underlying GALEX observations used in SkyView are 3840×3840 pixel images where only  a circle with a radius of about 2900 pixels is actually exposed.  In earlier releases the exposed region seemed to be centered with the image square but that is no longer always the case for GR4.  The observation center can now be hundreds of pixels offset from the nominal image center.

SkyView tries to mask out the non-observed reqion when it combines GALEX observations, but when the mask is off-center this masking may go awry.  We’re downloading information from MAST giving the actual centers for the observations rather than the nominal image centers and we will update the survey description file for MAST appropriately within the next few days to accommodate this, but in the meantime GR4 images may show unexpected blank patches.

CLI users of SkyView can play with the image finder setting to try to mitigate this, but this is unlikely to help in most cases.

The 2MASS survey data are available again via SkyView.

The SDSS server connection problems that were reported earlier this week have been resolved. However again due to connection problems with the remote system that provides the data, 2MASS survey data are unavailable via SkyView. We have been in touch with personnel at IPAC and hope to have the connection issues resolved soon. We apologize for the inconvenience.

Sloan Digitized Sky Survey images are not available at this time due to problems connecting to the remote server that hosts the SDSS data. We are investigating the problem and apologize for any inconvenience.<p>

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The operational version of SkyView now supports the full GALEX GR4 release. This includes a total of about 170,000 UV images of the sky. Total coverage is about 25,000 square degrees or well over half the sky.

As discussed in earlier posts, GALEX uses a slightly different method of image selection than most other surveys. For a given output pixel, we find the input image which is within a cutoff radius and has the largest exposure. Thus where there are both survey pointed GALEX observations, the pointed data should be used but the output should transition to survey coverage outside the region of pointed coverage. It’s usually easy to see these transitions since the survey data is significantly noisier (due to the shorter exposures).

We’re cleaning out the GALEX cache in SkyView, so you may see a slight decrease in performance as SkyView extracts files from the MAST archive that were already in our cache.

One consequence of this release is that the SkyView jar has gained a little weight — it’s now about 17 MB. Almost all of that is survey descriptions and survey data. Ultimately we may need to separate the code and survey documents into separate files.

The Java 1.5 compatible jar has also been updated to the same version. Previously this had still been pointing to SkyView release 2.4.