[Passband] a useful self-contained model?

[David Berry]

Martin,
       Just got back to work after a while away (hope you all had a
good time in Boston!) and noticed this. I like the "divide and conquer"
notion - define general purpose independent component models and then
aggregate them into more specific-purpose higher level models. So a
passband would be used as a component in a more specific higher level
model. To be consistent, though, should not the same approach be
used to design the passband model itself? That is, a passband could be
seen as a specific use of general purpose component models.

A passband is basically a recipe for transforming an input scalar
value (a spectral axis value) into an output scalar value (transmission).
Now we have the general purpose Mapping model which can be used to
describe such numerical transformations, so why not use a Mapping as a
component of a passband? Compound passbands are then simply catered for by
using a SeriesMap/CompositeMap as the Mapping.

In addition to the Mapping from spectral axis value to transmission value,
a passband model would need to specify the exact details of the spectral
axis being used - does the Mapping require input wavelength values, or
input frequency values, or what? And in what units? For this, you would
use a Frame, which would encapsulate a complete description of the spectral
axis.

So a passband could be modelled as a combination of a Frame and a Mapping.
Of course, you may need some extra stuff as well, such as a passband name,
etc.

David




On Mon, 31 May 2004, Martin Hill wrote:

> I originally put up the Passband model example (at my mini course in OOD at the
> IVOA Interop) as an exercise after fiddling about with some ideas on the twiki
> (see DataModelPassband) but after talking to people I think it might be an idea
> to develop it.
>
> The intention is 1) to provide a passband model that can be 'plugged in' to the
> SED model (from SSA) and ObsData when it is ready, and 2) to practice modelling
> a relatively self-contained concept that is familiar to everyone, yet likely to
> be viewed and stored and used in different ways by different disciplines.
>
> I've read several documents from several people (and will hopefully be receiving
> some more over the next few days) and I think this would be a good opportunity
> to get this rather useful bit right and written down and agreed for the VO,
> without all the other baggage that is sometimes associated with passbands (such
> as velocity...)
>
> If you think this is a waste of time and there is a model you are all happy
> with, then obviously feel free to say so (I don't know why I said that, I can't
> think of any shy people here).
>
> If not, here are some questions:
>
>   - I'm using Jonathon's term 'Fravergy' (or 'Photon' as the same thing on some
> twiki pages) for the 'Frequency/Wavelength/Energy' property of a photon.  I like
> it, but please let us know if you don't and have some term that sounds a little
> less Monty Python...
>
>   - Who needs errors on passrate()?
>
>   - Is 'passrate(Fravergy)', returning a value 0-1 probability, sufficient for
> Radio, X-Ray, etc?
>
>   - Is a simple Min/Max sufficient for the limits of the passband?
>
>   - Does anyone tend to use Passbands as just an error on the frequency on a
> measured flux? ie, should Passband also be an Accuracy, or be able to provide an
> Accuracy, for things like plots?
>
>   - What else?
>
> I expect we will end up with something far too comprehensive in our requirements
> to be useful, so the next set of arguments (er, discussions) are likely to be
> about what gets modelled for EverydayPassband and what gets moved to
> RadioPassband or XRayPassband or ReallyComplicatedScaryPassband...
>
> Similarly we may also find a split in purpose; a Passband used in 'metadata'
> *might* be different from one associated with a Flux for an SED.
>
> I mention these so we don't get too caught up in the existing model. However I
> would like to use it as a starting point (the second iteration in my approach
> slide) but want to concentrate on first writing down a summary of what we all
> need to know about passbands (and only passbands...).
>
> Cheers,
>
> Martin
>
>
> --
> Martin Hill
> www.mchill.net
> +44 7901 55 24 66
>
>
>

----------------------------------------------------------------------
Dr David S. Berry    (dsb at ast.man.ac.uk)

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