Re: [Celestia-developers] stars.dat

[Andrew T. <ajt...@go...>]

>From links on the CDS, I found http://fr.arxiv.org/abs/0708.1752
"Validation of the new Hipparcos reduction" by van Leeuwen which
includes discussion of the negative parallaxes.

Seemingly therefore it would make sense to use the new reduction for
position and parallax information despite the fact this would lead to
a smaller number of stars included. Spectral types are not listed in
the new reduction, so would have to be sourced from hip_main.dat.

Would it be better to use the Vmag in the original hip_main.dat (which
appears to be a catalogue-derived value, rather than measured by
Hipparcos), or transform from Hp and B-V using
http://www.tass-survey.org/tass/catalogs/tycho.html ?

Andrew

2008/7/14 Fridger Schrempp <fri...@de...>:
> I think Selden's arguing in the right direction. First of all, someone should
> UNDERSTAND what the physical basis was that allowed to reduce the initial
> uncertainties so significantly. Was it merely statistically based or was there
> more new input? Only then could an independent judgement be made about the
> reliability of particular reevaluated parallaxes that have changed even in
> SIGN! Certainly, to me this fact already signals a considerable instability,
> which I would want to first understand before making a decision!
>
> If I was involved with that analysis, I'd get into email contact with the
> author. That's always the safest recipe.
>
> Fridger
>
>
> Selden E Ball Jr wrote:
>> The appearance of negative parallaxes when improved data reduction
>> algorithms were applied suggests to me that the previous corresponding
>> positive parallax values may have been incorrect.
>>
>> I suspect that it would be inappropriate to mix values from the two catalogs.
>>
>> I also suspect that there might be some discussion in the book
>> about the new negative parallax values. I'm sure they won't have
>> been overlooked.
>>
>> s.
>>
>>> It's nice to see that new reduction is finally available. The
>>> disturbing thing is that quite a few formerly "good" parallaxes are
>>> now negative.
>>
>>> Option 2 gives us the largest number of useful parallaxes, but it
>>> seems difficult to defend on theoretical grounds. Shouldn't we always
>>> use the value with the lowest error?
>>
>>> --Chris
>>
>>> On Sat, Jul 12, 2008 at 8:36 AM, Andrew Tribick
>>> <ajt...@go...> wrote:
>>>> I've been investigating the new reduction of the Hipparcos data (now
>>>> available at http://cdsarc.u-strasbg.fr/viz-bin/Cat?I/311)...
>>>>
>>>> It looks like the parallax errors have either decreased or remained
>>>> the same, which is good.
>>>>
>>>> Defining "good" parallaxes as those >0.4, the changes original (HIP1)
>>>> -> new (HIP2) are:
>>>>
>>>> Changes for the better:
>>>> negative->good = 2387
>>>> negative->low = 1858
>>>> low->good = 1294
>>>>
>>>> Changes for the worse:
>>>> good->negative = 4994
>>>> good->low = 1780
>>>> low->negative = 525
>>>>
>>>> The question now is what to do about the latter three cases.
>>>> 1) Use HIP2 regardless
>>>> 2) In the cases where HIP1 has a +ve parallax and HIP2 has -ve
>>>> parallax, use data from HIP1.
>>>> 3) In the cases where the change is for the worse, use HIP1.
>>>> 4) other
>>>>
>>>> I'm leaning towards option (2) myself.
>>>>
>>>> Andrew
>>>>
>>
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