Description: The FOBOS guide system is housed in the focal plane
module. Its purpose is to allow fine adjustment of target star alignment on
the science fiber, and to monitor guiding during exposures. The system path
within the focal plane module is highlighted in pink in the figure below.|
The guide fiber assembly consists of 42 1/16'' ferrules. Two sets of 6 ferrules are mounted in an array and viewed by a camera. Each of these sets surrounds a single science fiber in the telescope's focal plane. When a target star is slightly misaligned with the desired science fiber, some of the stellar light will be seen in one or more guide fibers, and therefore tell the observer in which direction the telescope needs to be moved to center the target star. Because a relatively large area must be imaged onto the guide system camera (SBIG STV), an optical system was desined to provide approximately ~3.3X demagnification of the guide fiber ferrules. The remaining 30 guide fibers are mounted out of the way. Their corresponding science fibers are used to collect sky background.
To minimize expense, 2 off-the-shelf Edmund Industrial Optics achromatic doublets were used for this design. The layout can be viewed here.
A spot diagram is available here.
A ZEMAX file is also available.
System/Prescription Data Title: Guide Fiber Monitoring System (2 achromats) Date : WED AUG 27 2003 LENS NOTES: This design uses Edmund Industrial Optics lenses and provides ~3.3X demagnification from the guide fiber mount plane to the STV CCD. GENERAL LENS DATA: Surfaces : 8 Stop : 4 System Aperture : Float By Stop Size = 11 Glass Catalogs : SCHOTT HOYA Ray Aiming : Off Apodization : Uniform, factor = 0.00000E+000 Effective Focal Length : 7184.62 (in air at system temperature and pressure) Effective Focal Length : 7184.62 (in image space) Back Focal Length : -2133.203 Total Track : 180.9378 Image Space F/# : 77.58252 Paraxial Working F/# : 1.362138 Working F/# : 1.320633 Image Space NA : 0.3445884 Object Space NA : 0.1094593 Stop Radius : -11 Paraxial Image Height : 3 Paraxial Magnification : -0.3 Entrance Pupil Diameter : 92.60617 Entrance Pupil Position : -510.8895 Exit Pupil Diameter : 27.30249 Exit Pupil Position : -35.93992 Field Type : Object height in Millimeters Maximum Field : 10 Primary Wave : 0.5875618 Lens Units : Millimeters Angular Magnification : -3.391858 Fields : 3 Field Type: Object height in Millimeters # X-Value Y-Value Weight 1 0.000000 0.000000 1.000000 2 0.000000 5.000000 1.000000 3 0.000000 10.000000 1.000000 Wavelengths : 3 Units: Microns # Value Weight 1 0.486133 1.000000 2 0.587562 1.000000 3 0.656273 1.000000 SURFACE DATA SUMMARY: Surf Type Comment Radius Thickness Glass Diameter Conic OBJ STANDARD Infinity 90.41489 20 0 1 STANDARD (-)EDSCI 45353 446.01 4 SF10 50 0 2 STANDARD 46.95 13.92 BAFN10 50 0 3 STANDARD -69.28 123.0444 50 0 STO STANDARD Infinity 5 22 0 5 STANDARD EDSCI 45211 21.17 11.04 BAFN10 25 0 6 STANDARD -16.08 3 SF10 25 0 7 STANDARD -118.66 20.93339 25 0 IMA STANDARD Infinity 6.414694 0