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Michael K. wrote:
>Every program I have lets you display positional information with J1950,
>J200, or JCurrent (JNow). I don't know what this means exactly in terms of
>epoch and equinox. None of these (amateur) programs address both epoch and
>equinox. By "middle of nowhere" I mean that if I use JCurrent there is no
>star within a few arcsecs but with J2000 it is smack on GSC 00393 00221.
Ah, so those are the units of "middle of nowhere." :-)
Unfortunately, you are talking to someone who presumably is a
photometric expert, but who also works at the acknowledged mecca
of astrometry. It is "B1950" (for Besselian) and "J2000" (for Julian),
and those are the only recognized (recent) equinoxes. "Jnow" is an undefined term;
what you really mean is "current epoch". Most astronomical dictionaries
give the difference between equinox and epoch; I leave this as an
exercise to the reader!
>I have been using GSC for the zeropoint.
Two comments. GSC is known to have large errors, both in the zeropoint
and because it is a mix of blue, yellow and red plates (all different bandpasses).
In general, it matches "V" pretty well, *but it cannot be trusted.*
On the other hand, Tom's data as processed by Michael R.'s pipeline, is pretty
good. I would definitely recommend you change the magnitudes you use for
your comparison stars to be the magnitudes from Tom's reductions.
Second, as I have mentioned numerous times, the TC237 does not have a V-like
bandpass. Please read the M67 paper! If you don't have JAAVSO or
access to that volume through ADS, it is available on my ftp site
in the directory:
Use CR (adjusting with R magnitudes) to better indicate what you are
actually measuring. You can calculate Rc for your comp stars from
V and Ic; an approximate relation is just to take R = (V+I)/2.
I want to explain my assumption:
>Unfiltered = B + V + R + I +/- overlap or gaps, no?
>How could unfiltered get less signal than if I had an Ic filter on?
It can't get less, but it might get *more*. Think about it a while, then
get back to me.
>Apparently the I does not stand for infrared?
Back in the days when the system was first set up by Johnson, the filter
names were chosen to be "Ultraviolet", "Blue", "Visual", "Red", "Infrared",
where "Infrared" meant _photographic infrared_. These full names are not
used anymore, and especially with the difference between photographic and
astronomical infrared, I highly recommend you not use such names. Note
that the red extension filters have no obvious names: J,H,K,L,M,N. In fact,
Johnson actually just defined J,K.L,M,N, obviously the next letters after "I".
"H" was added later when that atmospheric window was recognized. So using
names for part of the Johnson broad-band filter set is not wise.