[daip] POLARX/Y in FITSfiles (was: forwarded message from Harro Verkouter)

Fri Aug 11 06:25:40 EDT 2006

Hi John,

Thanks for sharing your insights in this issue.

I agree it is more of a geodetic thing than anything else and I don't 
think you've oversimplified.

Maybe the easiest is to implement a change in FITLD such that for JIVE 
FITS files the value of POLARX/Y is recomputed to be in units of arcsec.

This means that JIVE would continue to write the values as-is so there 
would be no time-dependency: all past and future JIVE FITS files 
would/will be equally wrong and the fix equally simple... no subclauses 
or ifs/buts/whens... just recompute unconditionally (apart from the 
origin==JIVE condition obviously...).

I agree that perpaps in this case the IDI document is in error so 
propably it's best to change it.

We would document though that for historical reasons, JIVE wrote/writes 
the values in different units.

I think there is already some JIVE specific code in FITLD so adding a 
bit to that shouldn't be that difficult I hope.

Let me know if this might be a workable solution.

Cheers,

harro verkouter

Jon Romney wrote:
> 
> Thanks for the forward, Eric.
> 
> This clearly falls in the astro/geodetic area for which Craig
> Walker is the VLBA's contact person.  However, during his
> current prolonged absence I will make the following points --
> 
> --  This is not "just" a matter of convention among VLBI people.
> There is a convention in the earth-rotation community, which all
> VLBI systems should adhere to.  I believe it is to express the
> polar X/Y coordinates in arcseconds.  However, I'm not sure,
> and have made a point of *not* checking so that my suggestions
> below may seem less self-serving.  If my belief is correct, then
> the fundamental error is in the IDI spec.
> 
> --  The proper fix should involve changing whichever data are
> inconsistent with the earth-rotation conventions.
> 
> --  Perhaps I've missed something, but I don't see why it would
> be so difficult to operate on archive data.  The fix is very
> simple, so it could be included in FITLD as Harro suggests.  It
> could be applied every time at no significant computing load,
> without any need to retro-edit existing files.
> 
> It would "only" be necessary to identify the files that need to
> be fixed.  I believe the IDI files include sufficient informa-
> tion to do that: the correlator or organization that wrote the
> file, and possibly the date it was written.
> 
> Apologies if I've oversimplified the issue.
> 
> Jon
> 
> 
> Eric Greisen wrote:
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>> From: Harro Verkouter <verkouter at jive.nl>
>> To: Eric Greisen <egreisen at nrao.edu>
>> CC: Huib Jan van Langevelde <langevelde at jive.nl>
>> Subject: Re: [daip] bug in AIPS 'tst"/ POLARX/Y
>> Date: Thu, 10 Aug 2006 08:30:30 +0200
>>
>>
>> Ok so I changed the subject to incorporate POLARX/Y.
>>
>> To summarize: it is a matter of units. IDI says it should be 'meters', 
>> VLBA s/w writes in units of 'arcseconds'. This breaks (at least) the 
>> UVFIX task - UVWs are way off since apparently UVFIX *expects* 
>> arcseconds rather than meters.
>>
>> Our original question to you was: do you know if AIPS has an internal 
>> unit associated with the POLARX/Y values and if so, which one is it? 
>> Other tasks besides UVFIX may be affected by this.
>>
>> Depending on if/what AIPS internally thinks we may or may not have to 
>> fix FITLD to recompute the value of the POLARX/Y keyword in order to 
>> represent it in the units which are internally used.
>>
>> Browsing through 31DEC06 sources I found POLARX/Y referenced in a few 
>> places and mostly in the comments it is taken to be in units of meters 
>> save in the file APL/SUB/JPOLAR.FOR where it is assumed to be in units 
>> of arcsec....
>>
>>
>> The fix in itself would be a no-brainer for any of us - VLBA, JIVE, 
>> AIPS   - I'd say.
>> However, the more difficult thing we're faced with is the question of: 
>> how can we fix this in such a way that it is somehow consistent and 
>> hopefully not break existing archive data... it would be quite a 
>> nuisance to have to retro-edit FITS files to modify the values of the 
>> POLARX/Y keywords. That would definitely not be a no-brainer I hope 
>> you'll agree with me.
>>
>> By the way - answer these issues at your own convenience...
>>
>> Cheers,
>>
>> harro verkouter
>>
>> Eric Greisen wrote:
>>
>>> I can get the info eventually if you will remind me of the POLARX/Y
>>> problem.  The VLBA is run by expert scientists not me - all aips does
>>> is use whatever the correlator writes which may or may not be
>>> correct.  Craig Walket, John Romney are ones who might change what is
>>> written.  I cannot find the previous e-mail - either I lost it or it
>>> was not from you and it does not say POLAR in its subject.
>>>
>>> Eric Greisen
>>
>> ------- end -------
>>
> 
> 