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Re: downloading data
I'm dumb, but isn't there an ensemble solution for each frame?
Wouldn't the error in the fit be a suitable error for each star in
that frame? (i.e. the zeropoint error).
Maybe that's what you said...?
On Sep 28, 2005, at 9:42 AM, Michael Richmond wrote:
> 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
> just as it currently does, but ...
> - use this new table to estimate the uncertainty
> to be reported with each magnitude
> instead of using the current uncertainty
> This means, for example, that a star with V=10.34 would
> yield a big list of individual measurements, as it
> 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?