Publications of the Astronomical Society of the Pacific 102:1086-1091, September 1990 MYLAR AS AN OPTICAL WINDOW LAIRD A. THOMPSON Astronomy Department, University of Illinois, Urbana, Illinois 61801 Received 1990June 4 ABSTRACT Optical tests have been made to determine whether Mylar film has high enough optical quality to be used as a window for a telescope dome or as the entrance window for a closed-tube telescope. The results are encouraging. The thinnest Mylar film available from DuPont (thickness 1.5 jxm) has reasonably high optical quality and, if properly mounted, sufficient tensile strength to resist destruction by gusts of wind. An attempt to use 1.5- xm Mylar to reduce dome-induced turbulence is described. Key words: seeing-optical testing-mylar film 1. Introduction Ground-based optical telescopes and their domes are generally not in thermal equilibrium with the outside air. Turbulence induced by the lack of thermal equilibrium can degrade the quality of astronomical images. In a quest to reduce this source of image degradation, several imaginative solutions have been suggested and/or implemented. Note that most of the following examples are elaborate and costly. At Pic du Midi, a 2-meter telescope was built with a solid telescope tube and a closed dome the tube and dome are connected with a moving shroud to prevent warm air from flowing out of the dome. Only the top of the telescope tube is open to the outside air (Rosch 1987). The very successful New Technology Telescope (NTT) at the European Southern Observatory has an open-tube telescope located (when the dome slot is open) in clear air sandwiched between two individually enclosed halves of the dome. The NTT dome is conceptually more advanced but similar to the older Multiple Mirror Telescope dome. Racine (1987) proposed an even more elaborate scheme for a high-resolution telescope. The telescope tube would be attached directly to the dome, similar to the Pic du Midi design, but an optical window would be placed at the top of the telescope tube to prevent thermal mixing of the cool nighttime air with air in the closed telescope tube. At many observatories image quality might improve if an optical window were used to seal the entrance slit of the dome and/or the top of a closed telescope tube. The window would prevent the wind from mixing outside air with air inside the dome and/or telescope tube. Unfortunately, a glass optical window large enough to cover the dome slot or the top of a closed telescope tube would be expensive and so massive that it would be difficult to support. If Mylar film can be used as an optical window both of these problems would be easily solved. Mylar film is very inexpensive and simple to mount. Amateur astronomers have reported reasonable success in closing off small domes with thin sheets of Mylar (Sahula 1978). The purpose of this paper is to describe optical tests of various types of Mylar film both in a laboratory setting and at an observatory to determine how Mylar might function as a large optical window. 2. Mylar Characteristics Mylar polyester film is manufactured by E. I. DuPont de Nemours & Company, Incorporated (Wilmington, Delaware) in a wide range of types. Not all of these are optically clear, yet there are two types that raised initial interest: type-c Mylar which is produced to meet the needs of the capacitor industry and type-d Mylar which is touted as a film of great clarity. Type-D Mylar is sometimes termed optical grade. While type-d sounded quite interesting for astronomical purposes, the first optical tests showed it to be inadequate. Product engineers at DuPont later said that type-d Mylar is microscopically embossed on one side to enhance its slip characteristics, and in this process its optical properties are compromised. In the remainder of this paper only type-c Mylar will be discussed. Type-C Mylar is manufactured in six standard thicknesses that range from 1.5 xm to 23 xm. Data sheets provided by DuPont show that it has excellent optical transmittance. The following statements apply to the thickest version (23 jxm) of type-c Mylar. Transmittance turns on sharply at 320 nm, rises to 87% at 400 nm, remains at 87% transmittance until 900 nm, and then rises to 91% transmittance and remains high to 2.3 pan. Thinner samples of type-c Mylar are expected to transmit somewhat better. Based on the DuPont data sheets, in the visual portion of the spectrum the transmittance of 1086
MYLAR AS AN OPTICAL WINDOW 1087 the thinnest Mylar samples approaches the theoretical limit imposed by two air-dielectric surfaces. Mylar has an index of refraction of approximately 1.65. There is a reasonable expectation that the thinnest type-c Mylar will be optically flat. DuPont technical representatives suggest that the thickness of this product is controlled to within ± 1% 2% at the time of manufacture. If this tolerance is met, the 1.5-pan thick Mylar should be flat to approximately X/10 at 500 nm. Thicker Mylar films are held to a manufacturing standard of only ±5%, so for these one would predict decreased optical performance. The widest rolls of L5- xm thick type-c Mylar produced by DuPont are 61 cm wide. Large samples of the thinnest Mylar are moderately easy to handle. However, it is an excellent dielectric so it does accumulate electric charge and attract dust. Metallic strips can be attached to the Mylar to help keep the accumulated charge at a minimum. 3. Initial Tests DuPont provided a full set of small (21 cm X 28 cm) samples of both type-c and type-d Mylar that were used in the initial tests. The gross optical properties of these samples were tested by placing them in front of the objective of a small-aperture telescope while viewing a star. This survey technique was used to show that all but the thinnest type-c Mylar samples introduced significant aberrations in the star image. Two laboratory tests were made to investigate the optical properties of the Mylar. First, a standard Foucault knife-edge test was assembled with a 21.6-cm diameter spherical mirror (mirror aberrations < X/10 at 500 nm). The Mylar samples were placed in front of the spherical mirror in such a way that the test beam passed through the Mylar twice. For the thicker Mylar samples it was clear that the optical aberrations consisted of strong linear striations in one direction, presumably in the direction of manufacture. Only the 1.5-pan thick sample of type-c Mylar showed an excellent pupil image with slight linear striations. All other samples were rejected as potential optical windows. The second laboratory test setup consisted of a spatially filtered He-Ne laser (633 nm) uniformly illuminating the same 21.6-cm diameter spherical mirror. Laser light reflected off the spherical mirror produced a diffractionlimited image at the mirror focus, and a laboratory photometer was placed at this focal point. The 1.5-pan thick type-c Mylar was placed in the beam close to the spherical mirror, so again the light passed through the Mylar twice. With a large (with respect to the diffraction limit) aperture in the photometer, a measurement of the double-pass transmittance of the system was made with the Mylar in the beam and with the Mylar out of the beam. The double-pass transmittance of the 1.5-pan thick Mylar was measured in this way to be 86% (±1.5%). A single pass through the Mylar would therefore transmit 93% of the laser light. By stopping the spherical mirror down to a diameter of 13.6 cm, it was possible to match the first minimum in the Airy diffraction pattern to the diameter of the smallest aperture in the laboratory photometer. Measurements of the central Airy peak in the diffraction pattern were made with and without the 1.5-pan Mylar sample. This test showed that only 9% of the light in the central Airy peak was scattered by the double pass through the Mylar. Using the standard definition of the Strehl ratio (cf. Born and Wolf 1964), this measurement can be used to infer that the Mylar introduces a wavefront variance of approximately X/20 at 633 nm. 4. Mylar Tests at the Observatory Given the success of the laboratory tests with the 1.5- xm thick type-c Mylar, a 61-cm-wide roll was ordered from DuPont. This sample was taken to the 1-meter telescope at Mount Laguna Observatory where, on three separate occasions, the Mylar was tested under different seeing conditions. Mylar windows were mounted in two positions along the telescope beam: immediately in front of the closed telescope tube (see Fig. 1) and in a window in the dome shutter (see Fig. 2). The frame placed over the closed telescope tube has a center support which was aligned along the secondary-mirror support strut. Two pieces of the 61-cm-wide Mylar were wide enough to cover the 1-meter aperture. The 1-meter Mount Laguna Observatory telescope was configured at//13.5 (rather than our optional//7.6) and the TI 800 x 800 CCD of the observatory was mounted bare at the focal plane. With this configuration the CCD gives 0.20 arc sec/pixel. First, CCD images were obtained of the telescope pupil with the Mylar window mounted at the top of the telescope tube. The exposures were repeated with the Mylar window removed. Figure 3 shows the results. Linear striations can be seen in the pupil image with the Mylar in the beam. However, the amplitude of the striations appears to be small relative to the aberrations intrinsic to the primary mirror. With the same telescope and detector configuration, focused images of stars were obtained on several occasions with and without the Mylar window mounted at the top of the telescope tube. In all cases there was no significant change in the FWHM of stellar images produced with and without the Mylar window. During these tests, image FWHM ranged between 1.1 and 2.5 arc sec. Since the pupil images both in the laboratory and at the telescope show little evidence of strong Mylar aberrations, this result was expected. However, there is a potential problem that the Mylar film may produce a halo of scattered light. To demonstrate that this is also not a problem, image profiles are presented here for one set of data
1088 LAIRD A. THOMPSON Fig. 1-Mylar window mounted on top of the 1-meter telescope. A crude shroud blocked air flow between the window and the top of the telescope tube. obtained 1989 December 2. On this particular night, to reduce problems of image wander and to improve the angular resolution for the test, very short exposures (0.010 sec) were obtained and later centroided and coadded. The individual images in both data sets (Mylar and no-mylar) showed considerable variance in image FWHM ranging from 5 pixels = 1.0 arc sec to 9.5 pixels = 1.9 arc sec. Figure 4 shows a comparison between the radial intensity profiles of the coadded images with and without the Mylar in place. The two profiles are close to being identical. There is a slight difference in the FWHM between the two profiles: 1.3 arc sec with the Mylar window and 1.1 arc sec without the Mylar window. However, this difference is probably not significant relative to the variance in image FWHM that was occurring that night. The elapsed time between the two sets of exposures was approximately 1 hour, and atmospheric seeing can easily change by 20% during an hour of time. The fact that the two profiles are identical at large radii shows that the Mylar does not scatter light from the image core into the wings. 5. Suppression of Thermal Exchange with Mylar Windows Quick tests were made with the Mount Laguna 1-meter telescope to demonstrate the effect of mounting a Mylar window to cover the closed telescope tube and another Mylar window to cover the dome slot. Resistive thermometers were used to measure the air temperature at three locations: (1) inside the telescope tube adjacent to the primary mirror, (2) 5 cm above the cement floor of the dome in an open area to the west of the telescope pier, and (3) outside the dome on the catwalk. Figures 5 and 6 show temperature measurements on two nights, the first without the two Mylar windows in place and the second with both windows mounted. Clearly, the Mylar acts to impede the exchange of heat. Without the Mylar in place (see Fig. 5) the temperature inside the dome dropped to within 0?7 C of the outside air temperature after the dome had been opened three hours. The air near the primary mirror also dropped over the same time span but remained 2 C warmer than the outside air. With the Mylar in place (see Fig. 6) the inside dome temperature remained more than I o C warmer than the outside temperature throughout the test, and the air near the primary mirror cooled less rapidly. 6. Discussion While the Mylar windows were doing their job of reducing the mixing of warm and cool air, there was no
MYLAR AS AN OPTICAL WINDOW 1089 Fig. 2-Mylar window mounted in dome slot. The dome shutter stopped air from passing above the window, and the wind screen blocked air from passing from below. Each of the three panels in the window were approximately 0.5-m wide. evidence of any improvement in image quality with the windows in place. There are two possible explanations. First, the poor image quality at Mount Laguna Observatory may simply be a characteristic of something other than dome-induced seeing (local orographies and/or upper-atmospheric turbulence). Second, it is abundantly clear from the data displayed in Figures 5 and 6 that the Mount Laguna 1-meter telescope is many degrees C out of equilibrium with the surrounding air. Simply trapping the very warm air inside the telescope tube and inside the dome may be an inadequate way to solve dome-seeing problems. It is likely that a telescope and dome have to be much closer in thermal equilibrium to the outside air before Mylar windows provide any advantage. Anyone who attempts to proceed further with these experiments should be aware that the first attempts to mount the Mylar as a dome window were halted by a gusty wind that tore the Mylar window to shreds. Mylar has a tensile strength high enough to withstand quite large static air pressures (up to 21,000 psi and a tear strength of 250 Newtons/mm). However, for the experiments described above the Mylar was relatively loosely attached with double-sided tape to its frame, and the wind whipped the thin Mylar until it snapped. If the Mylar were held in a frame by pinching it between two rubber gaskets, and if constant pressure were applied to the Mylar window from inside the telescope tube or from inside the telescope dome it is likely that the Mylar would remain taut thereby preventing it from whipping in the wind and tearing. 7. Conclusions The thinnest grade of type-c Mylar which is 1.5- xm Fig. 3-Photographs of the 1-meter telescope pupil, (a) The telescope pupil with no Mylar, (b) The telescope pupil with the Mylar in place.
1090 LAIRD A. THOMPSON 00 z LU Fig. 4-Radial profiles of stellar images without the Mylar (open circles) and with the Mylar (filled circles). One profile was scaled in intensity to superpose it on the other profile. There is no evidence in this profile for scattered light produced by the Mylar. 10 12 14 16 18 20 22 24 26 28 30 TIME (hrs) Fig. 5-Temperature variations obtained 1990 March 21 with no Mylar windows mounted. Triangles show outside temperature, circles the air temperature inside the dome, and squares the air temperature adjacent to the primary mirror. The dome was opened at 16:30 and the primary mirror cover at 18:45. thick provides excellent optical quality as an optical window. Aberrations are less than X/10 and no scattered light from the Mylar is apparent. Type-C Mylar is produced regularly in widths of 61 cm. If it is mounted on a dome or over a closed-tube telescope and pressurized from inside, it is likely to withstand moderately heavy gusts of wind. If used to isolate (moderately) warm air inside domes and/or closed-tube telescopes, it may help to re- duce the effects of dome-induced seeing. The laboratory tests of Mylar samples were started by Mike Svec and brought to excellent completion by Mike Corn. The Mylar window attachments to the 1-meter telescope at Mount Laguna Observatory were built by Jay Grover, and the temperature probes were provided by Dr. Ron Angione. Numerous product representatives at DuPont provided technical information on Mylar and
MYLAR AS AN OPTICAL WINDOW 1091 Fig. 6-Temperature variations obtained 1990 March 23 with Mylar windows in place. Symbols are the same as in Figure 5. The dome was opened at 17:50 and the primary mirror cover was opened at 19:15. Notice how the inside air temperatures fall less rapidly in Figure 6 than in Figure 5 after the dome is opened. generously sent samples free of charge. Special thanks to the American Astronomical Society Small Research Grant awards committee and to NASA for providing iunds to carry out this work. Mount Laguna Observatory is operated jointly by San Diego State University and the University of Illinois at Urbana-Champaign. REFERENCES Born, M., and Wolf, E. 1964, Principles of Optics (Oxford, England: Pergamon Press), p. 464. Racine, R. 1987, private communication. Rosch, J. 1987, in Identification, Optimization, and Protection of Optical Telescope Sites, ed. R. L. Millis, O. Franz, H. D. Abies, and C. C. Dahn (Flagstaff, AZ: Lowell Observatory), p. 146. Sahula, P. 1978, Sky and Tel., 56, 67.