Transcript feasibility of A wide-field instrument for the NRO telescopes

Erin Elliott, Sr. Astronomical Optics Scientist
John MacKenty, WFC3 Team Lead
Space Telescope Science Institute
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Optical Design by Erin Elliott

Instrument shown here is a proof-of-concept
design only!

Design issues that remain are engineering
challenges, NOT fundamental limitations.
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Instrument feasibility

The as-is configuration of the NRO
telescopes can support wide-field
instruments.

Physical geometry doesn’t preclude
fields of larger than ~2 degrees.

Adequate space for instrument packages
exists behind the primary mirror support
structure. (~ 2.4 m in dia x 1+ m).

Could extend downwards further.

On-axis wavefront performance could
potentially support an on-axis instrument
– 1.6 arc min FOV without tertiary mirror.

Wavefront error of telescope system is
reported as < 60 nm RMS. Will limit
performance at wavelengths < 600nm.
volume
potentially
available for
instruments
Layout of a telescope similar to the
delivered units.
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Proof-of-concept wide-field imager design

Instrument consists of
two folds, two
powered mirrors, and
a spherical corrector
plate.

Covers a field of view
of 0.375 square
degrees.

Optics occupy a
volume of 1.9 x 1.0 x
0.65 m (1.24 m^3).

Possible to include
two such instruments
(Note: Primary ID and OD are not to scale in this plot.)
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Layout details

Order of reflections: fold 1, fold
2, tertiary, pupil & spherical
corrector plate, quaternary,
image.

Provides an accessible pupil for
filters. (Currently 5.5 inches in
diameter.)

Image plane configured in three
squares, for good tiling efficiency.

(Each ray bundle shown
represents a different field point.)
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Additional views
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Infrared Instrument Considerations

NRO Telescope “use as is”
 Design assumes room temperature telescope
 Similar to HST situation
 Silver mirror coating  lower emissivity than HST

IR Detector
 WFC3
○ 1.7 mm cutoff at 145°K  dark <0.04 e-/s/pix
○ Zodi limited in broad filters (readnoise ~ 12e-)
○ Filters at -30°C
 NRO Telescope
○ Zodi limited ~2 mm cutoff  100-120°K detector
○ Filters must be ~ -<50 °C

Current design accommodates
 Cold enclosure for filters/corrector plate/cold stop
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Performance and image plane layout





Instrument performance contours of RMS WFE (assuming 0 WFE for OTA).
Total field of 0.375 degrees square.
27 4k x 4k arrays with 0.11 arcsec pixels.
10 micron pixels (Hawaii-4RG10) – larger FOV possible with 15 micron pixels
Bottom field region is inaccessible because of the beam clearances required for a
reflective system.
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
Corrector plate has a spherical aspheric
term.
 Plate is thin and unpowered, so doesn’t
introduce significant chromatic aberration.

Mirror sizes:






Fold 1, rectangular, 0.37 x 0.39 m
Fold 2, three-square config., ~ 0.38 x 025
Tertiary, rectangular, 0.58 x 0.38 m.
Pupil & corrector plate, circular, ~ 5.5 inches
Quaternary, rectangular, 0.4 x 0.26 m.
Image plane, three-sqr. config., 0.36 x 0.2 m
Footprint plots (not to scale), showing beam position on the
mirrors, for 12 field points at the corners of the image plane.
fold 1
fold 2
Tertiary and quaternary are conics with a
coma aspheric term. Both are convex.
tertiary

quaternary
Mirror details
image
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A 6 x 3 Pointing Mosaic with ErinCAM
Conclusion

The as-is configuration of the NRO telescopes can
support wide-field instruments with good image
quality.

The proof-of-concept design presented demonstrates
a FOV of over 0.375 square degrees in a single
instrument.

Thermal requirements for cooling of detectors and
optical elements and thermal stability of telescope
require careful trade for long wavelength cutoff.
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