[fitsbits] Comments on the TNX distortion convention

[LC's No-Spam Newsreading account]

I have read also the HTML doc of the TNX convention (AFTER I sent the 
previous comment on the SIP convention).

Again, formally, the TNX convention fulfills the requirements for 
inclusion in the registry (it is clearly documented, and it has been in 
use for a long time).

I must confess that I regard some of its formalism as inelegant (I do 
not like the fact coefficients are stored in sorts of "long strings" 
(the WAT1_* and WAT2_*) instead of having individual keywords, but this 
is of course irrelevant for inclusion in the registry.

Apparently the TNX convention can include the SIP (type=3 ?) with a 
different formalism.

This again raises the problem of the coexistence of different 
conventions (already raised for SIP vs WCS IV in the previous post)
with the respect to the standard.

Very honestly the TNX documents starts with "The TNX World Coordinate 
System is a non-standard system ...", so there should be no objection
to include it in the registry at least for "historical" purposes.

One usage of the registry could in fact be to keep track of "old" 
conventions even when superseded by new standards (to preserve backward 
compatibility). The registry should foresee an editable status flag to 
mark a convention as obsolete (I dare not using the word "deprecated" 
:-)) and therefore discouraged from further use.

My last statement is of general nature, and it does not imply that the 
TNX or SIP conventions are to be considered obsolete NOW.

Lucio Chiappetti

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