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Update readme for 1p8 #182

Merged
merged 3 commits into from
Jul 25, 2024
Merged

Update readme for 1p8 #182

merged 3 commits into from
Jul 25, 2024

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javierggt
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@javierggt javierggt commented Jun 26, 2024

Description

This PR makes minimal changes to readme (the one in github.io). I tried to leave the readme as unchanged as possible, but I actually believe it should be improved more. I think as it stands, the readme is hard to understand.

Still, these changes would bring it up to date.

questions:

  • should we document here that slight changes to COLOR1 to avoid 0.7 and 1.5?
  • What is the alpha in the MAG_BAND description?

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@jeanconn
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Regarding "should we document here that slight changes to COLOR1 to avoid 0.7 and 1.5?" - I'm thinking that's a tiny processing detail that maybe doesn't make the final public doc for COLOR1 (which currently has)

COLOR1 - float variable expressing the cataloged or estimated B-V color,
        used for mag_aca, in mag.  If no colors are available, the default
        value is 0.7000.  If Tycho-2 data show colors redder than
        (B-V)=1.5, then the default value is 1.5000.  True cataloged
        color values are stored in COLOR2.

The details about 0.7000 and 1.5000 remain true.

@jeanconn
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Regarding the "what is alpha" question in the MAG_BAND section - I'm not clear about how those values were determined. In the old agasc 1p5 comments there is a reference about how it was used.


In AGASC1.5, MAG_ACA is derived from V and B-V (generally COLOR1) via
the polynomial color equation 
     MAG_ACA= V + C0 + C1*(B-V) + C2*(B-V)^2 + C3*(B-V)^3 
where
     (B-V)= beta*COLOR1
and
     V= MAG - alpha*(B-V)
Values of alpha are tabulated for every published magnitude individually.

For published magnitudes from Tycho catalogs, we use
        beta=  0.85
        C0=    0.428638   
        C1=   -0.774029   
        C2=    0.283002   
        C3=   -0.267284

If published magnitudes are from the PPM catalog, we use
        beta= 1.0
        C0=   0.478573
        C1=   0.221899
        C2=  -0.550461

While for stars with no color information (generally stars with only
GSC1.1 magnitudes) we use only
        C0  -0.65

From an analysis of the Tycho colors of observed ACA mags, we find
an RMS dispersion in color of just 0.5mag, which is the new value used
in AGASC1.5 for the MAG_ACA_ERR calculations of all stars with no
(B-V) color information, or with (B-V)>1.5 from Tycho-2.  The
old color error value of 1.5mag contributed to large MAG_ACA_ERR
values in earlier versions of AGASC that reduced the efficiency of
guide star selection inefficient.  

@jeanconn
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I figure we should just ignore alpha in the context of the AGASC 1.8 readme update.

@javierggt
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javierggt commented Jul 23, 2024 via email

@jeanconn
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I would say that I think the AGASC 1.8 catalog update is "proper" - you just have the code in the agasc-gaia repo showing how MAG_ACA was derived instead of these old/defunct polynomials.

@jeanconn
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Though that kinda raises the question - should we make the agasc-gaia repo public and link to it from the README?

@taldcroft
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Though that kinda raises the question - should we make the agasc-gaia repo public and link to it from the README?

I would prefer not since there is a lot of uncurated material there.

@taldcroft
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About:

 If Tycho-2 data show colors redder than (B-V)=1.5, then the default value is 1.5000.

That is not precisely correct if the AGASC catalog color is now from Gaia but Tycho-2 shows B-V > 1.5 . I would suggest:

If the color is derived from Tycho-2 (C1_CATID=5) and that color is redder than (B-V)=1.5 then
COLOR1 is set to 1.5000. This is the case for about 21,000 stars in AGASC 1.8. True cataloged 
color values are stored in COLOR2.

@taldcroft
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About the alpha issue, I think we don't care and that leaving it blank as in this PR is fine.

@taldcroft
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And I didn't see the I figure we should just ignore alpha in the context of the AGASC 1.8 readme update. bit. So we are in agreement.

@taldcroft
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Apart from my one comment, I think this is good enough.

@jeanconn
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OK I updated the COLOR1 text per Tom and a tiny typo while I was in the neighborhood.

@jeanconn jeanconn merged commit 718146b into master Jul 25, 2024
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@jeanconn jeanconn deleted the readme branch July 25, 2024 15:48
@javierggt javierggt mentioned this pull request Nov 7, 2024
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3 participants