[coords] Question - Time domain coordinates and frames

[Arnold Rots]

No, the timeOrigin is only REQUIRED in a TimeFrame if it is referenced by a
TimeOffset.
So, both the TimeOffset and the timeOrigin can (and should!) refer to the
same TimeFrame.
(they should, because specifying the origin in a different TimeFrame is
asking for trouble)

It does mean that TimeOffsets that use different timeOrigins need to
reference different TimeFrames.
That is not a problem, since most TimeOffset-based time series will use the
same timeOrigin.

  - Arnold

-------------------------------------------------------------------------------------------------------------
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 cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------------------------------------------


On Thu, Apr 12, 2018 at 3:54 PM, CresitelloDittmar, Mark <
mdittmar at cfa.harvard.edu> wrote:

> Thanks Arnold..
> I agree with how you described just about everything in terms of the
> current types.
> The one part that causes me confusion is below..
>
> So.. timeOrigin
>   1) should be provided in the Frame ONLY if the referring coordinate is a
> TimeOffset.
>   2) is expressed as a non-TimeOffset TimeStamp
>   3) 'referenced to the same TimeFrame' (as the original TimeOffset?)  <<<
> this I don't understand
>        If the TimeOffset.frame contains a timeOrigin expressed as a MJD
>        and that MJD refers to the same frame, that would be an MJD
> referring to
>        a frame with a timeOrigin, which opposes #1.
>        If, the MJD refers to a different frame (with no timeOrigin), then
> you have a short train of types/frames.
>
> You are asserting that if I have 2 TimeOffset-s with different offsets,
> they are in different frames.
>    T1{ time=t1, frame=TimeFrame1{ timescale='TT',
> refPosition="TOPOCENTER", timeOrigin=MJD{ date=50814.0, frame=TimeFrame1} }
> }
>    T2{ time=t2, frame=TimeFrame2{ timescale='TT',
> refPosition="TOPOCENTER", timeOrigin=MJD{ date=51179.0, frame=TimeFrame2} }
> }
>
> or maybe
>   TimeFrame0{ timescale="TT", refPosition="TOPOCENTER"}
>    T1{ time=t1, frame=TimeFrame1{ timescale='TT',
> refPosition="TOPOCENTER", timeOrigin=MJD{ date=50814.0, frame=TimeFrame0} }
> }
>    T2{ time=t2, frame=TimeFrame2{ timescale='TT',
> refPosition="TOPOCENTER", timeOrigin=MJD{ date=51179.0, frame=TimeFrame0} }
> }
>
> but they are NOT in the same frame, with different offsets..
>   TimeFrame0{ timescale="TT", refPosition="TOPOCENTER"}
>    T1{ time=t1, frame=TimeFrame0, timeO=MJD{ date=50814.0,
> frame=TimeFrame0} }
>    T2{ time=t2, frame=TimeFrame0, timeO=MJD{ date=51179.0,
> frame=TimeFrame0} }
>
> Mark
>
>
> On Thu, Apr 12, 2018 at 2:49 PM, Arnold Rots <arots at cfa.harvard.edu>
> wrote:
>
>>
>> The time stamp can be specified as:
>> JD (no units required)
>> MJD (no units required)
>> ISO-8601 (no units required)
>> TimeOffset (units required, as well as a value for timeOrigin in the
>> frame)
>>
>> The timeOrigin should expressed as a TimeStamp (JD, MJD, or ISOTime, but
>> NOT TimeOffset),
>> referenced to the same TimeFrame.
>>
>> So, yes, this is recursive, but there are strict constraints on the
>> timeOrigin which should prevent runaway looping.
>>
>>
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