[cube access] minutes from telecon (2013-07-03)

[Arnold Rots]

Similarly, the AIPS MOMNT task applies a mask derived from a smoothed
(in 3-D) version of the cube, but usually it takes some fiddling with its
parameters to get an optimal result.
SUM c.s. (including unmasked moments) are pretty useless, because you
are mainly adding noise.
That's why I am skeptical about the general usefulness (and urgency) of
adding these kind of services.

Cheers,

  - Arnold

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Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Wed, Jul 24, 2013 at 11:26 AM, Tim Jenness <tjenness at cornell.edu> wrote:

>
> On Jul 23, 2013, at 15:47 , Douglas Tody <dtody at nrao.edu> wrote:
>
> > Interesting discussion.  My view, having read through all the postings,
> > follows.
> >
> > *** SUM, AVG, INTEG, moments, etc.
> >
> > This is an important issue (especially for the more general AccessData
> > problem, not just the cutout prototype).
> >
> >
>
> …
>
> >
> > For the simple "cutout" prototype this is too ambitious, it should be
> > left to the AccessData prototyping, which we need for our full up cube
> > access prototyping.  The most we might want to do is define more
> > precisely what is permitted for something like SUM, e.g., if the
> > channels are not equal in size, compute the integral instead.
> >
>
> I'm not sure I understand. So if you have equal-sized channels you get the
> SUM but are lacking the information to be able to convert it to an
> integral. If you have unequal channels you do get the integral. Wouldn't it
> be easier to give you the integral every time? It seems that the difference
> between SUM and INTEG is so small that adding explicit support for INTEG is
> not much of an issue (but make sure the units on the output image are
> correct). The other moment maps are almost as easy if you use the naive
> formula.
>
> More importantly, I find that naive SUM/INTEG are not as useful as you
> might think because you still can't see that weak line in the sum if there
> is a lot of baseline adding in lots of 0+/-noise to the SUM. The JCMT
> pipeline runs a clump-finding algorithm first and masks the baseline
> regions before calculating all the moment maps. This gives very nice
> results but I imagine is not something you want to be doing in a general
> service.
>
> --
> Tim Jenness
>
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