Aligning the spectrometer

The basic alignment has to be done by measurement, because visible light cannot be used to align an RC designed for EUV. The angle of the beam entering the cham ber and the RC is 84.5°, whereas in the flight GIS the angle is 84.75°, because o f a late design change. This means that the calculations performed in Chapter 2 (see Figure 16) to find the angles and positions o f the spectral lines had to be repeated for this different entrance angle.

It is very difficult for ju st one person to set up the alignment. The two most im portant pieces of equipm ent for the alignment comprise a knife edge jig which accurately brings the RC edge up to the height of the optical beam and a microscope which is small enough to put into the optical beam, but can be looked into from above, so that it is not necessary to put the eye in line with the beam— in an ultra clean cham ber with little room for manoeuvre. The microscope was built by the author by com bining a collim ator and microscope, since it was im possible to buy a suitable device.

The knife edge jig was also specially made with two studs which can be pushed up against the RC jig to accurately locate the vertical knife edge on the RC. It is placed at the required position of the entrance slit (see Figure 16) by m easurem ent from the edge of the cham ber. The microscope is focused onto the knife edge at the height of the optical beam (indicated by a mark on the edge) and the cross hairs aligned with the edge. The knife edge jig is then removed and replaced with the adjustable slit. The slit position is adjusted until it is in focus and aligned with the cross hairs of the

microscope, without moving the microscope or changing its focus. This ensures the slit is placed on the RC.

The chord length from the entrance slit is used to find the position around the RC of the zero order. The same knife edge and microscope method is employed to position a zero order slit accurately on the RC.

Once the two slits are set up and adjusted to about 50 pm width, then it is possible to position the grating. A diffuse light is shone in through the lamp port (with the UV lamp removed). The grating is placed in approximately the correct position, again using the knife edge and microscope. Fine adjustment to its angle is obtained by rotating it until the diffuse light coming through the entrance slit is focused on the zero order slit. The light falls through the slit and can be seen on the opposite chamber wall.

The next stage is to align the UV source. The lamp is designed so that a laser beam can be shone right through it. Before re-assembling the lamp with the main chamber, a laser is set up to shine through the lamp port, through the entrance slit and to reflect off the grating through the zero order slit. Again the laser light can be seen on the opposite chamber wall.

At this point the lamp is re-joined to the main chamber and moved about bodily until the laser light can be seen to have travelled right through it and arrived in the main chamber. The lamp is now aligned.

But, when the lamp is evacuated, it moves. For this reason, the laser is left in position until the lamp has been evacuated, so that the lamp can then be moved again to re-align it. The problem here is that in order to allow the lamp light into the main chamber, the baffle between the chambers needs to be open. Therefore, the main chamber has to be evacuated with the lamp. With the lid closed on the main chamber it is very difficult to see whether the laser is shining through the zero order slit when moving the lamp around to align it. The zero order box, which is designed to absorb the zero order light and stop it from reflecting round the chamber giving scatter, is instead used to reflect the laser light from behind the zero order slit, out through the viewing port. In this way, the alignment can be checked provided that the laser has not been moved.

Unfortunately, if the entrance slit is narrowed down to 50 pm or less, there is not enough laser light to be seen from outside the chamber. The chamber therefore has to be opened up again for the entrance slit to be narrowed after the alignment has been set up. To set up the entrance slit, feeler gauges may be used to calibrate the vernier adjuster. Due to the difficulty of fitting feeler gauges in such a confined space, an alternative

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method is to set up the zero order slit to the required width using the feeler gauges and then reduce the entrance slit width until all the light goes through the zero order slit.

500 400 <u X CL c 300 C 0) > <u

I

Figure 62. Spectrum obtained using the UV spectrom eter

with the detector Jeremiah in position 4 on the RC and argon used in the lamp. A few of the identified lines are labelled according to wavelength in nm.

The detector is mounted on the RC using a specially made mount, again with location studs for the lower RC and an accurately machined upper RC location for the detector studs to be pushed against. The lower RC jig plate had to be machined away where the detectors were to be mounted to allow room for the detector connectors. The detector can be isolated from the m ount by using the fibre glass spacer manufactured for the flight GIS. The detector alignment studs then have to be set back by the 0.5 mm and pushed against the mount jig using a 0.5 mm spacer, ju st as in the flight GIS.

Baffles are required to cut down scattered light. A slit baffle is used on the cham ber port to reduce the width of the beam arriving at the entrance slit. Another is mounted on the back side o f the entrance slit to cut down the angle and height of the beam leaving the slit, reducing extra orders o f diffraction. The grating is baffled with a glass slide on its edge, to stop UV light from hitting the edge or passing straight over the

top. Baffles are also used to stop any light from the grating except that which falls in the required region of the spectrum.

Figure 62 shows a spectrum obtained by placing the detector in the fourth (longest wavelength) position on the RC. The gas in the lamp was argon. Some of the lines are identified.