Design of a Compact Wide Field Telescope for Space Situational Awareness

1. Design of a Compact Wide Field Telescope for Space Situational Awareness David Lee, Andy Born, Philip Parr-Burman, Peter Hastings, and Naidu Bezawada UK Astronomy Technology Centre

2. Introduction • ESA Space Situational Awareness programme – network of ground and space based sensors and infrastructure. • ESA funded design study. • Consortium partners: Ground based telescope Space based telescope Scheduling simulator & ground based support infrastructure

3. Space Situational Awareness • Use a network of ground based sensors to: • Detect and identify man-made objects in orbit. • Characterisation and orbit determination. • Track near Earth objects. • Ground based sensors • Ground based optical telescopes (passive). • Ground based radar (active). • Some useful orbits are becoming crowded. • Collision warning, regulation, intelligence, etc.

4. Driving requirements • The telescope target requirements were informed by a previous ESA studies (PDAOSS). • They represent a starting point for this work. • The driving requirements are:

5. Telescope requirements

6. Telescope system characteristics • The driving requirements lead to the following telescope parameters:

7. Review of existing wide field telescopes • A review was performed to identify any existing telescopes that are close to the main telescope characteristics. • This survey concluded that there are no existing telescopes that satisfy the field of view and throughput required for the SSA application.

8. Trade study analysis • A number of trade studies were performed to analyse best choice of: • Telescope optical configuration. • Type of mount. • Type of enclosure. • Choice of detector.

9. Optical trade study • Compared performance parameters of various types of wide field telescope • Schmidt telescopes. • Three Mirror telescopes. • Unobscured Three Mirror Anastigmatic telescopes. • Two mirror telescopes with field corrector. • Prime focus telescope with field corrector.

10. Wide field telescope designs Two Mirror with field corrector Folded Schmidt Prime focus with corrector Three Mirror Anastigmat Unobscured TMA

11. Wide field telescope designs 0.8 m 1.6 m 1.1 m 1.0 m 2.2 m

12. Comparison of three designs • Comparison of • Three mirror design. • Unobscured TMA. • Folded Schmidt. • Drawn to same scale. • Folded Schmidt selected • Better optical performance. • Easier tolerances. • Good stray light performance.

13. Mechanical trade study • Choice of enclosure • Calotte style enclosure chosen (commercially available). • Type of mount • Altitude-Azimuth. • Equatorial. • Altitude-Altitude. • Types of shutter • ‘Bonn’ shutter chosen (commercially available).

14. Mount Trade Study • Three mount types were evaluated. • The Alt-Az was selected due to the advantages: • Compact and stiff. • Design independent of site latitude. • Disadvantages are: • Requires field derotation (addressed by camera rotation). • Existence of “blind spot” - axes must be moved rapidly to track objects moving past zenith (but blind spot is assessed as acceptable). Alt-Az Alt-Alt Equatorial

15. Detector trade study • Considered Charge Coupled Devices • Full frame CCDs. • Frame transfer CCDs. • Interline transfer CCDs. • Also considered CMOS sensors • Monolithic active pixel CMOS sensors. • Hybridised CMOS sensors.

16. Detector trade study • The following detector types were considered. All relevant requirements were assessed. The principal differences are: On the basis that a mechanical shutter is not an onerous requirement the full frame CCD was selected.

17. Wide field telescope – Optical design Corrector Lens 1 Corrector Lens 2 3.0 m Primary Mirror Fold Mirror Shutter 1.2 m Focal Plane Field Corrector Lens 2.1 m

18. Mechanical design – Telescope assembly • Principal Design Features: • Folded Schmidt Optical Design. • CCD based Camera. • Mechanical shutter to control exposure and allow low noise readout. • Rotating camera assembly to remove effect of sky rotation during sidereal tracking.

19. Altitude-Azimuth Drive: Compact. Stiff. Well-understood by potential manufacturers. But requires rotating camera. Mechanical design – Telescope Mount

20. Mechanical design – Camera subsystem • Camera Features: • Cooled, rotating assembly. • Non rotating parts: • Closed Cycle Cooler. • Rotating parts • Detector. • Focus mechanism. • Shutter (not shown – rotates with detector to ensure accurate epoch registration).

21. System performance • Performance does not quite meet the specification, even with the idealised assumption of all energy collected in one pixel. • Following this analysis a model was developed to estimate performance given different streak geometries.

22. System performance • This analysis illustrates performance given a sky background of 22 mag/arcsec2 (typical for La Palma). Exposure time is 0.07 sec. • 17th magnitude objects are detected with a SNR of >3 for most streaks and with a SNR of 5 for some streaks.

23. System performance improvement • The analysis repeated with 2 x 2 binning. • On-chip binning results in a significant detection performance improvement due to reduced level and reduced effect of readout noise. • This is at the expense of reduced angular resolution and spatial discrimination in survey mode

24. Summary & Conclusions • A telescope design has been proposed that is a step beyond what has been manufactured to date, but is: • Feasible within existing manufacturing facilities. • Uses a detector that is available. • The cost has been estimated. It is subject to a wide range of estimated costs for the optics. • This is due to the challenges imposed by the large F/1.3 design. • The system performance does not quite meet the requirements defined at the start of the study, but.. • Methods to improve detection performance are proposed.

25. Contact information • Thank you for listening • David Lee e-mail: david.lee@stfc.ac.uk