150-foot tower system upgrades 1996

Between 1993 and 1996 an upgrade project was implemented at the 150-foot tower to provide multiple spectral samples from different spectral lines and to cure problems of spectrograph grating allignment. Many details of this were published by Ulrich, R.K., et al., 2002ApJS..139..259U, Mt. Wilson Synoptic Magnetic Fields (ADS Abstract). This page provides some additional details as well as access to a number of photographs of critical components. These are from black and white prints of photos taken on Feb. 27, 1996. The upgraded system was put into operation on April 15, 1996 so these images are of the components prior to their installation or upgrade.

		This is a view of the observing room as you enter from outside. The main spectrograph head assembly is the circular pillar at the center. It rotates so that either of two exit slit configurations can be used. The one for magnetograms is in the operational position as shown here. The reversed position allows for the visual measurement of sunspot magnetic field strength using Hale's optical analyzer. Field strengths from this optical setup are recorded on the sunspot drawings.
		This is the observing room looking north. The spectrograph head is seen from an angle that makes the entrance slit visible -- the small black box on a raised enclosure at the center of the image. The guiding ring is the white circular rail above the head. A setting circle with angular markings can be seen just above the white ring. This is used to adjust the scanning direction so that the sun's equator is along the x-axis of the magnetogram. The scanning screws carry a frame that moves the guider pickoff mirrors. These provide a closed loop servo that controls the second flat mirror and thus the solar image position. Along the back wall are portraits of people and events important to the project. The door directly behind the spectrograph head opens to the lift that provides access to the tower top.
		Closeup view of the entrance slit and polarimetry components. From the top down along the optical axis leading to the entrance slit are found a circular polarizer that can be swung into the beam for calibration,
	The entrance slit is below the black circular housing at the bottom of this photo. The Walraven image slicer is just above inside the rectangular box. The entrance aperture is visible on the top of this box. The KDP modulator is in the silvery cylinder just below the larger black box that contains a filter wheel.
		The older exit slit assemblies. Blue is on the left, red is on the right. The red stage fiber-optic image reformattor contain two bundle while the blue stage reformattor has ten-bundels of which only two were used in the older system. The old configuration provided a total of 4 spectral samples. The output of the reformattors is used to illuminate two photomultiplier tubes on each stage.
		This is the new red stage with the older 10-channel fiber-optic reformattor. The stage is seen here from the rear. In current operation, the 20-channel fiber-optic reformattor is used for 6768 and the outputs are combined in pairs each of which is used to illuminate a separate photomultiplier tube like the one laid out below the assembly.
		This photo shows the electronics inside each of the stage housings. The voltage dividers and amplifiier/Voltage to Frequency converters are in the chamber a the end of the stage toward the top of the photo. Two rows of six photomultipliers are inside the cylinder-block housing at the lower side of the photo. The clamps on this cylinder-block are used to capture the output end of the fiber-optic bundles and hold them aligned so that their output hits the photocathodes.

		The spectrograph grating box with the Littrow lens housing above. A grating drive motor is to the right with its associated gear reduction box. This view is from the northeast. The front of the box is completely off and the grating tray is visible inside.
	View of the grating box from the southeast. In this image the side with a porthole access hole is shown. The assembly on the left side of the box is a grating clamp system. This system has been disabled since the friction and balance of the grating tray is adequate to hold the grating at a fixed angle and the absence of the brake allows for a quicker settling of system drifting following a grating move.
Final assembly of the grating box by Nick Magnone. The porthole cover is shown here in place. This has a watertight gasket to protect the grating in case of flooding. The 75-foot deep spectrograph pit becomes a well during wet years and water can easily rise to the level of the grating box should the pit pump fail. The box is normally dried with a common desicant to reduce the humidity inside the box and slow the degradation of the grating surface.