[vsnet-chat 3365] RE: V382 Vel comparison stars

[Sebasti若n Otero <varsao@fullzero.com.ar>]

Dear vsnet friends:
                                 I totally agree with Bruce on his
considerations. And about the sequences, Tycho 2 and UCAC1 are the most
suitable according to what I observe. As Bruce says,Tycho 2 is generally ok
but near 11th magnitude it also shows uncertainties: UCAC1 shows a 11.13V
for GSC 0810-01225 instead of 10.82. Well, that was the only problem I had
with the Tycho2 sequence.
                                 It's evident that it's very difficult to
work if we have to make a mixture of every catalogue available and deduce
which star is the best checking one by one, so I think we have to choose two
or three of the most reliable sources and make a comparison in order to
adopt one of them. First of all, having Tycho 2 available, we should not use
Tycho 1, at least for stars fainter than 10th magnitude. And then, we could
make a comparison for another star field between Tycho 2 and UCAC1. I insist
in the latter having the best v seq. for V382 Vel (As Berto Monard said,
this is just MY opinion).
                                 For example, we could choose the field of
V348 Sgr, which has very strong discrepancies between Tycho 2 and AAVSO
magnitudes and see what is shown in UCAC1.  Some stars are:

GSC 6858-02252  10.49, 0.93 Tycho2;
GSC 6858-02278  10.53, 1.25 Tycho2;
GSC 6858-01964  10.87, 0.62 Tycho2;  110 AAVSO
GSC 6858-01727  --------------------------;   109 AAVSO
GSC 6858-02920  11.23, 0.58 Tycho2;
GSC 6858-02438  11.42, 0.41 Tycho2;  115 AAVSO
GSC 6858-01425  11.42, 0.54 Tycho2;
GSC 6858-02962  11.70, 0.41 Tycho2;
GSC 6858-02176  11.75,-0.31 Tycho2;  106 AAVSO !!!!

The latter seems to be very blue and problematic.
Bruce, I would appreciate if you could provide us with UCAC1 magnitudes to
make a comparison.
Finally, we should go and look through the eyepiece and make an opinion.

Best regards,
Sebasti睹 Otero.



----- Original Message -----
From: Bruce Sumner <b.sumner@bom.gov.au>
To: <vsnet-chat@kusastro.kyoto-u.ac.jp>
Sent: Wednesday, July 05, 2000 1:21 AM
Subject: [vsnet-chat 3354] V382 Vel comparison stars


> Mati rightly raises (again) the concern regarding the choice of
> comparison stars. Faced with the lack of real photometry, we often
> have to 'make do' with derived magnitudes extracted from the various
> sky surveys now in existence. Lets look at the four stars Mati
> discusses in the field of V382 Vel. The magnitudes currently used
> originate from Tycho-1, which is known to have large errors at the
> faint end, nominally fainter than 10.0. This essentially covers the
> four stars under discussion. However since Tycho-1 there have been
> several other surveys that include photometry, albeit rather
> inaccurate photometry. Do these calalogues help with the V382 Vel
> sequence? Here is a table:
>
> GSC 0810-   Tycho-1      Tycho-2        USNO-A2.0          UCAC1
>         V    B-V     V     B-V     B     R    'V'     r     'mV'
> 01423     9.94  0.51   10.00  0.46   10.4  10.0  10.1    9.92  10.08
> 00801    10.43  1.27   10.54  1.49   11.7   9.7  10.4   10.05  10.43
> 00897    10.77  0.38   11.00  0.25         10.8  10.8   10.81  10.98
> 01225    10.77  0.42   10.82  0.42   11.1  10.9  11.0   10.87  11.13
>
> The USNO-A2.0 'V' values are from the 'standard' equation
> V=R+0.375(B-V) and the UCAC1 'mV' values are from my own calibration,
> and includes a colour correction for star 00801. All the magnitudes
> listed under V, 'V' or 'mV' are all derived V estimates, and can be
> intercompared between catalogues.
>
> So where does this lead us? Not very far unfortunately. It is fairly
> obvious that Tycho-2 should be used in preference to Tycho-1
> magnitudes, as Tycho-2 is known to be more accurate at the faint end
> than Tycho-1. However even Tycho-2 magnitudes fainter than about 10.5
> need to be used with suspicion (or care, if you rather). The other
> surveys, USNO-A2.0 and UCAC1, provide useful (rough) magnitudes, with
> UCAC1 having the potential to extend Tycho-2 to fainter magnitudes
> with reasonably small errors.
>
> The main problem with this sequence is the red comparison star 00801.
> As Sebastian Otero states, the red colour of this star makes it look
> fainter. In fact using the equation v=V+0.2(B-V) leads to an
> estimated visual magnitude of 10.8 for this star, considerably
> diferent from the 10.4 stated in Tycho-1.
>
> Is there a moral to all this. Yes, there are several things to
> consider if you are trying to create a comparison sequence without
> real photometry:
>
> a) don't use red stars in a sequence. I cannot do better than quote
> from Charles Scovil "In order to keep as close to standard as
> possible we (AAVSO) will use only comp stars with B-V of < 1.0  -
> unless there is absolutely no other star available than one with
> greater than B-V = 1.  Even then we plan not to exceed B-V of 1.5".
>
> b) Use Tycho-2 for stars brighter than 11.0, but beware that the
> absolute errors are larger than the stated errors for stars fainter
> than about 10.5.
>
> c) To extend fainter than Tycho-2, use UCAC1 in the Southern
> Hemisphere, suitable calibrated to V. This will take you to about
> magnitude 16 with errors in the order of 0.1 to 0.2 magnitude for
> 'ordinary' i.e. non-red stars.
>
> d) Use USNO A2.0 stars, using the V calibration equation, to confirm
> UCAC1 magnitude estimates in the South, and as a source of faint
> comparison stars in the Northern Hemisphere. However be aware that
> the accuracy of these derived V magnitudes is generally quite poor.
>
> Question - when will some group do a deep whole sky two colour
> photometric (CCD) survey? Wouldn't that be useful?
>
> Cheers,
> Bruce

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