FastCam optomechanical system design and

FastCam optomechanical system design and manufacture Gaizka Murga*a, Rubén Sanquircea, Ramón Campoa, Alex Oscozb, Roberto Lópezb, Rafael Rebolob a

IDOM, Av. Lehendakari Aguirre 3, 48014 Bilbao, Spain

b

Instituto de Astrofísica de Canarias, c/ Via Láctea s/n, 38205 La Laguna, Spain ABSTRACT

FastCam is an instrument jointly developed by the Instituto de Astrofísica de Canarias (IAC) and the Universidad Politécnica de Cartagena (UPCT), designed to obtain high spatial resolution images in the optical wavelength range from ground-based telescopes (http://www.iac.es/proyecto/fastcam and http://www.iac.es/telescopes/Manuales/manualfastcam.pdf). The instrument is equipped with a very low noise and very fast readout speed EMCCD camera which provides short exposure images to an FPGA-based processor which performs the selection, recenterg and combination of images in real-time (applying Lucky Imaging techniques) to provide diffraction limited resolution images in 1-4 m class telescopes from 500 to 1100 nm. IDOM has contributed to this new state-of-the-art instrument with the design of an optomechanical system conceived to maximize the image scale stability of the system for astrometry. The combination of aluminum plates, carbon fiber (CFRP) rods and stainless steel mounts in the optical bench defines an athermalized and stiff design to meet the requirements of thermal and mechanical stability. This work has been done with the support of the Aerospace Subprogramme of the Spanish Centre for the Development of Industrial Technology (CDTI) and the INTEK programme of the Basque Development Agency (SPRI). Keywords: FASTCAM, lucky imaging, stability, athermal, optical bench,

1.

INTRODUCTION

FASTCAM has been conceived as a high spatial resolution imaging camera for high accuracy astrometry in 1-4 m class telescopes. High resolution –diffraction limited images- is achieved by means of Lucky Imaging techniques [1] while the image scale stability is obtained by means of a mechanically stiff and athermalized optical bench. A FASTCAM prototype [2] has been successfully tested on the sky during tens of nights on several telescopes at the Canary Islands Observatories: 1.52m Telescopio Carlos Sánchez (TCS), 2.5m Nordic Optical Telescope (NOT), 4.2m William Herschel Telescope (WHT) and even the 10.4m Gran Telescopio de Canarias (GTC). The implementation described in the present paper is planned to be installed as a common user instrument in the Cassegrain focus of 1.52m Telescopio Carlos Sánchez (TCS) at the Observatorio del Teide (Canary Islands). These observations have confirmed the incredibly potential of the instrument which allows obtaining, regardless of the seeing conditions and without needing adaptive optics, extremely high resolution images. This prototype, although not finished yet, has already leaded to the first scientific results in different fields, such as brown dwarfs, exoplanets or black holes.

*[email protected]; phone +34 94 479 76 00; fax +34 94 476 18 04; www.idom.com

Ground-based and Airborne Instrumentation for Astronomy III, edited by Ian S. McLean, Suzanne K. Ramsay, Hideki Takami, Proc. of SPIE Vol. 7735, 77353E · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.856736 Proc. of SPIE Vol. 7735 77353E-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/26/2013 Terms of Use: http://spiedl.org/terms

2.

OPTICAL BENCH REQUIREMENTS

FASTCAM main optical system is composed by a collimator lens, a camera lens and an ultrasensitive low light EMCCD detector. Additionally, two filter wheels and a wide field camera feed by a folding mirror and the provision of an atmospheric dispersion corrector (ADC) are considered to complete the system. The main function of FastCam Optical Bench is to stiffly support all the aforementioned optical components enhancing the image scale stability during the observation. This stability requirement is formulated as less than 10μas distortion for a 5arcsec distance over a field of view of 10arcsec. Additionally the Optical Bench shall be compact, easy to maintain and shall provide features for the optical system alignment.

Figure 1 FastCam instrument 3D view.

3.

TOLERANCE ANALYSIS

The first step of the design process was to perform a tolerance analysis to determine the impact of the different sources of optical components displacement for the selection of the most suitable structural system. The effect of lens decenter, defocus and tilt on the image stability has been studied and the corresponding sensitivity determined by means of simulations with the software ZEMAX SE.

DET defocus DET defocus (μm) 144.818143 144.820143 144.828143 144.918143 145.818143 allow. displ.

0 2 10 100 1000 10.00

Image motion 0.00000 0.00000 0.00000 0.00000 0.00000

Image size 4.15897 4.15897 4.15897 4.15905 4.15976

Scale change Field increase

1.000000382 1.000001912 1.000019123 1.000191218

0.001911532 0.009557662 0.095612691 0.956090839 0.01

mas mas mas mas mas

Figure 2 Analysis of the impact of CCD detector defocus in image scale stability (part of the tolerance analysis)

Proc. of SPIE Vol. 7735 77353E-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/26/2013 Terms of Use: http://spiedl.org/terms

The effect of lens/detector defocus is highly mitigated by the telecentric design of the optical system. This way a displacement of 10microns in the detector is required to reach the defined stability limit. The effect of lens decenter causes a higher impact on the system stability (only 5micron displacement is required to reach the stability limit), while the system not highly affected by lens tilt. Considering this sensitivities the optical bench shall by stiff enough to minimize decenter caused by the structural flexion (when telescope points to zenith), but also thermally stable and stiff to minimize defocus caused by thermal expansion and axial loads (when telescope points to low elevation).

HA:γ

δA:θ

g

g declination angle: 30º hour angle: 0º

g

declination angle: 30º + θ hour angle: 0º

declination angle: 30º + θ hour angle: 0º + γ

Figure 3 Instrument orientation at different telescope declinations/hour angles.

4.

OPTICAL BENCH PROPOSED DESIGN

In order to increase thermal stability without adding a complex thermal control system an athermalized design composed by aluminum plates joined by means of adhesive-bonded carbon fiber reinforced plastic (CFRP) rods and stainless steel lens supports was designed. The combination of materials with different coefficient of thermal expansion and the appropriate selection of angles ensure that the relative positions of both lenses and the detector is constant no matter the temperature changes, while the stiffness provided by the CFRP rods limits lens decenter and defocus caused by mechanical deflections. CAM lens

intermediate aluminium plate

CAM lens DETECTOR

DETECTOR

independent of T increase

y x

z

non-deformed structure deformed structure

Figure 4 Athermalized configuration between camera lens and detector.


The aluminum plates are conceived to be manufactured in lathe and mill, the CFRP rods are commercially available pultruded rods and the stainless steel spiders for lens support are easily manufactured on lathe and electrical discharge cutting. The lens mounts were designed to be simple and compact, allowing a focusing range of 5mm with a pitch of 0.5mm in a backlash free arrangement locked by means of a screw with PTFE head. The lenses are fixed to the cell with a RTV sealing compound capable of compensating the difference between aluminum and glass coefficient of thermal expansion (CTE) difference. FOCUS LOCKING SCREW ON PTFE PAD LENS SUPPORT

LENS MOUNT FOCUS CELL

RTV SEALING COMPOUND is xxx with a syringe

LENS MOUNT

FIXTURE to center and hold the lens

WAVE SPRING to preload the focus thread

FOCUS THREAD M24x0.5 (±2.5mm)

Figure 5 Proposed design for the lens mount

Additional supports have been defined for the support of EMCCD camera, wide field camera, folding mirror and filter wheels. Regarding the control system, a hardware architecture compatible with the image acquisition system was proposed for the control of the different components: filter wheels, folding mirror, EMCCD and wide field cameras. The power supply system and the cable harness for power and control were also defined.

Figure 6 Power and control cable harness


5.

DESIGN VERIFICATION

So as to verify the optical bench stability performances a finite element model of the optical bench was created and the effect of mechanical and thermal loads analyzed. Differential deflections caused by gravity at different telescope orientations and residual thermal expansions were quantified for the worst case scenario during an observation. The effects of these deflections on scale stability was determined in ZEMAX, image scale stability was kept in all cases below 8μas complying with the image scale stability requirement (10 μas).

- 0.23 μm - 2.58 μm - 5.17 μm

Thermal analysis

g

gravitational direction perpendicular to optical path axis (x & z component equal to zero)

- 0.10 μm - 0.10 μm (