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

[Jose Enrique Ruiz]

Hi Tim
I agree with you on what INTEG is more interesting than SUM, from the
physical point of view. Actually, I think it should be worth to consider
functionalities for each of the Moments Maps of a velocity cube.

Zero Moment:  <I> = SUM[I] * Channel interval (in mapunits.channel) (e.g. K
km/s)
First Moment: <V> = SUM[I.V] / SUM[I]
Second Moment: <sigma> = SQRT[ <V*V> - <V><V> ]

I also support the addition of MAX access method :)



----
Jose E. Ruiz
Instituto de Astrofisica de Andalucia
Glorieta de la Astronomia s/n
18008 Granada, Spain
Tel: +34 958 230 618



2013/7/23 Tim Jenness <tjenness at cornell.edu>

>
> On Jul 22, 2013, at 21:46 , François Bonnarel
>  <francois.bonnarel at astro.unistra.fr>
>  wrote:
>
> > Hi Tim,
> > Le 16/07/2013 21:31, Tim Jenness a écrit :
> >> On Jul 16, 2013, at 12:15 , Patrick Dowler<
> patrick.dowler at nrc-cnrc.gc.ca>
> >>  wrote:
> >>
> >>> Below are my notes from short telecon we had to discuss some details
> of the cube access prototypes to be developed this summer.
> >>>
> >>> Executive summary: a few simple params that define cutouts and
> flattening:
> >>>
> >>> CUTOUT=<stc-S>
> >>> SUM=<axis>
> >> wouldn't the integral be more useful than the sum? (i.e. taking into
> account the width of each pixel that is being collapsed). Is there scope
> for a client to do a better implementation of INTEG that will mask out
> areas of zero emission? At CADC in the JCMT Science Archive the
> "representative images" are the integrated intensity images of the
> associated data cubes. You could short cut by selecting the pre-computed
> image and running cutout on that.
> > If you have calibrated data: Isn't that problem solved in advance ? your
> unit is something  like energy per wavelength unit or something ? Do I miss
> something ?
>
> In the radio or submm you tend to have a data cube with RA/Dec/Velocity
> and your lines are in kelvin. If you collapse it and SUM you just know the
> total of all the channels in each spectrum in, say, kelvin. You need to
> know the integral before you can do anything useful with the answer. If you
> get the answer in "K km/s" you can actually do things like work out the
> column density of the molecule at that transition. Maybe the submm is a bit
> weird in that we don't tend to have spectral lines in flux units.
>
>
>
> >>
> >>> AVG=<axis>
> >>>
> >> Is the assumption here that the spectra have continuum information? AVG
> for JCMT data will give very little information as the data are baseline
> subtracted and the average of lots of baseline and a weak line is
> essentially zero. MAX is more helpful in that it tells you the maximum
> value in the spectrum (assumes that noise artifacts from the edge of the
> band have been clipped off) and that is hopefully an emission line.
> > MAX could be an intersting feature in the full functionality
> >>
>
> Submm tools use INTEG and MAX all the time. Peak line strength and
> integrated intensity are what matter. Average and sum don't have any
> scientific content.
>
> --
> Tim Jenness
>
>
>
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