The use of innovative ideas and the latest technology have undoubtedly brought down telescope costs substantially. However, there are still ways to further reduce the cost of optical ground-based telescopes and make them affordable to much larger and wide spread astronomical communities. In this and subsequent papers we are presenting our studies carried out towards building affordable mid-size telescopes of 4.0-6.0m in size. In the present era, segmented mirror technology has become the first choice for building moderate to large-size telescopes. In any Segmented Mirror Telescope (SMT) the most important part is its primary mirror control system (M1CS). The conventional M1CS is based on edge sensors and actuators, but such a system introduces many design and implementation complexities. In this paper, we propose to make use of an Off-axis Alignment and Phasing System (OAPS), which is an active mirror kind of control system working in real time to maintain the figure of a segmented primary mirror without the use of edge-sensors. The alignment and phasing system which is an integral part of any segmented telescope can be used in the real time at the off-axis. Through extensive simulations we have explored the feasibility of using an OAPS for co-alignment, co-focusing as well as co-phasing of segmented mirror telescopes. From our simulations we find that the co-alignment and co-focusing of the segments can be achieved with a guide star as faint as 16-18 th magnitude. This implies that seeing limited performance for any segmented telescope can be easily accomplished without use of a complex edge sensor based control system. Whereas, to attain diffraction limited performance, mirror segments need to be cophased with an accuracy of few tens of nanometers. In our simulations we have used a dispersed fringe sensor based phasing scheme, which can effectively work up to guide stars of 14 th magnitude.
The use of innovative ideas and the latest technology have undoubtedly brought down telescope costs substantially. However, there are still ways to further reduce the cost of optical ground-based telescopes and make them affordable to much larger and wide spread astronomical communities. In this and subsequent papers we are presenting our studies carried out towards building affordable mid-size telescopes of 4.0-6.0m in size. In the present era, segmented mirror technology has become the first choice for building moderate to large-size telescopes. In any Segmented Mirror Telescope (SMT) the most important part is its primary mirror control system (M1CS). The conventional M1CS is based on edge sensors and actuators, but such a system introduces many design and implementation complexities. In this paper, we propose to make use of an Off-axis Alignment and Phasing System (OAPS), which is an active mirror kind of control system working in real time to maintain the figure of a segmented primary mirror without the use of edge-sensors. The alignment and phasing system which is an integral part of any segmented telescope can be used in the real time at the off-axis. Through extensive simulations we have explored the feasibility of using an OAPS for co-alignment, co-focusing as well as co-phasing of segmented mirror telescopes. From our simulations we find that the co-alignment and co-focusing of the segments can be achieved with a guide star as faint as 16-18th magnitude. This implies that seeing limited performance for any segmented telescope can be easily accomplished without use of a complex edge sensor based control system. Whereas, to attain diffraction limited performance, mirror segments need to be co-phased with an accuracy of few tens of nanometers. In our simulations we have used a dispersed fringe sensor based phasing scheme, which can effectively work up to guide stars of 14th magnitude.
The Prototype Segmented Mirror Telescope (PSMT) is a 1.3m segmented mirror telescope that aims to develop and demonstrate the segmented mirror technology indigenously. The telescope design includes a spherical primary with seven hexagonal segments of size 500mm each and an ellipsoidal secondary. Since the telescope has a spherical primary, hence it suffers from spherical aberrations as well as large off-axis aberrations, thus limiting its field of view. In order to improve the image quality over relatively larger field, an aberration corrector is required. The Faint Object Spectrograph and Camera (FOSC) is a widely used back-end instrument for any telescope 12 and PSMT is also supposed to be equipped with such an instrument. Therefore, we have designed the optics of the FOSC in such way that it meets dual requirements i.e. it works as a science instrument as well as an aberration corrector. The FOSC instrument consists of multi-element collimator and camera lenses and a grism is used as a dispersive element. The FOSC optics design is optimized for the visual wavelength range of 4500-8500A o and up to 10 arc-min field of view. Here, we present the optical design of the FOSC and outcome of the analysis carried out using the ZEMAX optical design software.
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