Cassegrain Focus Design

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Transcript Cassegrain Focus Design

Preliminary Foreoptics Design for
FASOT of 2nd Generation
L. Chang,
X.M Cheng
[email protected]
 Cassegrain Focus Design
 Coude Focus Design
1.Cassegrain Focus Design
Design Requirements:
Telescope Dia:800mm
Work Wavelength:400nm-1085nm
Sample size (sky):0.5 arcsec
FOV:First step is 0.5 arcmin;
Final:1 arcmin
Solar image
monitor system
Image Slicer
Optical Design Scheme of 2nd generation FASOT
1.Cassegrain Focus Design
FOV:9 arcmin
Made By
1.Cassegrain Focus Design
Folding Mirror
Focal Plane
SP Beam
Image Slicer System
UV Filter Cal. Wheel
IFU and
Polarization Splitter
Image Lens
PS Beam
Image Slicer System
2.Ideally, the sky image from the telescope focal plane should
be absolutely telecentric over the whole field of view.
1.Cassegrain Focus Design
Advantage of Telecentricity:
** “The magnified sky image on the microlens surface must be
telecentric in order to avoid pupil image shift on the microlens, which
may result in light loss that varies across the field.”
** “Magnification M is independent of the distance between the object
(here it is telescope focal plane) and the foreoptics. Conventional
optical systems produce images with higher magnification when the
object is closer to the lens. A telecentric system acts as if it has an
infinite focal length. An object moved from far away to near the optical
system goes into and out of sharp focus,but its image size is constant.”
Deqing Ren and Jeremy Allington-Smith,
Publications of the Astronomical Society of the Pacific, 114:866–878,
2002 August
1.Cassegrain Focus Design
FOV:1 arcmin
1.Cassegrain Focus Design
FOV :0.5 arcmin
1.Cassegrain Focus Design
Design Result:
Focal length of Collimator:45mm
Image size on telescope focal plane(0.5 arcsec): 23.27um
A Lenslet size:200um
Magnification from telescope focal surface
to image slicer/lenslet array:200/23.27=8.594
Focal length of image lens:45*8.594=386.73mm
F/# to Image slicer/Lenslet Surface:12*M=103.12
Fiber array in First Step:30”/0.5”=60
Fiber numbers:60*60=3,600 for SP Beam or PS Beam,Total:7,200
Final Step:60”/0.5=120
Fiber numbers:120*120=14,400 for SP Beam or PS Beam,Total:28,800
for 4 image slicers, each one for 3,600 fibers for SP Beam or PS Beam
1.Cassegrain Focus Design
Design Result:
 Fiber Core Dia: 60um
 F/# to fiber: f/7
 Lenslet focal length:1400um
 Pupil image on fibre core: 1400/(103.12*1.51) =8.95um
 F/# of fiber output: 1/ (2*(1/7)^2)^(1/2)=f/4.95
1.Cassegrain Focus Design
Spot Diagram
Maximal area error :4.84%
Minimum area error: 0.1%o
1.Cassegrain Focus Design
1.Cassegrain Focus Design
Equatorial mounting
2. Coude Focus Design
Equatorial mounting
Rotator: K mirror
Declination axis
Right Ascension axis
Optical Bench
1. Cassegrain Focus: fiber system following telescope to move,fiber may
be pulled, twistied……
may be Broken
2. Coude Focus Design:
**Optical Path is 4470mm,alignment is difficult
**Rotator precision must be higher
SP and PS Beam coupled to lens array
must be co-spatially aligned or
The End