Colours and Google Sky

I've said many times before that I wanted a version of Google Maps/Sky/Universe that allowed you to change the wavelength that you were looking at. Earlier in the week Rob over at Orbiting Frog hacked the Google Sky time slider by attaching a different wavelength layer to each month. Hence, just like that, the time slider became a wavelength slider. Rob still hasn't worked out how to change the label on the time slider so any advice/hints are welcome in the comments to his post. Rob has done a great job including wavelengths from optical (H alpha - a nice red) through infra-red, sub-mm and well into the radio (including the 'famous' Haslem et al. 408 MHz map).

Not to be outdone, Google themselves have finally launched a Google Maps version of the sky (spotted via Dave P); those without the desktop software Google Earth can now experience the sky too. The web-based Google Sky also has a multi-wavelength view which has a little more functionality than Rob's version but without the all-sky spectral coverage. In web-based GoogleSky you can choose between optical images (using the DSS/SDSS), microwave (which looks like WMAP although there is nothing saying that) and infra-red (which looks like IRAS/COBE but again no specific credit). Each of those options has its own slider that allows you to fade between them. That is quite neat but I have a couple of criticisms of web-based Google Sky.

My first issue is the poles problem. This relates to the way Google have converted a sphere into squares in order to efficiently serve their maps to us. When they created Google Maps, they took the decision to not care about the regions around the Earth's poles. This meant that a square gridding worked well for all the populated parts of the planet but caused huge distortions near the poles. For practical purposes this makes a lot of sense on the Earth because people don't usually want directions to Dome C or the Scott Base. The problem comes when you use the same gridding for the sky as it means you don't get to see the area around the celestial poles. We shall not see Polaris in web-based Google Sky. The desktop version of Google Sky doesn't have the same problem so Rob can show the whole sky.

The second issue I have is really over semantics and is a little esoteric. The 'microwave sky' seems to show a WMAP image of the cosmic microwave background radiation (CMB) but it doesn't look quite right to me as the plane of our galaxy seems obvious but rather narrow and the Large Magellanic Cloud is visible. I probably need to explain what is odd here. The WMAP images you usually see show either the sky as it is - warts and all - or try to show only the ripples in the CMB. This second type of image, which usually looks like lots of random red/green/blue splodges over the sky has the main CMB temperature, the effect of our movement in the universe, our galaxy and other bright objects removed as much as possible. This version on web-based Google Sky seems to still have a small amount of the galaxy and the Large Magellanic Cloud still visible but our galaxy looks far too narrow to me. It may be that they have applied some kind of squashing of the colours to make the microwave Milky Way look a similar thickness to optical one. Whatever they have done to the image, it isn't the microwave sky as you would see it if you had huge microwave eyes.

Despite its limitations, the web-based Google Sky does have the advantage that it is easy to use for other web-based applications. I expect mash-ups to start appearing over the next few days. If you find any, or make one yourself, please post them in the comments below.

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Posted in astro blog by Stuart on Friday 14th Mar 2008 (10:17 GMT) | Permalink
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