[fitsbits] Start of "Foreign File Encapsulation" Public Comment Period

[William Thompson]

Here are some comments on the Foreign File Encapsulation proposal.

1.  I'm not familiar with the "MEF" acronym.  From the context, I assume it
means something like "Multi-Extension FITS".  This acronym should be explained.

2.  I'm uncomfortable with the statement

 >     ... Writing of
 >     the PHU may be disabled even if a file is being written to disk (e.g.
 >     when writing a sequence of extensions to be concatenated).


This seems to refer to temporary files used in generating the final FITS
product, but could also be interpreted as allowing delivery of FITS files
without PHUs.  This should be clarified.

(By the way, the PHU acronym should be spelled out the first time it's used, as 
should EHU.)

3.  Along the same lines, I'm a bit confused by the statement

 >     fits file
 >         Any FITS file or FITS extension, regardless of the extension type.
 >         This has to include MEF files as well.


Since the intent seems to be to encapsulate files, does it make sense to 
encapsulate bare extensions without a primary HDU?  Would such extensions still 
have XTENSION='FOREIGN', or would it have something like XTENSION='BINTABLE' (or 
whatever) together with the keywords of section 5?

4.  I feel that the following statement should be clarified.

 >     MEF files in the input are written unchanged except that keywords are
 >     added to the first HDU to identify the MEF group (subsequent extensions
 >     are merely copied to the output stream unchanged).


Exactly which keywords are added to the header?  Which ones are required and
which ones are optional?  Are all the keywords in Section 5 other than FG_MTYPE 
required?  (I guess FG_COMP isn't required either, since it doesn't appear in 
the example.)

What happens to the FG_LEVEL keywords as groups are concatenated together?  The
parenthetical part of the sentence above implies that they're unchanged, but
that doesn't sound right to me.

5.  I don't understand how directories or symlinks would be encapsulated.  At 
one point the document talks about a TOC, which I assume means Table of 
Contents.  How would a TOC be encoded?

6.  The example FITS file seems pretty straightforward.  What would a more 
complicated example with multiple hierarchies look like?

7.  I'm not sure I understand the point of encoding the user and group ID of the 
file.  Surely that's platform specific.

-- 
William Thompson
NASA Goddard Space Flight Center
Code 612.1
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson.1 at gsfc.nasa.gov