Goodman Laboratory Technical Report

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Nov 15, 2013 (3 years and 4 months ago)


Goodman Laboratory Technical Report

Optics for the PROMPT Mid
Infrared Camera

J.Christopher Clemens

UNC Physics and Astronomy

Chapel Hill, NC 27599


Scope of this Study

This report
presents a solution to two problems inherent in the mid
camera prop
osed for

The first

inadequate back
focal distance to

the camera

assembly, the secon
d is the absence of a good location

to place a cold stop for reducing
infrared background from the wa
rm telescope
. Both of

these problems can be addressed by introducin
g 1:1 re
imaging optics after the telescope
focal plane.

To achieve a simple and effective design, we applied the following a priori design

The design shall

incorporate only commercial off
shelf optics

To avoid outgassing and CTE mismat
ch issues, any optics in the Dewar must be
singlets or air spaced multiplets.

The spacing from Dewar wi
ndow to focal plane array shall

be as s
mall as possible
to minimize De
war volume and maximize hold time, except that adequate space
must be provided to a
ccommodate the vendor’s filter wheel

The design wavelength range

must be 0.9 to 1.9 microns, though in operation the
bandpass will be restricted to either Y

1.07 microns)
, J
1.33 microns)
or H

1.78 microns).

The optics shall degrade the n
ative image quality by no more than 0.3”
(approximately 10 microns) when their contribution is added in quadrature


scale shall be 1:1 within 10%

The total distortion at field corner shall not exceed 2%

The focal plane array is a 256x 256 pix
el device with 40 micron pixels


Baseline design parameters

After some unsuccessful experimentation with designs incorporating only a single lens
group and no subsequent powered elements in the Dewar, we began a series of models
using achromats and
Hastings triplets for the collimator, and a variety of positive singlets
for the camera lens.
The required short focal lengths makes image quality wit
h a singlet
camera problematic
a negative power field flattener is usually required in such systems
keep the image quality acceptable at the field corners. Moreover, the requirement to


use off
shelf component means the surface curvatures are not entirely free

Nonetheless, it has proved possible to meet o
ur basic specifications using

n IR
achromat collimator and a positive meniscus singlet camera. The
negative surface
partially addresses the field curvature, and the re
maining curvature

is minimized

allowing non
telecentric performance
, so that the path length for marginal bun
dles is
somewhat shorter.

This will introduce
scale changes under re
focus, and future
studies of
the thermal and mechanical sensitivities

should address whether the amount is
acceptable for the astrometric requirements of the camera, which were not

Figure one below shows the Zemax layout and op
tical ray trace for a design

that uses
Edmund IR achromat (P/N NT45
) to collimate and a positive meniscus from

JML Optical Industries, Inc.

) to re
image. In practice the
collimation is intentionally imperfect to avoid ghosts from the flat

window and the
filters. The telescope used in this exercise is a perfect paraxial lens with 16” diameter
and F/17.32. Subsequent studies should verify that including the actual te
prescription yields acceptable image quality, especially in the field corners where the R
field curvature may further diminish the image quality.

These same studies should
incorporate a fold lens in a location that allows appropriate mechanical
s for the

: Layout of the PROPMT mid
IR camera re
imaging optics

The Dewar window and filter
are shown as flat elements in arbitrary locations. Their positions can be changed as long as the order
of elements is preserved.


The spacing between the Lyot stop and the positive meniscus is probably not sufficient
in this design to accommodate the vendor’s filter wheel. This can be resolved either by
incorporating the lens into the filter wheel or by increasing the spacing betwe
en the stop
and this element. The latter solution will degrade image quality below the permitted
specifications in the corners of the field only, while making the design more telecentric.

Distortion will improve in such a design, and the image scale will

be closer to 1:1 than in
the current design, where there is a magnification of 1.075.

In neither design have we
considered the limits on chromatic aberration, or examined monochromatic image quality
through the 3 filters, so these should be part of the f
inal report.

If incorporating the lens
into the filt
er wheel is preferred

to increasing the stop
camera spacing
, then the filter
holders must be deep enough to accommodate the 4.3 mm thick lens in addition to the

Figure 2 shows the through fo
us spot diagram for the
proposed design, with
polychromatic spots everywhere smaller than ~40 microns rms. It is possible to refocus
to improve center field image quality at the expense of field edge. The combination of
seeing, Nyquist blur, and diffract

yield an image with
90.8 microns = 2.6"
. The worst
degradation in this design, when combined in quadrature, yields ~2.9” spots, within our
specification for image degradation.

Figure 2: Through focus spot diagram for the PROMPT mid

stem. The field
positions are center field at top, edge of field 4 down and field corner at bottom. Boxes are 2x2


Finally, figure 3 shows the total distortion and the encircled energy plots. Total
distortion is below the 2% specified, while EE80

is everywhere smaller than 2 pixels. In
the opinion of this designer, it is not possible to do any better without intensive design
effort, custom optics, or a more complicated lens system in the camera position.


Figure 3:
Field curvature, distort
ion and encircled energy for the PROMPT mid
IR re