Proper motion, parallax and aberration

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Akarsh Simha

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Aug 9, 2021, 4:46:52 AMAug 9
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Hi

I'm curious to know how astrometry.net handles proper motion, stellar parallax and aberration. Whereas precession, nutation and the like affect all stars uniformly through a rotation, and hence do not change the skymarks, the aforementioned effects affect different stars differently and would hence alter the skymarks. As we move further from the year 2000, I imagine that the indexes will need to be regenerated for the present epoch to account for proper motion. But parallax and aberration having a period of a year, I imagine this is not a useful strategy in this case. Assuming a "perfect" imaging system producing a perfect plate of the sky, I presume that parallax and aberration would be the primary sources of error in the solution?

Regards
Akarsh

Dustin Lang

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Aug 13, 2021, 8:53:01 AMAug 13
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Hi,
The short answer is that we ignore them :)  For most stars and most images, the motions are tiny enough to not matter in terms of locating an image on the sky.  In previous work, we showed that we can use the proper motions to guess the date an image was taken (https://ui.adsabs.harvard.edu/abs/2008AJ....136.1490B/abstract).
And yes, at some point this would become a limiting factor.
cheers,
--dustin

John Murrell

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Aug 13, 2021, 9:17:14 AMAug 13
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If you use the indexes based on the Gaia positions these are not J2000 but are in a mid mission frame of reference so are different for DR2 & EDR3. Details of the reference epoch are on the Gaia website. This allows for the proper motions upto the date of the epoch. Also programmes like Aladin precess star positions to the current date. You will see this sometimes with HPM stars where the DSS image has an orange & cyan image showing the star positions when the Red & Blue plates were taken but the star position as now shown from the Simbad data has now moved.

The other thing to catch people out are the IAU constellation boundaries which were defined in the Epoch of 1875 so no longer align with the J2000 RA & Declination lines. This is particularly noticeable if you view the boundaries near the North or South celestrial pole.

John Murrell
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