A word about our galaxy images
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Profile David Thilker
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Message 1164 - Posted: 22 Mar 2013, 10:00:04 UTC

As volunteers on this project you may have noticed a wide variety in the telescope source and 'quality' of the galaxy images being processed. We think you deserve to know more about this.

Our project is rooted in the Pan-STARRS1 mission, a panoramic survey telescope now operating at the top of the Hawaiian island of Maui (on Mt. Haleakala). This innovative telescope uses the world largest digital camera (1.4 billion pixels) to repeatedly take photos of the sky through five filters ranging in color over the visible to near-infrared part of the spectrum. The tremendous speed with which PS1 can cover the sky enables it to find objects which move or change in brightness (e.g. you may have heard of Comet Pan-STARRS...) by looking repeatedly to the same areas. It also allows us to combine all these multiple exposures into one 'deeper', more sensitive view. The later approach of effectively staring is the one taken by our extragalactic project, though PS1 scientists have found time-variable things like cosmologically distant supernovae and super-massive black hole flares. The final, summed up PS1 images will have substantially more sensitivity and better resolution than even the world-famous Sloan Digital Sky Survey (SDSS), over a sky area much wider than imaged by SDSS.

However, as I said above, the PS1 telescope is still operating. The mission is now 75% complete, and we hope to continue observing until January 2014. The fact that the digital camera is built as a checkerboard mosaic of >3800 CCD separate detectors means that single (even multiple) exposures of the sky leave gaps on the sky. These are deliberately filled in over time as each part of the sky is observed, up to a maximum of 12 times per filter. The PS1 data now being processed (in our latest BOINC run) originates from a period when the PS1 survey was only about 60% finished, so gaps do remain. If you look at some of the multi-color galaxy data you've worked on (using the main page 'Images you have processed' link), it is not hard to find odd looking, frequently stripe-like regions of missing data in one filter or more. Rest assured that these will be long gone when the last photons are collected by PS1. As an example of what the eventual images will look like, check out our Astronomy Photo of the Day from earlier this year: http://apod.nasa.gov/apod/ap121012.html

So, you may wonder why we are processing the interim data from PS1. The answer is simple. We need to prepare our distributed-computing analysis system now, in advance. This means both the BOINC related and database management aspects (heroically headed by Kevin V. and team), plus the imaging and astrophysical interpretation components (to which the entire international team contributes). An illustration is understanding the degree to which our model parameters describing local galactic environmental conditions like regional stellar mass and star formation rate can be constrained with the available filters. This is one reason we also processed a large set of galaxies observed by SDSS. That project included a filter modestly bluer than the PS1 spectral window. It turns out this filter allows extra leverage in particularly active star-forming regions, dominated by hot young stellar populations. PS1 has the distinct edge in the red though, helping to quantify the spatial arrangement of old stars and peer through obscuring interstellar dust.

Our eventual goal is to combine image data reaching even further across the spectrum, adding ultraviolet light and infrared radiation detected by the space-based GALEX and WISE telescopes, respectively. We should be able to do this truly panchromatic analysis for about 30,000 galaxies large enough to be shown in adequate detail by the lower-resolution UV and IR data. Soon we will be conducting a test run of this GALEX-PS1/SDSS-WISE type for a small set of galaxies. As the PS1 survey is finished, we will ask you to crunch through our truly panchromatic data and process an entire sky full of a few-hundred thousand smaller, yet still resolved galaxies with visible light data only. The upcoming months will also witness some experiments with improvements such as: enhanced data preparation (spatially adaptive smoothing), hopefully GPU processing, plus consistency checks against a fully independent, second spectral energy distribution modeling code.

As ever, thanks for your help with our project! We could not do this without you! Even with the continually improving quality of PS1 imaging, we hope you enjoy looking at the targets you process. You may truly be one of the first people to ever inspect a digital color image for some of these galaxies (particularly if they occupy the southern sky).

David T.

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Message 1165 - Posted: 22 Mar 2013, 19:31:12 UTC - in response to Message 1164.

Thanks for sharing! The info is just great! Have a better understanding and appreciation of this project. Looking forward to the future with bigger and better results!

Bill

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Message 1166 - Posted: 22 Mar 2013, 22:50:07 UTC

A fascinating post. Thank you for taking the time to explain it in such a detailed manner!

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