FITS Header Issue

In some cases SkyView has been including newline characters in FITS headers. This is a violation of the FITS standards and apparently confuses some tools about subsequent header records. We’ve put in a fix for the problem this morning (10/25) so this should be resolved. It looks like it mostly affected surveys where we had put in links to other other surveys (e.g., the ROSAT PSPC surveys).

Thanks to Todd Hunter for bringing this to our attention. If you see any problems with the system you can use our E-mail (skyview at to let us know about it.

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SkyView system downtime scheduled for October 27-28

A significant downtime is  planned for the weekend of October 27-28 to upgrade computer facilities.  HEASARC/SkyView services may be down for a substantial fraction of the weekend.

The downtime will start at 12:00pm EST Saturday October 27 and will last for about 24 hours. We apologize for the inconvenience.

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SkyView system downtime has been postponed

The system downtime previously scheduled for this weekend has been cancelled and tentatively rescheduled for no earlier than October 13-14, 2012.

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SkyView system downtime September 21-23

A significant downtime is currently planned for the weekend of September 21-23 to upgrade computer facilities.  HEASARC/SkyView services may be down for a substantial fraction of the weekend.  We apologize for the inconvenience.

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Features in the Gallery: The GEICO Gecko?

One recent gallery entry shows — to me at least — a remarkable resemblance to the GEICO gecko.

or you can see it a lot bigger in its gallery entry.

This is a DSS blue image, and we don’t see anything at the same point in the DSS red.
So what is this If we zoom out a little we see this as an intense black region surrounded by a couple of bright spots. The bright spots don’t really look like stars — they have rings. My guess is that a couple of drops of some chemical got dropped on the plate and dissolved the plate emulsion. That left a just the glass in the central spot where we see the very black region. The DSS plates are scans of photgraphic negatives — so what we see as black is actually the most transparent region. As the drops dried out they became more circular and deposited the dissolved emulsion back on the glass plate, so the bright ‘stars’ are just the ‘sky’ that should have been seen in the black regions.

Or maybe someone’s selling flying saucer insurance for a savings of 15% or more.

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WISE survey interruption

No access to WISE survey data was possible for much of today due to problems at IPAC. These have been resolved and WISE data should again be available through SkyView.


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Approximate Projections and Raw Cutouts

One feature that we’ve added to the underlying SkyView code recently is the ability to create approximate projections.  Several surveys in SkyView, the DSSn and NEAT include distortions in the projection plane that mean that the data cannot be accurately represented as a simple projection.   However the standard FITS  WCS of an image allows for a general affine transformation so even though we cannot do make an exact WCS we can use the affine transformation to build an approximate WCS for the image that is correct to first order at the center of the image.  It’s relatively straightforward to compute the Jacobian of the distortion at the center of the user requested image and we simply need to add this to the existing WCS.  For a  DSSx the resulting WCS can be quite accurate over an image with a size of a thousand or more pixels.  For NEAT data the fit is almost always good at least to several hundred pixels.  Here I’m using very good to mean that the error is less than an arcsecond.

One use for this is to allow SkyView to return the raw pixels from a survey.  Of course this limits the ability to mosaic, but users sometimes want to get the data exactly as taken.  One concern about providing raw cutouts was that we couldn’t describe their geometry appropriately for some of the most commonly used surveys.  We’ll we looking at providing a ‘Raw’ projection in SkyView over the next few months.

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Problems accessing WISE data

For the past couple of weeks SkyView has not been able access data from the WISE archive. SkyView queries for  WISE images in regions already in our cache worked fine, but users who requested a new region got back a blank image.    This behavior seems to have started around June 27.

The problem arose when WISE inadvertently changed the URL’s they published the data under (using the VO Simple Image Access Protocol) to a format that was really only intended for internal use.  In particular it used a non-standard port number, rather than the standard HTTP port 80. The firewall that our web servers are behind does not permit access to unregistered, non-standard ports, so our servers were unable to download new data.

The WISE team fixed the problem immediately upon being notified of it and all should be back to normal.  We apologize for the time it took to discover this problem.  The normal behavior requests in cached regions concealed the problem from our regular checks.

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SkyView cache problems

A data disk used to cache survey data filled up over the weekend resulting in the inability to generate images for many surveys. We have cleared space on the disk and SkyView is now back to normal.  We apologize for the inconvenience.

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Features in the Gallery: Klingon Forcefields?


Since there are more than 100 surveys in SkyView, it shouldn’t be surprising that we are not experts in the characteristics of each one. A blog reader noted that there is a very unusual feature in the WISE data near around 8 55 39, 19 36 44 and asked what it was. Off hand I couldn’t say. Here’s a one degree image from the WISE 3.4 micron data from the Gallery — though I admit that I put it there.

Feature in WISE 3.4 data
Unusual Feature is WISE 3.4 Survey (1 degree centered at 8 55 39, 19 36 44)

It’s really quite an extraordinary object — but what is it, where does this grid pattern come from? It may be helpful to show how we can try to track this down.  [The vertical boundary towards the right of the image is a failure of the algorithm that tries to match the backgrounds of adjacent images.  That often has problems when there are very bright objects in the field but otherwise has nothing to do with the anomaly.]

The first step is to try to see if it’s real, i.e., could it just be some kind of glitch. Fortunately WISE has four different bands, so we can look at each of those. And indeed each of those shows something similar overall though the details vary quite a bit. It suggests that there really was something there.

If there was a real object there, then the only way it could cause this kind of weirdness was if was so bright that it overwhelmed the  optics and electronics of the detector. Our first step is to check the bright star catalog. We can do that in SkyView itself, or at the HEASARC (using the BSC5P catalog). There don’t seem to be any objects in the field so the anomaly isn’t something like Sirius. By the bye that indicates that the apparently very bright star we see in the top left of the image is actually fainter than about 7th magnitude or so.

If it’s a bright natural object we’re running out options but we still have moving objects: planets, asteroids and such. The elongation of the object suggests a moving object. Only a few planets are likely to be bright enough though. If it is a planet then it should be close to the ecliptic plane.  We convert the RA,Dec input to ecliptic coordinates (You can use the coordinate converter at the HEASARC for example.. We find that the ecliptic latitude is only about 2 degrees, very reasonable for a planet. Now we need to go to find detailed ephemerides for the bright planets. Let’s try the Horizons service at JPL. Infrared satellites don’t look close to the Sun usually, so we’ll skip Mercury and Venus. We really only need to look at Mars, Jupiter and Saturn.

The survey metadata indicate that the WISE data were taken in 2010, so we generate an ephemeris for Mars for that year. With a bit of luck we find a hit immediately. On April 27, 2010, Mars was at exactly the location we see the anomaly. At that time is was about apparent magnitude 0.5, so it was likely a thousand or more times brighter than the bright star we see in the image. That star is already showing very serious diffraction spikes, so its not surprising that something vastly brighter causes real problems.

Where does the grid come from? It seems like the WISE data is a sum of many images taken in quick succession. We get diffraction spikes on each one. As Mars moves the spikes form the pattern we see. Or maybe it’s something else entirely. With such a bright object very weird things may be happening in the electronics. But at least we know what we’re seeing.

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