[vsnet-chat 5963] Re: [AAVSO-DIS] CCD-V Vs visual observations = U ORIONIS

[Arne Henden <aah@nofs.navy.mil>]

Not to pick on Gary, but a couple of comments need to be made here.
This particular field has a terrible sequence, especially when
the Mira is bright.  The brightest comp is star 3, with V=9; the
next brightest comp is 3 magnitudes fainter.  When you are working
with a variable at V=7, you have too wide a dynamic range necessary
to get good signal/noise on more than the brightest comparison star.
This gets even worse when you look at the B magnitudes of the comp
stars and of the variable.  So first, Gary should *only* be using
star 3 in his reductions when the variable is this bright.
This is probably the cause of most of Gary's problems (other
than the mentioned typos, etc.).
   Miras in general are a poor example of whether CCD observers are
more or less acccurate than visual observers.  These stars have
very red colors and lots of molecular bands.  You cannot transform
them properly and different filters from different vendors, especially
where molecular bands come into play (Ic), will be difficult to
match between observers.  CCD V-band measures will differ markedly
from visual measures due to the redness and different response between
the Johnson V filter and the human eye.
   Finally, Gary's comment that these are unevaluated (raw) measures
is entirely appropriate.  I bet that you can find wide variations
in photometry for a given star, visual or otherwise, reported to any
variable star organization; it is not just the province of the AAVSO.
What is usually missing is the feedback from the data collecting group
to the observers to let them know that their data has problems.  This
is not easy for photometry, as seemingly smoothly varying stars can
often have bumps and wiggles that are real yet look like an incorrect
measure (for example, an eclipse that is rarely seen).  Sebastian
has been useful in this regard for U Ori; I doubt he is willing to
spend the time to do this for a few thousand more stars!  You have to
leave all the warts in the raw data until some later time when comparison
with other photometry might help in validation.  The quick-look plotting
facility of the AAVSO site is marvelously useful for the individual
observer in comparing his/her data to others.
   For minor planet astrometric work, there is an easy computerized
feedback mechanism since the errors of each observation with
respect to the current astrometric orbit can be calculated and
returned to the observer; it is rare that an orbiting body will
suddenly jump by thousands of kilometers from its predicted path!
   The real bottom line is that it is up to the individual photometrist
to look at his/her data, compared with others, and decide whether it
seems reasonable or has possible error.  If the latter, then you need
to do what Gary did:  see if there is an obvious reason for the potential
error (and correct it if so), or if no obvious reason, leave it in
the database for analysts to decide for themselves.  If corrections
are necessary, the database maintainer has to be able to replace
the incorrect observation and not leave it in the file.  As a sidebar,
I should note that if you find that you are making many errors, you
should look at the seemingly ok data as well as the deviant points
since these could also have errors that just happen to balance out.
Arne
> 
> The next observation in your email was about an observation done 9 days later 
> by (WGR), which is myself, at V = 6.97 and B-V of 3.69---actually I reported 
> B of 10.66.  This does seem unusual.  I went back and looked at the 
> photometry sheets for these two.  I did find that my chart showed comp star 5 
> with a magnitude of 12.991, while the latest chart on the web shows 13.991.  
> I also had a note on my chart that Ron Zissell had found this error, but I 
> used the old value.  I have corrected my chart.  This would change my value 
> to V= 7.31.  I gave the B observation two different days of scrutiny, and 
> could find nothing obvious.  The comps are in the right places and are the 
> same ones used in the V observation.  The 10.66 is the average of 3 
> observations which gave 10.18, 10.73, and 11.09 for comps 5, 4 and 3.  This 
> gives a Std Dev of .37 mag (I know that with 3 samples the Std Dev is not 
> appropriate, but I use it to measure the scatter of my observations on every 
> one).  Good numbers may be as los as .03 mag, and rarely better.  So No 
> Closure on the B observation.

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp