SkyView downtime October 4, 2016

SkyView was down from about 9:00am to 2:30pm today due to a problem encountered while updating the SkyView servers.  SkyView operations should now be back to normal. We apologize for the inconvenience.

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NASA SkyView vs SkyView

We often get email requesting  help downloading and using SkyView on the iPhone and other devices.  We thought we would try to clear up some apparent confusion.  These questions refer to a smart phone app that is not associated with NASA SkyView but has the same name – SkyView (or SkyView Free).  Information about this app can be found at the Apple App Store and the Terminal Eleven site.

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SkyView V3.2.1: SDSS and AKARI fixes

On Wednesday we released SkyView version 3.2.1 to fix the SDSS survey. It includes a new capability where when we are using an SIA service we can apply a regular expression transformation to the URLs being returned. We use this to transform SDSS image tile URLs from http://www.sdss3.org/… to https://www.sdss.org/…. The setting SiapUrlTransform expects two comma-separated fields which are interpreted as arguments to the Java String.replaceAll(String,String) method in modifying the input URL string.

We’ve had this fix for several days but were unable to release it. SkyView was unable to see remote HTTPS URLs due to GSFCs firewall settings. As soon as that was addressed by our system managers we made the release.

Shortly after we communicated some concerns with the publishers of the AKARI data regarding issues with their SIA service. They were using Ecliptic coordinates in places where ICRS coordinates were required. SkyView had implemented special code to address this. The AKARI team very quickly implemented fixes on their end, and this morning we removed the special handling we had been performing for AKARI. We delivered this as a modification to the v3.2.1 survey file rather than a new release to minimize the time that AKARI was off-line. SkyView in a Jar users who are using version 3.2.0 or later should get the new survey description next time they access AKARI data. Users of older versions of SkyView will need to upgrade to access AKARI data properly.

Users may have seen problems with accessing AKARI data for several hours this morning until we updated our software to recognize their fixes.

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Continued SDSS issues

While remote access to the UKIDSS server was restored shortly after our last message we are continuing to have problems with the SDSS. In addition to the problems with the metadata found in the SDSS SIA service, there has been some overloading of SDSS servers and it was recommended that we switch from http://data.sdss3.org/… urls to http://data.sdss.org urls which points to a different and more robust set of servers.

We enabled this capability and switched to the new servers in our test version 3.2.1. This failed initially. The new URLs were in turn being redirected to https://data.sdss.org. Java network connections do not follow redirects to a different scheme (i.e., from http to https) and so we weren’t getting the data. We put in some code to handle this kind of redirection and also tried simply making the initial request to the https addresses. All of this worked fine in our test environment and failed miserably when we tried to run it on the operational systems.

The current problem seems to be that the GSFC network firewall is quite restrictive and is not allowing SkyView general HTTPS access to the outside world. It does allow general outgoing HTTP. It may take a few days before we can get this switched.

In the meantime we have a dilemma. The SDSS3 links fail frequently when used from our web site, e.g., as I write this the initial SIA requests are failing due to some database error, but they do work some of the time. The SDSS links work very nicely when used in the JAR file but don’t currently work at all from the web site…

So for the nonce we are leaving SkyView at v3.2.0 but recommend that Jar users extract the jar from v3.2.1 (i.e.,
http://skyview.gsfc.nasa.gov/v3.2.1/jar/skyview.jar) where they are more likely to get a version of SkyView that addresses all of the problems we have seen.

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Problems with UKIDSS and SDSS remote surveys

Over the past couple of days we have discovered problems in accessing two of our remote survey data sets. The UKIDSS simple image access service has been non-responsive. We’re hoping it comes up again soon. You can see the red warning on the SkyView home page where we assess the aliveness of remote resources.

A user has alerted us to a more subtle and serious problem with the SDSS surveys. If they requested a large (0.5 degree) region they found that many of the pixels in their image were blank. If they requested the same image later most of those pixels were filled in. A few missing pixels might require three image requests to be filled in. It appears that the SDSS is supplying incorrect descriptions of the coverage of images in their SIA service. We have informed the SDSS of the issue but while it is being worked on we have created a version of SkyView, v3.2.1, which has a workaround for the issue. You can try it out now — and if you are using the SDSS for large images we strongly recommend that you do — and anticipate that it will be made the default version in the next few days.

The SDSS problem arises from how SkyView mosaics data from multiple images together. In the first stage an image generator defines a set of candidate images that are to be considered. For remote services, where we are using the SIA protocol to get data, all of the images returned by the SIA service are considered as candidates. We then consider each pixel in the output image to be produced and decide which of the candidates it should be sampled from. The usual approach — and the one used for the SDSS — is to pick the candidate where we will be furthest from the edge of the image.

To make this determination we need to know the sky coverage of each candidate. If we have already downloaded an image so that it exists in the cache we can and do use information in the FITS file itself. If we haven’t downloaded the image yet, we normally use the coverage information provided in the SIA metadata returned with the image URL. Typically there will be a lot of candidate images that aren’t going to be used in the mosaic and we don’t want to have to download them all.

Since the SIA metadata is wrong, when we chose the candidate image, it often turned out that that image could not be resampled at the given pixel. However the SDSS SIA metadata generally did point us to include the right images, just not the proper boundaries for each one. So after we downloaded the data we could recompute the best source image and that would work. Internally the class we use to find the image to resample a given pixel is called an ImageFinder. The default ImageFinder is the Border ImageFinder. For the SDSS we have created a special ImageFinder, LocalBorder, which iteratively reruns the image finding operation until all of the images that are used in the mosaic are downloaded before we do the image finding. Typically this requires three iterations for large images. However we still only download the images that we actually need, and only once. The image finding is generally pretty fast, so the time penalty for this kludge is modest. We only do the actual resampling with the final data.

However for this to work, the SIA information has to be at least in the correct ball park. Generally it appears that the SIA-specified center of the image is deep within the actual image. In this case
we anticipate that our workaround will at least greatly mitigate if not always cure the problem.

Note that the SDSS problem is particularly acute for those using SkyView-in-a-Jar. Since many popular regions are already available in the cache for the web site, the problem may not show up there. However jar users are building up their own caches from scratch.

The jar file that addresses these issues can be found at http://skyview.gsfc.nasa.gov/v3.2.1/jar/skyview.jar. Note that to get the correct survey description file users will need to use the setting:
xmlurlprefix=http://skyview.gsfc.nasa.gov/v3.2.1/jar/
until this version is released since the default for this setting points to the current release which currently has the older version of the SDSS survey description.

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SkyView v3.2.0: Displaying quantitative survey information

In the home page in the new v3.2.0 you may notice a new line under the Survey Documentation link:

Summary: CSV or Plot

You can click on CSV to download a table of the information on all SkyView surveys. If there are multiple ranges of epoch, the epoch given will be from the beginning of the first range till the end of the last.

If you click on Plot you can easily make plots of the information like

Plot of Resolution versus Frequency

You can just mouse around in the plot to see which survey corresponds to which point. Here the mouse happened to be near the GALEX NUV survey point. Or you can pick a different X or Y axis and see how SkyView surveys are distributed in time, energy, resolution and coverage.

These plots use the Plotly plotting package.

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SkyView v3.2.0: Quantitative metadata

Version 3.2.0 of SkyView includes a new section, Quantitative, in the survey description file. This section is intended to capture machine-friendly quantitative characteristics of SkyView surveys. Diverse sources have been used for these and notes are included giving the source when it is not in the standard reference for the survey.

Currently all surveys include a central, minimum and maximum frequency in Hertz.

A central epoch is given and one or more ranges for the epoch are available. Note that the central epoch is a kind of average of the ranges and occasionally the central epoch is actually outside of the constituent ranges. Epochs are given in Modified Julian Days.

The coverage of each survey is given as a fraction of the sky. For some of the GOODS survey datasets these can be quite small.

Both a nominal resolution and scale are given. For surveys where the resolution may be variable a typical good value is given. Units for these quantities are arcseconds.

Lastly an estimated sensitivity of the survey in Janskies is given. Often this involves significant estimation and the numbers should be treated with caution. Note that a better gauge of the relative sensitivity of two surveys may be the product of frequency and sensitivity.

These data may be obtained as a CSV file or explored as a plot using new capabilties in v3.2.0.

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SkyView V3.2.0: Survey descriptions downloaded on demand

The biggest change in v3.2.0 of SkyView is that survey description files are no longer part of the SkyView jar. In v3.1.21 we added a small file that retained the mapping between survey names and the description files they referred to. In that version we only used the file to skip reading all of the survey description files — the user only needed to read the survey descriptions for the surveys they were interested in.

In v3.2.0 we took further advantage of this file by adding the information about when the survey description files were created to it. When a user requests a survey, SkyView looks to see if the survey is available as a local file (e.g., as it would be for the web site). If not it checks to see if there is a cached version of the file and if the cached version is newer than the version at the SkyView web site. If so this version is used. Otherwise the file is downloaded from the SkyView web site, placed in the cache and then processing continues. So users will always get have the latest version of the survey to work with.

There remains one possible problem. If a survey updates tile files without changing their names, then SkyView will not be able to tell that the cached data files are out of date. Fortunately most dynamic surveys change the names of the files (e.g., using a version identifier) when they update their tile data.

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SkyView v3.2.0: Some survey data removed from jar

SkyView-in-a-Jar users will note that the new v3.2.0 SkyView jar is much smaller than the earlier versions, under 4 megabytes compared to over 15 megabytes for v3.1.21. Part of the reason is that the way we access some older surveys has been changed.

The SkyView software can look for survey data in three distinct locales: over the web using URLs, in the local file system using file names, and in the SkyView jar using resource names.

Almost all surveys use URLs. There are two special features for these. First, SkyView can notice that the URL is pointing to data on the local file system. The SkyView web site uses this to translate SkyView URLs to local file references. Second, most URLs are cached. When a user or the SkyView web site asks for a URL from, say, the SDSS, it downloads it only once. The next time that URL is requested it is retrieved from the cache. Skyview-in-a-Jar users will also cache the data from surveys stored on the SkyView machines, since for them those data are not local.

In previous versions a few survey’s data (EGRET, CompTel, nH, 1420 Mhz, 408Mhz, HEA01A2) were actually included in the jar file directly. We’ve now moved these to use URLs as above. Since these will be cached, SkyView-in-a-Jar users will only download the data once. All of these surveys are quite small and we don’t anticipate anyone will really notice the difference. The one exception is that this did allow users to test out SkyView with no network connections. This is still easy to do using the UserSurvey settings. More sophisticated
users can also provide survey description files that point to local data.

A few surveys are still provided as references to files in the local file system. This includes the Mellinger survey where our agreement is that the underlying data will not be directly visible as a URL. The other survey is the GOODSIRAC data. These data are available as URLs but it is difficult to characterize the image WCSs in a way that works with our survey description files. When specifying image URLs SkyView tries to provide enough geometry information in the description to enable the program to decide which candidate images are needed for a mosaic before downloading them. That was hard to do for this survey. As local files rather than URLs, SkyView can always use the WCS in the actual FITS file when looking at their geometry.

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SkyView v3.2.0 Preview: Summary

Next week we will be releasing version 3.2.0 of SkyView. You can play with it right now if you specify the version v3.2.0 in the SkyView URL, but it’s still being tested. There are several major changes in this release and we’ll be putting out a series of posts describing them in detail. Major changes include:
– Survey description files are now downloaded (and cached) as needed from the SkyView web site. This means that SkyView-in-a-Jar users will no longer need to update the their local copy of the JAR file to take advantage of updates to surveys.
– The SkyView jar used to contain the data for a few smaller generally fairly old surveys. To further reduce the size of the jar, these have been taken out.
– Survey description files now contain a set of fields that give simple, uniform, quantitative information on surveys (frequency, epoch, resolution, sensitivity) in a format easily used by a machine.
– There are new capabilities to extract and display these quantities as CSV downloads or in interactive plots.

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