Document web address: http://andromeda.roque.ing.iac.es/~af2/af2_tech_note_1.htm
Version: 1.0
Author: Ian J. Lewis
Date: 08/1996
1.0 Introduction
2.0 Gripper optics
3.0 Sky viewing probe optics
4.0 Coherent bundles
5.0 CCD autoguider interface
optics
6.0 Westinghouse TV
interface optics
7.0 Final notes
Abbreviations
Melles griot - MG
Spindler and Hoyer - SH
This technical note is intended to provide information about the various optical systems within the Autofib-2 robotic fibre positioner system. The layout of the various optical systems may be more exactly determined by examining the Autocad R12 drawing set although it should be noted that these drawings were made before the optical elements were aligned. However no significant changes have thus far been made.
The optical systems may be slit into 4 sections, the gripper optics
(fibre viewing system), the sky viewing probe optics, the CCD autoguider
interface optics and the Westinghouse tv interface optics. Information
is also given on the coherent bundles used within Autofib-2.
The gripper optic system is used to feed a magnified image of the back illuminated fibre core to a video tv camera mounted on the top of the gripper unit.
A triplet lens is used to relay the image of the fibre core to an accessible point through the jaws and centre of the gripper body. A folding prism with internal silver coating is then used to fold the light through 90 before a magnifying lens projects the magnified image onto the video CCD.
Transfer lens MG 01 LAT 015 f =
30mm = 17mm
Folding prism MG 01 PRA 011 12.712.712.7mm
Magnifying lens MG 01 LAO 001 f = 10mm = 6mm
The sky viewing probe optics are based on P. Ellis’ original plans for the PFIP autoguider probe to avoid fringing effects between the CCD pixels and the individual fibres in the coherent bundle.
There are two sky viewing probes which are optically identical, one is mounted on the XY carriage the second is mounted on a radial probe and may access only the edge of the prime focus field.
The incoming f/2.8 beam is folded by a 45 front silvered mirror. The f/2.8 focal plane is then reimaged at f/7.4 onto the input face of the coherent bundle using a triplet relay lens which gives a magnification of about 2.65.
Folding mirror SH 34 00 57 15202mm
Relay lens MG 01 LAT
013 f = 20mm = 12.5mm (u = 27.6mm v = 73mm)
Two coherent optical fibre bundles are used inside Autofib-2 to relay the image from the sky viewing probes to the CCD autoguider. The coherent bundles were manufactured by Dolan-Jenner and supplied by Oriel Scientific.
The coherent bundle from the mobile sky viewing probe is approximately 96 inches long with a throughput of about 41%, the coherent bundle from the off axis probe is approximately 36 inches long with a throughput of about 45%.
Each coherent bundle has entrance and exit apertures of 55mm consisting of 10m optical fibres. The lengths of the coherent bundles are 36 inches (off axis probe) and 96 inches (mobile probe).
Mobile sky viewing probe Dolan-Jenner ISO596
Off axis sky viewing probe Dolan-Jenner ISO536
5.0 CCD autoguider interface optics
The CCD autoguider interface optical system has two functions, firstly to relay the output face of the two coherent bundles onto the CCD and secondly to optically place the output faces of the two coherent bundles adjacent to each other on the same CCD. The latter is not possible mechanically as the stainless steel terminations of the coherent bundles are too large.
The off axis probe coherent bundle is reimaged directly onto the CCD autoguider by a pair of achromat lenses. Note that this lens group is not symmetrical, the lens holder has a ridge at one end to aid identification (the orientation should be noted on disassembly).
lens closest to coherent bundle MG 01 LAO 124 f = 100mm
= 30mm
lens closest to
CCD MG 01 LAO 079 f = 60mm = 30mm
The mobile sky viewing probe coherent bundle is reimaged with no magnification by a triplet lens and two folding flat mirrors so that the final image is optically adjacent to the of axis probe coherent bundle. The last mirror in the sequence is a special thin mirror to get the images as close together as possible without vignetting. This image is then projected onto the CCD using the same achromats as for the off axis probe.
triplet relay lens MG 01 LAT 015/66 f = 30mm = 17mm
mirror
SH 34 00 57 20152mm
thin mirror
MG 01 MFG 007 25251mm
The overall magnification of the CCD interface lenses for both coherent
bundles is 0.6
which means that the 105mm image of the coherent bundles are demagnified
to 63mm on the CCD. This does not allow for the small gap between the coherent
bundles which in reality is necessary.
6.0 Westinghouse TV interface optics
Two Canon 50mm camera lenses are used to provide 1:1 imaging over a wide field in order to project the guide fibres onto the detecting area of the Westinghouse TV unit. Slight changes of magnification are possible by moving or reversing one of the lenses and refocussing the system.
There appears to be a difference in the depth of the camera detector
behind the mounting face of the various Westinghouse tv units which means
that the interface unit is set up for one particular camera unit.
All distances and magnifications given are theoretical, in practice the optics were assembled and focussed in sequence to ensure that all the optical systems were parfocal. The technical manual contains more details on this procedure.
The technical manual also contains some layout diagrams of the optics, if the original Autovcad versions of these diagrams (or the original sub assembly drawings) are examined they can be zoomed to show more detail than is possible in an A4 diagram.