Talk:International Celestial Reference System and its realizations

Untitled

This article is marked as "stub" but considering that it contains a link to

http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/icrs-narrative

which is a perfect and complete description there is really no reason expand the article! All what should be said is said there!

Stamcose (talk) 08:08, 11 June 2011 (UTC)

Merger discussion

System and frame cannot be described without one another. fgnievinski (talk) 02:45, 2 May 2021 (UTC)

I also notice International Terrestrial Reference System and Frame. fgnievinski (talk) 22:48, 14 July 2021 (UTC)

@Mark viking: as you've undone the move, could you please comment. fgnievinski (talk) 22:48, 14 July 2021 (UTC)

That edit was was more than two years, so I don't remember the full context. But my edit summary gives the gist of it: reference system and reference frame are different concepts and I think ideally they should have separate articles. I envision the reference system article talking about the protocol or system for creating and coming to consensus on a reference frame. Then the reference frame article would talk about the history, impact and uses of the set of reference frames that have been created. How a book is manufactured and how that book is used are obviously related, but that doesn't mean that the publisher and the book should be merged into the same article.

That said, both articles are fairly short at present and a merge could be successful if both topics are clearly distinguished in the merged article. International Terrestrial Reference System and Frame is also two separate topics sitting in the same article, but it is so stubby that it doesn't make sense to split it at this point. --{{u|Mark viking}} {Talk} 00:53, 15 July 2021 (UTC)

@Mark viking: I've started Draft:International Celestial Reference System and Frame. fgnievinski (talk) 04:42, 15 July 2021 (UTC)

I also note that the official publication about the subject is titled "The International Celestial Reference System and Frame"[1] fgnievinski (talk) 04:50, 15 July 2021 (UTC)

The US Naval Observatory site is being re-organised so the links to this site including references [1], [2] & [3] are broken. Replacements for these pages do not seem to exist but may be restored when the work on the website is complete (some time in 2022). — Preceding unsigned comment added by John Murrell (talk • contribs) 10:20, 22 January 2022 (UTC)

Suggested Revisions due to Optical Reference Frames?

I recently added a discussion of the IAU's adoption of the GAIA Celestial Reference Frame for Optical wavelengths and the restriction of the use of the ICRF3 to radio wavelengths. It doesn't fit all that well in the current structure of the article. I suggest discussing the Radio and Optical Reference Frames separately. The present discussion of the various versions of the ICRF could go under a new heading of Radio Wavelengths and a new parallel discussion of the Hipparcus and Gaia reference frames would go under a new heading of Optical Wavelengths. The following outline is what I have in mind.

• Versions
  • Radio Wavelengths
    • ICRF1
    • ICRF2
    • ICRF3
  • Optical Wavelengths
    • Hipparcos Celestial Reference Frame (HCRF)
    • Second Gaia celestial reference frame (Gaia–CRF2)
    • Third Gaia celestial reference frame (Gaia–CRF3)

Since there are now two accepted Celestial Reference Frames, I also suggest changing the title of this article to "International Celestial Reference System and Frames." Comments on these suggested changes are welcome.

--SteveMcCluskey (talk) 19:44, 10 June 2022 (UTC)

As I understand it, there is just a single ICRF3 standard based on multiband radio-wavelength VLBI observations. Gaia–CRF3 is accepted as a realization of the ICRS, but it isn't part of the ICRF3 standard, so changing the the title to "frames", where frame implicitly refers to the ICRF, would misleading. Perhaps "International Celestial Reference System and its realizations"? Implicit is in this is the subtle point While ICRS may spawn multiple reference frames, only one realization at a time is given the name ICRFn, for some value of n, and n=3 is treated as the current standard against which other realizations/frames are aligned.

An organization along the lines

• Reference system and its realizations -- Briefly discuss concepts behind system, realization, and frame
  • Sources -- Briefly discuss that these extragalactic sources come in multiple wavelengths, so far three radio bands and one optical band.
• Versions
  • Radio Wavelengths
    • ICRF1
    • ICRF2
    • ICRF3
  • Optical Wavelengths
    • Hipparcos Celestial Reference Frame (HCRF)
    • Second Gaia celestial reference frame (Gaia–CRF2)
    • Third Gaia celestial reference frame (Gaia–CRF3)

may be worthwhile. That is, having an intro section would set up the context in which the optical frames could be understood as deriving from the ICRS just as the ICRFn have. I am just an amateur in these matters, however, and would be happy to be corrected if I'm misunderstanding things. -- {{u|Mark viking}} {Talk} 21:26, 10 June 2022 (UTC)

Thanks for the comments. I'd be a little cautions in saying "the optical frames could be understood as deriving from the ICRS." The Optical frames are derived from observations that are independent of the ICRFn, although the various optical versions were rotated to agree with corresponding radio positions used for the version of the ICRF at the time of their creation. Both the ICRFs and the Gaia-CRFs meet the technical specifications of the ICRS.

I've just begun working on this project and notice that there are three different versions of ICRF3, depending on the particular wavelength bands used as position sources for a particular version. ICRF1 and ICRF2 use only one waveband as sources so there are single solutions. For the Optical wavelength realizations, the Hipparcos HCRF is based, primarily or exclusively?, on stellar sources, while Gaia-CRF2 and Gaia-CRF3 are exclusively based on extragalactic sources. Since the sources vary for each realization, it seems more convenient to place separate discussions of the number and characteristics of the sources used in the discussions of each version. It would also be useful to compare the three versions of ICRF3 and Gaia-CRF3, assuming there are published sources to be found for such comparisons. --SteveMcCluskey (talk) 20:10, 12 June 2022 (UTC)

Thanks for taking a look. I agree, saying that Gaia-CRFs meet the technical specifications of the ICRS is more accurate than "deriving." Comparison and alignment of a subset of the optical GAIA-CRF3 sources and the ICRF3 radio sources was part of the development of GAIA-CRF3. [2] I'm certain that comparison of sources among the three radio bands was part of the ICRF3 development. It's a good idea to describe within each reference frame's subsection the sources the reference frame used. --{{u|Mark viking}} {Talk} 02:55, 13 June 2022 (UTC)

I've finished my edits for now, but there's one remaining thing that I can't quite make sense of. In the Lede there's a passage comparing the ICRF to some other system of equatorial coordinates:

Coordinates in the ICRF are approximately the same as equatorial coordinates, and the differences at J2000.0 are:

• The mean pole in the ICRF lies at 17.3±0.2 mas in the direction 12 h and 5.1±0.2 mas in the direction 18 h.
• The mean equinox is shifted from the ICRF right ascension origin by 78±10 mas (direct rotation around the polar axis).

Looking at its first appearance in the article, the best sense I can make of it is that it was intended to describe the offset of the ICRS (not the ICRF as the text has it now) from some other system (Perhaps FK5 was intended??). Since it doesn't make much sense as it is now, I suggest deleting the entire passage. --SteveMcCluskey (talk) 20:54, 17 June 2022 (UTC)

Thanks for your improvements. It could be stated better, but this passage reflects the fact that ICRF1 axes were rotated to correspond the optical FK5 J2000.0 optical system to maintain continuity with a reference frame previous to ICRF1. That is, ICRF1 was designed to replace FK5.[3] This historical fact may be better placed in the ICRF1 section. --{{u|Mark viking}} {Talk} 21:31, 17 June 2022 (UTC)