SED Data Model: Questions and Comments

[Igor Chilingarian]

Hello,

On Thu, 3 Feb 2005, Gilles DUVERT wrote:

> Hi,
>
> DM is not just the sum of its parts. One has to make choices. The need
> is to insure data QUALITY and MAINTAINABILITY. Keep things simple. DM
> should describe the physics of the data, that unite, not the habits of
> the data producer, that divide.
>
> So:
>
> A measure is a value and an error. If there is no flux calibration, the
> measurement does not exist, and a data model shall not be concerned by this.
> DO NOT PERMIT TO WRONG OR UNRELIABLE DATA TO BE OUT IN THE VO!!!

Value and error have nothing to do with the flux calibration. And this doesn't
always affect the physics. Detector of any type (at least those I know to be
used in optical astronomy) doesn't give an output in a physical flux, but only
in counts per channel (or pixel, or just total counts), and no error is given:
usually it is assumed __by astronomer__ to correspond to Poisson photon
statistics or some other primitive model. And probably everybody agrees on the
fact, that it is NOT POSSIBLE to have reliable absoulute measurements for flux
while you're below the atmosphere. All the calibrations are completely
MODEL-DEPENDENT: you estimate some extinction law, the same atmosphere
transparency during observing object and calibration frames, and so forth.

Beside this there is a problem of calibration for photometry as well. If you
take a photometric point, say Johnson J magnitude with some error-bar
(favourite example by Pedro), it doesn't bear you any useful information,
because nobody knows what Johnson J is! There is __a kind of__ transparency
curve for a filter, but it varies quite a lot in different realization of the
filters used in different observatories. What astronomer will do to find out
the details (zero-point, transmission curve, and so on) -- he'll give a phone
call to the person who published the data.

Does it mean that according to your proposition, we must reject all the
ground-based spectral data, and photometry except SDSS :)? What will remain?
The processed contain of MAST + SDSS + some other space mission archives? And
all the astronomers will be happy with this?

> USE FREQUENCY, this is the core of reality, insensitive to media. Does a
> bracket gamma line takes a cold because the weather that day was cold
> and humid and it did not take its scarf? Leave the gory details to the
> producer of the data. Leave the end-user convert metadata frequencies to
> whatever unit it pleases himself.
I'll make the comments concerning this point in the next message.

> >4. Resolution and line spread functions
> >
> Let the DM support standard functions (sigma, gauss, lorentz) that fit
> 99% of needs, and leave the transform of the data to this functions to
> the  expertize of the data producer. He knows best.
It is absolutely impossible, because it requires deconvolution that never
works in practice. So, Jonathan's idea about representing the functions and
evaluating them is a good solution. For a moment, the DM allows only FWHM - an
integral characteristic of the line-spread-function, that is unsufficient in
case you want to make detailled analysis -> you again have to give a phone
call (or e-mail) to the data provider to find out the details.

With best regards,
						Igor

P.S.: Sorry, if you have this message more than once.