[daip] Re: Aips problems

Eric Greisen egreisen at nrao.edu
Mon Sep 19 12:27:10 EDT 2005


Enno Middelberg writes:

 > I have tried to get some help with an Aips-related problem, but an email 
 > sent to daip at nrao.edu on 31 August remained unanswered. Can you please 
 > forward my email to the relevant person (or possibly look into this 
 > yourself)?

   We had a bad computer problem with mail at that time.

 > I have a VLBA spectral line data set and encounter two problems which I 
 > am unable to solve. I have an experiment in which the water masers in 
 > NGC3079 were observed with the VLBA using 256 channels in each of 8 IFs. 
 > Each IF has a bandwidth of 8 MHz, and hence a channel has 31.25 kHz.
 > 
 > 
 > Problem 1
 > ---------
 > 
 > I am trying to use SETJY to be able to convert frequencies into 
 > velocities, but the way this has to be done seems laborious and 
 > redundant. Using the "SYSVEL" parameter and APARM(1) one defines the 
 > velocity of a given pixel. This requires that I calculate the velocity 
 > for, eg, channel one in each of the eight IFs separately and use SETJY 
 > eight times to set the velocity (this is the laborious part)! 
 > Furthermore, as the observing date, the observing frequency and the line 
 > rest frequency are known, the conversion to km/s in, eg, POSSM, should 
 > be straight forward, and, once tested, less error-prone than my own 
 > conversion. What am I missing?

     Normally the correlators provide this information.  Is there no
velocity information in your data set?  CVEL (if the velocity was not
tracked) does require the SETJY function.

     I agree that we should be able to compute this at some level but
issues like was the velocity tracked are harder to determine.

 > 
 > 
 > 
 > Problem 2
 > ---------
 > 
 > This is related to making an image cube from VLBA spectral line data. I 
 > want to make an image cube from (nearly) all 2048 channels, with one 
 > channel per plane, using IMAGR. I specify
 > 
 > BCHAN         2
 > ECHAN         255
 > CHANNEL       0
 > NCHAV         1
 > CHINC         0
 > BIF           1
 > EIF           8
 > 
 > and would expect that IMAGR takes channel 2 in IF1, then channel 3 in 
 > IF1, and so on, until all channels in IF1 are done, and then starts with 
 > channel 2 in IF2, and so on. But IMAGR averages channel 2 from _each_ 
 > IF, then channel 3 from each IF, etc, resulting in a cube with 255 
 > planes. Here is some output from the message server:

The problem is that you are looking at your data the way you would
like to see it, not the way it is.  It contains 256 spectral channel
and 8 IFs, not 2048 spectral channels.  You must run IMAGR 8 times to
make 8 spectral cubes.  The task MCUBE may then be used to construct a
single 2048-channel cube.

Eric Greisen




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