SkyView access to UKIDSS survey data

SkyView users may now generate images using the public UKIRT Infrared Deep Sky Survey (UKIDSS) data. While UKIDSS data is available in only a fraction (14%) of the sky, it goes much deeper and has higher resolution than the 2MASS data in the same wavebands. E.g., here is a comparison of 3-color images of the Galactic Center region (1000 pixels with a 0.1 degree field of view) in 2MASS and UKIDSS generated by straightforward SkyView requests (using a Clip resampler is the only non-default).

Galactic Center: 2MASS

A 3-color image of the Galactic Center in 2MASS KHJ colors

Galactic Center: UKIDSS

A 3-color image of the Galactic Center in UKIDSS KHJ

It’s easy to see how much deeper the UKIDSS data goes. SkyView uses the latest public release for the UKIDSS data. This is the DR8 release for most of the sky, and the DR7 for the UKIDSS Galactic Plane Survey. Many thanks to the UKIDSS team including Mike Read and Mark Holliman in setting up access for us. UKIDSS coverage is different in the different bands while the survey is ongoing and the Y band coverage will always be limited.

UKIDSS data is served by the Wide Field Astronomy Unit (WFAU) at the Institute for Astronomy in the University of Edinburgh. SkyView accesses the data using the Virtual Observation Simple Image Access protocol and caches images as users request them. As the cache of images at SkyView grows the time to generate images will diminish.

Internally UKIDSS data required that SkyView add support for the ZPN, zenithal polynomial, projection which is what UKIDSS uses. Another article will discuss use of the ZPN projection.

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SkyView support for ZPN projection

As part of integrating UKIDSS data into SkyView, we have added support for the Zenithal Polynomial, ZPN, projection. In this projection the angular distance between a point on the sky and the reference position is transformed to a distance from the center of the projection plane using an arbitrary polynomial where the polynomial may be of any order up to 20. For UKIDSS a third order polynomial is used (but the second order term is nil). The azimuth of the point with respect to the reference position is preserved. Users may be more familiar with other azimuthal projections like the TAN and SIN projections where the projected distance is a trigonometric (tangent or sine respectively) rather than polynomial function of the distance on the sphere.

The ZPN projection is now fully supported for both input — the projection used in survey source images — and output — the projection users request. The ZPN projection is the first projection supported in SkyView where additional parameters beyond the reference coordinates are needed to define the projection. For input images, these parameters are looked for as the values for PROJPn keywords in the FITS header. On output users may specify _PROJPn settings (note the leading underscore).

One aspect of the implementation of the ZPN projection may be of note. The inverse projection is generally not soluble algebraically and is done using an iterative procedure, so that transformations from the ZPN plane to the sphere are less efficient than from the sphere to the plane. In a typical request, SkyView will transform from the user projection plane, into a standard reference system on the sphere and then into the projection plane of the source data. So the ZPN projection will be a bit more efficient as an input projection than for output.

We have not added fields in our query web forms that allow users to specify the projection parameters, but users can set these in GET or POST requests to the web interface.

In general if you would like to see support for a projection (or coordinate system) in SkyView which is not already there, please let us know.

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WMAP data accessible using SkyView Jar

We’ve made a small change to the way the WMAP data is described to allow it to be accessible either through the standard Web interface or to remote users using our SkyView Jar.

Access isn’t quite the same as for most surveys. Normally when you access a SkyView survey using the jar, any raw files you want are downloaded and cached on your machine. That allows subsequent queries to run much faster. The WMAP data is treated rather like the DSS datasets. You can access it over the web using the SkyView jar, but no caching will be done.

The reason is similar to what happens in the DSS. Our caching framework assumes standardish FITS images, i.e., a 2 or 3 dimensional pixel array in the primary header of a FITS file. Each DSS image is broken up into hundreds of image tiles. The WMAP data uses HEALPix images where the pixels are stored as a table rather than an image. The caching code can’t handle these image types. If this is something you’d like to see, let us know and we can look at supporting caching for these surveys.

One other survey is not available at all through the jar. The Mellinger surveys are not public and so we do not distribute raw data for these at all so the jar cannot access the raw Mellinger data when run remotely.

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SkyView includes WMAP Nine Year Data

We have updated SkyView to use the new nine year maps for WMAP. These represent the full mission dataset for the Wilkinson Microwave Anisotropy Probe. The structure is unchanged from the earlier 7 year data. We provide a total of six surveys. These include five frequency bands and the WMAPILC survey which uses multiwavelength data to remove any contribution from the Galaxy.

WMAP data internally uses HEALPix data formats and so currently WMAP may only be accessed through our web interface. However a remote user can download the raw data from the LAMBDA where users can download both the frequency maps and the ILC image and can then use those images as user-defined surveys with the SkyView jar.

This is expected to be the last major change to the WMAP mission data.

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UKIDSS surveys to be added to SkyView

Early in January 2013 we shall be adding access to the UKIRT Deep Infrared Sky Surveys (UKIDSS). Their web site describes these surveys as the successor to 2MASS. Ultimately they will cover about 7,000 square degrees, or a bit more than one sixth of the sky. UKIDSS goes about 3 magnitudes deeper than 2MASS (a factor of ~15 in brightness) and has substantially higher resolution.

SkyView will use the public UKIDSS data which currently are release DR7.

The UKIDSS data that we will include to includes four bands, Y, J, H, and K which range in wavelength from about 0.97 to 2.37 microns, or in frequency from 310 to 126 THz. Only limited Y band coverage is available. Survey resolution is 1″ and pixels are 0.4″ on a side.

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SDSS update

The FITS headers generated by the SDSS surveys will now reflect that these are using DR9 data rather than DR8 results. The SDSS site switched to using DR9 a couple of weeks ago. We’re told the primary difference is improved astrometry in the DR9 data.

Note that future SDSS releases are not expected to have significantly greater sky coverage.

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SDSS DR9 release

It appears that the SDSS servers have begun serving SDSS DR9 data. SkyView accesses the SDSS through the Virtual Observatory Simple Image Access protocol (SIAP). In this protocol a user first requests a list of the images available in a region and a set of available URLs is returned.

As discussed in the previous article, the initial call for the list of URLs was working, but it appears that the URLs that were being returned have been a format they no longer supported. This may have been the case since September. The SDSS servers are now returning valid URLs but even though we are still querying using the URL for the DR8 SIAP service, the URLs being returned are for DR9 files. Thus images produced in the surveys labeled SDSSx in the SkyView interfaces are now DR9 although the documentation and internal FITS contents suggests that they are DR8. These will be updated early next week.

Access to the SDSSx surveys will likely be slower than normal until the SDSS cache is rebuilt using DR9 files.

Access to the SDSSDR7x surveys has been unaffected by these problems. It uses a different set of servers than the more recent releases.

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WISE and SDSS problems

Recently we’ve had some problems with access to the WISE and SDSS surveys (but not the SDSS DR7). We access these remotely using a standard protocol where we first find a list of the images in the region of interest and then download the ones we want. For WISE the initial queries were failing which meant that all WISE requests failed. We notified IPAC of the problem and they quickly rectified it.

For the SDSS the initial query for the list of images works fine, but if we try to download any image file the request fails. This means that some SDSS SkyView requests will succeed and some may succeed partially. SkyView caches the results from remote queries, so if anyone had requested a particular image previously, we don’t try to download it again, we just use our cached copy.

Update: (12/13/2012)
Both the SDSS and WISE data have been restored. It appears that the SDSS changed their output format several months ago so that SDSS queries that requested new regions have been failing for some while — but since we had rather large amount of data in the cache many SDSS queries succeeded.

The SDSS surveys are now pointing to DR9 data and since we do not have these cached SDSS queries will generally slow down as we rebuild our local cache.

Only the surveys labeled SDSSx are affected. The SDSS DR7 release which uses different servers and returns data in a somewhat different format, seems to be working without any problems. However it has somewhat less total sky coverage than the DR8 release.

The SDSS team are aware of the problems are are working to fix them. We apologize for any inconvenience to our users.

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Can you help me find a star?

We often receive email messages asking for help finding a star. In this post we will give instructions for displaying an image of a star in SkyView, as well as provide further tips for finding the star on a star chart and determining the star’s constellation and one final note about purchasing a star.

Instructions for generating an image of a star:

1. To generate an image in SkyView you’ll need a set of coordinates – the star’s right ascension and  declination.  These are the stellar equivalent of the latitude and longitude on the Earth. These may be in various formats but the RA usually is given as three numbers – hours, minutes and seconds.  The first is between 0 and 23, the second between 0 and 59 and the third between 0 and 60.  The third number, the seconds, may  contain a fractional part.  The Declination is similar, but the the first number can range between -90 and 90 and is called degrees not hours.

For SkyView these coordinates may need to be converted to a format containing just hours, minutes, seconds or decimal numbers.  They can be separated by a comma or the sign for the declination (more information)

An example of a set of coordinates:    RA: 6h 39m 15.70s DEC: 2 16′ 22.7″

The SkyView format of this set of coordinates would be:

6 39 15.70, 2 16 22.17   or   6 39 15.70 +2 16 22.17

Coordinates in this format can be used in the SkyView Query form.

Enter the coordinates in the Coordinates or Source input box.


2. Next a SkyView Survey needs to be selected.  The SkyView Query form provides many surveys in many wavelengths.  Typically someone trying to find a star will want to see how it appears in the night sky so an optical survey is best.  The best survey to start with is the Digital Sky Survey (DSS) which is at the top of our DSS survey category.


3. Once at least one  survey is selected and coordinates are entered the user can click the Submit button and an image of the sky centered on the coordinates will be displayed.

Below is a partial  image of the SkyView Query Form showing the coordinates entered in the Coordinates or Source box and the DSS survey selected.  Below the form is an image of the star that displays after the form is submitted.

 SkyView Query Form

SkyView Query Form

DSS image at 6 39 15.70 +2 16 22.17

SkyView Image centered at 6 39 15.70 +2 16 22.17

The SkyView Query Form has many other options for manipulating the SkyView image to change image size, image colors, resampling, projections, coordinate systems or to add contours and other overlays.  These are just a few of SkyView‘s features.

Related Tips:

There are many websites that provide sky charts for a particular location and information about constellations.  Here are two of these sites  we have found useful:

http://djm.cc/constellation.html: A simple form that displays the name of the constellation for specified coordinates

http://vizier.u-strasbg.fr/vizier/VizieR/constellations.htx#q: Displays boundaries for constellations.  Enter coordinates below the list of constellations.

http://www.fourmilab.ch/yoursky: Get images of entire sky as viewed from a given location at a specified time and date.  Their Virtual Telescope shows constellations and other objects at specified coordinates.

http://www.heavens-above.com:  Get sky charts and satellite schedules as seen from specified locations. This site also gives the time in various time zones/formats (local time, GMT, UTC)

A Final Note:

Many of these requests for help finding a star come from people who have purchased or received a gift of a star from a registry service. No star registry services are recognized by any political or  scientific authorities.  More information can be found at http://www.iau.org/public/buying_star_names/

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VLSS Redux available through SkyView

The VLSS sky survey has been updated to use data from the VLSS redux analysis described in Lane et al 2012. There is a slight increase in sky coverage and resolution and a substantial improvement in the noise due to better handling of bias due to the cleaning algorithm, atmospheric effects, radio interference and bright sources.

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