1. What is FITS? FITS = Flexible Image Transport System
Invented ~1980 to transfer Radio Astronomy datasets from one observatory to another
Format now used by most astronomical software
Widely used as data archive format
FITSIO and other libraries developed to read/write FITS efficiently using C, Fortran, Perl, Python, Java…
Always big-endian (so portable)
Format defined to the last bit in primary literature.
Images and binary tables fairly space-efficient.

2. FITS Structure in outline
Any number of Header-Data units (HDUs)
Header has metadata: lines of 80 characters
NAME = value /[units] other commentary
A few reserved names, e.g. DATE-OBS, no rules otherwise
Data section can be Image or Table
Image:
8, 16, 32-bit integers, 32 or 64-bit floats
Up to 999 dimensions (mostly 2 or 3 dimensions)
Table
up to 999 columns, up to 2 billion rows
8/16/32-bit integers, 32/64-bit floats, strings, logical, 64/128-bit complex.
Vector columns (optional variable length vectors)
Null values can be defined

3. What’s wrong with FITS
Flat structure for metadata, single name-space, only 8-character names
Blocked in units of 2880 bytes
Length of data unit must be declared in header unit, so cannot stream output
No structure to HDUs, just a simple list
No direct access to HDUs, have to decode each header in turn to find size of data unit.
Always big-endian, requires byte-swap on most popular platforms.
Not quite fully 64-bit clean

4. What’s wrong with VOTable
Three alternative ways of including binary:
Pure XML but very verbose, 4-byte real may turn into ~24 bytes, e.g.
<TD> E-04</TD>
Blob encoded in Base64
Pointer to separate FITS file
Possible loss of linkage
Duplication of header data
Tabular datasets only, no format yet for images, spectra, etc.
Cannot use direct offset to required row or field

5. What we want Flexible header to contain metadata
XML very suitable
Data section to contain heterogeneous data (table) or homogeneous (image etc) but to be efficient, evenly spaced
Binary formats as in FITS very suitable
Endian-neutral
Suitable for streaming data

6. VOTable++ or FITS++?
Use MIME/DIME to have separate sections of file, as proposed for BinX, and have:
XML-based header, very similar to VOTable
Binary data units to follow, similar to FITS
Use BinX/DFDL in header to describe data unit.
Can choose big or little-endian when file created
Allow streaming format for tables, no pre-defined length.
Allow FITS HDU as special case of data unit for compatibility.
Make fully 64-bit clean for tables >2Grows.