Cornell University
Memo:
TM-FOR03-66
Subject:
Warm Optics Testing: Folded Collimator-Camera System at 0.633
microns
Distribution:
FORCAST Team
Authors:
Marc Berthoud, Luke Keller and
Mansi Kasliwal
FORCAST
Date:
20-November-2003
Version:
1.1
Orig.
Date:
20-November-2003
Posted:
Summary
We have tested the FORCAST mirror assembly using a single (f/20) laser beam at the
center of the SOFIA focal plane. The measured spot FWHM is about 20 m at the location of the FORCAST focal plane, which is consistent with our ray tracing predictions.
Figure 1. Warm, folded optics test set-up. The set-up on the right creates a (f/20) beam focused on
the SOFIA focal plane. The FORCAST optical assembly is the box on the left.
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Introduction
Testing of the FORCAST optical system has continued with warm testing of the folded
collimator-camera system with HeNe laser light ( = 0.6328 m). We are using a digital
camera (Olympus MIC-D digital microscope) to re-image and measure the point spread
function (PSF) at the FORCAST focal plane. The tests that we describe here, the first in a
series, are for the PSF at the center of the FORCAST field of view only.
Test Set-up
We simulate the (f/20) SOFIA telescope beam with a HeNe laser and laser beam expander (Figure 1). Two adjustable mirrors (Mirrors 2 and 3) allow alignment of the beam
with the FORCAST optical assembly. Neutral density filters are used to dim the laser.
All mirrors used inside the FORCAST assembly are actual FORCAST diamond turned
aluminum optics, except the dichroic where a flat, adjustable mirror is used instead. We
use an adaptor flange to attach the digital microscope assembly to the FORCAST camera
block at the nominal focal plane position.
Test Detector Array Format
The digital camera is 640 x 480 x 8.4 m pixels. We set the microscope to maximal magnification where the array covers approximately 0.61 x 0.46 mm, or 12 x 9 pixels of the
FORCAST BIB array (0.05 x 0.05 mm per pixel). The test array over-samples the FORCAST pixel scale by a factor of ~50 at this magnification and the FORCAST predicted
PSF by a factor of ~100.
Figure 2: Microscope Image of Calibration Lines. The microscope is at highest magnification. The
distance of 223 pixels/line was determined by autocorrelation. Dark lines are laser printed at 1200
dpi.
Re-imaging Optics
The Olympus MIC-D microscope objective lens system, for which we do not have an optical prescription and therefore cannot model, re-images the FORCAST focal plane with
adjustable magnification. The near-field operation of the microscope allows us to focus
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on the position of the FORCAST focal plane. However, the optical axis of the microscope does NOT coincide with the FORCAST optical axis since the FORCAST focal
plane is tilted with respect to its axis to minimize distortion. The result is that, although
the microscope can be focused and its focal plane is parallel to the FORCAST focal
plane, the microscope zoom walks the image out of the field of view fairly quickly and
the final image is astigmatic when out of focus. This astigmatism is due to the misalignment of the FORCAST camera and microscope and is NOT intrinsic to the FORCAST
optics. We have set the microscope to maximal magnification for the tests and do not adjust zoom during our tests. To determine the pixel scale of the microscope we imaged laser printed lines 0.212 mm apart as shown in Figure 2 at the magnification we used.
Test Procedure
We image the PSF after focusing and co-aligning the imaging camera and FORCAST
optics.
Focus and Alignment Procedure
Before mounting the microscope onto the FORCAST camera block we mark two positions on the microscope focusing knob: for the mounted microscope to focus onto the
nominal FORCAST focal plane and for it to focus 0.066” towards the collimator (below
focus. This distance was chosen because of our particular setup.
To obtain the (f/20) beam we adjust the positions of mirrors 2 and 3 to obtain the correct
beam travel length. A screen at the position of the SOFIA focal plane allows focusing the
beam expander. Alignment is performed by temporarily placing pinhole masks at the location of the SOFIA focal plane and in front of the FORCAST collimator mirror. Mirrors
2 and 3 are adjusted to align the beam so it passes through both pinholes.
After mounting the microscope we align the beam to the center of the image using the
adjustable mirror that replaces the dichroic mirror. The beam we study is therefore not
exactly at the center of the FORCAST focal plane.
Data Acquisition and Analysis
We take images of the central spot with the microscope focused 0.066” below, 0.033”
below, at, 0.033” above and 0.066” above the FORCAST focal plane. The views are recorded as JPEG images transferred to an IDL procedure that fits a two-dimensional gaussian profile to determine the widths in x and y direction. The pixel scale described in the
previous section allows to find FWHM (full width at half-maximum).
A word about colors
To investigate the response of the microscope CCD to saturation images with various filters were taken. All images were taken with the shortest exposure time for the microscope. As the pixels get saturated signal starts to leak from the red channel into the green
and the blue one and the peak has an abnormal horizontal surface at the top. To avoid
saturation we used fairly dark images, such that the maximum in the red image was about
half the possible maximum.
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FORCAST
Test Results
-0.066”
-0.033”
+0.033”
+0.066”
In Focus
Figure 3: The images of the laser beam at the FORCAST field center for different focus positions.
The first image is from the lowest focus position. The images show the red channel of the images
taken with the microscope and have been scaled to use the full greyscale range. Each image shows a
field of 610m wide and 460m high.
Figure 4. Detail of the “in focus” laser spot at the center of the FORCAST focal plane. The square
grid indicates a 2 x 2 array of FORCAST detector pixels.
Figure 3 shows the images of the central spot taken with the microscope. As expected
the spot shifts in the field of view due to the misalignment between the beam direction
and the optical axis of the microscope. The horizontal line pattern is due to diffraction
from the diamond tool marks in the FORCAST camera mirrors. Dimmer diffraction lines
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are visible above and below the main spots as can be seen particularly in the last image.
At the lowest FORCAST wavelength, this effect will be attenuated by at least an order of
magnitude. It is unclear what causes the extended signal to the upper right of the main
spot in the last image. It may be internal reflections within the microscope. Of importance is that for most of the images almost all the signal is close to the laser spot. A
close-up of the “in focus” image is shown in Figure 4. Again the diffraction streaks are
seen above and below the maximum.
Figure 5: Horizontal cross-section of the central spot shown in Figure 4.
Figures 5 and 6 show cross sections of the laser spot at the “in focus” position. The horizontal cross section looks not unlike a gaussian but the vertical cross section clearly
shows the interference pattern from the mirror toolmarks.
The results of the fits are shown in Table 1. Over the range of foci that we measured the
size of the spot varies by about a factor 2. As expected the spots closer to the “in focus”
position are smaller. The narrowest spot is when the focus is 0.033” below the nominal
FORCAST focal plane. The narrowest spot size is about 20m or 2/5 of a FORCAST
pixel.
Due to the fringe pattern the widths in vertical (y) direction are less indicative of optical
properties of the telescope and may be ignored.
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Figure 6: Vertical cross-section of the central spot shown in Figure 4.
Focus
0.066” above
0.033” above
In focus
0.033” below
0.066” below
Filter Peak Value
(intensity)
31
94.9
34
74.4
38
57.2
38
95.4
38
52.7
Xcenter
(pix)
114
199
306
429
540
Ycenter
(pix)
320
300
278
239
193
x
(pix)
17.6
13.8
10.8
8.8
10.8
y
(pix)
8.9
8.4
8.8
8.3
10.8
FWHMx
(m)
39.5
30.9
24.2
19.7
24.2
FWHMy
(m)
20.0
18.8
19.7
18.6
24.2
Table 1: Fitting results for the central spots. The first column indicates the focus position of the microscope relative to the FORCAST focal plane. The following columns indicate the filter values used
for the images, the maximum value for the fitted gaussian and its location in the image The last 4
columns show the width of the fitted gaussians in microscope pixels and the FWHM in m.
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Comparison Predicted PSF
Figure 7: Simulated spot diagram of the central beam onto the FORCAST focal plane. The simulation was done using the ZEEMAX ray tracing program.
Figure 7 shows the results of the ray-trace analysis using ZEEMAX. The simulation includes the SOFIA telescope. The rms radius is about 20m. For a Gaussian distribution
with σx=σy the FWHM is 2*sqrt(ln(2))=1.665 the rms radius. The simulations would imply a FWHM of 30μm. Smaller aberrations and better focusing of our laser beam compared to a beam created by the SOFIA telescope are probably responsible for the better
quality of our measurements.
Conclusions:
On-axis warm testing of the folded FORCAST optical system using visible light indicates
that the performance exceeds that required to deliver diffraction limited images down to
7.5m.
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Supporting Files
File
Description
lwc_FINAL.ZMX FORCAST optical system final design (ZEMAX)
Revision History
Version
Date
Comments/Changes
1.0
31-October-2003
First release
1.1
4-November-2003
Checked version
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