Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 01/09/2016 - 22:06
I was attempting to compare magnitudes used for calibration stars in the PEP program with data in the Hipparcos catalog (http://vizier.u-strasbg.fr/viz-bin/VizieR-3?-source=I/239/hip_main). I was rather surprised to see how many of them had only ground-based magnitudes listed. What was the rationale for putting so much terrestrial photometry in the Hipparcos catalog? What was the methodology for choosing the sources? My hope was to get systematically measured magnitudes for comparison (I realize that Hipparcos V and B-V are synthesized).
Tom
[quote=tcalderw] What was the rationale for putting so much terrestrial photometry in the Hipparcos catalog? What was the methodology for choosing the sources? My hope was to get systematically measured magnitudes for comparison (I realize that Hipparcos V and B-V are synthesized).[/quote]
I think the best source to look for the answers to these questions is this guide to catalog fields profided by ESA:
http://www.cosmos.esa.int/documents/532822/553204/sect2_01.pdf/88f60038…
Look for the section on column H5 on page 107-108.
CS
HBE
Hi Tom,
I do not see well what you mean ? From what I know, all of the catalog rows have HPmag, VTmag and BTmag. A few rows have no VT or BT and a dozen have no Hp.
The sattelite had two photometry experiments: first was the Hp sensor. Its highly accurate photometry was non filtered and very difficult to transform to Johnson. The second experiment, said Tycho, had two channels: blue ( BT ) and green (VT). The filters were different than Johnson V and B and need a transformation to be used for Johnson photometry. I would note that the VT filter is very similar to the response curve of the green channel of the Canon DSLR and could be used directly as reference, it's an option. The BT and VT are less accurate than Hp but generaly good enough for what we do. You can access to the measures scatter plot that is often very useful to understand the possible variability of our comp stars.
I have to re-check but the B-V should be the BT-VT transformed. Information on transformation of Tycho data is in:
http://www.rssd.esa.int/SA/HIPPARCOS/docs/vol1_all.pdf
section 1.3 appendix 4
Clear Skies !
Roger
I guess Tom was referring to column H5 which gives a magnitude in Johnson V. However, as the document I linked above explains:
CS HBE
Hi Heinz-Bernd, nice to read from you !
I didn't see you answered when I sent my own, this is a little bit a missing function of this forum, some other put an alert when someone answer just before you do.
You are right, the question is probably about that H5/Vmag personnaly I am not considering ! The true product of Hipparcos being Hp, VT and BT. In fact most of the H5 cases seems transformed from Hp that is not ideal, as they say there are serious systematic errors in transforming unfiltered Hp mag (H flag), I suspect more than the said 0.01 mag !
For me it's better to use the Tycho 2 catalog having much more star, those 118000 Hip are not often in our FOV.
Clear Skies ! (here, very bad for a month now... )
Roger
I am referring to the
Gents:
I am referring to the data available at http://vizier.u-strasbg.fr/viz-bin/VizieR-3?-source=I/239/hip_main, which is identified as the "Hipparcos Main Catalog." For example, if you look up HIP 7918 and select the r_Vmag field, you will see that the V magnitude is from ground photometry. Am I missing something here?
Tom
Greetings to Roger! Horrible weather most of the time here as well...
Tom, if you click on the link under the column number in the catalog query result for this star, you'll get the full entry (all fields) from the catalog.
http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-ref=VIZ5692bcbab043&-out.a…
As Roger already mentioned in his reply, the Hipparcos satellite simply had no Johnson V filter available, instead it used a filter called VT for the "green" part of the spectrum (and also another BT filter for the blue part).
So you cannot get V, B and henece B-V values from the satellite directly to include in the catalog, which left the makers of the catalog with two possible alternatives for each star:
a) use ground based photometry data done in proper Johnson V and B filters, or
b) use the VT and BT values from the slightly different filters onboard Hipparcos and derive a transformed V magnituide and B-V color
The second alternative introduces some additional error into the derived value, since the transformation cannot be perfect. How big this error is depends (among other things) also on spectral type of the star in question.
So, as explained in the section I quoted in my earlier reply from the ESA guide, the makers of the catalog decided to use either a) or b) depending on which would give the smaller error for each star.
So almost all stars in the catalog have VT and BT mag measurements from the satellite and if you wish you can do the transformation to the Johnson V filter yourself to get a "space based" Vmag, but if you then find that the Hipparcos catalog has a ground based Vmag given in field H5 for that star, this was done for the reason that probably this ground based value will be more accurate than the transformed value.
One should keep in mind that the main mission of the Hipparcos satellite was astrometry, not photometry, We get magnitudes as a by-product and to help in the identification and cross-matching of stars.
CS
HBE
Actually the HIPPARCOS magnitudes are one of the most accurate Vmag values I've seen even when derived from a transformation from a different system.
About:
a) use ground based photometry data done in proper Johnson V and B filters, or
b) use the VT and BT values from the slightly different filters onboard Hipparcos and derive a transformed V magnitude and B-V color
Only a very small handful of stars have Vmags derived from Tycho Vt.
They are derived from Hp magnitudes using V-I or B-V colors (and Bt-Vt when there are no B-V from the ground)
The number of stars per source is:
Vmags from the ground = 23 139; Vmags derived from Hp = 94 669; Vmags derived from Vt = 409 (information from the HIPPARCOS catalogue).
Transformation from Hp to V is pretty good for normal stars, the problems arise for late type giants (M0 and later), emission line stars and some double stars with combined photometry.
Cheers,
Sebastian