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<H2><A NAME="SECTION001835000000000000000">
Influencing the plan</A>
</H2>
The program will propose reasonable default choices, and will provide some
approximate estimates of the magnitudes where photon noise becomes excessive,
and where photon and scintillation noise are comparable.
Because the crossover between photon and scintillation noise depends on both
zenith distance and azimuth, a set of values will be presented for your
inspection.
Table <A HREF="node269.html#tbl:crossover">13.1</A> shows what a typical crossover table looks like.
<P>
<BR>
<DIV ALIGN="CENTER"><A NAME="tbl:crossover"> </A><A NAME="12309"> </A>
<TABLE>
<CAPTION><STRONG>Table:</STRONG>
A sample crossover table</CAPTION>
<TR><TD><IMG
WIDTH="627" HEIGHT="149"
SRC="img555.gif"
ALT="\begin{table}
\begin{tex2html_preform}\begin{verbatim}SCINTILLATION = PHOTON NOI...
...
V 10.7 & 9.8 7.5 & 7.4 11.3 5.9\end{verbatim}\end{tex2html_preform}\end{table}"></TD></TR>
</TABLE>
</DIV>
<BR>
<P>
There's a lot of useful information in this table, so let's go over it
carefully.
First, notice the similar pairs of columns at the left, under the title
<TT>SCINTILLATION = PHOTON NOISE</TT>.
The left-hand pair of columns gives crossover magnitudes for an airmass near
2.36; the right-hand pair, for an airmass of 1.10.
These are the maximum and minimum airmasses at which the planning program
expects to schedule observations of extinction stars.
The smaller airmass will be adjusted by the program to provide more or
fewer extinction stars, as needed; you can push it back and forth a little
if you want.
<P>
For each airmass, there is one pair of columns.
These contain the magnitudes at which scintillation noise and photon noise are
expected to be equal, for two different lines of sight: one looking along the
projected wind vector in the upper atmosphere, and the other looking at right
angles to the wind.
When we look at right angles to the wind, the scintillation shadow pattern
moves with the full wind speed across the telescope aperture, and we have the
maximum possible averaging of scintillation noise in a given integration time,
the least scintillation noise, and thus the brightest possible crossover
magnitude.
But when we look directly along the wind azimuth, the motion of the shadow
pattern is foreshortened by a factor of sec <I>z</I>; then the scintillation noise
is maximized (for a given zenith distance), and the crossover does not occur
until some fainter magnitude, where the photon noise is big enough to match the
increased scintillation.
<P>
As we do not know the wind azimuth in advance, we can only say that the
scintillation and photon noises will be equal somewhere in the interval between
these two extremes.
We would generally prefer to have the extinction measurements limited only by
scintillation noise, so the initial faint limit for extinction stars is set
1.5 magnitudes brighter than the brightest of the crossover values.
This makes the photon noise half as big as scintillation, near the zenith.
(Our sample table shows these initial values.)
These are conservative values.
<P>
However, we may need to set the actual faint limit used in selecting standard
and extinction stars (shown in the rightmost column of
the table) somewhat fainter.
For example, we may be using such a large telescope that we cannot observe such
bright stars.
(In particular, if you are doing pulse counting, the program will impose a
bright limit as well as a faint one.)
In this case, both photon and scintillation noise will be quite small, and we
can safely use considerably fainter stars without compromising our requested
precision.
<P>
In the example above, the user has requested an accuracy of 0.01 magnitude.
The planning program divides this error budget into four parts:
scintillation, photon noise, transformation errors, and instrumental
instabilities.
If these are uncorrelated, each can have half the size of the allowed total; in
our example, that's 0.005 mag.
So the table gives the magnitude at which the photon noise reaches its allowed
limit (in the next-to-last column), for the adopted integration time (5
seconds, in our example).
This magnitude
should be regarded as an absolute limit for extinction and standard stars.
<P>
Obviously, we could actually push the extinction stars close to this
photon-noise limit, without exceeding the requested error tolerance.
However, between the photon-noise limit in the right half of the table and the
crossover values to the left, there is a substantial contribution of
photon noise to the total, and hence a substantial advantage to using brighter
stars.
If we use this advantage, we provide some ``insurance'' -- a little slack in
the error budget.
<P>
Whenever the crossover table appears, you will be given an opportunity to
change the actual planning limits, whose current values are given in the last
column.
The columns to its left provide you with the information you need to make a
good choice: the crossover magnitudes, and the pure photon limit, for each
band.
<P>
Although these values
are given to 0.1 mag precision, you should be aware that the
scintillation noise can fluctuate by a factor of 2 or more
within a few minutes, so that only a rough estimate is really possible.
Furthermore, the photon-noise estimates are only as good as the estimates
available to the program for the transmission of the instrument and the
detective quantum efficiency of your detector.
So all these numbers are a little ``soft''; you should not take that last
digit literally.
Just bear in mind that the photon noise varies with magnitude, and that the
scintillation varies with airmass and azimuth, by the amounts
shown in the table.
<P>
Now, let's consider adjusting the circumzenithal airmass.
If the high-altitude, low-airmass almucantar is too close to the zenith,
only a few standard stars will be available in the small zone it intercepts
as the diurnal motion carries the stars past it.
Then it may be necessary to choose a larger minimum airmass to expand the
region of sky available for choosing extinction stars.
Conversely, if too many extinction stars are selected, it makes sense to
reduce the zone width a little, thereby getting not only a more reasonable
number of stars, but also a little bigger range in airmass for each star.
<P>
The planning program will make coarse adjustments in the minimum airmass (i.e.,
in the width of this zone)
to get about the right number of extinction and standard stars, but you can
also make fine adjustments yourself.
The program gives you this option after making a preliminary selection of
standards.
When it asks whether you want to adjust the sky area or magnitude limits, reply
``sky,'' and it will make a small change and try again.
You may need to make several adjustments to get what you want.
<P>
If you expect from past experience that you will have excellent photometric
conditions, you may be able to reduce the number of extinction stars a little.
However, this is risky: if the weather turns against you, you may need more
stars than you expected!
Conversely, if you know the observing run is at a place or time of year that
usually has mediocre conditions, you will surely want to play it safe and add
some extra extinction stars.
<P>
You can also alter the number of candidate stars by adjusting the
magnitude limits.
By adjusting magnitude limits separately in different bands,
you can manipulate the range of colors available.
For example, because signals tend to be low in ultraviolet passbands, the
photon noise is high there, so
it often happens that the default faint-magnitude limits do not
allow enough very red stars.
In this case, making (say) the faint U limit fainter and the V limit brighter
biases the selection toward redder standards.
<P>
Thus, you can manipulate the region of sky and the magnitude limits to select
a reasonable number of standards with a good range of colors.
You can make
such adjustments iteratively until you are satisfied with the selection.
At each stage, the program will show you the distribution of the selected stars
in a two-color diagram, as well as on the sky, and ask if its selection is
satisfactory.
If it is not, you reply ``no'' to the question ``Are these stars OK?'', and
then have another chance to manipulate the zone width and magnitude range.
When you finally reply ``yes'', the program will make up an observing schedule
for each night of the observing run, using the set of stars you approved.
<P>
Keep in mind the possibility that a star previously certified as a photometric
standard can still turn out to be variable!
A surprising number of bright eclipsing binaries continue to be discovered
every year.
Furthermore, stars of extremely early or late spectral type, and stars of high
luminosity, tend to be intrinsically variable by small amounts.
Therefore, it is important to use a few more standards than would otherwise be
absolutely necessary, just in case of trouble.
A little redundancy is cheap insurance against potential problems.
We also need some redundancy to find data that should be down-weighted
(see section <A HREF="node274.html#reductions">13.5.3</A> to see why.)
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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