Edge Sensors for the Schwarzschild-Couder Telescopes

David HannaMcGill University

UCLA AGIS MeetingJune 26-27, 2008

The Requirement:

facets on the S-C telescopes need to be positioned to a tolerance of the order of 10 microns

light-weight optical support structure will undergodeformations of at least this scale while tracking sources

a system of actuators to dynamically correct the facets’ positions and orientations is planned

a related system to measure the facets’ positions and orientations must be developed

Edge Sensors

to measure the displacement of one mirror facet with respect to its neighbour we plan to use a system of position-sensitive detectors (PSDs) illuminated by diode lasers or collimated LEDs.

relative motion of the two facets causes the position of the beam spot on the PSD to be displaced; the job of the actuatorsis to move the facets in order to put the spot back to itsnominal position

light sourcePSD

beam

Mirror 1 Mirror 2

for the PSD, we are evaluating a lateral-effect photodiodeof the pin-cushion type ( Hamamatsu S5991).

- electrodes on the edges of the silicon photodiodecollect charges produced by the illumination of the PSD.

- ratios of currents from electrodes on opposite sides are used to calculate the centroid of the light pool.

- thus the device is non-imaging.

- shaped electrodes increase linearity near the edges and produce the pin-cushion-shaped sensitive area.

figure from Circuit Cellar(www.circuitcellar.com)

Position Sensitive Detectors (PSDs)

15 mm

- this PSD model is available for $30 each in large quantities.

- resolution is 1.5 m

Hamamatsu S5991

Light Sources:

we have two options: laser diode and light-emitting diode (LED)

LED – blue(465 nm) Optek Technology ~ $0.70

(shown here with collimator, 1 mm diameter 10 mm long)

LASER diode- red (650 nm) BEA lasers ~ $60

The LED can be made bright enough for our purposes but the forward current(80 mA) is larger than the suggested maximum (20 mA)

Probably OK since - collimator acts as a very effective heat sink- we can use the LED in a low-duty-cycle mode (needs study)

Hamamatsu provides a circuit for reading out the PSD.

We have carried out tests on accuracy and stability using the setup shown at the right;

the collimated LED is attachedto an X-Y-Z translation devicewhich is used to make controlledscans across the PSD

- the blue spot below the PSDshows the size of the beam spot obtained using the simple collimator

System Evaluation

Scan in x and y

use micrometer translator to change laser spot position on PSD

compare PSD reading with translator position

repeat to test for reproducibility

Overnight Stability Test

leave LED on for 16 hours

record x, y and current

sunrise

+/- 10 um

Electronics Developments

single PSD circuit

op-amps turn currents into voltages for ADCsto digitize

4-PSD op-amp board

Present status and future plans

PSDs and op-amp circuits now at UCLA awaiting tests on mechanical setup

new circuit which puts ADC near the op-amp (ie send digital information over cables to improve noise rejection) to be developed this summer at McGill