Alessandro Feb 2, 2013
--- In find_orb@yahoogroups.com, Bill Gray wrote:
>
> Hi Alessandro,
>
> What Rob just said.
>
> If you load up the observations for this object and shut off the 1950 data
> and solve for an orbit, and then click on either 1950 observation, you'll
> see that the summary of data for the observation says "dT = -33.1 minutes;
> cross 0.38." (And close to that for the other observation.) Meaning that
> the object passed within .38 arcseconds of that location 33.1 minutes earlier.
>
> I notice also that the two data points are 31.5 minutes apart. (Easy
> way to do this is again to click on each observation; the time is given
> in HH:MM:SS form in the observation summary area.)
>
> Ideally, you'll be able to dig up a corrected time for those observations,
> and things will fit Just Fine.
>
> This is a pretty handy thing to keep in mind. When I see an observation
> with bad residuals, I'll usually check the dT and cross-residual data;
> it's amazing how often I'll see that it's off by exactly an integer number
> of hours, but with a sub-arcsecond cross-residual. For NEOs, it's often
> off by just a few seconds, i.e., data taken by somebody who got good
> astrometry, but either didn't set their clock well enough or had problems
> with trailed data.
>
> (Though why your two 1950 data points fit in the first place is a little
> strange. Maybe you had the right times in the file fed to Find_Orb, but
> not in the file sent to MPC?)
>
> -- Bill
>
> On 02/02/2013 03:12 AM, Alessandro wrote:
> > I think I found a 1950 DSS precovery image of this asteroid.
> > When I used Find_Orb to check the residuals of these observations, I got
> > a reasonable result: they were well below 0".5, and the overall mean
> > residual was 0".784
> >
> > So I sent the measurements to MPC and apparently all was fine, the
> > observations appear in the MPC database. However, yesterday I
> > realized that the 1950 observations are not used in the orbit
> > calculation because their residuals are incredibly high.
> > MPC calculates:
> >
> > 19500422 675(29.6- 10.2+)
> > 19500422 675(29.6- 10.3+)
> >
> > I can not understand this.
> > Did I make a big error when using Find_Orb?
> >
> > Thanks!
> >
> > Best wishes,
> > Alessandro Odasso
>