[AZ-Observing] Re: WDS J14171+0103A
- From: Brian Skiff <bas@xxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Sat, 19 Dec 2015 13:10:02 -0700
On Sat, 2015-12-19 at 12:07 -0700, Michael Collins wrote:
On 12/18/15 19:52, Brian Skiff wrote:
Well, the Hipparcos _limit_ is around mag 11 or so,
but the survey completeness was only around mag 8.
This was my general recollection, though I didn't remember the
magnitude limit. I looked at the Hipparcos home page yesterday, where I
found the claim that "The Tycho 2 Catalogue, completed in 2000, brings
the total to 2,539,913 stars, and includes 99% of all stars down to
magnitude 11..." From that, I errantly inferred that there should be a
good chance the pair in question would be in the catalog.
The Tycho instrument on the spacecraft _did_
do continual sweeps of the sky as the spacecraft
spun around. After a couple iterations on the
reduction process, the Tycho-2 catalogue was
produced, but again working from a much larger
ground-based input catalogue. They had to work out
the spacecraft orientation moment-by-moment,
then using that predict from the star catalogue when
the 2.5 million stars would transit through the
instrument field, and reconstruct the detections
from here. Even in the 90s of course this was
an immense computing problem (a colossal multi-
dimensional matrix), which is why it took
so long to get the catalogue out. Details are here:
http://adsabs.harvard.edu/abs/2000A&A...357..367H
The first sections up to the first parts of
section 3 are enough to let you recognize how big a
problem this was. Figure 6 is also revealing.
It says that the photometry errors reported in
Tycho-2 are too small by 50 percent for stars
fainter than mag 9 (i.e. most stars). So whatever
the error is that's given in the catalogue needs
to be increased by half to give the real external
error. "Some sources of additional noise unaccounted"
they admit in the text. Nevertheless, it is an
amazing work.
The revised Hipparcos catalogue, by the way is
item I/311 in VizieR.
After a bit of fumbling about, I think I know how to submit a query
correctly, but still don't find an entry. So, not in the revised catalog
either, right?
Right, the star was too faint for the Hipparcos
side of things. Only 120,000 bright(er) stars in
Hipparcos versus 2.5 million for Tycho-2.
So for this pair, HD 125057, here's what I did using
JD's image. First thing is ....
I saw the spectral type in one of the references and briefly
considered going through the exercise you detailed, but I was already
off on a tangent by that point. That said, thanks not only for chasing
this down but also for providing a comprehensive description of the
process. It's a great example of the work involved in turning disparate
measurements into information.
...which was kinda the point! B-) Despite the
huge worldwide effort to try to create software to allow
researchers to dig into the current and upcoming immense
databases, there's seems as yet no way around "experience"
to provide the holistic view and ability to sort the
useful/not useful and good/bad data. You still have to
know a lot of astronomy!
The story behind the story. One of the reasons I never pass up the
chance to have a beer with an astronomer.
The USNO folks are very good about providing
these files, as Richard Harshaw noted, but it seems they
shouldn't have to waste any time on it. There's just
two guys maintaining the WDS catalogue, who try to
get some original work done in addition, and one of 'em
counting the days to retirement.
Thus I also know that they appreciate amateurs
like Richard supplying them with results, and myself
submitting measurements for new and known binaries
(about 20,000 measures so far, including data for 2500
new common-motion systems), all of which helps clean up
the WDS.
You can get a hint of the activity in this area
amongst amateurs by having a look at the Journal of
Double Star Observers:
http://jdso.org
...the latest issue includes a contribution by Richard.
There are similar semi-amateur journals in the UK,
Spain, and Italy that I know of, possibly more.
This is another aspect of the CCD astronomy revolution
that doesn't get much visibility.
\Brian
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