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<H2><A NAME="SECTION000411000000000000000">SLA_ATMDSP - Atmospheric Dispersion</A>
<A NAME="xref_SLA_ATMDSP"> </A><A NAME="SLA_ATMDSP"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Apply atmospheric-dispersion adjustments to refraction coefficients.
<DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_ATMDSP (TDK, PMB, RH, WL1, A1, B1, WL2, A2, B2)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>TDK</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>ambient temperature at the observer (degrees K)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>PMB</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>pressure at the observer (mB)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>RH</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>relative humidity at the observer (range 0-1)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>WL1</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>base wavelength (<IMG WIDTH="26" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img21.gif"
ALT="$\mu{\rm m}$">)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>A1</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>refraction coefficient A for wavelength WL1 (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>B1</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>refraction coefficient B for wavelength WL1 (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>WL2</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>wavelength for which adjusted A,B required (<IMG WIDTH="26" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img21.gif"
ALT="$\mu{\rm m}$">)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>A2</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>refraction coefficient A for wavelength WL2 (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>B2</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>refraction coefficient B for wavelength WL2 (radians)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>To use this routine, first call sla_REFCO specifying WL1 as the
wavelength. This yields refraction coefficients A1, B1, correct
for that wavelength. Subsequently, calls to sla_ATMDSP specifying
different wavelengths will produce new, slightly adjusted
refraction coefficients A2, B2, which apply to the specified wavelength.
<DT>2.
<DD>Most of the atmospheric dispersion happens between <IMG WIDTH="50" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img34.gif"
ALT="$0.7\,\mu{\rm m}$"> and the UV atmospheric cutoff, and the effect increases strongly
towards the UV end. For this reason a blue reference wavelength
is recommended, for example <IMG WIDTH="50" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img35.gif"
ALT="$0.4\,\mu{\rm m}$">. <DT>3.
<DD>The accuracy, for this set of conditions: <BR>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>height above sea level</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP>2000m</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>latitude</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="26" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img36.gif"
ALT="$29^\circ$"></TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>pressure</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP>793mB</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>temperature</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="34" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img37.gif"
ALT="$290^\circ$">K</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>humidity</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP>0.5 (50%)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>lapse rate</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="86" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img38.gif"
ALT="$0.0065^\circ m^{-1}$"></TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>reference wavelength</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="50" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img35.gif"
ALT="$0.4\,\mu{\rm m}$"></TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="RIGHT" NOWRAP>star elevation</TD>
<TD ALIGN="CENTER" NOWRAP> </TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="26" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img39.gif"
ALT="$15^\circ$"></TD>
</TR>
</TABLE>
<BR>
<BR>
is about 2.5mas RMS between 0.3 and <IMG WIDTH="50" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img40.gif"
ALT="$1.0\,\mu{\rm m}$">, and stays
within 4mas for the whole range longward of <IMG WIDTH="50" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img41.gif"
ALT="$0.3\,\mu{\rm m}$"> (compared with a total dispersion from 0.3 to <IMG WIDTH="45" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img42.gif"
ALT="$20\,\mu{\rm m}$"> of about <IMG WIDTH="25" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img43.gif"
ALT="$11\hspace{-0.05em}^{'\hspace{-0.1em}'}$">). These errors are typical for ordinary
conditions; in extreme conditions values a few times this size
may occur.
<DT>4.
<DD>If either wavelength exceeds <IMG WIDTH="53" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img44.gif"
ALT="$100\,\mu{\rm m}$">, the radio case
is assumed and the returned refraction coefficients are the
same as the given ones.
<DT>5.
<DD>The algorithm consists of calculation of the refractivity of the
air at the observer for the two wavelengths, using the methods
of the sla_REFRO routine, and then scaling of the two refraction
coefficients according to classical refraction theory. This
amounts to scaling the A coefficient in proportion to <IMG WIDTH="53" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img45.gif"
ALT="$(\mu-1)$"> and
the B coefficient almost in the same ratio (see R.M.Green,
<I>Spherical Astronomy,</I> Cambridge University Press, 1985).
</DL></DL>
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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