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> Like Pandora's box, the Tycho 2 can o' worms has been opened.
;-)) But now we can close it with more confidence.
It's a shame that most catalogues or software packages or whatever include
the "official" convertion....
Another proof that "official" doesn't mean "right" ;-))
Sebastian.<
Sebastian,
I went through this discussion in several iterations with Henden, Sumner,
Skiff and other people involved in creating the guidelines for sequence
selection for the International Chart Working Group. That is where I came up
with my practice of using Tycho 2 V for stars brighter than 10.5, taking the
errors for each star into account on a case by case basis.
As to the software issue, Bill Gray has expressed a willingness to include
or add the Bessel transformation to Guide as a user selectable function,
provided there is enough interest shown to warrant the work.
Bill has a web page devoted to the conversions and the way it is handled in
Guide 8.
http://vsnet.projectpluto.com/photomet.htm
I think I'll put in my formal request to him to add the Bessel
transformation to Guide again.
Maybe if several other people involved in chart making did the same it would
be available soon...hint, hint.
Copied below is a message I received from Bill quite a while ago stating his
willingness to do this.
Mike
Hi all,
I'm sending this out to people who have indicated an interest
in how Guide 8.0 handles photometric conversions (Tycho to
Johnson, and B-V to/from V-R to/from V-I). I've just updated a
page discussing how it's currently done and possible ways it
_might_ be (perhaps ought to be?) done:
http://vsnet.projectpluto.com/photomet.htm
In particular, there is some discussion of an algorithm for
conversion of B-V to V-R and V-I that may be of interest (one
that is a bit more accurate than a simple linear conversion).
I am very open to comments on this. I've seen suggestions that
variable star folk, for example, might adopt particular conversion
formulae to match the way Guide 8.0 does them. I STRONGLY recommend
against this; in my opinion, it would be having the tail wag the
dog. Guide 8 can easily be revised to use any algorithm. (This can
be done as a "software-only" solution, with an update posted on my
Web site. Guide only does the photometric conversions at runtime,
so no new CDs would have to be issued; the CDs have the raw,
"original" magnitude data.)
If there was an interest, I can even have the method be
user-selectable ("convert Tycho to Johnson B and V using Bessell's
method", vs. "convert using a linear relationship", for example.)
-- Bill
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