Wed, 09/14/2016 - 17:59
Dear all,
the GAIA first catalog release is now available at Vizier and several other data centers.
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I%2F337
Could it be of use for us is unclear to me ? The scatter seems very good but in a wide band-pass very like CV or Hipparcos.
Clear Skies !
Roger
Reading about the photometric DR1 here - http://gaia.esac.esa.int/documentation/GDR1/index.html
1. It goes down to magnitude 20, which is excellent coverage beyond what APASS provides.
2. It states standard errors are under 0.03 magnitude, but not sure how they determine the errors, include systematic along with the random statistical? How realistic are the errors?
3. It has a single G band for photometry. This raises similar issues that we had in the past with other space surveys, converting them to V. Especially a problem here, its only a single passband, how can you do a proper conversion?
So, it seems some good things, but some issues as well.
Mike
Hello Michael
All is not lost. If one follows the link, and then goes to the Documentation for DR1, there are many interesting sections. Section 5.3.5 shows how to transform to all the other major systems, including Johnson-Cousins, Hubble, SDSS, Hipparcos, etc, etc. The bad news is its not so simple, they include different transform coef for blue stars, red stars, etc. for instance. Its going to take some study. Arne's help here is definitely welcome.
Its interesting that it goes down to mag 20.7 with an error of .03 with the "clear" filter. A BVRI image would cost about 2 magnitudes and would make the error .06 mags for the same exposure, I believe.
Gary
Yes the different transformation for different color stars. So, how does one find out the color of a star, if it is fainter than APASS? You cannot from the single G band data, so you would be forced to go to another catalog, possibly GSC or USNO, but those dont always have multiband data to get colors from either.
Mike
I'll try to clarify a bit here. Gaia is actually not doing photometry. It has two low resolution spectrometers one for red (640-1050 nm, dispersion 7-15 nm pixel-1) and one for blue (330-680 nm, dispersion 3-27 nm pixel-1) light. So all that photometry is actually synthetic photometry done on flux-calibrated low resolution spectra. So in principle, Gaia pipeline could reproduce almost any mid-to-broadband photometric system. Just that DR1 contains Gaia's own instrumental system.
It's still somewhat complicated, though. Namely, the line spread function of Gaia is not a nice and clean one.. so very narrow spectral lines will be smeared (in non trivial way) along dispersion. And that definitely will affect photometry in a systematic way.
General properties of red and blue "photometer" are discussed in the overview paper http://www.aanda.org/articles/aa/pdf/forth/aa29272-16.pdf in paragraph 3.3.6 and references about spectrophotometric calibrations are given on page 23.
Unfortunately, papers:
van Leeuwen et al.
Carrasco et al.
Evans et al.
Riello et al.
are not public yet, they should be available anytime soon. See http://www.cosmos.esa.int/web/gaia/dr1 A&A papers section at the end of the page.
Best wishes,
Tõnis
Hi Gary et al.,
As Tõnis mentions, the GAIA photometry is derived from low-resolution spectra (essentially like our diffraction gratings). Right now, all they've released is what I'd call a rough G-band photometry, with many caveats regarding the defects at this processing stage. I think the DR2 release in Q4 2017 is supposed to have better photometry. This initial GAIA release is really concentrating on astrometry. Note that no proper motion is taken into account, and so the positions are mean 2015 and so should not be used for reporting coordinates in the J2000 system yet.
What is interesting is their epoch photometry release, highlighting many thousands of interesting variable stars. It will take many months to see the papers coming out of this massive flood of data. I hope that the A&A papers will be publicly available and not hidden behind their paywall.
Arne
The issue of lack of proper motion information and the fact that Gaia positions are for J2015.0 will prevent us from doing a massive VSX coordinate update until the next release in late 2017.
Wouter van Reeven said in the Amastro list that they will include ppm in that release because they need more data than currently available.
So let's wait a little longer.
For now, let's continue using UCAC4, PPMXL, 2MASS for positions.