[fitswcs] Draft WCS Paper V: Time
Arnold Rots
arots at head.cfa.harvard.edu
Fri Apr 3 16:46:58 EDT 2009
Bill,
Thank you very much for your comments. They are helpful (of course).
I'll respond more thoroughly later, but can you clarify your third item:
Do I understand you to say that you have an n-dimensional image where
one axis is time as well as, say, a spatial coordinate?
On EXPTIME: The list of keywords in 3.6 is obviously incomplete (and
will always be, since it is there for illustrative purposes), but it
would be good to add EXPTIME and some others. I also know plenty of
FITS files with the keyword EXPTIME, but unfortunately it does not
have the same meaning: it is exposure time, alright, but the single
frame exposure time.
So, it can't very well rise to the level of a standard :(
Cheers,
- Arnold
William Thompson wrote:
[ Charset ISO-8859-1 unsupported, converting... ]
> 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/
> > --------------------------------------------------------------------------
> >
> > _______________________________________________
> > fitswcs mailing list
> > fitswcs at listmgr.cv.nrao.edu
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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)
>
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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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