Re: TASS Database, plans and progress.

[aah@nofs.navy.mil]

Michael replied to Chris' latest ideas about the database, and his reply
sounds good to me.  Some comments on Michael's comments:

>  I've worked my way through most (but not all) of the data which
>Glenn, Tom, Mike G., and the JHU guys have sent in.  The number
>of entries in the "observ" table (the one containing one entry per
>measurement of a star) is about 6 million, and there are about 
>2 million distinct entries in the "tass_cat" table.  The largest
>file in the Postgres data area is 606 MB.
  Neat.  Lots of data!  I presume the tass_cat table then has many unmatched
TASS vs. mag16 stars?  mag16 has only 2.5M entries, and covers a much wider
declination range than the current mark III sites.  Are these unmatched
entries single observations, or multiple detections?

  Some reasons why astronomers have not used the color transformations:
>               V = a*v + b*r + c*i
>               R = d*v + e*r + f*i
>               I = g*v + h*r + i*i
  (1) if you do not have the R filter, this reduces to the same transformations
      as before, so the added complexity is not needed.
  (2) this assumes a single-valued function.
  (3) the best transformations are when you use a color that brackets the
      bandpass in question.  If you have V&R, the (V-R) color is much
      closer to correct than (R-I) and I bet you don't gain by using the
      longer lever arm, especially since you add in another error term.
  (4) A better solution has always been to use quadratic terms to handle
      second order effects.  I don't recommend it since the TASS filters
      appear to be pretty close to the standard system.
Bottom line:  Chris' added complexity might help Tom's 3-band camera with
no gain for any other mark III site.  The extra terms might be useful for
the mark IV.  There is no harm in including them outside of the extra
calculation time, and you might gain by having a generic formula that can
be used with any of the mark III sites.  I probably would be conservative
and set c,f,g to zero even for Tom's camera.
  Michael, what do you mean by suggesting that standard stars other than
Landolt standards might be used?  Be careful here!

>  It would be convenient to calculate the scatter between the
>"standard catalog" magnitudes and the transformed TASS magnitudes,
>after the fit has been made; this gives some indication of the quality...
  I'd assume the transformation file itself would include these error
quantities. Since these would be mean transformation errors, they would be
the same for all stars that night and need not be carried as extra fields
in the main catalog.
Arne