SkyView Blog

If you’ve tried the new GALEX survey from the command line, you may have noticed some messages

Warning: Multiple occurrences of key: EPOCH

popping up. Many of the GALEX FITS files have two EPOCH keywords in their headers. Fortunately they both have the same value, so this is innocuous enough. Still the FITS reader gets a bit nervous when it sees this.

It’s a little more embarrassing since the EPOCH keyword is deprecated in FITS. Epoch usually means the time of the observation, but it was explicitly defined to be the epoch of the coordinate system in the original FITS definition. To relieve the confusion this engendered EPOCH was deprecated. We’re supposed to use EQUINOX for the epoch of the coordinate system and DATE-OBS for the epoch of the observatoin.

So having even one EPOCH keyword is bad form much less two!

SkyView now includes data from GALEX. This is a major new survey for us. GALEX provides high resolution images in the ultraviolet for about a quarter of the sky. This fills a substantial hole in the resolution/regime coverage for SkyView. The GALEX data are accessed through the Web from MAST. Currently the fourth release of GALEX data (GR4) is being ingested and we will update the survey description to include new data as it becomes available.

By default GALEX uses a different image finder than other surveys. When it needs to choose which input image to sample for a given output pixel, it looks only at images whose center is under some maximum radius from the position of the output pixel and then chooses the one of those with the longest exposure time. The maximum radius is currently set to 0.58 degrees. These settings can be overridden if you use the SkyView-in-a-Jar locally, but are fixed for the Web interface.

A three color image of M81 with Red=DSS2R, Green=GALEX Near, Blue=GALEX Far

In the past week we’ve begun the process of adding the GALEX near and far UV data into SkyView. Assuming we don’t run into unexpected problems it should be available sometime next week. One issue that did come up is that GALEX images are circular not rectangular. Normally when we look for which image to sample at a given pixel we use the candiate source image that we would sample furthest from the edge of the image. That’s the Border image finder. For GALEX a more appropriate choice is to take image whose center is nearest the pixel. There’s a new Radius image finder for that. Since the exposure and characteristics of the observation don’t vary very much within the observed circle, a still better approach would be to find the image where the pixel is within some fiducial radius of the center, but which has the longest exposure. That way we get the best image over the largest field of view. That’s a combination of the Radius and Exposure image finders in the current release. By design it’s very easy to add in an image finder with exactly these characteristics and that’s what we’ll be doing.

You may wonder why this didn’t come up in the much older SkyView ROSAT PSPC surveys — they also have circular images. If we were to build images from the PSPC the same way we do from GALEX, by dynamically combining observations in response to a user request, that’s exactly what would have happened. However SkyView ran through all of the PSPC data and created a set of rectangular tiles that added the exposure from all observations that overlapped the tile. It’s these pre-coadded tiles that are used for the PSPC surveys. An advantage of this approach is that in regions where more than one observation was made, data from multiple tiles is added together. We’ll want to make that possible for GALEX data someday too.