During the Summer 2017, the Department of Physics and Astronomy decided it was time to clean Goodsell’s 16.2 inch Brashear refractor lens. The lens was last cleaned over 30 years ago, in 1987. Since this cleaning, there was long a question of whether the lens had been reinstalled correctly, as the image quality seemed to suggest it could have been installed backwards. Lastly, there was also an interest in measuring the exact material that each piece of the objective lens was made of.
Instrument project manager Mark Zach, as well as Professors Cindy Blaha and Joel Weisberg, carried out most of the disassembly, cleaning, and reassembly processes. Photos were taken by Bruce Duffy, Geoffrey Mo, and Mark Zach.
Removing the lens
The first step in removing the lens for cleaning was to fix the telescope’s objective end down towards the ground. Mark did this with two cargo straps (Fig. 1). Notice that the lens cap (a garbage can lid) is held on with brass retainers that surround the screws holding the lens in place. After removing the lens cap, a lift cart was placed underneath the lens with cardboard protection (Fig. 2). With the cart bed raised underneath the lens, the brass screws and retainers were removed so that the lens could be separated from the telescope (Fig. 3).
Cleaning the lens
In order to clean both glass pieces which make up the lens, Joel and Mark carefully turned over the assembly and removed the retaining ring (Fig. 4). The pieces were separated using industrial suction cups and placed on cloth protection (Fig. 5). The glass pieces were first brushed off to remove the majority of the dirt and dust, lightly dabbed with cleaning fluid (ask about the composition of the cleaning fluid, isopropyl alcohol and something else I think?), and then rinsed with deionized water (Fig. 6).
Finding the density of the glass pieces
A spherometer was used the measure the precise radius of curvature of each glass piece (Fig. 7). The mass of each piece was also found (Fig. 8). Using this information, precise calculations of the density of each glass piece could be made, so that the materials could be determined. The reason the lens is made of two pieces of glass of different materials (Crown and Flint Glass) is that the combination of two indices of refraction allows the focusing of both blue and red wavelengths to reduce chromatic aberrations in the images produced by the lens (Fig. 9).
Reassembly
With the cleaning and measurements done, the lens was reassembled carefully (Fig. 10). The conclusion was also made that the lens was in fact installed correctly. This suggests that the image imperfections may be the result of abnormalities in the lens shape instead.