[Aavso-photometry] Arne's M67 Finder Chart Sequence Values

[Tim Crawford]

On Mon, Jan 19, 2009 at 11:47 PM, Bill Perkins
<billperkins at birdlandobservatory.net> wrote:

> 4 of the 64 stars in the list of stars that Arne has generously made available to others prior to publication at
>
> http://binaries.boulder.swri.edu/binaries/fields/m67.html
>
> are variable stars (which takes nothing away from the measurements made at the time).
>
> Star 26 is EW Cnc
> Star 35 is HW Cnc

> Star 49 is Hv Cnc
> Star 55 is EV Cnc
>

Arne Wrote:

>Several of the stars are suspected variables, though their mean error doesn't indicate any large variability. 

I can easily confirm what Arne is saying for at least two of the above stars plus my "approach" to TC calculations, in so far as individual star selection occurs (excluding the question of mixing stars of different colors) might be of benefit to others, regardless of the FOV chosen for this purpose.

Each year when I re-calculate my Transformation Coefficientsusing Arne'sM67 sequence values I zero point the photometrytool such that the  instrumental mags will equal the sequence value for Star #1 (V=10.018) for all four filters (and this is done for three separate nights).

Once the photometry tool is zero pointed on this particularstar, for each filter, then the calculated IM'sshould pretty well match the sequence values of the other stars if they are stable (which presumes, of course that Star #1 has a very solid sequence, which it certainly appears to be, IMO, after a number of years of reliance).

Therefore, I then select ~ 25 stars and have my SS subtract each filters IMvalue from the Sequence value.  If this difference exceeds a plus or minus .08 magnitude for any single star for any single filter then I discard the star from my final mix. 

In looking at my SS for the last couple of years I note that I have included both Star #26 and Star #35 and see no reason to discontinue the use of either stars in my mix as long as they continue to meet my above criteria.

FYI, I usually end up with ~20 stars in the final mix with about half of those that were discarded occuring because of close doubles and my relatively ave poor seeing at my location.

Per Ardua ad Astra

Tim Crawford CTX
Arch Cape Observatory

PS- Yes, the .08 magnitude difference is a self-imposed limit based upon my previous experience with a lot of sequence values for a number of fov's giving consideration to the fact that each star is only a small part of the whole "ensemble" in this specific case.

When I use the same "technique" when selecting stars with a new FOV for differential photometry I tend/prefer to limit comp selection to those stars whose zero pointed IM's  agree within .06 magnitude of each other's sequence values, if at all possible.