Developing and testing an optical alignment system for SALT's segmented primary mirror

Abstract

The design of the Southern African Large Telescope (SALT), which is based closely on the Hobby-Eberly Telescope (HET) at the University of Texas but includes advances incorporating lessons learned from HET, is briefly reviewed. The flowdown of requirements from the optical error budget to the primary mirror control subsystems is presented. The techniques and algorithms used by the Center of Curvature Alignment Sensor (CCAS) to measure segment tilt and piston and estimate the global radius of curvature of the primary are discussed in detail. The steps in the process that allows CCAS to capture and identify segments misaligned by more than 70 arcsec and bring them into alignment with residual errors less than 50milli-arcsec is fully described. Next, the hardware and software designs of CCAS are presented, as well as the results of laboratory performance testing. CCAS has been installed and integrated with the primary mirror control system. Performance results of the integrated system over a range of environmental conditions will be shown. Finally, the overall results of this project are summarized and suggestions for future improvements presented.

Development of an alignment device for the Prototype Segmented Mirror Telescope

“…16 In the SALT telescope, the large alignment error is handled by moving the SH sensor perpendicular to the optical axis for a raster scan. 2 Whereas, the newly built Japanese segmented mirror telescope SEIMEI makes use of 18 reference LEDs fixed on the inner wall of the dome and a deflectrometry camera installed at the Nasmyth focus to bring the alignment errors within AE30 arc-sec. 17 For the M1CS test bed and PSMT, we plan to use a semitransparent screen on which the segment images can be viewed and the alignment errors can be reduced to a value within the coarse measurement range of the segment alignment device.…”

“…However, due to the effects of dome seeing, vibration, and wind shake, only pistons with an accuracy of 50 to 60 μm could be measured. 2,20 The global alignment of a telescope, which includes the segmented primary as a whole and the secondary, is equally important for achieving the desired telescope image quality. The Keck and SALT telescopes use a single SH sensor for the segment as well as for the global alignment of telescope optics.…”

“…The Keck and SALT telescopes use a single SH sensor for the segment as well as for the global alignment of telescope optics. 2,6 Thus, using a similar approach, in addition to segment alignment, the PSMT-SAD may also be used for global tip-tilt and focus adjustment of the telescope.…”

“…Then, the star images formed by different segments were manually brought within a 0.5 arc-min diameter area so that further the alignment could be taken over by the APS device 16 . In the SALT telescope, the large alignment error is handled by moving the SH sensor perpendicular to the optical axis for a raster scan 2 . Whereas, the newly built Japanese segmented mirror telescope SEIMEI makes use of 18 reference LEDs fixed on the inner wall of the dome and a deflectrometry camera installed at the Nasmyth focus to bring the alignment errors within ±30 arc-sec 17 .…”

“…Various telescopes have made use of the SH sensor based alignment device for both coarse and fine alignment of the segments. Telescopes such as the SALT 2 and HET 3 make use of this device by placing it in the primary mirror ROC plane (where an artificial point source is used to illuminate the primary), whereas for the Keck telescopes, the alignment device is placed at the telescope focal plane, and natural star light is used for alignment. The telescopes working in the diffraction limited regime, such as the Keck and the upcoming TMT 4 and MSE, 5 uses an SH sensor based alignment device for measuring the segment figuring errors, which in turn are dynamically corrected by the warping harness.…”