[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: downloading data

  Andrew wrote:

> One degree boxes now mean Sec(Declination) degrees in
> RA so my boxes now overlap except on the equator. Just
> another minor change to my code to ignore the stuff I have
> read twice.

  I'm the guilty party who asked Michael S. to make this
change.  Most of the other tools I use to request data
from big databases (SIMBAD, NED, etc.) use the convention
that a one-degree box cover a square one-degree region
on the sky.  The old TASS interface yielded a square
region near the equator, but an increasingly elongated
rectangle as one moved towards the poles.  I think
that the new convention will make analysis simpler for
most users.

> But some things have not changed. One is still given the
> incorrect error estimates:

  I'm guilty of not moving quickly on this point.  I have
been wanting for a long time to provide two tables of

     1) table of scatter of Mark IV database magnitudes from 
             their mean value, as function of magnitude,
             something like this (I'm making up values here)

                 V mag         7       8      9     10    ....
                 V uncert    0.03    0.05    0.07  0.10   ....

             These values would be simple internal estimates
             of the consistency of measurements which have run
             through the ordinary pipeline.

     2) a similar table, but this time using magnitudes based
             on an ensemble solution of all the stars within
             a region considerably smaller than a full 4x4 degree
             frame; perhaps a 1x1 degree area.

  We know that there are systematic errors in the photometry as 
a function of a star's position on the frame  -- see TN 97.
The first table would be dominated by those errors.  It would
warn users of the scatter they might reasonably expect for a
random star of a given brightness in a random field.

  The second table would require that a new set of measurements
be added to the database -- magnitudes based on ensemble solutions.
I believe that these values would have somewhat smaller scatter
from the mean, and so would be more useful for some purposes.

  I've been grabbing data in 1x1-degree blocks from the Mark IV
database for the past few weeks, and just yesterday, finally 
finished (there are a few spots where the file transfer had
problems, so I'll have to try again, but that's a minor issue).
I have a script which has succeeded in running the ensemble
photometry code on a few sample blocks -- so I could start that
going on the entire set.  It will take several days to several
weeks, I _think_.  It should be possible to create a very
quick and only slightly dirty table of the second sort
by examining the ensemble output for many blocks.

  So, I think we might be able to move forward in a few
little steps:

       a) very soon: someone calculates the simple scatter from mean
             for a large set of stars in the Mark IV database
             (called "Table 1" above), and posts it to the
             TASS E-mail list.

       b) less soon: Michael Sallman (sorry, Michael, I don't mean
             to force this on you, but I'm not sure anyone else
             can do it easily) inserts this table into the database,
             and creates a slightly modified query form (as an option,
             without destroying the current form) which will

                 - grab magnitudes and positions and dates and so forth
                          just as it currently does, but ...
                 - use this new table to estimate the uncertainty
                          to be reported with each magnitude measurement
                          instead of using the current uncertainty values

             This means, for example, that a star with V=10.34 would
             yield a big list of individual measurements, as it currently
             does, but that the "uncertainty" value attached to each
             line would be identically 0.06 mag (or whatever the 
             Table indicates).

             No, this isn't the "right" way to estimate uncertainty
             for some purposes; but it is probably the method which
             will help users interpret and use the data best,
             especially casual users.

          c) sometime in future: I finish the ensemble analysis for 
             all (or most) of the blocks in the northern sky.  I then
             create a big list of ensemble mag and uncertainty for
             all stars.  Actually, there could be one big list
             of just "mean mag" and uncertainty, and a second
             enormous list of individual magnitude measurements
             for each star.
          d) further in future: somehow, these two lists are
             made available for query by users.  Probably the
             first list would be done first, since it would
             be only a few gigabytes (at a guess).  It could
             be placed into the existing Mark IV database,
             or this information could be served from another
             site via a similar interface -- whatever is 

  Andrew, would these two tables, and a single "typical" uncertainty
value based on them, satisfy some or all of your needs?