Engineering Test Results:

Calibration of the Effective Wavelength for the LGS-AO Tip-Tilt Sensor, STRAP

R. Campbell, P. Stomski

14 Sep 2004

  1. Introduction. The AO system corrects for differential atmospheric refraction, DAR, with the purpose of keeping the science object position in a particular location relative to the science instrument. The correction can be applied in a variety of ways depending on the mode of operation (P. Stomski, 2002). The inputs to the calculation include the telescope position and the effective wavelengths of the two detectors: the tip tilt sensor and the science detector. The science detector, which is in the infrared, is accurately determined from the instrument�s filter bandpass and is automatically updated when a filter is selected. The tip/tilt sensor is a visual wavelength broad band sensor and its effective wavelength can depend on the color of the guide star. The purpose of this engineering test is to calibrate the LGS-AO STRAP avalanche photo-diode (APD) tip-tilt sensor as a function of guide star color. The wavelength calibration is needed to achieve better than 10mas position accuracy for LGS-AO science performance, a specification determined by the adaptive optics working group, AOWG.
  2. Test. The test was comprised of observing guide stars with a variety of colors at a variety of air masses to empirically determine the effects of DAR. The observations were made with the tip-tilt stage, TSS, fixed and with the AO loops closed so that any relative shift on the science detector would be solely due to the effects of DAR. Please see the test procedure (Stomski 2004) for a detailed description of the test setup and procedure.
  3. Data. Data were acquired using NIRC2 with the AO setup in dual NGS mode on 2 Sept. 2004 (UT). K band observations were made at elevations of approximately 80�, 65�, 50� and 30� with stars that varied in spectral type from A0 to M0. The stars were chosen from the SAO catalog and thus should have well determined B-V indices of color. For each of the samples, centroids of the star where measured and offsets from a reference point were measured. The reference point is simply the data point at the highest elevation. Table 1 summarizes the data.

file

star

spec type

Vmag

B-V

Zentith angle

Centroid

angle error

Shift �amplitude arcsec

xCoord

yCoord

5

SAO 74309

A3

9.24

0.34

6.05

602.3

295.8

0.0

0.000

8

SAO 74149

G0

9.57

0.85

10.08

601.2

293.3

0.0

-0.025

10

SAO 73936

M0

8.64

1.48

14.73

599.2

289.2

0.0

-0.066

12

SAO 73419

A2

10.32

0.05

26.65

603.2

276.1

0.0

-0.197

13

SAO 73271

G5

8.96

0.58

29.68

602.9

276.8

0.0

-0.190

15

SAO 73385

K2

8.25

1.51

29.19

601.0

280.0

0.0

-0.158

17

SAO 93956

A2

8.70

0.10

40.75

603.8

266.9

0.0

-0.289

19

SAO 94023

F8

8.94

0.53

41.15

604.2

264.9

0.0

-0.309

20

SAO 94080

K5

8.27

1.73

40.83

602.4

272.4

0.4

-0.234

22

SAO 94852

A0

9.29

-0.01

55.01

591.8

234.8

-13.8

-0.624

27

SAO 94959

K7

8.85

1.87

53.68

580.1

259.0

-11.9

-0.429

29

SAO 95065

G0

9.52

0.41

53.20

594.3

240.9

-12.4

-0.555

                   

Table 1

A summary of STRAP effective wavelength test observations. Note that a rotator fault resulted in a rotation angle error in the last 4 images. Measured centroids and the relative shift from the effects of DAR are included in the table.


  1. Analysis. Theoretical DAR curves were calculated over a range of zenith angles using the formula from Filippenko (1982). The infrared wavelength used was based on the NIRC2 Kcont filter which is centered at 2.27 μm. The visual wavelength was adjusted to best fit the measured shift of the stars. The resulting best fit and measured data are plotted in Figure 1. The 3 DAR curves of Figure 1 vary in visual wavelength by approximately 0.1� μm. The star B-V colors vary by approximately 2.0 magnitudes and if we assume the DAR effect is a linear function of guide star color we get the following equation for effective wavelength of strap:

References

1. Filippenko, A. V., PASP vol. 94, Aug. 1982, p. 715-721

2. P. Stomski, D. Le Mignant, P. Wizinowich, R. Campbell, R. Goodrich:� Compensating for Differential Atmospheric Refraction in the Keck II AO system.� SPIE Proc. Volume 4839, 943-953 (2003).

3. P. Stomski, R. Campbell . STRAP DAR Test procedure