Robert H. Koch’s Work on Lightweight Medium-Aperture Mirrors

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  • ABSTRACT

    After a visit by Peter Waddell from the University of Strathclyde, Glasgow, UK in 1991, Robert H. Koch launched a program at the University of Pennsylvania to build lightweight pneumatic membrane mirrors, initially for balloon flight observations where weight is at a premium. Mirror cells were fabricated from sizes 0.18 m to 1.77 m, and experiments conducted to characterize the mirror figure and stability. Most of the work stopped after Prof. Koch’s retirement in 1996 until 2006 when the authors expressed an interest in building an array of medium-aperture portable telescopes. The program re-started in earnest at Gravic, Inc. in Malvern, PA in 2008 with Koch using his extensive observational astronomy experience to guide the fabrication of a fully operational 1.07 m membrane mirror telescope with an optical tube assembly weigh-ing under 45 Kg. Residual wavefront aberrations remediation resulted in Koch and the authors investigating membrane tensioning techniques with different cell designs, active secondary wavefront correction, photometric algorithms for ab-errated images, and the use of additional lightweight mirror substrates from the Alt-Az Initiative Group, such as foamed glass. The best result for the lightweight mirrors was a point spread function spot size of several arc seconds. A lightweight 1.6 m cast aluminum cell alt-az telescope was subsequently designed by Koch and the authors for prime focus use.


  • KEYWORD

    membrane mirrors , optical aberrations , telescope arrays

  • 1. Genet RM, Holenstein BD, Genet R, Johnson J, Wallen V 2010 Alt-Az light bucket astronomy, in Lightweight alt-az telescope developments. P.57-69 google
  • 2. Holenstein BD, Mitchell RJ, Holenstein DR, Iott KA, Koch RH, Genet R, Johnson J, Wallen V 2010a Experiments with pneumatically-formed metalized polyester mirrors in Lightweight alt-az telescope de-velopments. P.381-394 google
  • 3. Holenstein BD, Mitchell RJ, Koch RH, Genet R, Johnson J, Wallen V 2010b Figures of merit for light bucket mirrors, in Lightweight alt-az telescope de-velopments. P.271-289 google
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  • [Fig. 1.] The reflector dome at the Flower and Cook Observatory wascapable of housing a 1.0 m or larger telescope. This picture was takenin the late 1990’s looking towards the southwest. The trees to the south needed trimming and were a constant observing hazard.
    The reflector dome at the Flower and Cook Observatory wascapable of housing a 1.0 m or larger telescope. This picture was takenin the late 1990’s looking towards the southwest. The trees to the south needed trimming and were a constant observing hazard.
  • [Fig. 2.] Recent image of Koch holding a 0.30 m (12 inch) aperture pneumatic mirror built at University of Pennsylvania in the early 1990’s.
    Recent image of Koch holding a 0.30 m (12 inch) aperture pneumatic mirror built at University of Pennsylvania in the early 1990’s.
  • [Fig. 3.] Recent image of Koch holding a small carbon fiber optical tube assembly containing a 0.18 m (7.25 inch) pneumatic membrane mirror. Carbon fiber was chosen for its light weight, strength, and low coefficient of thermal expansion.
    Recent image of Koch holding a small carbon fiber optical tube assembly containing a 0.18 m (7.25 inch) pneumatic membrane mirror. Carbon fiber was chosen for its light weight, strength, and low coefficient of thermal expansion.
  • [Fig. 4.] Preparing for a launch at Wallops Island in 1995. Robert Koch is in the center foreground working on the dual charge-coupled device cameras at prime focus, and Kenneth Lande is on the left working on the balloon payload.
    Preparing for a launch at Wallops Island in 1995. Robert Koch is in the center foreground working on the dual charge-coupled device cameras at prime focus, and Kenneth Lande is on the left working on the balloon payload.
  • [Fig. 5.] Completed telescope payload about to be launched at Wallops Island in 1995. Koch is walking towards the carbon fiber telescope truss tubes that Lande is holding. The electronics were located in the silver-colored, sealed and pressurized cylinder in the lower part of the image.
    Completed telescope payload about to be launched at Wallops Island in 1995. Koch is walking towards the carbon fiber telescope truss tubes that Lande is holding. The electronics were located in the silver-colored, sealed and pressurized cylinder in the lower part of the image.
  • [Fig. 6.] Koch is observing the wooden mirror cell constructed by a carpentry student from a local trade school. Variable length truss tubes are temporarily setting inside the mirror cell. An epoxy surface later added to the cell to reduce leaks added weight, but the structure still remained under 35 Kg (77 lbs.).
    Koch is observing the wooden mirror cell constructed by a carpentry student from a local trade school. Variable length truss tubes are temporarily setting inside the mirror cell. An epoxy surface later added to the cell to reduce leaks added weight, but the structure still remained under 35 Kg (77 lbs.).
  • [Fig. 7.] Koch is peering at the pre-tensioned Mylar polyester film that has been attached to the mirror cell before the top retention ring has been attached. A vacuum of about 30 millibars for 2 mil film would produce an f/2 focal ratio.
    Koch is peering at the pre-tensioned Mylar polyester film that has been attached to the mirror cell before the top retention ring has been attached. A vacuum of about 30 millibars for 2 mil film would produce an f/2 focal ratio.
  • [Fig. 8.] Koch taking a break on the hot July, 2009 day just before first light of the 1.07 m pneumatic mirror telescope. Rich Mitchell is seen in the mirror reflection. The pneumatic control system and battery are on the ground below the Kevin Iott-designed and built IPI 393 GEM mount. A custom-built high-speed electrometer photometer is at prime focus.
    Koch taking a break on the hot July, 2009 day just before first light of the 1.07 m pneumatic mirror telescope. Rich Mitchell is seen in the mirror reflection. The pneumatic control system and battery are on the ground below the Kevin Iott-designed and built IPI 393 GEM mount. A custom-built high-speed electrometer photometer is at prime focus.
  • [Fig. 9.] Robert Koch is pondering a solution to remedy aberrations arising from the biaxial Young’s modulus of Mylar which was preventing the 0.3 m pneumatic mirror located on a test stand from operating at the expected performance level.
    Robert Koch is pondering a solution to remedy aberrations arising from the biaxial Young’s modulus of Mylar which was preventing the 0.3 m pneumatic mirror located on a test stand from operating at the expected performance level.
  • [Fig. 10.] Disassembled CNC machined cell for a small pneumatically-formed mirror. The edges contacting the membrane were precision machined and polished to minimize the cell design as a source of aberrations.
    Disassembled CNC machined cell for a small pneumatically-formed mirror. The edges contacting the membrane were precision machined and polished to minimize the cell design as a source of aberrations.
  • [Fig. 11.] Cartoon of exaggerated edge-on views of two mirrors with the same peak-to-valley, root mean square (RMS) surface height, and Strehl ratios measures of their aberrations. The bottom mirror scatters photons into a larger diameter point spread function because the RMS gradient norm (RMS slope) is much larger.
    Cartoon of exaggerated edge-on views of two mirrors with the same peak-to-valley, root mean square (RMS) surface height, and Strehl ratios measures of their aberrations. The bottom mirror scatters photons into a larger diameter point spread function because the RMS gradient norm (RMS slope) is much larger.
  • [Fig. 12.] Calibrated flats and mirrors which have been cold silvered with Peacock Labs solutions. The upper right flat and the front row of mirrors have been overcoated with an extra thin layer of Peacock’s Permalac solution causing a slight yellowing in the visible appearance and a reduction of about 7% in the reflectivity.
    Calibrated flats and mirrors which have been cold silvered with Peacock Labs solutions. The upper right flat and the front row of mirrors have been overcoated with an extra thin layer of Peacock’s Permalac solution causing a slight yellowing in the visible appearance and a reduction of about 7% in the reflectivity.
  • [Fig. 13.] One point six meter alt-az telescope design for a pneumatic mirror with a cast aluminum cell. Cutout images of Koch and Mitchell are included for scale.
    One point six meter alt-az telescope design for a pneumatic mirror with a cast aluminum cell. Cutout images of Koch and Mitchell are included for scale.
  • [Fig. 14.] This picture of Robert Koch hard at work is typical of how many of his students and colleagues recall him. Koch liked to graph experimental measures in real-time and this provided immediate insight into the phenomena under study.
    This picture of Robert Koch hard at work is typical of how many of his students and colleagues recall him. Koch liked to graph experimental measures in real-time and this provided immediate insight into the phenomena under study.