ISO Spectral access and QUANTITY discussion
posuna at iso.vilspa.esa.es
posuna at iso.vilspa.esa.es
Mon Nov 17 01:17:45 PST 2003
Dear all,
some days ago, we posted a mail to the DAL announcing our extension of
SIAP to Spectrum access to the ISO data.
In that mail, we made an exposition of how we managed the problem of
the axes/units in the representation of spectra using some sort of
dimensional equations.
In view of the recent discussions on the QUANTITY issue, we thought it
might be of interest to the DM group, and thus we attach the mail
below.
Our dimensional analysis trick might be applied to other "quantities"
and models as it is a very general way of describing physical
quantities.
Hope this is of interest.
Best regards,
P.
======================================================================
From: posuna at iso.vilspa.esa.es
To: dal at ivoa.net
Cc: Pedro.Osuna at esa.int, Jesus.Juan.Salgado at esa.int,
Christophe.Arviset at esa.int, Alberto.Salama at esa.int Reply-To:
posuna at iso.vilspa.esa.es Subject: Simple access to ISO Spectra (+images)
using ISO AIO in SIAP mode Date: Fri, 7 Nov 2003 11:23:25 +0100
Sender: owner-dal at eso.org
X-Mailer: Sylpheed version 0.9.2 (GTK+ 1.2.9; sparc-sun-solaris2.8)
Dear all,
at the Strasbourg IVOA meeting, we proposed a simple access to spectra
using a similar protocol to the SIAP. We have modified our system in
this line for accessing both ISO images and spectra in a SIAP-like way.
You can access an example at:
http://pma.standby.vilspa.esa.es:8080/aio/jsp/siap.jsp?id_target=M51&imageType=spectrum
(as usual in our system, a "format=html" can be input to see a
human-readable html page with the outputs)
We have included an "imagetype" parameter that can either be:
- spectrum
- image
- all
with obvious meanings.
By default (i.e., not including the imagetype), the answer will only be
images, fully SIAP compatible thus.
For the metadata information for the spectra in the VOTable output,
this is what we have put in:
- FIELD_ID="AXES" ucd=VOX:Spectrum_axes datatype="char" arraysize="*"
This one indicates the axes names (in our case corresponding to the
keyword names in our fits files) that ought to be used from the
spectrum product to do a display of the spectrum
- FIELD ID="UNITS" ucd="VOX:Spectrum_units" datatype="double"
arraysize="*"
This one indicates the units in which each of the axes is represented
- FIELD ID="FORMAT" ucd="VOX:Spectrum_Format
This is a mime format for spectrum products in fits format.
As the aforementioned quantities would not be enough to be able to
overlap or represent different spectra coming from different
instruments, etc., we believe that we have found a solution by
introducing the following fields:
- FIELD ID="DIMEQ" ucd="VOX:Spectrum_dimeq" datatype="char"
arraysize="*"/>
This one represents the dimensional equation of the units in each of
the axes. More on this later.
The dimesnional equation is based on the fundamental dimesnionless
quantities M,L,T,Q for Mass, Length, Time and Charge respectively.
We represent the equation by, e.g:
MLT-2
where any number after the quantity means (exponential), i.e, the
equation above should be read MLT^-2
(this could be changed if it does not result too clear)
- FIELD ID="SCALEQ" ucd="VOX:Spectrum_scaleq" datatype="char"
arraysize="*"/>
This one represents the scaling factor needed to transform the
dimensional equation (dimensionless by definition) to the
International System of Units. More on this later.
With these two quantities, one can always transform from any system of
units to other, and with the scaling factor it is possible to display
and overlay diffrent scaled spectra. We give some explanation of what
we mean below with a working example.
--------------------------------------------
Use of dimensional equation for the metadata
--------------------------------------------
Our data fluxes, in the case of LWS, are given in "w/cm^2/um", whereas
the data for SWS are given in Jy.
We thus need a way to convert from one to the other. Let's try
to convert the LWS spectrum units (w/cm^2/s) to the SWS spectrum units
(in Jy) and then display both in the same browse tool.
According to our server, the SWS flux (in Jy) dimensional equation is:
[SWS] = MT-2
and the LWS (w/cm^2/um) dimensional equation is:
[LWS] = MLT-3
So, to represent one with respect to the other, we will have to divide
both dimensional equations:
[SWS] MT-2
----- = ------- = LT
[LWS] ML-1T-3
as the only quantities we can use in the transformation are those
coming from the conversion between "wavelength" and "frequency", to go
from one to the other we will have to use a combination of "LAMBDA" and
"c" (wavelength and light velocity). So the above equation has to be
equal to:
LT = [LAMBDA]^n [c]^m = L^n [LT-1]^m = L^(n+m) T^-m
Which means (by equaling the exponents):
n+m=1
-m=1
and thus:
m=-1
n=2
so to go from SWS spectrum to LWS one, we will have to multiply by:
[LAMBDA]^2/[c]
which is as expected from physical reasonings (please check page 82 of
the ISO Handbook - Vol 1. MISSION OVERVIEW at
http://www.iso.vilspa.esa.es/manuals/HANDBOOK/I/gen_hb/).
(In case the above 2x2 system of equations is not solvable, then the
two spectra could not be comparable).
This would give the dimensionless equation. To get to International
System units, we would simply have to do:
FLUX(SWS units) = Flux(LWS units) [LAMBDA]^2 [c]^-1 [SWS scale] / [LWS
scale]
FLUX(SWS units) = Flux(LWS units) [LAMBDA]^2 [c]^-1 10^-26 10^-10
= 10^-36 Flux(LWS units) [LAMBDA]^2 [c]^-1
where obviously, LAMBDA and c should be given in international system
units (LAMBDA in International Units is deducible from the LAMBDA given
in the spectrum times its Spectrum_scaleq factor).
This procedure might look cumbersome, but once the algorithm is
understood a computer can do it without problems.
------------------------------------------------------------------------
We believe this way of presenting metadata in this type of simple
access a la SIAP makes it easy to access our spectra and might be used
for other spectra as well, at least for those in fits format.
This does not clash in our view with the more complicated and general
SSAP work which is going thoroughly on, but simply tries to give access
to our spectra products in an easy way.
Please let us know if you have any comments.
Best regards,
Pedro Osuna and Jesus Salgado.
--
Pedro Osuna Alcalaya
SOFTWARE Development Group
XMM-Newton Science Archive
e-mail: Pedro.Osuna at esa.int
Tel + 34 91 8131314
European Space Agency
VILLAFRANCA Satellites Tracking Station
P.O. Box 50727
E-28080 Villafranca del Castillo
MADRID - SPAIN
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