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Subject: Re: Star location accuracy, Part 2
From: Philip Hill
Reply To: mapug@shore.net
Date: Tue Dec 20 02:14:00 1994

> > Some concern about Regulus' location accuracy was expressed. I find
> > that its location is 0.51 arc minutes off (in 2000)

The FK5 position of Regulus for equinox and epoch J2000 is

RA: 10 08 22.315 Dec: +11 58 01.89

The position in my manual (#1697 CDS for the German Polar Mount, Ver 0693-1) for star 100, Regulus, is

RA: 10 08.5 Dec: +11 58

This makes the error 1.88 arc minutes.

If the RA were correctly rounded to 10 08.4 the error would be 0.41 arcmin. If the positions are correctly rounded to 0.1 min of time for RA and 1 arcmin for Dec the maximum error would be 0.90 arcmin. Anybody with the latest software, which I believe gives positions to seconds, might care to check.

FK5 stands for Fundamental Katalog 5 and is the basis for the whole system of astrometric star positions. The mean errors quoted for the position of Regulus are about 10 milliarc seconds. Equinox J2000 refers to the coordinate system. Epoch J2000 includes proper motion and specifies where the star will actually be in 2000. However, you shouldn't need to worry about proper motions unless you are setting on Barnard's star or Alpha Cen.

The gross errors in position and identification are indeed a matter of concern. I reported one some time ago, but obviously that was but the tip of the iceberg. If they have not done so already, I hope Meade will take this on board urgently. This is certainly a matter of quality control, but it only has to be checked once, not for every telescope.

Phil Hill (pwh@st-andrews.ac.uk)