[Passband] a useful self-contained model?

[Alberto Micol]

I do not know for the radio domain, but certainly in the optical
I would claim that the difference between an SED and a spectrum is
that to describe the spectrum one needs a dispersion relation; i.e. not 
all the spectra
are regularly spaced in "fravergy". A dispersion relation is not 
available for a SED;
the SED points come from different catalog/images/publications etc.; 
the spectrum comes
from dispersing the light using a grating or a grism or a prism or a 
combination of those,
whose characteristics are known and described via the dispersion 
relation.
I do not think that it would be that useful to assign a passband to 
each channel.

>>> - Who needs errors on passrate()?

I have never seen an error on the throughput (that's how we call the 
passrate).

Other things needed?

1.- distinguishing between e.g. a filter transmission curve and the 
overall bandpass:
the most useful thing to the astronomer is the multiplication of the 
transmission
curves of all the elements in the instrument: optical elements 
(mirrors/lenses/fibre channels),
filter, detector quantum efficiency.
We need to indicate whether it is the overall thing or just one of the 
components.

2.- Names: There are various names for each bandpass; the obvious one 
is the name of the filter
as known within the observatory (e.g. F555W for HST/ACS), but sometimes 
an alternate name (e.g. "V") is used to indicate that the filter is 
similar (but almost never identical) to a certain standard filter. I do 
not like to see a filter named V when it is not really V.

3.- Min/Max: in the RM (if I remember) these two numbers are not 
mathematically defined
but are chosen by the data provider in a way to ensure that no search 
by fravergies
will miss the given filter;

4.- Wavelengths: different observatories use different characteristic 
wavelengths.
The one I remember now are: central wavelength, pivot wavelength,
and someone uses the Schneider, Gunn, and Hoessel (1983, ApJ 264, 337) 
definition
which has the property that such "mean" wavelength equals the speed of 
light over
the corresponding "mean" frequency, and is half way between a "mean" 
wavelength, and a "mean" frequency.


5.- Filter efficiency: someone characterises the filter by giving the 
transmission at one of the characteristic wavelengths (see 4. above)

6.- Red-leak; some filters exhibit a red-leak. The min/max above (see 
3) might purposely ignore
the red leak. Probably it would be just good to add a flag to know 
whether there is such a leak or not, leaving to the full transmission 
curve the details about the importance of such leak.

7.- Zeropoints; for space based detectors the zeropoint tells how to 
translate detectors counts
into magnitudes in some system. On the ground it is a bit more 
complicated because of varying atmospheric conditions. Nevertheless 
each measurement should bring along its own zeropoint
quantity (in the sense of value and error <- it is here that the error 
is very relevant).

8.- Atmospheric conditions; airmass and seeing are needed to 
characterise a SED measurement; should they end up modelled in the 
bandpass?

9.- When playing with SEDs (a la AVO demo) it is extremely important to 
pass around the bandpass info; not only for describing the SED but to 
allow for more sophisticated analysis, like for example
when an SED is to be compared with a library of template spectra for 
classification purposes.
AVO and GAVO are putting together a system to actually classify SEDs of 
planetary nebulae
by matching the SED with models of stellar atmospheres. The models are 
to be multiplied by
the filter throughputs before the comparison can take place.
The SEDs should hence always bring along a URI(?) defining (if not 
pointing to) the
bandpasses of each of the SED points.
It would be nice to have a service able to resolve those URIs to return 
all the bandpass
characteristics and throughput.


BTW, I remember a Jonathan's document describing a bandpass datamodel.
What is the status of that document? Is this exercise a way to continue 
Jonathan's work?
I should read it again now, after few years ...

That's all I can think of, after a nice free day spent with a dozen of 
screaming 7to9 years old
kids, playing bowling and trying to make sure they get back to their 
parents safely (for the record, I almost succeeded :-).

Alberto