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

[Tim Jenness]

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