[fitsbits] Start of the 'INHERIT' Public Comment Period

[Rob Seaman]

Bill wrote:

> The INHERIT convention on the other hand only reduces the size of  
> the FITS file by a small fraction of 1% in typical cases.

There is an assumption here that only images will use such a feature,  
and further, that all images are large.  A file containing reduced  
MOS data might have one or more bintables or even as many extensions  
as spectra.  In general, we shouldn't assume that headers are smaller  
than data units.

A notion of FITS compression is to preserve readable headers.  To the  
extent that this discussion is about minimizing the size of headers  
(and not about the correct data model for FITS objects), INHERIT is a  
natural complement to the tile compression convention.

> Nobody has suggested that the inherit convention shouldn't be  
> documented
> in the registry.

Bob started the discussion with:

	"I have to express some concern about registering the INHERIT  
convention."

My apologies if I misunderstood.

> If the IAUFWG decides this would be useful, then a mechanism for  
> adding usage comments or recommendations could be added to the  
> Registry.

By all means, comment away.  Feel free to append mine.

Bob wrote:

>> a reader that encounters INHERIT, and manipulates headers with  
>> even simple copy operations, could make a bit of a mess.

A reader that does not understand INHERIT will copy the extensions  
verbatim, including the INHERIT keyword itself.  The only trouble  
that might arise is if the primary header is disconnected from the  
extensions, but similar trouble might afflict any FITS file that is  
naively split apart.

A reader that does understand INHERIT may trigger inheritance, of  
course, in the copy.  In this case, the extensions will contain all  
the keywords.

Or a reader may more subtly implement INHERIT and choose between  
these two correct behaviors on an application specific basis.

I think concern about "confused users" is inevitable, but overstated  
in this case.

>> It is good to document what is out there, but we do not  
>> necessarily want to encourage its further adoption.

One might consider a prerequisite to discouraging the use of unique  
conventions to be the adoption of similar functionality within the  
standard.  A broader discussion of a coherent FITS data model and of  
how individual HDUs are related to one another sounds interesting,  
but beyond the scope of the registry.

Rob