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\begin{document}
{\centerline{\bf{NOTES ON OPTICAL ALIGNMENT AT GHRIL}}}
\bigskip
\rightline{Peter Gray, Isabel Escudero \today}
\vskip 5mm
{\bf Background}
\vskip 3mm
The combination of the WHT's ALT--AZ mounting and the versatility of its
computer control system provide a range of possibilities in the alignment
of GHRIL optics with the telescope. The alignment of optics at GHRIL
has three main aspects:
\begin{enumerate}
%
\item {\bf Pointing Alignment}: To enable a star to be acquired onto the instrument aperture and then
tracked for extended periods. 
%
\item {\bf Internal Alignment}: To align the various optical components within the GHRIL optical
experiment with each other. 
%
\item {\bf Vignetting Alignment}:To ensure the telescope beam once acquired onto the instrument aperture
passes through the optical system with minimum vignetting and aberration. 
%
\end{enumerate}

In respect of the first requirement, the CALIBRATE procedure in the control
system allows the part telescope pointing to be temporarily adjusted to cope
with slight changes in telescope collimation. This procedure is usually carried
out using a TV camera mounted on the instrument rotator where the reference
mark to which the calibration is done is the centre of rotation of the
instrument rotator (found by tracking a star and rotating by $180^{\circ}$).
When the GHRIL focus is used without any form of instrument rotation or
derotator optics, no such rotator centre can be established. However, if
instead a nominal reference mark is established on a TV and the CALIBRATE
procedure run, the telescope pointing and tracking is adjusted such that
subsequent objects will be acquired onto this mark and tracked with the field
rotating about this point. Note that, if instead, the default GHRIL pointing
model was used and no CALIBRATE done, a star could be moved onto the instrument
aperture with a BEAM beamswitch command, however because the field rotation
would occur about some other point (the true instrument rotator centre), an
on--axis star would not be tracked because of rotation. The axis about which a
GHRIL optical system should be assembled can therefore be any arbitrary axis on
the GHRIL table as long a CALIBRATE procedure is carried out using the optical
system's axis as the TV reference. This will enable a star to be acquired and
tracked. 
 
To satisfy the last 2 criteria, GHRIL system's optical axis needs to be
aligned with the telescope pupil so as to minimise vignetting and
aberrations due to offaxis rays. However,  this vignetting alignment is 
somewhat less critical than the pointing alignment since most optical systems
will have optical components which are slightly oversized and can tolerate
some degree of misalignment of the beam passing through them.
\vskip 2mm
{\bf Suggested Alignment Methods}
\vskip 3mm
As described above, the alignment procedure does not necessarily require the
optical system axis to be exactly coincident with the telescope axes. However
some sort of repeatable reference is helpful. The telescope defines an number
of axes which can, but need not necessarily, be coincident. 
\vskip 3mm
1. {\bf Secondary Mirror Optical Axis}:\hfil\break
\vskip 2mm
The optical axis of the secondary mirror is commonly used as an optical
reference, however it is the combination of the primary and secondary mirrors
which actually forms the optical axis of the complete telescope (ie a
translation misalignment of primary and secondary can be cancelled out by a
tilt of secondary with respect to primary). A laser beam can be aligned aimed
at the centre of the secondary can be adjusted in translation and tilt to
reflect back on itself. This beam then defines the axes of the secondary mirror
and can be used as a reference, however since it is not necessarily coincident
with the defined pointing axis, it is important also to perform a CALIBRATE at
this position. 
\vskip 3mm
2. {\bf Altitude Mechanical Axis}:\hfil\break
\vskip 2mm
Alternatively, the mechanical axes of the telescope can be used. The most
convenient to use is the altitude axis. The Nasmyth instrument rotator axes
have been adjusted to be coincident with the altitude axis and since the
default pointing centre has been defined to lie on the rotator axis centre, it
is the most convenient axis to work from. 
 
A pair of alignment plates can be accurately fitted to dowel locations on the
centre section inside the altitude bearings. They have been pre--aligned to
coincide with the altitude axis and repeat to $\pm$0.25mm. Each plate has a
central glass alignment target with accurately concentric rings which can be
sighted with the alignment telescope or aligned with a laser beam (less
accurate). The targets are only aligned in translation to the altitude axis
centre, they are NOT adjusted in tilt to be square to the axis. 

Whilst it is possible to remove and replace the glass targets from their
holders and retain the alignment setting, NONE of the other adjustments or
clamping screws should be disturbed. With the Nasmyth flat in the stowed CASS
position it is possible to sight through the entire altitude axis through a
hole in the side web of the Nasmyth flat support. 

*** Isa, Check this section and add your experiences of doing it. I haven't 
done it anyway! ***
A laser mounted on the GHRIL table can be adjusted in tilt and translation 
to pass through the centre of both targets. The best method of doing this is
to arrange a white screen (piece of paper) behind the second target on the
far side. The shadow of the concentric rings can then be seen, the aim is to
get the laser beam centred on the ring pattern AND the pair of ring shadows
coincident. The advantage of using a laser is that once it is aligned with 
the altitude axis, it can be used to square--on the optical components on
the GHRIL table by aligning the back reflections. The disadvantage is that
the relatively large diameter of the laser beam makes it difficult to
accurately align translation. The present mounting for the GHRIL alignment
laser also makes it difficult to precisely translate and tilt the laser
to achieve exact alignment. 
 
An alternative to the laser is the Rank Taylor--Hobson alignment
telescope (MAT). The MAT is mounted on a precision base which positions
it at the correct height and enables fine tilt adjustment. With the
MAT at the back of the table at the approximate position of the 
altitude axis, the base can be moved by hand to align the MAT with the
altitude targets, then clamped. The final tilt alignment can be done
with the MAT's tilt screws and the final translation alignment with
the cross--hair micrometers. Once set up this way, the MAT can be used to
align the remainder of the GHRIL optical components, assembling them from the
front of the table backwards.
 
A problem common to both the laser and MAT alignment techniques from the
BACK of the table, is that the alignment of each component must be done from
the REAR. This is especially a problem with the alignment of the detector since
it is probably to view a suitable reference mark from the back.
\vskip 3mm
2. {\bf Alignment of GHRIL Components}:\hfil\break
\vskip 2mm
*** Isa, Can you do this section. Describe how you go about setting up
the alignment of the lens, etc using the laser. ***

\end{document}
\end{article}
