[alma-config] parameters for IMAGR

[Bryan Butler]

with regards to some of steven's IMAGR inputs (in his email from
11aug2000):


>DPARM(5)=1
>DPARM(6)=4
>FUNCTYPE 'LN'

i presume that these are for LWPLA (for printing)?

>SELBAND 1000
>SELFREQ 2.99793E+05

these shouldn't really be necessary.  they should just be set at
the defaults, which allows IMAGR to get it from the input dataset.
maybe there is a practical reason for doing this though?

>CPARM(1)=12

ahh.  this implies that you have an old version of IMAGR.  you
should probably import the newest one, in which CPARM, and some
other parameters, are replaced by IMAGRPRM.  in the newer IMAGR,
you can, for instance, do multi-scale CLEAN if you want.  very
powerful...  in any case, setting the aperture size should be 
unneccessary.  this is only important for multifrequency synthesis, 
where you can make a frequency dependent PB correction if you know
the aperture size.  in fact, i'm not even sure what PB correction
is implemented in IMAGR (uniform illumination maybe?).

>CMETHOD 'DFT'

while this minimizes the errors, in practice, i don't know anybody
that does straight DFT with VLA data.  the number of visibilities
prohibits it.  so gridded FFTs are always used.  on the one hand,
we might say that we should do the most accurate thing available,
but on the other hand, we might say that we should do what most of
the observers (and the pipeline) will do with ALMA data.  DFT is
more accurate, but gridded FFT is what will be used, of necessity,
until computers get *alot* faster (try doing a 1024X1024 image of
1 million visibilities - you'll have a long wait!)...

>FACTOR 0

for complicated images, you will probably want to make this something
like FACTOR=-.5 or so.  this makes minor cycles CLEAN less deeply.
costs CPU, but gets you much better results on complicated extended
structure.  you might also want to set PHAT=.02 or so, but this is
probably not a big issue since the u-v coverage for all of the ALMA
configurations is so good that this shouldn't be much of an issue
(and its use is 'deprecated' in the current IMAGR).

>APARM(6)=0
>APARM(8)=1
>APARM(9)=1
>APARM(10)=0.05

unclear to me which tasks this is for...

>GUARD(1)=-1
>GUARD(2)=-2

i would just leave these at default.

>UVWTFN 'NO'

as i mentioned yesterday, i would use 'NA', but i think that in our
case the two are equivalent...

>GAIN 0.25

this is too high a loop gain.  i would use the default 0.1, or perhaps
even lower (0.05?).

>MINPATCH 51

again, for CLEANing complicated extended structure, you will want to
increase this.  it's not as big an issue as for VLA data, because we
don't have sidelobes nearly as bad, but just to be safe, i would set
this at some large fraction of the total image (like 256 or so).

>UVWTFN 'NA'

i'm a bit confused by two specifications of UVWTFN...

>ROBUST 5

again, using UVWTFN='NA' (or 'NO' for that matter) makes ROBUST useless.



	-bryan