[fitswcs] Draft WCS Paper V: Time
William Thompson
William.T.Thompson at nasa.gov
Fri Apr 3 18:18:07 EDT 2009
Arnold Rots wrote:
> 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?
An example of the kind of instrument that I'm talking about, and one I'm
particularly familiar with, is the Coronal Diagnostic Spectrometer on SOHO.
Here is an example set of images from that instrument.
http://solar.bnsc.rl.ac.uk/gallery/cds_039.shtml
These images are built up by scanning a linear slit from right to left across
the field. Hence the data on the left side of the image was taken at a
different time from the data on the right side of the image.
Here is how I would envision this data being encoded in a full WCS treatment:
NAXIS = 3
NAXIS1 = 15 /Wavelength direction
NAXIS2 = 60 /Solar-X
NAXIS3 = 143 /Solar-Y
WCSAXES = 4 /To include time as a degnerate axis.
CTYPE1 = 'WAVE'
CTYPE2 = 'HPLN-TAN'
CTYPE3 = 'HPLT-TAN'
CTYPE4 = 'UTC'
CUNIT4 = 's'
CRPIX4 = 1
Plus other keywords describing axes 1,2,3. The PV (or CD) matrix would be
formed so that PV4_2 (or CD4_2) would describe the relationship between the
spatial and temporal dimension.
(The way we handle this now is simply to put a separate array in the binary
table giving the time of each exposure.)
A similar example would be that of synoptic maps, such as the STEREO COR1 maps
found at
http://cor1.gsfc.nasa.gov/synoptic/
Note that in these maps, time increases from left to right, but Carrington
longitude increases in the opposite direction. I envision these being handled
in a similar fashion as above.
A more complicated example would be the Grazing Incidence Spectrometer portion
of CDS, which uses a pinhole instead of a slit, and builds up an image by
scanning in both X & Y directions simultaneously. For that, I think you would
need something like the -TAB projection.
> 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 :(
Understood.
> 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
>>> http://listmgr.cv.nrao.edu/mailman/listinfo/fitswcs
>>>
>> --
>> 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)
>>
> --------------------------------------------------------------------------
> 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/
> --------------------------------------------------------------------------
>
--
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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