EVLA Spectral Line Imaging Analysis IRC+10216

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Revision as of 18:30, 2 June 2010 by Cbrogan (talk | contribs) (UV Continuum Subtraction and Setting Up for Self-Calibration)
This tutorial is under construction. There are several things still to be added 

For the time being (until release candidate is built) you want to run this on casapy-test

This tutorial picks up where EVLA Spectral Line Calibration IRC+10216 leaves off.

UV Continuum Subtraction and Setting Up for Self-Calibration

UV-plot of the spectral line signal in both spw for IRC+10216.

Now we can make a vector averaged uv-plot of the calibrated target spectral line data. It is important to note that you will only see signal in such a plot if (1) the data are well calibrated, and (2) there is significant signal near the phase center of the observations, or if the line emission (or absorption) is weak but extended. If this isn't true for your data, you won't be able to see the line signal in such a plot and will need to make an initial (dirty or lightly cleaned) line+continuum cube to determine the line-free channels. Generally, this is the recommended course for finding the line-free channels more precisely than is being done here due to time constraints, as weak line signal would not be obvious in this plot.


in the Display tab, choose colorize by spw and change the Unflagged points symbol to custom and Style of 3.

You should see the "horned profile" typical of a rotation shell. From this plot, you can guess that strong line emission is restricted to channels 18 to 47 (zoom in if necessary to see exactly what the channel numbers are).

In the Data tab you can also click on "all baselines" to average all baselines, but this is a little harder to see.

Now we want to use the line free channels to create a model of the continuum emission that can be subtracted to form a line-only dataset. We want to refrain from going to close to the edges of the band -- these channels are typically noisy, and we don't want to get too close to the line channels because we could only see strong line emission in the vector averaged uv-plot.


The "want_cont=T" will produce two new datasets, IRC10216.contsub is the continuum subtracted line data, and IRC10216.cont is the continuum estimate (note however, that it is still a multi-channel dataset).

If you want to try self-cal: Unfortunately, at the moment, uvcontsub2 doesn't leave the continuum subtracted line dataset in the state you need *if* you think you might want to self-calibrate the data later. This is because clean always looks at the "corrected" datacolumn, while gaincal (also used for self-calibration) always looks at the "data" column. You also need to know that unless the imagename supplied to clean already exists, clean always overwrites the model column with the clean model (this will be the model supplied to the self-calibration process).

The files produced by uvcontsub2 will have the following in their data columns:
                       Data       Model     Corrected
IRC10216.contsub     Line+Cont    Cont        Line
IRC10216.cont          Cont       Cont        Cont

Now the imaging task clean will clean both of these files fine, and correctly overwrite the model data column with the correct clean model. However, if you try to self-cal (i.e. run gaincal) on the continuum subtracted line data (.contsub), it will use the Line+Cont as its input along with the line only clean model. If the line and continuum have significantly different morphology (almost always) the self-cal process will fail. In contrast the continuum dataset (.cont) will work fine because the Data and Corrected columns agree.

To fix things up, we must split the "corrected" column (which places "Corrected" in the "Data" column of the new dataset. Then you can either run clearcal to reinitialize the "model" and "corrected" columns or let clean do it for you. We put this step here explicitly for clarity about the process.


Now IRC10216.contsub.data' will have the Line in the "data" column, 1 in the "model" column, and the Line in the "corrected" column as desired.

We expect that these extra shenanigans will be unnecessary in the future.

Image the Spectral Line Data

Here we make images from the continuum-subtracted, calibrated spectral line data. Because the spectral line emission from IRC+10216 has significant extended emission, it is very important to run clean interactively, and make a clean mask. To make the cube a bit smaller and stay away from noisy edge channels we restrict the channel range using the spw parameter.

Channel 28 shown for the HC3N cube shown in the interactive viewer with the white contour showing the mask contour drawn with the polygon tool.


It will take a little while to grid the data, but the viewer will open when it's ready to start an interactive clean. Use the "tape deck" at the bottom of the Viewer display GUI to step through to the channel with the most extended (in angular size) emission, select "all channels" for the clean mask, select the polygon tool (second in from the right) and make a single mask that applies to all channels (see example in thumbnail). Once you make the polygon region, you need to double click inside it to save the mask region -- if you see the polygon turn white you will know you succeeded. Note, that if you had the time and patience you could make a clean mask for each channel, and this would create a slightly better result.

After making the mask you should check that the emission in all the other channels fits within the mask you made using the "tape deck" to move back and forth. If you need to include more area in the mask, you can chose the "erase" toggle at the top, and then encircle your existing mask with a polygon and double click inside. Then go back to "add" toggle at top and make a new mask. Alternatively, you can erase a part of the mask, or you can add to the existing mask by drawing new polygons. Feel free to experiment with this a bit.

note: If you start an interactive clean, and then do not make a mask, clean will stop when you tell it to go on because it has nothing to clean. There is no default mask.

To continue with clean use the "Next action" buttons in the green area on the Viewer Display GUI: The red X will stop clean where you are, the blue arrow will stop the interactive part of clean, but continue to clean non-interactively until reaching the stopping niter or threshold (whichever comes first), and the green arrow will clean until it reaches the "iterations" parameter on the left side of the green area. When the interactive viewer comes back use the tape deck to recheck that you mask encompasses what you think is real emission. The middle mouse button by default controls the image stretch.

Note that for this example, threshold has been set to threshold='3mJy' to protect you from cleaning too deeply. With a careful clean mask you can clean to close to the thermal noise limit (note here I mean the actual observed rms noise limit and not the theoretical one you calculated for the proposal, as flagging, weather etc can affect what you actually get). It is ALWAYS best to clean each channel in a cube to a specific threshold than to stop using the niter parameter, which can leave each channel cleaned to different levels. There are many ways to determine a suitable threshold. One way is to make a dirty image (niter=0), open the cube using the viewer, go to a line free channel, select the box region tool, make a box near the field center about the size of your source, double click inside. The rms noise of that channel will appear in a pop-up window (rms noise for whole cube will go to terminal). Try a few different boxes, average the results and this is a good estimate of the rms per channel assuming your data are not dynamic range limited (i.e. noise can be higher in channels with strong signal). This is the absolute minimum for threshold. With no mask you probably shouldn't clean deeper than 3x this rms.

Channel 16 shown for the SiS cube in the interactive viewer with the white contour showing the mask contour drawn with the polygon tool.

Keep cleaning, by using the green Next Action arrow until the residual displayed in the viewer looks "noise like". To speed things up, you might change the iteration parameter in the viewer to something like 300. This parameter can also be set in the task command. You will notice that in this particular case, there are residuals that cannot be cleaned -- these are due to the extended resolved out structure on size scales larger than the array is sensitive to (the "Largest Angular Scale" or LAS that the array is sensitive to can be calculated from the shortest baseline length), and potential residual phase and amplitude calibration errors. We will explore this in a few sections with self-calibration.

Repeat the process for the SiS line using the call below, note that the emission for this line is less extended than the HC3N -- this has to do with the different excitation requirements of the two different lines. The SiS is excited closer to the central star than the HC3N.


Image the Continuum data

Below the use of mode='mfs' will make a single multi-frequency synthesis image out of the specified spw. Again you should make an interactive clean mask. Since no threshold is set, you will need to stop cleaning when the residual looks noise like using the red x "Next Action" button (it will be done when the viewer comes back the second time).


Now look at the result in the viewer if you like


Image Analysis and Viewing

Next make integrated intensity maps (moment 0) and integrated velocity maps (moment 1). To do this, we'll want to know what channels the line emission starts and ends on, and also the rms noise in a single channel. So first lets open the viewer:

# In CASA 

Then use the Viewer tape deck to see which channels have significant line emission. For HC3N, the line channel range in the cube is 11 to 40, and it is the same for SiS.

Then use the tape deck to go to a line free channel, select the box region tool and make a box. When you double click in the box, the image statistics for the whole cube will print to the terminal and for the channel you are on, it will print to a pop up window. Move the box around a bit to see what the variation in rms noise is. You should get something like 2 mJy. Note that the rms is much worse in channels with strong emission because of the low dynamic range of these data. If you want the box tool to go away (i.e. if you want to make a new one), hit the escape key.

Now lets make the moment 0 and moment 1 maps. For moment zero, its best to limit the calculation to image channels with significant signal in them, but not to apply a flux cutoff, as this will bias the derived integrated intensities upward.

HC3N moment 0 map with white continuum contours superposed.
SiS moment 0 map with white continuum contours superposed.

For moment 1, it is essential to apply a conservative flux cutoff to limit the calculation to high signal-to-noise areas. Here we use about 5sigma.


Now user the viewer to further explore the images you've made.


--Crystal Brogan