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tart-telescope / tart2ms

Convert TART data to measurement set format
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Rephase to arbitrary user provided position #47

Closed bennahugo closed 1 year ago

bennahugo commented 1 year ago

Needed to rephase to e.g. Fornax A for mapping

tmolteno commented 1 year ago

Agree that this needs to be done. I think adding a

  --rephase {obs-midpoint, ICRS}
                        Rephase all visibilities to a new phase center (default: None)

and removing the SCP and NCP (since these are just special cases of ICRS coordinates). We need to be absolutely specific about which reference frame we are using e.t.c. if it is to be ICRS, we need to point to a reference document about how to generate these co-ordinates. What document (URL) do you propose?

bennahugo commented 1 year ago

I think I will keep all the options under one CMD flag - it is useful to recognise names from catalogs, special bodies and special positions. The solar system objects can only be tracked by name as they don't have a fixed ephemeris.

On Sun, 11 Dec 2022, 20:32 Tim Molteno, @.***> wrote:

Agree that this needs to be done. I think adding a

--rephase {obs-midpoint, ICRS} Rephase all visibilities to a new phase center (default: None)

and removing the SCP and NCP (since these are just special cases of ICRS coordinates). We need to be absolutely specific about which reference frame we are using e.t.c. if it is to be ICRS, we need to point to a reference document about how to generate these co-ordinates. What document (URL) do you propose?

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tmolteno commented 1 year ago

I was referring to getting rid of NCP and SCP since these are probably not targets to track. Having some objects by name isn't a bad idea, but I this issue is around specifying arbitrary positions, and so we need to be absolutely specific about which reference frame we are using . We therefore need to point to a reference document about how to generate these co-ordinates. Which celestial frame are you propose?

We then need to point to a reference document about how a user can generate these coordinates. This should probably be in the [docusaurus] (https://github.com/tart-telescope/tart_website), with references.

We can decide which objects to track by name in Issue #46, but here we should make the decision about which co-ordinate system and syntax for specifying it should be used.

tmolteno commented 1 year ago

I have added a placeholder at (https://github.com/tart-telescope/tart_website/blob/main/docs/tutorial-extras/measurement-sets.md) which can point to documentation about the co-ordinates and syntax for them.

bennahugo commented 1 year ago

Not sure I want to remove the poles as named positions, rather just augment with additional recognized objects and positions. I'm inputting 3CRR catalogs with names as filed. I will perhaps move them to a json somewhere and document them as requested

I think we should stick with J2000 positions throughout and use regex to recognize positions. So midpoint, ncp, scp are recognized as stands (plus and additional named positions to be read). Then special objects (Sun, Moon, Mars etc). Then catalog names from the 3CRR catalog (ASCII catalog as available online), then regex extracted positions J2000 as specified by casa

On Mon, 12 Dec 2022, 09:46 Tim Molteno, @.***> wrote:

I have added a placeholder at ( https://github.com/tart-telescope/tart_website/blob/main/docs/tutorial-extras/measurement-sets.md) which can point to documentation about the co-ordinates and syntax for them.

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tmolteno commented 1 year ago

This issue is about specifying celestial coordinates. Tracking objects by name is Issue #46.

  1. We have to choose a syntax for coordinates. This is simply not optional.
  2. We must provide a link to where this is defined (first in this issue, then in the code and also in the documentation), and how they are going to be represented.
  3. Technically J2000 is a reference epoch. (https://en.wikipedia.org/wiki/Epoch_(astronomy)#J2000), not a coordinate system. I suggest we use ICRS.
  4. Here is the astropy documentation (https://docs.astropy.org/en/stable/coordinates/index.html). We need to be clear which syntax and grammar are going to be used for coordinates on the command line?
bennahugo commented 1 year ago

@tmolteno I agree we should use ICRS as the system, but to be honest, there is no difference between FK5 and ICRS for the resolutions we are talking about (order few tens of mas whereas our resolution is at best degrees...). There is no need to overcomplicate the CMD arguments with additional system frame specifications, e.g. epoch (more important) ra dec as follows

J2000 9h25m00s -05d12m00s

the syntax will therefore be --rephase {obs-midpoint, ncp, scp, [other known bodies], [solar system bodies], [known 3CRR positions], radec} with appropriate documentation in the CMD with examples.

tmolteno commented 1 year ago

We need a no_space format for the arbitrary positions. The new IAU name - which corresponds to the co-ordinates is ideal for this purpose:

    tart2ms --rephase J0925-0512

This is both sufficiently accurate, from a TART resolution point of view, as well as allows people to use source designations from papers which commonly use the new IAU format.

Notes:

bennahugo commented 1 year ago

Excuse the brevity, not at my desk. Reason for the more elaborate format is to specify B1950 epoch and GAL coordinates.

On Thu, 15 Dec 2022, 00:13 Tim Molteno, @.***> wrote:

We need a no_space format for the arbitrary positions. The new IAU name - which corresponds to the co-ordinates is ideal for this purpose:

tart2ms --rephase J0925-0512

This is both sufficiently accurate, from a TART resolution point of view, as well as allows people to use source designations from papers which commonly use the new IAU format.

Notes:

  • This would mean that NCP is J0+90, and SCP is J0-90.
  • Higher accuracy might need more precision, but suggest doing this with a different PR.
  • I think this is pretty friendly, and would mean we don't need to try and build fixed source catalogues into tart2ms, but rather can read any catalogue (https://3crr.extragalactic.info/cgi/database) and easily enter the designator.
  • @rubyvanrooyen https://github.com/rubyvanrooyen might have useful input here.

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tmolteno commented 1 year ago

This does not preclude a more general method, however I think that this should not be done until a proper syntax is defined. At this stage:

The key is to have code that is easy to maintain, and this means doing the minimum necessary to do the job.

bennahugo commented 1 year ago

Ok I will compromise on J|B followed by twelve digits for now. The proposed arbitrary digits are a bit too confusing as they may not mean HHMMSS+/-DDMMSS. This therefore only supports either J2000 or B1950 epochs. I have implemented this and a PR is incoming

bennahugo commented 1 year ago

I leave SCP and NCP along with arbitrary specifyable lists of special coordinates and catalog defined coordinates as part of the options