[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
More information about the Daip
mailing list