feasibility of A wide-field instrument for the NRO telescopes

1. Erin Elliott, Sr. Astronomical Optics Scientist John MacKenty, WFC3 Team Lead Space Telescope Science Institute feasibility of A wide-field instrument for the NRO telescopes

2. 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.

3. 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.

4. 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.)

5. 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.)

6. Additional views

7. 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

8. 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.

9. Mirror details fold 1 • 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 m fold 2 tertiary quaternary Footprint plots (not to scale), showing beam position on the mirrors, for 12 field points at the corners of the image plane. image

10. A 6 x 3 Pointing Mosaic with ErinCAM

11. 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.