BITPIX = -64), which is so far beyond the normal depth of astronomical images that most astronomical software, including CASA, could not display it. Fortunately
kvis, from the karma package, could, so the first step was to view it in
kvisand export it as a
miriadimage, which downsampled it to
BITPIX = -32. At the time CASA had no task for adding arbitrary headers, so I used
miriadto specify the position, epoch, and brightness unit:
puthd in=inputthrukarmass.mir/epoch value=2000.0
puthd in=inputthrukarmass.mir/bunit value=Jy/pixel
puthd in=inputthrukarmass.mir/crval2 value=-23.0,dms(declination)
puthd in=inputthrukarmass.mir/crval1 value=18.0,hms(RA)
puthd in=inputthrukarmass.mir/cdelt1 value=0.00311,arcsec
puthd in=inputthrukarmass.mir/cdelt2 value=0.00311,arcsec
puthd in=inputthrukarmass.mir/ra value=18.0,hms
puthd in=inputthrukarmass.mir/dec value=-23.0,dms
puthd in=inputthrukarmass.mir/ctype1 value=RA---SIN
puthd in=inputthrukarmass.mir/ctype2 value=DEC--SIN
fits in=inputthrukarmass.mir op=xyout out=inputthrukarmass.fits
inputthrukarmass.fitsinto CASA with idlfitstocasaim.py, which also added frequency and Stokes axes. Wolf and D'Angelo produced a 900 GHz image from their simulation, which maximizes both the dust brightness for ALMA's frequency range, and the sharpness of ALMA's point spread function. However, ALMA's Tsys is also expected to go up with frequency, the effect of phase noise worsens, and there is always a tradeoff between surface brightness sensitivity and beam sharpness. Taking those effects into account led us to pick 672 GHz as the optimum observing frequency. This meant that two aspects of the input image had to be changed:
ia.open('input672GHz_50pc.image') ia.imagecalc(outfile='input672GHz_100pc.im', pixels='0.25 * input672GHz_50pc.image') ia.close()
Top left: Protoplanetary disk simulation provided by Sebastian Wolf, as "observed" at 672 GHz by ALMA's Y1 configuration for 8h, but without any noise or phase errors. The disk mass is set to that of the Butterfly Star in Taurus, with an embedded planet of 1Mjup around a 0.5M\u2299 star (orbital radius: 5AU). The assumed distance is 50 pc. The image of the star appears slightly off center because it was added later. The Y1 configuration is the most compact Y configuration, with a 0.025" x 0.020" beam (maximum baseline = 8km, Briggs weighting).
Top right: A same simulated observation, with only thermal noise (Tsys = 540K).
Bottom left: The same simulated observation (incl. thermal noise), with phase corruption from a 100m thick screen with 0.5mm PWV exhibiting Kolmogorov turbulence. (pwvtodelay = 300.0 # fs / mm)
Bottom right: The same simulated observation, also with 0.5 mm PWV, but with residual phase errors (using a specification found in ALMA memo 521) after correction from fast switching and WVR. (pwvtocorrecteddelay = 1.4142 * 0.02 * pwvtodelay + 0.01 / 2.9978e-4, for now see the memo and noisify_alma.py for more details.)
Each image is displayed with the same spatial and intensity scales, and is linked to its FITS file.
The observation simulations were performed by casapy's "almasimmos", which currently does not add thermal or phase noise. That was done with noisify_alma.py, by Kumar Golap and Rob Reid. It's not very general, but it's a start. This object is rather small and easily fits within a single primary beam, so I didn't worry about pointing errors after calculating that they should not make much difference.. This should be nearly the same setup as used by Wolf and D'Angelo 2005 (scroll to bottom), except for the change in frequency.