Simple access to ISO Spectra (+images) using ISO AIO in SIAP mode

[posuna at iso.vilspa.esa.es]

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