Gregoire Tremblay

Optical tests of a 3.7-m-diameter liquid mirror: behavior under external perturbations

We built and tested a 3.7-m-diameter liquid mirror that rotates on a ball bearing. Although the ball bearing is of a poor quality, the mirror is surprisingly good for one that comprises 1-mm-thick mercury layers. We found no evidence of the strong astigmatism that might have been expected from Coriolis forces. We did not detect effects of turbulence might or vibrations for thin mercury layers, illustrating the necessity of using thin layers: Large liquid mirrors would have had unacceptable optical qualities for layers much thicker than 2 mm.

GALLIUM LIQUID MIRRORS: BASIC TECHNOLOGY, OPTICAL-SHOP TESTS, AND OBSERVATIONS

We have investigated the use of gallium and low-melting-temperature gallium alloys for liquid mirrors. They have advantageous characteristics with respect to mercury. Simple knife-edge and Ronchi optical texts indicate that gallium mirrors ahve optical qualities similar to those of mercury mirrors. We have observed stars with a 1-m diameter liquid mirror telescope that used gallium- indium alloy. The instrument operated satisfactorily, within the limitations of a very simple telescope and instrumentation. Arguably, our most interesting finding is that the high melting temperature gallium isnot an obstacle since our experiments show that it is easy to supercool in large volumes and quite stable in the supercooled state. Furthermore, eutectic alloys of gallium have significantly lower melting temperatures than pure gallium and are also easy to supercool and stable in the supercooled state. The results of our experiments are encouraging and warrant further, more rigorous, continuation of this work.

Mechanical and Thermal Tests of the Containers of Liquid Mirrors

We give a generic description of a liquid mirror system and summarize some practical information useful in making them. We compare laboratory measurements of deformations, due to the weight of mercury on the container of a 3.7-m mirror and to temperature changes on a 1-m container, to finite element computer simulations. We find that the measurements agree reasonably well with the numerical simulations. The measurements on the 1-m container show no evidence of fatigue after a few thermal cycles. These results validate the computer models of containers described in a companion article.

Polarimetric active imaging in dense fog

Operation under degraded visual environment (DVE) presents important strategic advantages. 3D mapping has been performed under DVE and good quality images have been obtained through DVE with active imaging systems. In these applications, the presence of fog clouds degrades the quality of the remotely sensed signal or even renders the operation totally impossible. In view of making the active imaging method more robust against dense fog, the use of polarimetry is herein studied. Spherical particles typical of fog do not depolarize incident polarized light in the backscattering (180°) direction. So, in principle, there should be less dazzling caused by aerosols for active imaging systems operating using the secondary polarization. However, strong depolarization still occurs at angles close to 180°. The greater the ratio of size to wavelength, the closer to 180° will the depolarization occur. When the cloud optical depth is small, the major scattering events seen by an active camera are the single backscattering events. However, when the optical depth of the cloud is higher than 1, multiple scattering becomes more important and causes depolarization due to the backscattering around 180°. The physics of this process will be discussed. Experimental results supporting the analysis will be presented. Those experimental results were obtained under controlled environment using the DRDC-Valcartier aerosol chamber. The experimental method herein proposed is based upon the use of ICCD range gated cameras wherein gate width and gate location may be varied on the fly. The optimal conditions for the use of these devices in view of obtaining the best image contrast are experimentally studied and reported in this paper.

The shower curtain effect paradoxes

It is generally admitted that the relative location of an aerosol between an observation device and the observed scene will have an influence on the detected image quality. These effects are usually classified under the label “shower curtain effect” (SCE). The usual formulation describing it is as follows: an observer standing away from a shower curtain can detect the presence of a person standing just behind it whereas the opposite is not true. Starting from a discussion of experimental results which seemed to invalidate the SCE, we show that it is not the only mechanism at work and that thorough analysis of the measurement setup is required before reaching such conclusion. We base our discussion on four cases, two of them of the passive detection type, the two others being of the active type. We also show that the ratio of scattered to unscattered light at the detector is of utmost importance. We show this by further developing our model [10] of the point spread function (PSF) of the receiver. This model allows the discussion of the SCE in the frequency domain in terms of the cuton and cutoff frequencies of the receiver. In the end, we show that the apparent paradoxical results we had found cannot actually be placed under the “shower curtain effect” denomination because: 1-) the amount of unscattered light captured is higher than the amount of scattered light, and 2-) the receiver cuton frequency is much higher than the aerosol cutoff frequency rendering most mechanisms of the shower curtain effect ineffective.

Liquid mirror telescopes: a progress report

We review the present status of liquid mirror telescopes. Interferometric tests of liquid mirrors (the largest one having a diameter of 2.5 meters) show excellent optical qualities. The basic technology is now sufficiently reliable that it can be put to work. Indeed, a handful of liquid mirrors have now been built that are used for scientific work. A 3.7-m diameter LMT is presently being built in the new Laval upgraded testing facilities. Construction of the mirror can be followed on the Web site: http://astrosun.phy.ulaval.ca/lmt/lmt-home.html. Finally we address the issue of the field accessible to LMTs equipped with novel optical correctors. Optical design work, and some exploratory laboratory work, indicate that a single LMT should be able to access, with excellent images, small regions anywhere inside fields as large as 45 degrees.

Liquid Mirror Telescopes: A progress report

We review the status of the liquid mirror project. Interferometric tests of a f/1.2 2.5-m diameter liquid mirror carried out with a scatterplate interferometer show Strehl ratios of order 0.6, close to the value of 0.8 usually taken to signify that diffraction limit has been reached. The mirror is certainly better than implied by the data because the interferograms were taken with 1/500 second exposures and the wavefronts therefore include the effects of seeing in the testing tower. Correctable small variations of the rotational velocity account for another substantial fraction of the deviations from a parabola. We have videotaped hours of interferogram and PSF observations that show that those we analyze are representative.