# Difference between revisions of "N891 simdata (CASA 3.3)"

Old version: N891 simdata2.

To create a script of the Python code on this page see Extracting scripts from these tutorials.

## Nearby edge-on spiral

Roughly modeled after NGC891

Updated for CASA 3.3

• The cube is being binned to a coarser velocity resolution in order to speed the simulation. The fits file is grs-12kms.fits
```# In CASA
# Initializing sim_observe
# Laying down some basic ground rules
default 'sim_observe'
project = 'n891d'
skymodel = 'grs-12kms.fits'
```
• Units: K - first convert to flux surface brightness: Jy/Sr = 2x1023 k T / λ2, = 4x108T at 110GHz.
```# In CASA
# Setting the new frequency of the central channel
incenter = '110.1777GHz'
```
• Now we need to decide if this model data will work at the desired pixel scale
• The GRS resolution of 40" at ~10kpc is 0.04" at 10Mpc, so we should be able to do a simulation of observing at ~0.1-0.2". The resolution plot (See Figure 1) indicates that for ALMA at 100GHz, configuration 20 is appropriate.
• If we intend to set incell=0.2arcsec in sim_observe, then the cube needs to be multiplied by 4x108 * (.04/206265)2 = 1.4x10-5 to obtain Jy/pixel. The cube peaks at ~20K, so we can perform the simulation with inbright=3e-4, which should yield a peak of ~1mJy/bm.
• Will we be dominated by the noise in the input model? Input noise ~150mK or S/N~20, so at our scaled intensity, ~0.05 mJy/bm. The ALMA Sensitivity Calculator says that ALMA will achieve 2.5mJy/bm in 2 hours for the input 212m/s channel width (0.075MHz), so the noise in the input model should not affect our results.
```# In CASA
# Setting the new channel width
inwidth = '0.075MHz'
```
• We do have a sensitivity issue though - if we decrease the spectral resolution by a factor of 6 (bin the input channels in some other program - sim_observe will know how to do that in the future but not yet), and plan for 3 8-hr tracks, then the sensitivity calculator suggests that we'll get <0.25mJy rms, or S/N>10 per beam. Rather than simulate 3 days of observing, I'll increase inbright by sqrt(3) and simulate one 8 hour track. Figure 2: here's the cube with the sim_observe's scaling and World Coordinate System

```# In CASA
# Scaling the surface brightness
inbright = '1.4e-4'
```
• the ALMA 12m primary beam is 50" so we'd space a mosaic by 25", but the model cube has 326x357 pixels, or 13 arcsec with our small pixels. That's a lot smaller than the primary beam, so it doesn't matter much what output image size we ask for.
```# In CASA
# Finish up the image model, and setting up the pointing
indirection = 'J2000 7h00m34 -23d03m00'
incell = '0.2arcsec'
setpointings = True
integration = '300s'
pointingspacing = '25arcsec'
mapsize = '60arcsec'
```

There are 659 channels in the input cube, but as noted above we want to bin those to 109 channels of 1.2 km/s each.

```# In CASA
# Finish up the rest of the settings for this run of sim_observe
graphics = 'both'
verbose = True
overwrite = True
observe = True
antennalist = 'alma;0.5arcsec'
totaltime = '3600s'
sim_observe()  # Run sim_observe to create the simulated data we need
default 'sim_analyze'
project = 'n891d'
image=T
vis = project+'.alma_0.5arcsec.ms'
sim_analyze()  # All other default settings are OK in sim_analyze
``` Figure 3: a spectral profile in the box marked in green

Figure 4: Sample results

Last checked on CASA Version 3.3.0.