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Re: GJD to HJD
Chris and all,
While you can type in a few more lines of code and "think" you have done a
measurement better, testing that you have actually done it can be a major
Suppose you type in those extra lines. Now how are you going to test that
you are not 1 second (or ever larger) off somewhere? Well, you have to set
up some independent measuring apparatus. OK so suppose we borrow some
secondary clock standard and get it set up to dump into an independent
computer system. (We can't use the same one because we might have the same
error in both paths.) Then we rig an interrupter switch to the shutter, and
have it's signal record the time in computer #2. Now we do this a few
times and compare the results and generate an error statistic. Better
still we go out and measure some event. But that is hard since the shutter
is not fast. So we can't measure some occultation to get the time.
It is a lot more than the time it takes to write the few lines of
code. More of my time seems to be spent checking things than building them.
At 09:44 AM 6/14/02 -0700, you wrote:
>We _can_ know the shutter open/close times to a few milliseconds
>Tom's data may not be that good but it is very easy to make it
>that good. Microseconds are hard but anyone with a modem
>connection to the Internet can get milliseconds. If you record the
>time of the STAMP's confirmation message for the open and close
>commands and subtract the transmit time (at 9600 baud) you can get
>to milliseconds not that you really need it.
>I'm one of those nuts who thinks that if I can gain 0.02 seconds
>by only the cost of typing in a dozen lines of code, I'll type it
>in. You only do it once.
>So a one second error in HJD conversion _could be_ the largest error
>source in in the time.
>--- firstname.lastname@example.org wrote:
> > I am probably misunderstanding Chris. He states:
> > >It would appear that
> > >the JD -> HJD conversion is the largest source of timing
> > uncertainty.
> > >Our computer clocks are very easy to keep to <<1 second.
> > >I'd say that unless we can get the error introduced by the
> > conversion
> > >to HJD two orders of magnitude below other sources of timing error
> > >we keep UTC for the archive. I know we can live with a few seconds
> > >of error but it is just plain embarrassing that our data reduction
> > >process should be the largest source of error in a measurement.
> > The largest sources of experimental error remain the computer
> > clock setting, the knowledge of the shutter open/close times, and
> > the time smearing due to the finite exposure length.
> > Using SLALIB, you should be able to get the GJD->HJD conversion
> > to under a second. This conversion is important, as you will get
> > +/- 8minutes timing error due to the size of the earth's orbit.
> > Again, for long-period variables, an 8-minute error might not be
> > important, but for a 2hr eclipser, it can mean the difference between
> > seeing an eclipse and missing it.
> > The difference between assuming the observer is at the center of
> > the earth or somewhere on the surface is +/- 0.02sec, below the
> > sources of other error in the calculation. Ignore it.
> > Using UTC in the database is fine by me, but just remember that
> > you *have* to apply a heliocentric correction to the data before
> > using it for analysis.
> > Arne
> Home: 310-376-1029 email@example.com
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