Design and Evaluation of a Telecompressor for a Cassegrain

In order to make better use of their telescope's available field for imaging, avid telescope users make use of an optical device known as a telecompressor also known as a Shapley lens or a focal reducer. Rather than widening the field like a Barlow lens by increasing the telescope's focal length a telecompressor does the opposite compressing the field by shortening the telescope's focal length. This allows for shorter exposure times for film and the potential to fit a telescope's entire field onto a CCD camera. A simple telecompressor would consist of a single positive lens. Such a system would be in adequate for astrophotography, which is why telecompressors like many other refractive optical systems are corrected for certain wavelengths of light. Sometimes a telescope can benefit from having the telecompressor designed just for that particular telescope. The director of the observatory at Texas A&M University Kingsville has been contemplating whether or not a custom made telecompressor would hold any benefits. The report discusses the preliminary design and evaluation of an astronomical achromat according to the third order aberration theory. The achromat is then optimized by finite ray-tracing. The achromat's performance as a telecompressor is compared to the performance of a telecompressor designed just for the telescope at TAMUK. The results indicate that the achromat performs about as well as the custom telecompressor. Analysis also presented that the custom telecompressor was optimized for the wrong system could potentially perform much better if it were optimized on the correct system. However in the meantime an ordinary achromat should suit the system well enough for CCD astroimaging.