The ESPRI project: astrometric exoplanet search with PRIMA - I. Instrument description and performance of first light observations

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Table C.4

PRIMA-VLTI control loops acting on the stellar beams.

Function	Sensor	Actuator	Bandwidth^f	FB^a	Band	#^e
			(Hz)

Telescope tracking (Alt.+Az.)	Encoders	Torque motors	~5	1	...	2
Telescope guiding (Alt.+Az.)	CCD	Torque motors	~5	1	V	2
Image stabilisation telescope	STRAP^b	M6	~5–10	1	V	2
Image stabilisation laboratory	IRIS	STS-FSM	~1	1	H	4
Lateral pupil stabilisation^c	Quadcell	STS-VCM	~1	1	1.3 μm	4
Primary fringe tracking	FSUB/FSUA	One delay line	~10	1	K	1
Secondary fringe tracking	FSUA/FSUB	One DDL	~5–10	1	K	1
Longitudinal pupil position^d	...	DL-VCM	...	0	...	1
Image position on STS-M10	...	Derotator	...	0	...	2
Telescope focus^g (manual)	IRIS	M2	...	0	H	2
Angle tracking^h (optional)	IRIS	FSU-ACU	~5	0	H	4

Notes.

^(a)

The flag indicates whether it is a feedback loop.

^(b)

Quadcell sensor based on avalanche photo diodes.

^(c)

The prima metrology beams are stabilised.

^(d)

A linear model depending on the carriage position is used to blindly adjust the mirror curvature.

^(e)

Number of instances.

^(f)

Lowest frequency of –3 dB gain loss.

^(g)

The telescope focussing is done manually by the operator.

^(h)

The angle tracking (IFG) consists of offloading the residual image offsets measured with IRIS onto the image actuators of the FSU (Sahlmann et al. 2009). It is usually not used during astrometric observations.

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