You've probably seen the press release going around about the "most detailed map of the Milky Way". Strictly speaking it's the most sensitive, highest-resolution all-sky HI survey of the Milky Way, but I think we can forgive the simplified headline in this instance.
Here's a little comparison between the previous Leiden/Argentine/Bonn survey, the new HI4PI survey, and the GALFA-HI survey. It's a bit crude - HI4PI* is definitely better than LAB, but I probably haven't done it justice. More details would probably show up if I chose better settings. Still, GALFA-HI is clearly still far superior in terms of resolution.
* I can't help but read this not as "4 pi", as in 4 pi steradians as it's intended, but "for P. I." as in "principal investigator"...
I was hoping to use the new data to remake the Hydrogen Sky project - last time I relied heavily on GALFA-HI, because the low resolution of LAB doesn't give such nice results. The problem with GALFA is that it's an Arecibo survey, so it has fantastic resolution and sensitivity but limited coverage. Which makes it rather tricky to find photographs of the correct orientation to overlay the data with any kind of accuracy.
Although HI4PI is better than LAB, I'm not sure it's worth the effort. The LAB data is only 250 MB, HI4PI is 32 GB (high spatial and velocity resolution and 32 bit data files instead of 16). I'm not even sure my computer (even in work) could load a 32 GB file into memory and the improvement wouldn't be that dramatic. Not that the survey isn't much better for science, just not so much for visualisation, unfortunately.
... which software are you using to process your data? If it's dependent upon loading everything into memory before processing anything, I'd be willing to volunteer some time and analysis to looking at the problem.
ReplyDelete... a bit of digging and I found this:
ReplyDeletehttp://www.aanda.org/articles/aa/abs/2016/10/aa29178-16/aa29178-16.html
aanda.org - HI4PI: a full-sky H i survey based on EBHIS and GASS | Astronomy & Astrophysics (A&A)
Rhys Taylor Hmmm. Any hope of getting some time on a proper supercomputer? Let me cast this about amongst people I know.
ReplyDeleteContrast between two spots in an image is the same no matter what number of pixels devoted to it. Am I wrong about that Rhys Taylor? Aren't all imaging systems limited to the same contrast range?
ReplyDeleteBob Calder I think that would only be true if you had identical receivers performing the same observing setup (but with different integration times), the sources were both above the noise level but below the saturation limit, and the images were created using the same algorithm. In that case you'd measure different absolute fluxes but the relative contrast would be the same. However, what should be more resilient is the inferred intrinsic luminosity of the source after calibration - as long as it's a point source to both receivers, that should be identical (if it's detected and not saturated) regardless of the receiver size, integration time, observing and imaging setup.
ReplyDeleteBut, in general, this isn't the case. Telescopes are different sizes with different angular resolutions, so a point source to one telescope may be resolved to another, so the flux is spread out over more pixels. Or velocity channels for that matter. Instruments can have different dynamic range sensitivities. Images can be reconstructed in ways which favour point sources or extended emission, from the same raw data (number of pixels in the image doesn't usually relate to number of receiver pixels). Choice of observing setup can strongly affect the end result, especially (as in the OP) where there's very bright emission filling the entire beam and the data must be calibrated accordingly or things just don't work at all. In a word... no. :)
Thanks Rhys Taylor. I can imagine Instruments might have different dynamic range sensitivities, but I have no idea how difficult it is to get the engineering done.
ReplyDelete