[vsnet-chat 4688] Re: comp

[Brian Skiff <bas@lowell.edu>]

     Hans-Goran wrote:
>> but what criteria must it be for to pick a  useful
>> comparing star for visual observations for a MIRA star Sra,Srb etc....?
>> Example for TW LYR a star i have difficult to find useful comparing star
>> in the surrounding field?

     Ah, looks like I answered a different question!  I just had a look  at
the on-line AAVSO (e)-scale chart.  This is one where the sequence was
found from measuring a photographic plate on an iris-photometer.  Chaz
Scovil I'm sure did the best he could on these, but there are typically
modest zero-point errors (few tenths of a magnitude) at the bright end,
then almost always a scale error at the faint end for these, such that stars
are rated progressively too bright as you go fainter compared to "truth",
i.e. standard V.  If the stars you are interested in are fairly faint, then
Arne Henden might be willing to get a set of BV frames to allow the
sequence to be corrected down to some faint limit (V ~16 or 17) with stars
close to the variable.  For brighter stars (roughly V < 10.5), which are
necessarily farther afield, then it is probably okay to use Tycho-2 stars
after corection to the standard system.
     In general it is preferable for visual observing to choose stars
with B-V color < 1.5 or so if they are available in the field.  TW Lyr is
fairly far from the galactic plane, so the stars should not be much
reddened, although it is on the "dusty" side of the Milky Way in this
region.
     It is easy to check the bright stars in the field (I can do this if
you wish), but for fainter ones, new data are required.  The existing
survey catalogues (GSC, A2.0, UCAC, etc.) are not sufficiently accurate
and are not on the standard systems, so you can't (or shouldn't) use those
except 'in extremis'.  Unfortunately I am not a position to observe faint
stars regularly myself at the moment, although it is something I'd like
to be able to do.

\Brian

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