Etalon 1052 Measurements




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Etalon 1052 Measurements
I. History
Etalon 1052 is the lowest resolution etalon for SALT PFIS. It provides single-etalon tunable filter and low resolution modes, and also serves as the order-selecting etalon in medium and high resolution modes. The required characteristics for the etalon were developed in consultation with the SALT Science Working Group and through the PDR process, with input from the vendor, ICOS, about the feasibility of the proposed specifications. The final specifications for the etalon are listed in Table 1. ICOS submitted a quotation for the system on December 12, 2001. The etalon was ordered on January 14, 2002.

Table 1 Low Resolution Etalon Specifications


clear aperture 150 mm

surface quality λ/100 at 650 nm

finesse approximately 30 at 650 nm

reflector coating 90% ± 4% from 430 nm to 860 nm

spectral resolution tunable from 500 to 1000 at 650 nm

(implied gap) 5.4 to 10.8 microns

gap tolerance ± 1 micron

glass reference capacitor

outer surface AR coating <1% from 430nm to 680 nm
During the construction of the etalon, we entered into an extended discussion with ICOS about possible re-design of the etalon end plates to minimize the size of the mounting fixtures, in view of the limited space available in the axial direction in the collimated beam in PFIS. In the end, ICOS confirmed that their original specification of the height of the etalon was in error, so no re-design was necessary. This process added significantly to the time required to manufacture the etalon. The etalon was essentially completed in November 2003, but one of the anti-reflection coatings developed a stain from an unknown cause. Figure 1 shows an image of the stained coating. After monitoring the evolution of this stain, ICOS determined that the AR coating had to be stripped off and the etalon re-coated. This was done, the etalon was re-assembled and tested, and shipped to Rutgers on March 9 2004.
When the etalon was inspected on receipt at Rutgers, we discovered that one of the rubber pads located between the top glass surface of the etalon and the metal end plate of the etalon housing had slipped out of position during shipment. After discussion with ICOS, it was determined that we should not attempt to repair this problem, and that an ICOS representative would travel to Rutgers to inspect and repair the etalon. Chris Pietraszewski visited Rutgers from March 23-25 with a re-designed end plate and new pad system to install on the etalon. On inspection, it was discovered that vibration during shipment after the pad had slipped out of position caused one of the optical bonds between a piezo element and the etalon plate to open. Figure 2 shows the disassembled etalon: the rubber pads are to the left of the etalon; the broken optical bond is indicated by the colored fringes of the piezo element near the 12 o’clock position on the etalon. The broken bond required shipping the etalon back to ICOS for repair. The repaired etalon was received at Rutgers on May 13 2004.



Figure 1 Etalon AR coating stain


Figure 2 Disassembled Etalon at Rutgers

II. Test Apparatus

The test station at Rutgers consists of a pair of 1200 mm focal length 150 mm aperture telescopes arranged as a collimator and camera. The etalons are mounted on a slide that positions them in the collimated beam. The light source is a 100 w quartz halogen lamp. The transmitted light is fed to an integrating sphere that illuminates the entrance slit of a 0.5m spectrograph, with three gratings that provide FWHM spectral resolution of 2.5 Å, 0.8 Å, and 0.4 Å over the wavelength range 4000 – 9000 Å. The detector on the spectrograph is an 1100 x 330 pixel thinned back-illuminated Site ST001 CCD. The detector is cooled to 55 C below ambient temperature and run in MPP mode to minimize dark current. Figure 3 shows the test set and Figure 4 shows etalon 1052 mounted in the test set.




Figure 3 Test Set



Figure 4 Etalon 1052 in test set

III. Test Results

Since the gap of the low resolution etalon is designed to be very small, the piezo actuators have enough range to change the gap by about a factor of two, appreciably varying the etalon’s spectral characteristics. We plan to operate the low resolution etalon in two modes, near its smallest and largest gap, and have thus tested it at both settings. The two settings are distinguished by the Z axis coarse setting on the controller, Zc +4 for the larger gap and Zc +1 for the smaller. We plan to name these operating modes TF (tunable filter) for the lower resolution, smaller gap and LR (low resolution) for the higher resolution, larger gap. The measured properties of the etalon in these two modes are presented in Tables 2 and 3.


Table 2 LR Mode Properties

Wave

FWHM

FSR

Finesse

Resolution

Gap

X offset

Y offset

(Å)

(Å)

(Å)







(μ)







4264.80

11.73

89.3

7.6

364

10.18

-30

-85

4354.12

6.21

93.1

15.0

701

10.18

-30

-85

4446.05

6.60

100.0

15.2

674

9.88

0

-110

4549.46

7.03

106.5

15.1

647

9.72

0

-110

4658.86

6.99

112.2

16.1

667

9.67

-10

-110

4773.68

6.12

117.6

19.2

780

9.69

-10

-110

4894.06

6.30

123.6

19.6

777

9.69

-10

-110

5020.79

6.77

130.1

19.2

742

9.69

-10

-110

5153.71

7.10

136.9

19.3

726

9.70

-10

-110

5297.43

7.06

143.8

20.4

750

9.76

-20

-95

5442.13

6.98

150.8

21.6

780

9.82

-10

-105

5598.96

7.51

158.4

21.1

746

9.90

-20

-95

5759.64

8.58

166.3

19.4

671

9.97

-15

-100

5932.34

9.58

173.6

18.1

619

10.14

-30

-90

6111.67

8.78

179.3

20.4

696

10.42

-40

-80

6298.29

7.91

181.7

23.0

796

10.92

-65

-60

6495.52

8.34

182.8

21.9

779

11.54

-110

-30

6677.60

8.75

182.3

20.8

763

12.23

-100

-5

6866.46

9.29

184.1

19.8

739

12.81

-125

20

7054.92

9.66

192.1

19.9

730

12.95

-135

35

7248.41

9.77

206.6

21.1

742

12.72

-125

15

7467.82

10.25

224.1

21.9

729

12.44

-125

5

7697.36

10.95

240.7

22.0

703

12.31

-120

-10

7948.84

11.98

256.0

21.4

664

12.34

-125

0

8210.64

12.87

272.5

21.2

638

12.37

-120

-5

8492.25

15.88

294.0

18.5

535

12.27

-120

-5

8798.63

18.19

322.1

17.7

484

12.02

-120

-5

9131.92

27.41

337.9

12.3

333

12.34

-110

-15


Table 3 TF Mode Properties

Wave

FWHM

FSR

Finesse

Resolution

Gap

X offset

Y offset

(Å)

(Å)

(Å)







(μ)







4408.75

15.95

232.7

14.6

276

4.18

-112

40

4641.44

18.23

248.1

13.6

255

4.34

-100

25

4905.00

16.63

280.4

16.9

295

4.29

-112

33

5202.18

18.88

311.6

16.5

276

4.34

-112

33

5528.12

19.12

344.1

18.0

289

4.44

-120

40

5890.43

23.53

374.4

15.9

250

4.63

-112

45

6277.01

19.73

383.8

19.5

318

5.13

-165

85

6658.00

20.33

350.4

17.2

327

6.33

-250

140

6977.83

18.74

328.3

17.5

372

7.42

-290

180

7314.61

18.47

362.8

19.6

396

7.37

-280

160

7703.46

20.93

420.6

20.1

368

7.05

-245

140

8155.90

25.14

479.3

19.1

324

6.94

-255

150

8662.14

30.93

506.2

16.4

280

7.41

-220

150


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