Defaults in STC

[Anita Richards]

> There are two kinds of uncertainties, especially for historical data.
>
> One is "I don't know exactly when this plate was taken - it was in
> 1925 or 1926".
> In principle, the observing time range, coupled with the observatory
> location, should give one a huge uncertainty in Doppler velocities, so
> no special provisions are needed.  And inaccuracies in, say, positions
> can be handled through the error elements.
>
> The other is "unknown": "I don't know whether this velocity is radio
> or optical definition, etc".
> I think the only reasonable way to handle this, if we must, is to
> allow "Unknown", "Not available", or something like it, as an allowed
> value.  It does put the bruden on the user to figure out (a) whether
> the data are still useful, and (b) if needed, what would be a
> reasonable guess for a default.
>
> So, somewhat reluctantly, I am leaning toward accepting the concept of
> "Unknown" as allowed value for a number of coordinate system elements.
> If I understand your previous message correctly, you would agree with
> that solution.  Correct?

Thanks, yes, that is all exactly what I was trying to say, with the added
proviso that (as you mention in the original document), e.g. velocity
convention = "unknown"  would cause problems for software using the model
to deduce redshift from velocity _unless_ the application software
intrpreted "unknown" as e.g. optical convention and added the appropriate
error; knowing a redshift to a low accuracy might still be very useful.

That would be outside the province of the STC model but is something whcih
people writing applications could take care of.  Does that sound sensible?
(i.e. caveat emptor, and your model stays pure :)

cheers

a
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dr. Anita M. S. Richards, AVO Astronomer
MERLIN/VLBI National Facility, University of Manchester,
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).