I just realized that the dT difference depends heavily on weather the asteroid perturbation effect is taken into account or not: in our case if you shut off the 1950 obs but take the asteroid perturbation into account the dT difference goes down from 31 minutes to about 23 seconds.


Cheers
Alessandro

--- In find_orb@yahoogroups.com, "Alessandro" wrote:
>
>
> Thanks Rob and Bill for your reply.
>
> What is very strange is that if you download all the available obs from MPC including the 1950 ones ... When you use Find_Orb to calculate the orbit, provided that you do a lot of full steps taking into account planets and asteroid... the residuals for the 1950 obs are very little. This is what initially led me to believe that everything was fine!
>
> Maybe this is just an evil effect and as you say the problem here is simply to identify the correct time for those obs.
>
> Best wishes
> Alessandro
>
> --- 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
> >
>