![](Fig1a.jpg)
Figure 1a. This figure shows the secondary tilts required
to correct for transverse flexures of both the telescope truss and
the secondary mirror mounts as the telescope changes altitude. No
software corrections for these flexures were installed in the
telescope software at the time of these measurements. The circles
show the x-tilts of the secondary needed to minimize telescope coma
and the squares show the y-tilts required. X-tilts are rotations of
the secondary mirror about the Nasmyth axis. Positive x-tilts of the
secondary are in the direction of your fingers if your right thumb
points along the Nasmyth axis towards the NA2 port.
The small negative slope which seen in the y-tilt data is possibly
real. It is reproduced in the data from the next night. A linear
fit to these y-tilt data yields the function: y-tilt =-8.71 - 0.056 *
altitude. The standard deviation of the y-tilt points after this
linear trend is removed is 2.56". Below I will show that after the
fits for altitude dependence are removed, the scatter in both x- and
y-tilts on this night are identical. In terms of its effects on the
imaging quality of the telescope, this possible altitude dependence
of the y-tilt is not significant. At 60љ the y-tilt correction is
measured to be approximately -12".
The line shown is a least squares fit to the x-tilt data. The
functional form of the fit is shown at the top of the inset and was
choosen to correspond to that assumed by the 3.5m software. The fit
parameters m2 and m3 are in good agreement with those derived from
data taken on 14 April 2000. The parameter m1 represents a zero-point
offset to the tilt function which is not expected to agree with the
data taken on 14 April because the secondary mirror was removed from
the telescope between these two measurements. The expected errors to
the fit parameters are also shown in the inset.
![](Fig1b.jpg)
Figure 1b. This figure shows the the x-tilts required to
minimize coma on the 3.5m telescope measured on the night of August
15. Once again, the inset shows the fit parameters for the line
plotted. Notice that the data points on this graph show much greater
scatter than that seen in Figure 1a. Subtracting the fit from these
data and then computing the standard deviation of the residuals
yields a scatter of 6.8". A similar calculation with the x-tilt data
of August 14 yields a scatter of the residuals of only 2.4", nearly
three times smaller than the scatter seen in this graph.
Because of this increased scatter in the x-tilts, the y-tilts have
been excluded from this graph for clearity. However, the y-tilt data
are very consistent with the y-tilts measured on August 14 (Figure
1a). A linear fit to the y-tilt data on August 15 yields the
function: y-tilt =-6.48 - 0.097 * altitude. The standard deviation of
the y-tilt residuals after this linear trend is removed is 3.3". A
similar calculation yields a scatter of 2.6" in the y-tilt residuals
of the August 14 data. At 60љ the y-tilt correction is measured to
be -11", which is not significantly different from the -12" measured
on August 14.
The slight negative trend in the y-tilt data and the absolute
values of the y-tilts reproduce between the two night's worth of
data. One very interesting point is that the scatter in the y-tilts
on August 15 is nearly identical to the scatter seen in the y-tilt
data measured on August 14, despite the fact that the x-tilt data
showed an increased scatter by nearly a factor of 3! The "noise"
seen in the August 15 measurements is quite specific to the x-tilt
data. The source of this noise is not currently understood.
When one takes into consideration the increased noise in the
x-tilt measurements, the fits for the data taken on August 14 and for
the data taken on August 15 are consistent.