<div dir="ltr">
<span class="gmail-im">Steve Allen Wrote:<br></span>
<i>In historical data each catalog is its own reference frame,<br>
and the literature of constructing recent catalogs using the<br>
old data shows the painstaking measures needed to remove the<br></i><div><i>
systematic effects present in the original catalogs.</i><span class="gmail-im"><br></span></div><div><br></div><div>I will grant you that it is extremely important to get this right for the</div><div>positions that were used to derive the PM.</div><div>However, the PM itself is a different matter - with some notable</div><div>exceptions; Polaris comes to mind.<br></div><div><br></div><div> - Arnold</div><div><br></div><div><span class="gmail-im"></span></div><div><span class="gmail-im"></span></div>
<div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div><br></div><br><div class="gmail_quote"><div dir="ltr">On Sat, Dec 22, 2018 at 2:09 PM Steve Allen <<a href="mailto:sla@ucolick.org">sla@ucolick.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On Sat 2018-12-22T13:40:40-0500 Arnold Rots hath writ:<br>
> which reference frames apply?<br>
> Depends on what the provider of the data used; for modern data one would<br>
> hope ICRS, but historical data could be any FK<br>
<br>
In historical data each catalog is its own reference frame,<br>
and the literature of constructing recent catalogs using the<br>
old data shows the painstaking measures needed to remove the<br>
systematic effects present in the original catalogs.<br>
<br>
> On the other hand, I suspect that for proper motions with any reasonable<br>
> precision this won't make a bit of difference<br>
<br>
That statement would remove volumes of painstaking efforts from the<br>
literature.<br>
<br>
--<br>
Steve Allen <<a href="mailto:sla@ucolick.org" target="_blank">sla@ucolick.org</a>> WGS-84 (GPS)<br>
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855<br>
1156 High Street Voice: +1 831 459 3046 Lng -122.06015<br>
Santa Cruz, CA 95064 <a href="http://www.ucolick.org/~sla/" rel="noreferrer" target="_blank">http://www.ucolick.org/~sla/</a> Hgt +250 m<br>
</blockquote></div>