I'm having this problem with some archival data from 1990. I have two nights of data on a galaxy, spaced a few nights apart from one another. I calibrate the nights separately, and image them separately. It rapidly becomes obvious that the images from the two nights are shifted relative to one another, but not in any simple way. The galaxy center, at the image center, is at the same position in both, but as you go out radially, the first night's sources are further out, radially. It's as if the image is stretched one night, relative to the other.
AIPS thinks that both images have the same pixel scale, so that means a source will have different R.A. and Dec (by about an arcminute, pretty bad!) between the two nights' images. If I DBCON the two nights, I get lots of double sources.
The observational setup claims to be exactly the same between the two nights-- same pointing center, same correlator set-up.
Anyone have any idea what might be causing this? I think I've had a similar problem with GMRT data before, and gave up temporarily and went on to a different project. I'm guessing it's a calibration issue?
Friday, January 4, 2008
Double Sources-- and not the AGN kind
Monday, December 3, 2007
Setting up New Data Disks
My latest organizational scheme for AIPS is to give each project a new AIPS data disk. I just spent some time adding a new data disk to my AIPS installation on my laptop. Here's what I did to get it. (BTW, I've got a MacBook, but that shouldn't matter for this.)
In $AIPS_ROOT, cd to DA00. Save a copy of the DADEVS.LIST file as DADEVS.LIST.old. Then edit the DADEVS.LIST. I just copied the first uncommented line (set up by AIPS) and incremented the data disk number by 1, i.e., LOCALHOST_2. Now save a copy of the NETSP file in the same directory to NETSP.old. Again, edit the file by copying the first uncommented line and incrementing the data disk number by 1. Finally, cd to $AIPS_ROOT/DATA, mkdir LOCALHOST_2 (or whatever your new data disk is named), cd to LOCALHOST_2, and create a file named SPACE (touch SPACE). Now when you start up AIPS you should have additional data disks available.
Most of this tip was taken from the AIPS Manager FAQ (see "How do I configure new AIPS data areas?").
Thursday, October 4, 2007
Your friend, the tilde
Ever had to enter a lot of numbers into an array in an AIPS script and run out of room (like in this post)? Trying to figure out the mysteries of plcolor without typing 5 million numbers? In your time of need, the handy tilde will help you out. See below the fold for more details.
The popsym help file (type help popsym to access) gives you this handy tidbit of information on the tilde
A(i) ~ 1,2,3 Store values in A(i),A(i+1)... (change only as many as on RHS)
Now here's an example,
filename = '10208722.UVF','10208724.UVF','10208726.UVF','10208727.UVF'
filename(5) ~ '10208733.UVF','10303423.UVF','10303424.UVF','10303461.UVF'
Tah-Dah!! That's all there really is to it. Use the tilde, Luke -- your fingers will thank you.
Friday, September 21, 2007
Beam Squint and Self Calibration (relevant even if you don't care about polarization!)
Here's another tip from Juan Uson about a self-cal trick to get the very best dynamic range in your images.
The VLA has beam squint in Stokes V. What this means is that away from the center of your field, the RR and LL polarizations separate a bit (I don't really understand the details of this). This leads to an image artifact-- if you have a bright source off the center of your field, it'll seem like you can't clean it very well. Even if you've done your very best job at self calibration and cleaned very deeply, there will still remain rings around the source (which of course drives up your noise).
One easy fix for this problem is to do a normal Phase self calibration, but when it's time for your A&P self calibration, average your RR and LL. I believe the relevant parameter for this in CALIB is APARM(3) > 1. This should give you a self cal that is not affected by the beam squint, and models your bright sources better!
Clean Components and Self Calibration
Well, I just talked to Juan Uson from NRAO-Charlottesville, and he delivered some useful information for us burgeoning self-cal experts.
He says that you should ALWAYS use all your clean components for self calibration. Don't cut at the first negative! He says that when they were first learning about self cal, they were very timid and therefore advised people to cut at the first negative, but now they know better. You want to use your very best clean component model for self cal, which includes all components!
Here are some reasons why--
a bright source that is centered on the edge between two pixels will actually take an infinite number of clean components to model, because basically the clean algorithm keeps exchanging flux back and forth between the surrounding pixels. so, to model such a source, there are lots of negative clean components required.
if a source has any complex structure at all, you can't really effectively model it without using some negative components.
I told him that occasionally, the word on the street is "Just self cal off of one bright source that you know the structure of, and leave the rest of the flux alone." When I tried to do that once, the self cal created fictional ghost sources in my data and it scared me! Juan was not surprised; he said its a terrible idea to self cal on only a select few sources. Basically what self cal does is divide your real fluxes by your model fluxes, and try to make the residuals look like noise. If there is still real flux in your residuals (because your clean model is based off only a select few sources), then it will try to turn that real flux into noise and crazy things will happen.
So, clean out all your flux, and use all your clean components!
Wednesday, September 19, 2007
Filling data that includes midnight
When you get your data from the archive, it's normally in several files. For me, each file is usually a day's worth of data, so I fill and calibrate each data file separately. However, if you have observations that go from say 20:00 on 17-Feb to 6:00 on 18-Feb, the archive creates a separate files for the observations on the 17th and for the observations on the 18th. You need to read in both files to calibrate that series of observations.
Monday, September 17, 2007
Complications with Shamelessly Raiding the Archive
Amanda and I are spending the week in Socorro, so hopefully we will be learning little new AIPS facts every day! Here's my first one:
Sometimes I have dreams about raiding the VLA archive and combining like 6 different data sets. However, often times the data sets will have slightly different centers-- in my case, often one observer centers on the galaxy's center, while another observer center on a supernova that goes off in the outskirts of the galaxy, some 5 arcminutes away. I was wondering, can you just DBCON these two data sets together, and AIPS can handle it?
The answer is...
DBCON can adjust the phases, so yes, it can image data from two pointings with significantly different centers. What Miller Goss said you have to be wary of is that DBCON doesn't adjust amplitudes. So, if there are two different pointing centers, the center of your galaxy is going to fall in different locations of the primary beam in the different observations. If you are combining two observations separated by 5 arcminutes at 20cm, where the primary beam is 30 arcminutes, this shouldn't be a big issue...probably the primary beam sensitivity of the two pointings will only vary by a few percent. However, if the separation of your pointings starts approaching the size of the primary beam, then you can no longer use DBCON and need to think of some other way to combine the data.
If you have a question about AIPS that CANNOT be answered by reading the