[fitswcs] Draft WCS Paper V: Time

[William Thompson]

Arnold:

I'm very happy to see the time paper draft.  Time is a very important issue for 
solar physics, where we're examining dynamic phenomena.  On the whole, I'm 
pleased with the way this paper is forming up.  Below are some individual 
comments on the paper.

* First sentence:  If there's such a thing as astromomical data, is there also 
astropopical, or maybe astrodadical?  (I'm sorry, I couldn't resist.  ;-) )

* Section 2:  Time sampled data streams encompasses more than just light curves. 
  Other examples include magnetic field strength, solar wind speed, proton 
density, and other in situ quantities.

* Section 2:  Another very important use case is that of image maps which are 
built up from scans of a scalar or one-dimensional detector, such as with a slit 
spectrograph.  This is a particularly challenging use case because the time 
dimension is directly convolved with one or both spatial dimensions.

* In section 3.1.1 it's stated that the ISO-8601 data type can only be used in 
"pixel list" mode.  I think it's more accurate to state that it can only be used 
for lists, of which pixel lists are a typical example.  Another example would be 
a collection of images in a FITS binary table, where one column contains the 
images, and another column contains the datetime strings associated with each image.

* In Table 1, and several other points in the paper, it's stated that UTC 
differs from TAI by an integral number of seconds.  That is only true starting 
in 1972.  Prior to 1972, the UTC time scale was varied linearly relative to TAI.

* Section 3.6:  I was surprised not to see the keyword EXPTIME mentioned in this 
section.  This is the keyword used to express the exposure time in just about 
every FITS header that I'm familiar with, and we consider it a standard.

* I'm confused by the discussion of the time offset.  Is this an additional time 
which is added to the reference time before adding to the relative time to form 
an absolute time?

* Section 4.2.3:  I'm also confused by this section.  I believe that what's 
happening is that one uses CRPIX, CRVAL, CDELT, etc. to form the relative time, 
and then add DATEREF to convert this to an absolute time.

* There's no discussion of projections used with time axes.  An obvious 
projection applicable to time is the table (-TAB) projection from the 
spectroscopy paper.  This would be particularly useful for scanned slit 
spectrograms.

* It might be worth pointing out that relative times in UTC format must include 
any leap seconds which occur in the interval.  For example, if the reference 
date is DATEREF='2008-12-31T12:00:00', and the time value is 86400 seconds, that 
actually translates into '2009-01-01T11:59:59' because of the intervening leap 
second.  That may seem obvious, but I think it should be emphasized.

That's all I have for now.

Bill Thompson


Arnold Rots wrote:
> FITS WCS Paper V: Time
> ======================
> 
> Draft is available for comment
> ------------------------------
> 
> 
> I have the pleasure, on behalf of the team of authors, of (finally)
> announcing the draft of WCS Paper V on Time coordinates.
> 
> The group of authors has worked quite hard over the winter to get to
> this point where we feel it is ready for public discussion.
> During this time our dear and respected friend and co-author Pete
> Bunclark passed away. He put a lot of effort into this paper
> during the last months of his life and without him it would not have
> been where it is today.
> 
> You may find the draft (Version 0.71) and some useful links at:
> 
>     http://hea-www.harvard.edu/~arots/TimeWCS/
> 
> This version still needs some polishing and there are a few issues
> that are yet to be resolved. You will notiice editorial comments
> printed in red.
> 
> The intent is that this discussion be carried on through
> fitswcs at nrao.edu even though this announcement also goes out on
> fitsbits.
> 
> 
> Enjoy!
> 
>   - Arnold Rots
> 
> 
> --------------------------------------------------------------------------
> Arnold H. Rots                                Chandra X-ray Science Center
> Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
> 60 Garden Street, MS 67                              fax:  +1 617 495 7356
> Cambridge, MA 02138                             arots at head.cfa.harvard.edu
> USA                                     http://hea-www.harvard.edu/~arots/
> --------------------------------------------------------------------------
> 
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-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

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

(Note changed email address)