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FEASIBILITY OF A WIDE-FIELD INSTRUMENT FOR THE NRO TELESCOPES

Erin Elliott, Sr. Astronomical Optics ScientistJohn MacKenty, WFC3 Team LeadSpace 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 telescopeSimilar to HST situationSilver mirror coating lower emissivity than HST

IR DetectorWFC3

○ 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 accommodatesCold 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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Mirror details Tertiary and quaternary are conics with a

coma aspheric term. Both are convex.

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 mFootprint plots (not to scale), showing beam position on the

mirrors, for 12 field points at the corners of the image plane.fo

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image

A 6 x 3 Pointing Mosaic with ErinCAM

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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.